GSM ARCHITECTURE

Section 1 – GSM Architecture Overview

GSM Architecture Overview


Introduction

It provides an overview of the GSM network architecture. This
includes a brief explanation of the different network subsystems
and a description of the functionality of the elements within
each of the subsystems. Topics include:
• General architecture overview
• The Mobile Station (MS) Subsystem and Elements
• The Base Station Subsystem (BSS) and Elements
• The Network Subsystem (NSS) and Elements
• Introduction to network interfaces


A GSM network is made up of three subsystems:
• The Mobile Station (MS)
• The Base Station Sub-system (BSS) – comprising a BSC and
several BTSs
• The Network and Switching Sub-system (NSS) – comprising an
MSC and associated registers

The interfaces defined between each of these sub systems include:
• 'A' interface between NSS and BSS
• 'Abis' interface between BSC and BTS (within the BSS)
• 'Um' air interface between the BSS and the MS


Abbreviations:
MSC – Mobile Switching Center
BSS – Base Station Sub-system
BSC – Base Station Controller
HLR – Home Location Register
BTS – Base Transceiver Station
VLR – Visitor Location Register
TRX – Transceiver
AuC – Authentication Center
MS – Mobile Station
EIR – Equipment Identity Register
OMC – Operations and Maintenance Center
PSTN – Public Switched Telephone Network


Mobile Station
The Mobile Station (MS) consists of the physical equipment used by a
PLMN subscriber to connect to the network. It comprises the Mobile
Equipment (ME) and the Subscriber Identity Module (SIM). The ME
forms part of the Mobile Termination (MT) which, depending on the
application and services, may also include various types of Terminal
Equipment (TE) and associated Terminal Adapter (TA).


• The IMSI identifies the subscriber within the GSM network while
the MS ISDN is the actual telephone number a caller (possibly in
another network) uses to reach that person.

• Security is provided by the use of an authentication key and by
the transmission of a temporary subscriber identity (TMSI)
across the radio interface where possible to avoid using the
permanent IMSI identity.

• The IMEI may be used to block certain types of equipment from
accessing the network if they are unsuitable and also to check
for stolen equipment.


MS and SIM


The mobile station consists of :
• mobile equipment (ME)
• subscriber identity module (SIM)

The SIM stores permanent and temporary data about the mobile,
the subscriber and the network, including :
• The International Mobile Subscribers Identity (IMSI)
• MS ISDN number of subscriber
• Authentication key (Ki) and algorithms for authentication check

The mobile equipment has a unique International Mobile
Equipment Identity (IMEI), which is used by the EIR


Base Station Subsystem (BSS)


The BSS comprises:
• Base Station Controller (BSC)
• One or more Base Transceiver Stations (BTSs)

The purpose of the BTS is to:
• provide radio access to the mobile stations
• manage the radio access aspects of the system

BTS contains:
• Radio Transmitter/Receiver (TRX)
• Signal processing and control equipment
• Antennas and feeder cables


The BSC:
• allocates a channel for the duration of a call
• maintains the call:
monitors quality
controls the power transmitted by the BTS or MS
generates a handover to another cell when required


Network Switching System (NSS)

The NSS combines the call routing switches (MSCs and GMSC)
with database registers required to keep track of subscribers’
movements and use of the system. Call routing between MSCs is
taken via existing PSTN or ISDN networks. Signaling between
the registers uses Signaling System No. 7 protocol.


Functions of the MSC:

• Switching calls, controlling calls and logging calls
• Interface with PSTN, ISDN, PSPDN
• Mobility management over the radio network and other networks
• Radio Resource management - handovers between BSCs
• Billing Information


Interfaces

Um

VLR
Abis
A
BSC MSC
ISDN,
TUP


Exercise

Q1. Name the interfaces used between
Mobile and BTS
BTS and BSC
BSC and MSC

Section 2 – Access
Network

Access Network

Network

Objective

The Trainee will be able to understand:

• Different BTS configuration commonly used in the network
• Advantages of the configuration and optimal use of the trunks
• Abis mapping

Network

Introduction

Access network is a connection between MS and NSS, comprise of
BTSs & BSCs. It is responsible for radio management.

BSC looks towards MSC through single A-interface as being the
entity responsible for communicating with Mobile Stations in a
certain area. The radio equipment of a BSS may support one or
more cells.

A BSS may consist of one or more base stations, where an A-bis-
interface is implemented.

Network

BSS Configuration

• Collocated BTS
• Remote BTS
• Daisy Chain BTS
• Star Configuration
• Loop Configuration

Network

Collocated BTS: BTS is situated along with BSC or the MSC and no
additional E1 link is required.

BTS

BSC

Network

Remote BTS : BTS is situated in a stand alone position and additional E1
links are required to connect to BSC.

BSC
BTS

Network

Daisy Chain

BTS 3

BTS 1 BTS 4

BSC
BTS 2
MSC

Network

Star Configuration

BTS 3

BTS 1

BSC
BTS 4

BTS 2
MSC

Network

Loop Configuration

BTS 3

BTS 1

BTS 4

BSC

BTS 2 MSC

Network

Comparison of Different Configurations

• Daisy Chain: Easy to implement, effective utilization of
transmission links but if one of the link fails, all the BTSs
connected in the chain will went off.
• Star Configuration: Easy to implement but poor utilization of
links. Each BTS require one E1 to connect to BSC. But if link
goes down only individual BTS will be affected.
• Loop Configuration: Slightly difficult to implement but
effective utilization of E1 links. Even if one link goes off BTS will
continue to communicate with the network from the other side.

Network

BSS Interfaces

• Air Interface: Radio Interface between the BTS and
Mobile the supports frequency hopping and
diversity.

• A Interface: Interface carried by a 2-Mb link between
NSS and BSS. At this interface level,
transcoding takes place.

• OMC Interface: X25 Link.

Network

Network

Abis Interface (BTS - BSC)

If the BTS and BSC are not combined, A-bis interface will be used.
Otherwise, BS interface will be used. Several frame unit
channels are multiplexed on the same PCM support and BSC
and BTS can be remote from each other. Its main functions are:
• Conversion of 260 – bit encoded blocks (corresponding to 160x8
– bit samples for 20ms)
• Encoded block synchronization
• Vocal activity detection
• Alarm dispatch to BSC via PCM
• Test loop back operation

Network

TRX 1

TRX 2

Network

Exercise

Q1. In How many ways BTSs can be connected and which
configuration gives the optimal solution?

Q2. What is a difference between BS interface and Abis interface?

Q3. How many time slots are occupied by 1TRX on a PCM frame?

Section 3 – NSS Topology

NSS Topology


Objective


• Terminology used in Network Sub System
• Protocols and Interfaces inside NSS
• Call routing and circuit groups
• Switching modules
• Stand alone and integrated HLR
• Echo canceller and TRAU location
• Authentication, Ciphering, OMC, Billing center
• Transit Switch


Introduction

Network Sub System can be considered as a heart of the GSM
Network. All the major activities like switching of calls, routing,
security functions, call handling, charging, operation &
maintenance, handover decisions, takes place within the
entities of NSS.

Various kinds of interfaces are used to communicate between the
different entities. Different methods are used to optimize and
provide the quality network with the minimum operating cost.


Network Switching System (NSS)

Key elements of the NSS:
• Mobile Switching Center (MSC)
• Visitor Location Register (VLR)
• Home Location Register (HLR)
• Authentication Center (AuC)
• Equipment Identity Register (EIR)
• Gateway MSC (GMSC)

These elements are interconnected by
means of an SS7 network


NSS Identifier
IMEI – International Mobile Equipment Identifier.

The IMEI is an internationally-unique serial number allocated to the
MS hardware at the time of manufacture. It is registered by the
network operator and (optionally) stored in the AuC for validation
purposes.
IMEI = TAC + FAC + SNR +sp
TAC = Type Approval Code by central GSM body
FAC = Final Assembly Code, identifies the manufacturer
SNR = Serial Number, unique six digit number
sp = spare for future use


IMSI – International Mobile Subscriber Identifier

When a subscriber registers with a network operator, a unique
subscriber IMSI identifier is issued and stored in the SIM of the
MS as well as in the HLR . An MS can only function fully if it is
operated with a valid SIM inserted into an MS with a valid IMEI.
IMSI consist of three parts:
IMSI = MCC + MNC + MSIN
MCC = Mobile Country Code
MNC = Mobile Network Code
MSIN = Mobile Station Identification Number


TMSI –Temporary Mobile Subscriber Identity

A TMSI is used to protect the true identity (IMSI) of a subscriber. It
is issued by and stored within a VLR (not in the HLR) when an
IMSI attach takes place or a Location Area (LA) update takes
place. At the MS it is stored in the MS’s SIM. The issued TMSI
only has validity within a specific LA.

Since TMSI has local significance, the structure may be chosen by
the administration. It should not be more than four octets.


MSISDN – Mobile Station ISDN Number

The MSISDN represents the ‘true’ or ‘dialled’ number associated
with the subscriber. It is assigned to the subscriber by the
network operator at registration and is stored in the SIM.

According to the CCITT recommendations, it is composed in the
following way:
MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number


MSRN – Mobile Station Roaming Number

The MSRN is a temporary, location-dependant ISDN number
issued by the parent VLR to all MSs within its area of
responsibility. It is stored in the VLR and associated HLR but not
in the MS. The MSRN is used by the VLR associated MSC for
call routing within the MSC/VLR service area.


LAI – Location Area Identity

Each Location Area within the PLMN has an associated
internationally unique identifier (LAI). The LAI is broadcast
regularly by BTSs on the Broadcast Control channel (BCCH),
thus uniquely identifying each cell with
an associated LA.
LAI = MCC + MNC + LAC
MCC = Mobile Country Code, same as in IMSI
MNC = Mobile Network Code, same as in IMSI
LAC = Location Area Code, identifies a location area within a
GSM PLMN network. Maximum length of LAC is 16 bits.


Mobile Switching Center (MSC)

The Mobile services Switching Center (MSC) performs the
telephony switching functions of the system. It also controls calls
to and from other telephony and data systems, such as the
Public Switched Telephone Network (PSTN) and Public Land
Mobile Network (PLMN).

Difference between a MSC and an exchange in a fixed network is -
MSC has to take into account the impact of the allocation of
radio resources and the mobile nature of the subscribers and
has to perform in addition, at least the following procedures:


• required for location registration
• procedures required for handover

An MSC can be connected to only one VLR. Therefore, all mobile
stations that move around under base stations connected to the
MSC are always managed by the same VLR.

An MSC would communicate typically with one EIR. While it is
possible for an MSC to communicate to multiple EIRs, this is
highly unlikely since the EIR provides a centralized and
geographic independent function.


The MSC consults an HLR to determine how a call should be
routed to a given mobile station:
• For incoming calls to a mobile station, the MSC would typically
consult one HLR.
• For mobile-to-mobile calls in larger networks, a MSC could
consult HLRs of other systems to help minimize the trunk paths
to the other mobile station.

A given MSC can be interconnected to other MSCs to support
inter-MSC handovers


The following are typical MSC functions in a cellular system:

• Provide switched connections with PSTN
• Provide switched connections between mobile subscribers
• Provide coordination over signaling with mobiles
• Coordinate the location and handover process
• Provide custom services to mobile users
• Collect billing data


Protocols

MSC/BSC MSC/HLR OMC/MSC MSC/Fixed Network
MSC/VLR OMC/HLR
MSC/EIR OMC/VLR MSC/Voice
messaging
MSC/GMSC OMC/BSS
VLR/VLR
VLR/HLR
MSC/MSC
BSSMAP TCAP+MAP X.225 R2, ISUP other
Signaling
SCCP SCCP X.224

MTP MTP X.25 MTP

SS7 SS7


Switching In MSC

Signaling network is separated from the speech network and
consists of

• signaling Links (SL)
• signaling Point (SP)
• signaling Transfer Part (STP).


Telephony system contains:

• Group Switch to switch the calls,
• ST to perform signaling in accordance with SS7
• Trunk interfaces for interfacing the PCM.

Group switch provides a semi permanent connection between time
slot (PCM) and ST.


Signaling Point (SP)

SP provides the functions of signaling and transmit and receive
messages to and from different nodes. Each SP in the network will
have an identification code termed as signaling Point Code (SPC).


Signaling Transfer Point (STP)
Signaling Transfer Part is signaling point that only transfers messages
from one signaling point (SP) to another.

STP

SP SP
(SPC) STP (SPC)


Signaling Link (SL)

Signaling Link is the 64kbps link interconnecting two signaling Points
and provides the functions of message error control and message
sequencing. Each signaling Link has an SLC (signaling Link Code),
which identifies the signaling Link with in the signaling Link Set.


Service Switching Point (SSP)

The MSC contains:
• The Service Switching Point
• One or more radio control point

SSP handles the usual switching function and can be connected
via 2Mbps PCM link with:
• Other exchanges of fixed PSTN or mobile PLMN,
• Points on the SS7 signaling network,
• X.25 network

Continued…..


• The OA&M network,
• The Intelligent network,
• PSTN via user data channels and signaling channels using ISUP
and R2 protocols,
• Other elements of the GSM


Switching Function of SSP:

• Main control,
• Switching matrix,
• PCM multiplex connection,
• Service circuits
• Operation and maintenance
• Establishing and releasing section of the links from and to
mobiles,
• Finding circuits to the BSS; special circuit groups are created.
SSP selects an incoming and outgoing circuit.


Call Routing

• If a number received is a national or international number, the
address information is passed to the SSP.

• If the number received is an HPLMN (Home PLMN), the RCP asks
the HLR for a roaming number (MSRN). This MSRN is passed to the
SSP for routing.

• If the number received is an emergency service number, the
originating geographic area is attached to it and the combined
information passed to the SSP.

Continued…..


In the SSP the number received from RCP follow the standard
translation process:

• Preliminary analysis: Selection of a translator (national,
international),
• Translation: Determination of a routing depend on the first digits
dialled,
• Routing: Determination of an outing circuit group.


Circuit Groups

Call routes from the MSC through circuit groups. Different circuit
groups are created inside it:

• Group for the PSTN (according to the exchange)
• Group for the BSCs
• Group for the Supplementary services
• Group for the IWF


CG1 BSC1

CG2 BSC2

CGn BSCn

MSC CGa PSTN1

CGx PSTNx

CG Supplementary
Services

CG IWF


Interfaces


A-Interface (MSC – BSC)

The interface between the MSC and its BSS is specified in the 08-series
of GSM Technical Specifications. The BSS-MSC interface is used to
carry information concerning:

• BSS management;
• call handling;
• mobility management.


B-Interface (MSC – VLR)

The VLR is the location and management data base for the mobile
subscribers roaming in the area controlled by the associated
MSC(s). Whenever the MSC needs data related to a given mobile
station currently located in its area, it interrogates the VLR. When
a mobile station initiates a location updating procedure with an
MSC, the MSC informs its VLR which stores the relevant
information. This procedure occurs whenever an MS roams to
another location area. Also, when a subscriber activates a specific
supplementary service or modifies some data attached to a
service, the MSC informs (via the VLR) the HLR which stores
these modifications and updates the VLR if required.


C-Interface (HLR - MSC)
The Gateway MSC must interrogate the HLR of the required subscriber
to obtain routing information for a call or a short message directed to
that subscriber.


D-Interface (HLR - VLR)

This interface is used to exchange the data related to the location of the
mobile station and to the management of the subscriber. The main
service provided to the mobile subscriber is the capability to set up or
to receive calls within the whole service area. To support this, the
location registers have to exchange data. The VLR informs the HLR of
the location of a mobile station managed by the latter and provides it
(either at location updating or at call set-up) with the roaming
number of that station.

The HLR sends to the VLR all the data needed to support the service to
the mobile subscriber. The HLR then instructs the previous VLR to
cancel the location registration of this subscriber. Exchanges of data
may occur when the mobile subscriber requires a particular service,
when he wants to change some data attached to his subscription or
when some parameters of the subscription are modified by
administrative means


E-Interface (MSC - MSC)
When a mobile station moves from one MSC area to another
during a call, a handover procedure has to be performed in
order to continue the communication. For that purpose the
MSCs have to exchange data to initiate and then to realize the
operation. After the handover operation has been completed, the
MSCs will exchange information to transfer A-interface
signaling as necessary. When a short message is to be
transferred between a Mobile Station and Short Message Service
Centre (SC), in either direction, this interface is used to transfer
the message between the MSC serving the Mobile Station and
the MSC which acts as the interface to the SC.


F-Interface (MSC - EIR)

This interface is used between MSC and EIR to exchange data, in order
that the EIR can verify the status of the IMEI retrieved from the Mobile
Station.


G-Interface (VLR - VLR)

When a mobile subscriber moves from a VLR area to another Location
Registration procedure will happen. This procedure may include the
retrieval of the IMSI and authentication parameters from the old VLR.


H-Interface (HLR - AUC)

When an HLR receives a request for authentication and ciphering data
for a Mobile Subscriber and it does not hold the requested data, the
HLR requests the data from the AuC. The protocol used to transfer
the data over this interface is not standardized.


Switch Modules
Switch has three major types of equipment modules:
• Switching module (SM)
• Communication module (CM)
• Administrative module (AM)


Switching Module (SM):
All external lines, trunks, and special services circuits are
terminated at the switching module. The analog and digital
signals are converted to the digital format used inside the switch.
The SM performs almost 95% of the call processing and
maintenance functions including:

• Line and trunk scanning
• Tone generation
• Announcements
• Call progress supervision
• Routine maintenance and self-maintenance.


The SM also provides subscriber calling features including:
— call waiting
— abbreviated dialing
— call diversion
— conference calls.

SM further has two components:

9. Control units - Control all activities within the SM, such as call
processing and maintenance functions.
2. Peripheral units - Perform testing functions and provide
customers and other exchanges access to the switch.


Communication Module (CM):
The CM serves as the hub (focal point) for all inter module
communication in a switch. The CM has four main functions:

4. Call switching - The CM interconnects the paths between
modules to complete telephone calls and to relay data.

2. Message switching - The CM provides paths to send
information between processors to process calls, maintain
records, and perform system tasks.

Continued…..


3. Network timing - The CM provides accurate timing and
synchronization for the switch.

4. Fast pump - The CM provides resources to quickly download (pump)
an SM’s software if needed.


Administrative Module (AM):
The AM controls the CM and communicates with all the SMs
(through the CM). The AM monitors itself and the CM for
malfunctions. If there are any problems, they are reported to
maintenance personnel.

The AM performs resource allocation and processing functions that
are done more efficiently on a centralized basis such as:
• Call routing for inter module and intra module calls
• Administrative data processing/billing data
Continued…..


• Traffic measurement reports/system performance reports
• Memory management
• System maintenance
• Maintaining file records of changes to the system Software Release.
• Personnel interface/system monitoring
• Allocating trunks for call processing.


Switch

SM AM CM

Control Peripheral MSGS TMS
Unit Unit

Control I/O Disk Tape MCC
Unit Processor Unit Unit


Home Location Register

HLR is a database that stores subscription and set of functions
needed to manage subscriber data in one PLMN area. Any
administrative action by the service provider or changes made
by subscriber is first carried out on the HLR and then update the
VLR. Following are the subscriber data which frequently
changes:
- Identification number MSISDN & IMSI
- Service restriction
- Teleservices
- Bearer services
- Supplementary services


Beside the permanent data it also include dynamic data of home
subscriber including VLR address, call forward number and call
barring numbers.

Triplets are also stored in the HLR for the authentication purpose.

The HLR communicates with other nodes: VLR, AUC, GMSC & SMS – SC
via MAP (Mobile Access Protocol). To support this communication
HLR needs MTP and SCCP


MAP (Mobile Application Protocol)

The only way via which HLR communicates with other GSM nodes
is Mobile Access Protocol. Number of functional blocks exist to
support different MAP operations eg HLCAP is used for location
cancellation or HLUAP is required for location updating. Other
functions defined on the MAP are:
- Inter MSC Handover and subsequent handover
- Update HLR and VLR
- Fault Recovery
- Management and handling of supplementary services.

Continued…..


- Support of Short Message Services.
- Call establishment / delivery
- Security related data.
- Retrieval of subscriber data during call setup.

HLR also needs to communicate with GMSC, VLR, AUC and SMS-SC, for
which MTP and SCCP is essential.


SCCP (Signaling Connection Control Point)

All MAP messaging use SCCP to analyze the GT (Global Title) of
incoming information. If GT belongs to anther node, then SCCP
will use the services of MTP (Message Transfer Part) to reroute
the message.

SCCP must have the GT analysis to terminate and route MAP
messages from all nodes it communicates with.

To find out the DPC, SCCP looks in a routing case translation
table. The information about the DPC is then sent to MTP which
sends the message to the appropriate SP.


MTP (Message Transfer Part)

MTP must be defined to allow the nodes to communicate with each
other.

The MTP provides the means for reliable transport and delivery of
UP (User Part) information across the No. 7 network eg ISDN
User part (ISUP), the Telephone User Part (TUP), Signaling
Connection Control Part (SCCP), Interworking function User Part
(IWUP) and Data User Part (DUP)

Continued…..


MTP has the ability to react to system and network failure that
affect the user information.

MTP further has three functional levels:
4. MTP Level 1 – Signaling data link
5. MTP Level 2 – Signaling link
6. MTP Level 3 – Signaling network


HLR connects with MSC via C interface, VLR via D interface


HLR can be configured in two ways:
2. Integrated with MSC


• Hs

• Stand Alone HLR (External Database)


Integrated Vs Stand Alone HLR

The Integrated HLR is accessed by other MSC’s/ VLR’s via MAP, and
the switch can use MAP to query other off switch HLRs. The main
advantages with an integrated HLR solution at this early stage are:
• Efficient use of HW and lower HW investments
• Fewer physical connections required due to fewer physical nodes
• Less capacity required in No. 7 network as major part of HLR
signaling is internal within MSC/VLR/HLR


• A single fault will affect a smaller number of subscribers than if
standalone HLR is used

Major drawbacks are:
• Less processing capacity available for MSC/VLR.
Additional Switching capacity will be required earlier
• Migration to standalone HLR (which is to be preferred in a mature
larger network) will induce major changes in the network
• Administration of subscriptions
• Operation and maintenance for HLR geographically distributed


In Stand Alone HLR, call processing activities are not performed by the
switch. Only HLR queries are handled via the GSM standard MAP
messages coming over signaling links from other Mobile Switching
Centers (MSCs) in the wireless network.


Benefits:

• All HLR data is centralized, thus simplifying its ongoing
maintenance and operation
• High HLR Capacity
• High processing capacity
• On going enhancement

There are some drawbacks with standalone HLR
• A fault in a HLR will affect many subscribers
• A fault in a HLR will increase the signaling substantially in
the whole signaling network


HLR is responsible for:

• Connection of mobile subscribers and definition of
corresponding subscriber data.
• Subscription to basic services.
• Registration/deletion of supplementary services.
• Activation/deactivation of supplementary services.
• Interrogation of supplementary services status.

Continued…..


• Functions for analysis of mobile subscriber numbers
(MSISDN, IMSI, additional MSISDN) and other types of
addresses.
• Statistical functions for collecting data regarding the
performance of the system.
• Functions for communication with GMSC and VLR using
the No. 7 signaling system and MAP
• Handling of authentication and ciphering data for mobile
subscribers including communication with an authentication
center.

Continue…..


• Get Password/Register Password
• Alert Service Center
• Provide Roaming Number
• Send Routing Information for SMS
• Send Routing Information for GMSC
• Set Message Waiting Data


Visitor Location Register
It is a subscriber database containing the information about all the MS
currently located in the MSC service area. VLR can be considered as
a distributed HLR in the case of a roaming subscriber. If MS moves
into a new service area (MSC), VLR requests the HLR to provide the
relevant data and store it, for making the calls for that MS.

VLR is always integrated with MSC to avoid the signaling load on the
system.

It can also be viewed as a subset of a HLR.


VLR connects with MSC via B interface, HLR via D interface and with
another VLR via G interface.

G


VLR is responsible for

• Setting up and controlling calls along with supplementary
services.

• Continuity of speech (Handover)

• Location updating and registration
• Updating the mobile subscriber data.

• Maintain the security of the subscriber by allocating TMSI
Continued…..


• Receiving and delivering short messages

• Handling signaling to and from
- BSC and MSs using BSSMAP
- other networks eg PSTN, ISDN using TUP
• IMEI check

• Retrieve data from HLR like authentication data, IMSI,

ciphering key

Continued…..


• Retrieve information for incoming calls.

• Retrieve information for outgoing calls.

•Attach/Detach IMSI

• Search for mobile subscriber, paging and complete the call.


Security Feature

Both the users and the network operator must be protected against
undesirable intrusion of third party. As a consequence, a
security feature is implemented in the telecommunication
services. The following parts of the system have been
reinforced and provide the various security features:
2. Access to the network authentication
3. Radio part ciphering
4. Mobile equipment equipment identification
5. IMSI temporary identity


Authentication Center (AUC)

AUC is always integrated with HLR for the purpose of the
authentication. At subscription time, the Subscriber
Authentication Key (Ki) is allocated to the subscriber, together
with the IMSI. The Ki is stored in the AUC and used to
provide the triplets, same Ki is also stored in the SIM.

AUC stores the following information for each subscriber
4. The IMSI number,
5. The individual authentication key Ki,
6. A version of A3 and A8 algorithm.

Continued…..


Authentication is required at each registration, at each call setup
attempt (mobile originated or terminated), at the time of location
updating, before supplementary service activation, de-
activation , registration.

HLR uses the IMSI to communicate with AUC, triplets are
requested in sets of five.

Continued…..


In AUC following steps are used to produce one triplet:

4. A non- predictable random number, RAND, is produced
5. RAND & Ki are used to calculate the Signed Response
(SRES) and the Ciphering Key (Kc)
6. RAND, SRES and Kc are delivered together to HLR as one
triplet.

HLR delivers these triplets to MSC/VLR on request in such a way
that VLR always has at least one triplet.


Authentication Procedure:

The MSC/VLR transmits the RAND (128 bits) to the mobile. The MS
computes the SRES (32 bits) using RAND, subscriber authentication
key Ki (128 bits) and algorithm A3. MS sends back this SERS to AUC
and is tested for validity.


SIM Card RAND A4
IMSI
Ki SERS A4
=?
A3 IMSI
RAND Ki
A8 Ki

Kc Triplets
A3 A8
Triplets
Kc A2 Generation

Ciphering Ciphering RAND
Function Function SERS
A5 A5 Kc

MS BTS MSC/VLR HLR AUC OMC


Ciphering

The user data and signaling data passes over the radio interface are ciphered to
prevent intrusion. The ciphered key (Kc) previously computed by the AUC is
sent from the VLR to the BSS after the mobile has been authenticated. The
Kc is also computed in the MS and in the way both ends of the radio link (MS
and BSS) possess the same key.


Procedure:

For the authentication procedure, when SRES is being calculated, the
Ciphering Key (Kc), is calculating too, using the algorithm A8.

The Kc is used by the MS and the BTS in order to cipher and decipher the
bit stream that is sent on the radio path.


SIM AUC
Choice of random no
RAND (128 bits)

Ki RAND Ki

A3 A3 A3 A3

SERS
SERS
=?

A8 OK A8 A8
A8 Ciphering Command

Kc (64 Kc
Speech, data,sig
Speech, data,sig bits)
Ciphered Data A5
A5
Ciphering/Deciphering


Subscriber Confidentiality

The subscriber identity (IMSI), since is considered sensitive
information, is not normally transmitted on the radio channel. A
local, temporary identity is used for all interchanges. The identity
(TMSI) is assigned after each change of authenticated location.
For other cases:
• Call set-up
• Use of supplementary services
• Use of SMS

Continued…..


A TMSI is allocated when the one supplied by the MS is considered out of
date or when the MS does not provide the TMSI.

Transmission of the TMSI over the traffic channel is ciphered.


Equipment Identification Register (EIR)

Purpose of this feature is to make sure that no stolen or unauthorized
mobile equipment is used in the network.

EIR is a database that stores a unique International Mobile Equipment
Identity (IMEI) number for each item of mobile equipment.


Procedure:
• The MSC/VLR requests the IMEI from the MS and sends it to a
EIR.
• On request of IMEI, the EIR makes use of three possible defined
lists:
- A white list: containing all number of all equipment identities
that have been allocated in the different participating countries.
- A black list: containing all equipment identities that are
considered to be barred.
- A grey list: containing (operator’s decision) faulty or non-
approved mobile equipment.
• Result is sent to MSC/VLR and influences the decision about
access to the system.


EIR MSC/VLR MS
Storage of all number Storage of the
series mobile equipment equipment
that have been allocated identity IMEI
in the different GSM
-countries
Call Setup
Storage of all grey/black
– listed mobile equipment IMEI Request

Sends IMEI

Check IMEI
Continues/Stops
call setup
Access/ barring info procedure


Echo Canceller

In order to eliminate echo effects (noticeable by the mobile
subscribers while in conversation with PSTN subscribers)
caused by the time delay due to coding and decoding of signal
processing, group of echo cancellers are installed even for local
calls.

This is rarely a problem when communicating between two MSs.
However, when connecting to a PSTN telephone, the signal
must pass through a 4-wire to 2-wire hybrid transformer.

Continued…..


The function of this transformer is - some of the energy at the 4-
wire receive side from the mobile is coupled back to the 4-wire
transmit side and thus speech is retransmitted back to the
mobile.

As a result, all calls on to the PSTN must pass through an echo
canceller to remove what would otherwise be a noticeable and
annoying echo.

Continued…..


The process of canceling echo involves two steps:

• First, as the call is set up, the echo canceller employs a digital
adaptive filter to set up a model or characterization of the voice
signal and echo passing through the echo canceller. As a voice
path passes back through the cancellation system, the echo
canceller compares the signal and the model to dynamically
cancel existing echo. It removes more than 80 to 90 percent of
the echo across the network.
• The second process utilizes a non-linear processor (NLP) to
eliminate the remaining residual echo by attenuating the signal
below the noise floor.


Transcoder and Rate Adaptor Unit (TRAU)

The primary function of the TRAU is to convert 16kps (inc
signaling) GSM speech channels to 64kbps PCM channels in
the uplink direction and the reverse in the downlink direction.
The reason this process is necessary is because MSCs only
switch at the 64kbps channel level.


TRAU Locations

TRAU can be physically located in the BTS, BSC or MSC and hence leads
to a variety of installation configurations.


Advantages of Different Configurations

Case 1, TRAU at BTS: If the TRAU is installed at the BTS, each
16kbps GSM channel would need to be mapped to its own
64kbps PCM channel. This results in 75% of the transmission
bandwidth being wasted across both the Abis (BTS-BSC) and A
(BSC-MSC) interface.

Case 2, TRAU at BSC: If the TRAU is installed at the BSC, 16kbps
GSM channel mapped to 64kbps at the A (BSC-MSC) interface,
which increases the requirement of the Transmission trunks.


Case 3, TRAU at MSC: If the TRAU is placed at the MSC, as is generally
the case in current networks, a multiplexer can be placed at the BTS
which enables 4 x 16kbps GSM channels to be multiplexed onto one
64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this
configuration, only at arrival at the MSC is the 16-64kbps channel
conversion necessary, thereby maximizing the efficient usage of the
transmission medium by increasing the GSM channel throughput per
PCM 2048 bearer from 30 to 120 channels.


Operation And Maintenance Center (OMC)

The OMC centralizes all operations and maintenance activities for the MSCs and
BSSs using remote software control. It provides remote testing, operations,
and maintenance capabilities for the entire system from one central location.
Each BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled
from the OMC.


OMC Functional Architecture

Event/ Alarm
Management Security
Management
MMI

Operating Database
Fault System Configuration
Management Management
Communications
Handler

Performance
Management


The OMC supports the following network management functions:

• Event Management - General functions of the OMC include
operator input and output messages, application input
commands, and application output reports.

• Fault Management - The OMC provides fault management such
as diagnostics and alarms for the MSC and BSS. It provides the
means to isolate and minimize the effects of faults in the network
thereby enabling the network to operate in efficient manner.

Continued…..


• Security Management – It provides an extensive range of features to
ensure that access to the OMC functions is restricted to relevant
personnel.
The security features are as follows:
 Password Authentication of OMC operator
 Logging of OMC access attempt
 Configurable user access restrictions
 Automatic logoff


• Configuration Management - Configuration management for the
BSS consists of generic download, non-volatile memory
download, database administration, and translations download.
For the MSC, software release updates, database administration
(route analysis, IMSI analysis table), and subscriber
administration (connect/disconnect) are supported.

• Performance Management - Performance management supports
data collection (such as traffic data, handovers, statistics, plant
measurements, and volume data) and basic reporting.


Billing Center

Charging analysis is the process of analysing the Charging Case and then
ultimately generating the TT (Toll Ticketing) record so that an itemised bill can
be produced and then sent on to the customer.

The tariff structure consists of two parts:
• The network access component
• The network utilization component


The network utilization component is registered on a per call basis.
Charging starts at the moment the subscriber answers, or on connection
to an answering machine internally in the network.

The main elements are:
• Use of GSM PLMNs
• Use of national / international PSTNs
• Use of connection between different networks
• Use of the signaling system no.7


Depending on the type of call, one or more call tickets can be generated:

• Outgoing call to fixed network: a call tickets is generated by the
originating MSC.
• Incoming call from the fixed network: two call tickets are created: one in
the GMSC and another in the destination MSC. If a call forwarding
supplementary service is in operation, other call tickets are generated in
the MSC and the GMSC.

Continued…..


• Outgoing call from a mobile subscriber to another mobile
subscriber belonging to same PLMN: three call tickets are
created: one in the originating MSC, one in the GMSC (which
is in this case is the originating MSC) and another in the
destination MSC.

Call tickets mainly register the following information:
4. IMSI
5. Identity (MSISDN) and type (MSC or GMSC)
6. Mobile subscriber location identity


1. Other party’s identity
2. Call type (incoming, outgoing, forwarded etc)
3. Call status
4. Teleservices and bearer service
5. Date and time
6. Call duration


Call Detail Record (CDRs)

• Each call within the PLMN creates one or more call records
• These records is generated by the MSC/GMSC originating the
call
• The records are known as a ‘Call Detail Records’ (CDRs)
• CDRs contain the following information:
- Subscriber Identity
- Number called
- Call Length
- Route of call
• Often referred to as ‘Toll Tickets’


Call Charge Procedure

• Network supplies originating MS with CAI details
• MS calculates AOC record using CAI details
• This record acts as a ‘toll ticket’ which tracks the call on its route
through various networks
• Each call component can generate a separate CDR
• The record passes along the backbone to the home network
• Billing computer generates bills based on cumulative CDRs
• HPLMN collects the charges
• HPLM reimburses VPLMN using TAPs in accordance with
roaming agreement


The Transferred Account Procedure (TAP) is the mechanism by which
operators exchange roaming billing information. This is how roaming
partners are able to bill each other for the use of networks and services
through a standard process.


Gateway MSC (GMSC)

Gateway MSC (GMSC) connects the PLMN with other networks and the
entry point for the mobile subscriber calls having the interrogation
facility. It has the function to obtain the information from the HLR
about the subscriber’s current location and reroute the calls
accordingly.

In case of the network having only on MSC, the same MSC work as the
GMSC, while in the case having more than one MSC, one dedicated
MSC works as GMSC.


Roaming Number

A MSRN is used during the call setup phase for mobile terminating calls. Each
mobile terminating call enters the GMSC in the PLMN. The call is then re-
routed by the GMSC, to the MSC where the called mobile subscriber is
located. For this purpose, a unique number (MSRN) is allocated by the MSC
and provided to the GMSC.


Call Setup


1. GMSC receives a signaling message "Initial Address
Message" for the incoming call (MSISDN).
2. GMSC sends a signaling message to the HLR where the
subscriber data is stored (MSISDN).
3. The VLR address that corresponds to the subscriber location
and the IMSI are retrieved. HLR sends a signaling message
using the VLR address as the destination (IMSI).
4. VLR having received the message, requests MSC to seize an
idle MSRN and to associate it with the IMSI received. VLR
sends back the result to the HLR (MSRN).


1. HLR sends back the result to the GMSC (MSRN).
2. GMSC uses MSRN to re-route the call to the MSC. MSC performs
digit analysis on the received MSRN and find the association with
IMSI. The MSRN is released and the IMSI is used for the final
establishment of the call.


Transit Switch

When planning the trunk network architecture, it is important to
take into consideration the future expansion of the network.
Some factors that influence the trunk network configuration are:
• Number of MSCs
• Transmission costs
• Traffic distribution
• Traffic volume
• PSTN tariffs


In case of a medium networks (having 5 - 10 MSCs), some of the
MSCs are used as transits for the others and the number of
direct links between the MSCs are restricted.

In case of large networks (having more than 10 MSCs), separate
transit exchanges are used. These are connected to all MSCs
and are working with load sharing.

Transit functionality is used for passing on calls to another node.
This provides a hierarchical structured network.


High Usage trunk


Traffic between MSCs and from MSCs to other networks is routed over two
MSCs in a similar way as is used for the small network. The TGMSCs
are used as interconnecting exchanges, since they have trunks to all
MSCs in the operators PLMN.

MSCs located in the same city area or in close cities are likely to be
interconnected by high usage routes, while traffic between distant MSCs
is likely more economically routed over the TMSCs.


ADVANTAGES OF USING TRANSIT EXCHANGES

The use of transit exchanges implies a more stable network structure and some
of the most important benefits are:
• increased flexibility
• enhanced reliability
• easily expandable network
• platform for functional development
• lower handling costs
• improved signaling network


Value Added Services

Value Added Services includes the following:
• Point-to-Point Short Message Services
• Cell Broadcast Short Message Service
• Voice/Fax Mail
• Pre-Paid SIM

The products associated with each of these services as well as the required
interfaces into the core network elements are defined as:


Short Message Services (SMS)

The Point-to-Point and Cell Broadcast Short Message Services are implemented
using the Short Message Service Center (SMSC) and Cell Broadcast Center
(CBC).

SMSC is built around proven Open Systems Platforms from the UNIX based
computer platform to the MSC/HLR/VLR interfaces utilizing SS7.


Following are the services and functions for which SMSC is
capable of:
• Alerting services to indicate call or message waiting
• Paging interfaces providing full industry standard TAP
interworking
• Information services - subscription to financial, weather, traffic,
etc. services
• DTMF message entry via interactive voice prompts
• E-mail
• Network administration including bill reminders, statements on
demand, network
• service information and handset reprogramming.


The CBC product is based on the same Open Systems Platforms with an
X.25 interface to BSC components. It offers a wide range of
applications, which include advertising, general and specialist
information distribution services along with other non-mobile terminal
applications. The services and functionality that the CBC can provide
includes:
• Customer care information
• Weather and traffic reports
• Free advertising
• Variable re-transmission rates
• Distributed network interface units to handle varying network loads
• Local and remote message submission facility.


SMS Network Components


Callers which cannot reach the MS are given the option (by the VMS) to
leave either a short message or a voice mail message. Message
waiting notification will be delivered to the MS when the MS is
reachable. The VMS (voice mail system) communicates with the SMS
SC via TCP/IP or X.25.

The VMS has a trunk and signaling interface to the PSTN (e.g., R2, ISUP
signaling). The VMS has a trunk and signaling interface to the MSC for
mobile subscriber to access his/her voice mail.


SMS Applications

• SMS up to 160 alphanumeric characters.
• Alert services (MT-SMS)
— Voice Message Alert
— FAX/Telex Message Alert
— E-mail System Alert
— Paging Bureau Emulation Services.
• Information Services
— Financial Services (stock market queries and alerts)
— Weather or traffic information (e.g., from TV/radio station data
feeds)


• Network Administration
— Bill reminders (MT-SMS), bill payment
— Statements on demand (MO and MT-SMS)
— Handset re-programming and much more.


VMS

It supports a wide range of innovative applications including:
• Call answering
• Voice and fax bulletin boards
• Information on demand
• One number services
• Voice and fax messaging
• Interactive voice response
• Prepaid calling cards
• Voice activated dialing


Pre Paid SIM

The functionality of the Pre-Paid SIM feature includes:
• Provision of pre-defined limits based on air time or talk time
• Service provisioning including various provisioning options (point
of sale, service providers, etc.) and definitions of pre-paid
categories (throw away, top up, etc.)
• Service execution for air and talk time credit usage
• GSM MAP services, teleservice, bearer services and
supplementary services will all be available to the Pre-Paid SIM
subscriber, with possible limitations, as required by the network
operator.


Supplementary Services

Wide range available in GSM standard and Operators can also define their own

In GSM it is possible for the subscribers to check and modify
the parameters and status of their Supplementary Services


Some of the Supplementary Services are:

• Calling Line Identification/Restriction
• Connected Line Identification/Restriction
• Call Forwarding
• Call Waiting
• Call Hold
• Conference Calling
• Conference Calling
• Advice of charge
• Call barring


Exercise

Q1. Write a full form of following : IMEI, TMSI, MSRN, LAI, ST,
STP, SSP

Q2. How many circuit groups are required for 3 BSCs and 10
PSTN?

Q3. List down the three functions of each HLR & VLR.

Q4. Fill in the following:
E interface is used between ------
H interface is used between-------


Algorithm A8 is used for ----------
Algorithm A3 is used for ----------
Transit exchanges are used to reduce the ---------

Q5. List down the different locations of TRAU and explain the best
position.

Q6. What information is contained in the CDRs?

Q7. 2 advantages of transit switch.

Q8. Name some of supplementary services.

Section 4 – GSM Signaling

GSM Signaling


Objective


• signaling between MSC/VLR and BSS
• Concept of DTAP
• Concept of BSSMAP
• signaling between BSC and BTS
• Functions of LAPDm
• Functions of LAPD
• Frame structure of LAPDm And LAPD


Introduction

There are two different types of communication channels:
• Traffic channel at 64 Kbps, carrying speech or data for radio
channels.
• signaling channels at 64Kbps, carrying signaling information.

In PCM one time slot is reserved for signaling and remaining are
used for transmitting speech or data. As the entire siganlling is
done on 64Kbps , there should be special function converting
the information to 64Kbps format and back at the receiving end.


Protocols in GSM Networks

VLR AUC
MAP MAP
ISDN ISUP VLR HLR EIR
GMSC ISUP MAP MAP
MAP
MAP MSC
MSC
TUP
PSTN Switching System
BSSAP

BSC Base
LAPD Station
System
MS LAPDm BTS


GSM Signaling Matrix
Database

DTAP
BSSMAP MAP
BSS
DTAP
RR MAP
RIL3 RSM RSM
TCAP
BSSAP BSSAP
RIL3

ISUP

SCCP
SCCP

MTP2 &3 MTP2 & 3
LAPDm
LAPDm LAPD LAPD MTP1 MTP1
MS BTS BSC MSC


• MSC uses ISUP/TUP protocols for PSTN signaling.
• MAP siganlling for database applications like HLR, VLR, EIR,
AUC, SMS-SC, GMSC.
• GSM specific protocol as BSSAP, which comprises of DTAP and
BSSMAP.
• The BSC on layer 2 uses LAPD protocol, which is an ISDN.
• BTS has LAPDm as layer 2 protocol.
• Mobile has DTAP for MSC and RR for Radio Resource
signaling.


MAP (Mobile Application Part)

MAP is a protocol specially designed for GSM requirement. It is installed
in MSC, VLR, HLR, EIR and communicates in case of:

• Location registration
• Location cancellation
• Handling/management/ retrieval of subscriber data.
• Handover
• Transfer of security/ authentication data.


BSS Application Part (BSSAP)

BSSAP is used for signaling between MSC/VLR and BSS. Three groups of
signals belong to BSSAP

3. DTAP
4. BSSMAP
5. Initial MS messages


Transparent to BSS

M DTAP
M
Initial MS Message
S BSSMAP LAPDm
S
C
BSC/BTS


Direct Transfer Application Part (DTAP)

DTAP is a messages between the MSC and MS, passes through the BSS
transparently. These are call control and mobility management
messages directed towards a specific mobile.
3 main type of DTAP messages are:
• Messages for mobility management like location update, authentication,
identity request
• Messages for circuit mode connections call control
• Messages for supplementary services


BSSMAP

BSS management messages (BSSMAP) between MSC and BSS (BSC/ BTS),
which are necessary for resource management, handover control, paging
order etc. The BSSMAP messages can either be connection less or
connection oriented.


Initial MS Messages

These messages are passed unchanged through BSS, but BSS
analyses part of the messages and is not transparent like DTAP
messages.
Between BSS and MSC, the initial MS message is transferred in the
layer 3 information in the BSSMAP.

The Initial MS messages are:
• CM Request
• Location update request
• Paging response


LAPDm

Link Access Procedures on the Dm channel (LAPDm) is the layer 2
protocol used to convey signaling information between layer 3
entities across the radio interface. Dm channel refers to the
control channels, independent of the type including broadcast,
common or dedicated control channels.

LAPDm is based on the ISDN protocol LAPD, used on the Abis
interface. Due to the radio environment, the LAPD protocol can
not be used in its original form. Therefore, LAPDm segments the
message into a number of shorter messages.


Data exchanged between the data link layer and the physical layer
is 23 octets for BCCH, CCCH, SDCCH and FACCH. For SACCH
only, 21octets are sent from layer 2 to layer 1.

LAPDm functions include:
• LAPDm provides one or more data link connections on a
Dm channel. Data Link Connection Identifier (DLCI) is used for
discriminating between data link connections.
• It allows layer 3 message units be delivered transparently
between layer 3 entities.
• It provides sequence control to maintain the sequential order of
frames across the data link connections.


LAPDm Frame Structure

info length command address

N(R) P/F N(S) 0 0 0 1 SAPI CR 1


Sequence Number: N(S) send sequence number of the
transmitted frame. N(R) is receive sequence number.

P/F : All frames contain the Poll/Final bit. In command frames, the
P/F bit is referred to as the P bit. In response frames, the P/F bit
is referred to as the F bit.

Service Access Point Identifier: Service Access Points (SAPs) of a
layer are defined as gates through which services are offered to
an adjacent higher layer.SAP is identified with the Service
Access Point Identifier (SAPI).
SAPI = 0 for normal signaling of DTAP & RR
SAPI = 3 for short message services


LAPDm has no error detection and correction. It is used in two modes:
• Acknowledge &
• Unacknowledged

and having a different structure for both.


LAPD

All signaling messages on the Abis interface use the Link Access
Procedures on the D-channel. (LAPD protocol). LAPD provides two
kinds of signaling:
• unacknowledged information
• acknowledged information

LAPD link handling is a basic function to provide data links on the 64 kbps
physical connections between BSC and BTS.


Links are provided for operation and maintenance (O&M) of the
links, for O&M of the BTS equipment and for transmission of
layer 3 Abis messages.

Each physical connection can support a number of data links
(logical connections). On each physical connection each data
link is identified by a unique TEI/SAPI


LAPD has three sub signaling channels

3. RSL (Radio signaling Link), deals with traffic management,
TRX signaling.
4. OML (Operation & Maintenance Link), serves for
maintenance related info and transmission of traffic statistics.
5. L2M (Layer 2 Management), used for management of the
different signaling on the same time slot.


LAPD Frame Structure

Flag FCS info length command address Flag

N(R ) P/F N(S) 0 TEI 1 SAPI CR 0


LAPD Frame structure is made up of:
Flag: Indicates the beginning and end of each frame unit. Flag has
a pattern of 01111110.

FCS: Frame Check Sequence, provides the error checking for the
frame. If error is found frame will be retransmitted.

Command: It has two types of structure, in acknowledge mode it
has N(S) and N(R ). N(S) is a sequence number of frame sent
and N(R ) is the sequence number of the frame expected to
receive next.


C/R: This bit indicates whether it is command or response.

P/F: In command frames, the P/F bit is referred to as the P bit and
the other end transmits the response by setting this bit to F.

TEI: Terminal Endpoint Identifier, is a unique identification of each
physical entity on either side like each TRX within a BTS have a
unique TEI.


SAPI: Service Access Point Identifier, used to identify the type of link.

SAPI = 0 for RSL
SAPI = 62 for OML
SAPI = 63 for L2ML

Each LAPD link is identify by SAPI/TEI pair.


Exercise

Q1. Name the protocol which is transparent to BSS and what information is
used to transfer on this protocol?

Q2. Name the protocols used between
Mobile and BTS
BTS and BSC
BSC to MSC
MSC to PSTN

Section 5 – Call Handling

Call Handling


Objective


• Basic call concepts
• Location Area concepts
• Call setup in different scenarios
• SMS routing
• Intra and Inter MSC handovers


Introduction

Call setup is required to establish communication between a Mobile Station
and Network Subsystem (NSS). The NSS is responsible for establishing
a connection with the corresponded. Different types of calls require
different teleservices.

For the optimum utilization of the network, different location areas will be
defined to reduce the paging load on the system.


Basic Types of Calls

There are three basic types of call:

3. Mobility Management calls: Such as Location update. These
are used to collect information about the MS and only
signaling channels are used.
4. Service calls: Such as SMS. These calls passes very small
information, therefore signaling channels are used.
5. User traffic calls: Such as speech or data. Large amount of
data is exchanged hence traffic channels are used.


Basic Call Setup


Subscriber on switch A places a call to a Subscriber on switch B:

3. Switch A analyzes the dialed digits and determines that it needs
to send the call to switch B.
4. Switch A selects an idle trunk between itself and switch B
and formulate IAM
6. STP W receives a message, inspects its routing label, and
determines that it is to be routed to switch B.
7. Switch B receives the message. On analyzing the message, it
determines that it serves the called number and that the called
number is idle.
8. Switch B formulates an address complete message (ACM),
which indicates that the IAM has reached its proper destination.


2. Switch B picks one of its links and transmits the ACM over the
link for routing to switch A.
3. STP X receives the message, inspects its routing label, and
determines that it is to be routed to switch A.
4. On receiving the ACM, switch A connects the calling
subscriber
5. When and/or if the called subscriber picks up the phone,
switch B formulates an answer message (ANM),
6. Switch B selects the same link it used to transmit the ACM
7. STP X recognizes that the ANM is addressed to switch A and
forwards it over link


2. Switch A ensures that the calling subscriber is connected and
conversation can take place.
3. If the calling subscriber hangs up first switch A will generate a
release message (REL) addressed to switch B.
4. STP W receives the REL
5. Switch B receives the REL, disconnects the trunk from the
subscriber line, returns the trunk to idle status.
6. STP X receives the RLC, determines that it is addressed to
switch A.
7. On receiving the RLC, switch A idles the identified trunk.


Location Registration

When the mobile is turned on first time in the network, it has no indications
in its data about an old Location Area Identity. MS immediately inform
the network and request for the Location Update to the MSC/ VLR.
After registration MSC/ VLR will consider the MS as active and marked
the MS as “attached”.


Location Update

When the MS moves from one LA to another, it has to register. This
registration is performed when the MS detects another LAI than the
one stored. This is called location updating. This function provides
mobile subscribers with uninterrupted service throughout the GSM
coverage area so that they can:
• Be called on a permanent directory number irrespective of their
location at the time of call.
• Access the network whatever their position


There are four different types of location updating:

• Normal
• IMSI detach
• IMSI attach
• Periodic registration


Normal Update

• The Base Transceiver Station (BTS) of every cell continually
transmits the Location Area Identity (LAI) on BCCH.
• If MS detects LAI is different from the one stored in the SIM-
card, it is forced to do a location update.
• If the mobile subscriber is unknown in the MSC/VLR (new
subscriber) then the new MSC/VLR must be updated, from the
HLR, with subscriber information.
• It also consider the case of the location update timer runs out.


2. The MS requests a location update to be carried out in the new
MSC/VLR. The IMSI is used to identify the MS.
3. In the new MSC/VLR, an analysis of the IMSI number is carried out.
The result of this analysis is a modification of the IMSI to a Mobile
Global Title (MGT)
4. The new MSC/VLR requests the subscriber information for the MS
from the HLR.
5. The HLR stores the address of the new MSC/VLR and sends the
subscriber data to the new MSC/VLR.


5. The HLR also orders the old serving MSC/VLR to cancel all
information about the subscriber since the MS is now served by
another MSC/VLR.
2. When the new MSC/VLR receives the information from the HLR, it
will send a location updating confirmation message to the MS.


IMSI Detach

The MS must inform the network when it is entering an
inactive state (detach).
3. At power off or when the SIM card is taken out, the MS asks
for a signaling channel
4. The MS uses this signaling channel to send the IMSI detach
message to the MSC/VLR.
5. In the VLR, an IMSI detach flag is set for the subscriber which
is used to reject incoming calls to the MS.

The detach will not be acknowledged.


Only the VLR is updated with the “detached” information.


IMSI Attach

The attach procedure is performed only when the MS is turned on
and is in the same LA as it was when it sent the detach
message. If the MS changes location area while being
switched off, it is forced to do a normal location update. The
procedure is as follows

3. The MS requesting a signaling channel.
4. The MSC/VLR receives the IMSI attach message from the
MS.
5. The MSC/VLR sets the IMSI attach in the VLR, that is, the
mobile is ready for normal call handling.
6. The VLR returns an acknowledgment to the MS.


Periodic Location Update

To avoid unnecessary paging of the MS in case the MSC never got
the IMSI detach message, there is another type of location
updating called periodic registration.

The procedure is controlled by timers both in the MS and in the
MSC.

If the MS does not register within the determined interval plus a
guard time, then the scanning function in the MSC detects this
and the MS will be marked detached.


Paging

A call to MS is routed to the MSC/ VLR and send a paging message to
the MS. This message is broadcast all over the Location Area (LA),
which means that all BTSs with in the LA will send a paging message
to the mobile. The MS, moving in the LA and listening to the CCCH
information, will hear the paging message and answer it immediately.


Paging Capacity

Paging capacity is the number of
mobiles that can be paged per
second
This depends on:
• CCCH configuration
• AGCH blocks reservation
• Type of paging message used
• Paging message takes 4 bursts
(1 CCCH block)
• This can page up to 4 mobiles
depending on the message type
used


Paging Message Types

Type 1: can address up to two mobiles using either IMSI or TMSI

Type 2: can address up to 3 mobiles, one by IMSI and other 2
by TMSI.

Type 3: can address up to 4 mobiles using the TMSI only.

If the network does not use TMSI then only type 1 is used in the
network.


Calculation Of Paging Capacity

X = number of mobiles paged per paging message (1 to 4)
Y = number of possible paging messages per multiframe
Duration of channel multiframe = 0.235 seconds (235 ms)
• X depends on paging message type
• Y depends on CCCH configuration in the multiframe (e.g.
3 or 9) and the number of AGCH blocks reserved


PCH Dimensioning

Paging channel requirement in blocks per multiframe is given by:

Calls = Number of calls predicted for the location area during
busy hour
MT = Fraction of calls which are mobile terminated
PF = Paging Factor = number of pages required per call
M = safety margin
PMF = Paging Message Factor = number of pages per
message
Number of control channel multiframes per second = 4.25


Example

A particular location area contains 50 000 subscribers. It is predicted that
30% of these will receive a call during the busy hour. On average 2
pages are needed per call and only type 3 paging messages (TMSI) are
used.

This gives the following data:
Calls = 50 000
MT = 0.3
PF = 2
PMF = 4


A typical safety margin for peak variations in number of calls is 1.2

• 1 PCH block per multiframe will be adequate


Paging Control

The MSC has to initiate the paging procedure, as it holds the information
on the last MS location update.

MSC sends a paging message to BSC and sets a timer for response from
the MS, which is send as a part of service request message. The paging
message from the MSC contains a cell list identifier, identifying the cells
in which paging message is to be transmitted.


Call From MS (Mobile to PSTN)
cb

PLMN
VLR

Exch MSC
PSTN


Call From MS Overview

• Mobile is active and idle, wants to set up a call
• User dial the number and press send, at first moment it sends on
RACH
• MSC/VLR assigns a dedicated channel
• If the calling MS is allowed to make a call MSC/VLR
acknowledges the access request
• Depending on whether a fixed or a mobile subscriber is called,
number is analyzed directly in the MSC/VLR.
• Call setup message is acknowledged as soon as the link is
ready.
• MS is also assigned to move to a dedicated traffic channel TCH.


Signaling Interfaces

ISDN/ PLMN
PSTN
LAPDm

VLR
BSSMAP
LAPD
MSC BSC

DTAP
ISUP/TUP


Point Of Interconnect (POI) Location

In case of long distance mobile to PSTN call, circuits define to route a call in the
switch should be such that, call can travel maximum distance on the airtime
and minimum on the land line to enhance the revenue.

Call should handover to the POI as near as possible to the subscriber location.


Call to MS (PSTN to Mobile)

MSISDN
PSTN

GSM/PLMN

Link is setup from local
exchange to the GMSC GMSC


PSTN

GSM/PLMN

signaling No.7: Interrogation GMSC
function used by GMSC
MSISDN +
MSRN request
HLR


PSTN

GSM/PLMN

VLR GMSC
signaling No7: Request
for MSRN to VLR
MSC

HLR
IMSI


PSTN

GSM/PLMN

IMSI
MSRN in VLR.
VLR signaling No 7: MSRN GMSC
send to GMSC

MSC HLR

MSRN MSRN request + IMSI


PSTN

GSM/PLMN

VLR

GMSC
MSC HLR

Link is setup from GMSC to
MSC/VLR


PSTN

GSM/PLMN

VLR

BSC GMSC HLR
MSC

signaling No7: Paging
message is sent to the BSS


PSTN

GSM/PLMN

HLR

VLR
Air path signaling:
Paging message is
BSC GMSC sent over the air
MSC
path to MS. The MS
answers.


PSTN

GSM/PLMN

HLR

VLR
The link is setup from
the MSC/VLR to the
BSC GMSC MS, completing the
MSC
connection from
subscriber to
subscriber


Signaling Interfaces

ISDN/ PLMN
PSTN

HLR LAPDm

MAP VLR BSSMAP
LAPD
GMSC MSC BSC

ISUP/TUP
DTAP
ISUP/TUP


Mobile to Mobile (Mobile Originated)

MS BTS BSC MSC
Channel Request
rn Channel Request
rn+fn+TA
Channel Activation SDDCH Allocation
TA+SDDCH+power
Channel Activation Ack

Immediate assign commd
Immediate assign (AGCH)
rn+fn+TA+ SDCCH
Switch to
SDDCH SABM
Cm+Service Request Establish Indication

UA Service Request SCCP Connection Req
Service Request

SCCP Connection
Confirm


MS BTS BSC MSC
Setup (SDCCH) Layer 3CC
Layer 3CC
Tele/bearer service
called party no.
Layer 3CC Call proceeding
Layer 3CC

Assignment request

Channel type+cm
TCH allocation
Physical context request

Physical context confirm

Power+TA
Channel activation
SACCH TCH+TA+cipher+DTX
TA+power updates +power

Channel activation ack

Assignment command (SDCCH)

Release
SDCCH


MS BTS BSC MSC

SABM (FACCH)
Establish indication

Set
UA (FACCH) transcoder
Set switching
path
Assignment complete (FACCH)

Initiate SDCCH release
alerting
Layer 3CC
Layer 3CC

connect
Layer 3CC
Layer 3CC

Layer 3CC
Connect ack


Mobile to Mobile (Mobile Terminated)

MS BTS BSC MSC

Paging
Paging command TMSI/IMSI+cell list
Paging request (PCH)
TMSI/IMSI paging
group+ channel no
TMSI/IMSI

Paging request (RACH)

Channel required

Radio and Link Establishment Procedure


MS BTS BSC MSC
Setup
Layer 3CC Layer 3CC
Tele/bearer service

Call confirmed (SDCCH)
bearer service Layer 3CC
Layer 3CC

Normal Assignment Procedure for TCH

Ring tone alerting
Layer 3CC
Layer 3CC
connect
User answer
Layer 3CC
Layer 3CC

Connect acknowledge
Layer 3CC Layer 3CC


SMS Point to Point

The Short Message Service, SMS, provides means of sending text
messages, to and from GSM mobile station. SMS makes use of service
centre, which acts a store and forward center for short messages.


Mobile Terminated SMS

HLR

SMS - C SMS - GMSC MSC/VLR


SMS –C has the capability to transfer the short messages and also
provides the information about the delivery.

3. A user sends a message to an SMS – C
4. SMS – C sends the message to the SMS – GMSC
5. SMS – GMSC interrogates the HLR for routing information
6. HLR interrogates MSC/VLR for a roaming number
7. MSC/VLR returns a MSRN to the SMS – GMSC via HLR
8. SMS – GMSC reroutes the message to MSC/VLR

Continued…..


1. MS is paged and a connection is setup between MS and the
network.
2. If authentication was successful the MSC/VLR delivers the
message to the MS. It is transmitted on the allocated SDCCH
3. If the delivery was successful a delivery report is sent from
MSC/VLR to the SMS – C.

In the case of an unsuccessful delivery the service messages
waiting will provide the HLR and VLR with the information that
there is a message in the originating SMS – C waiting to be
delivered to the MS.


Mobile Originated SMS

MSC/VLR SMS - C


1. MS establishes a connection to the network, as in the case of normal
call setup. (This step is not performed if the MS is in busy mode,
since there already exists a connection)
2. If the authentication was successful MS sends the short message to
the SMS – C via MSC/VLR. The SMS – C in turn forward the short
message to its destination. This could be MS or a terminal in the
fixed network.


Handover

Changing to a new traffic channel during call setup or busy state is called
Handover. The network makes the decision about the change. After
receiving the information about the signal strength and quality the BSC
ranks the neighboring BTSs using the information.

After a evaluation of the situation and the decision to start the handover
procedure, the network is responsible for the setup of a link to the new
BTS.


Intra MSC Handover

VLR
BSC

MSC

BSC

New link
Old link


Intra MSC handover: Handover within the same MSC/VLR service
area but different BSCs.

• The BSC request for a handover from MSC/VLR
• New link (MSC/VLR to new BSC to new BTS) is setup and if a
free TCH is available, it must be reserved.
• MS receives the order to change to the new frequency and the
new TCH.
• If the BTS change has also change of location area, the MS
sends a request for location update after the call.


Flowchart

BSC2 MSC BSC1 MS
Measurement Report
H.O. Required
H.O. Request

H.O. Request Ack
H.O. Command
H.O. Command

H.O. Completed

H.O. Completed
Clear Command

Clear Completed


Inter MSC Handover

VLR

MSC BSC

VLR

MSC

BSC
New link
Old link


Inter MSC handover: handover between the two BSCs controlled by two
different MScs. Lot of signals exchanges are required before the
handover can take place.

• The serving exchange sends a handover request to the target exchange
• Target exchange will take over the responsibility for preparing the
connection to the new BTS.
• After the setup of a link between the two exchanges, the serving
exchange will send a handover command to the MS.


Flowchart
VLR BSC2 MSC-B MSC-A BSC1 MS
(MSC-B) Measurement
Perform H.O. H.O. Report
Allocate H.O.Number Required

Send H.O. Report

H.O. Request

H.O. Request RAD CH Ack
Ack
I AM (ISUP)

ACM (ISUP) H.O. Command H.O. Command

H.O. Complete

H.O. Send End Clear Command
Complete Signal

ANS (ISUP) Clear Complete


Exercise

Q1. Name the types of the location updates exists in the
mobile network?

Q2. Describe the different kinds of paging messages?

Q3. Calculate the paging capacity (mobiles paged per second) for
the following operator setting:
paging message type = 3
blocks reserved for CCCH and AGCH = 3


Q4. Calculate the PCH requirement for following:
Number of subscriber = 75,000
Busy hour calls = 40%
Assume on average 2 pages required per call
Safety margin for peak variation in number of calls =1.2
Paging message of type 2

Q5. Which part of the network allocates the MSRN to the call?

Section 6 – SS7

CONTENTS

• Introduction
• Signaling Modes
• CCS 7 Vs. CAS
• CCS 7 Link Types
• CCS 7 Signaling Network
• Signaling Network Components
• CCS 7 Architecture
• CCS 7 Functional Blocks
• MTP
• Signaling Data link (Level 1)
• Signaling Link Functions (Level 2)
• Signaling Network Functions (Level 3)
• MTP User Functions (Level 4)

Section 6 – SS7

• Functions of Signaling Link (Level 2)
• Organization of signaling Information
• Signal Units
• Signal Unit Delimitation/ Flag Imitation Prevention
• Error Detection
• Error Correction
• Basic Method
• Positive Ack
• Negative Ack
• Preventive Cyclic Re-Transmission
• Error Rate Monitoring
• Signal Unit Error Rate Monitor
• Alignment Error Rate Monitor

Section 6 – SS7

• Signaling Network Functions (level 3)
Service Information Octet
• Routing Label
• Signaling Message Handling
• Message Discrimination
• Message Distribution
• Message Routing
• Signaling link Management
• Link activation
• Link restoration
• Flow Control

Section 6 – SS7

Introduction

Common Channel Signaling System No. 7 (i.e., SS7 or C7 ) is
a global standard for telecommunications defined by the
International Telecommunication Union (ITU)Telecommunication
Standardization Sector (ITU-T). The standard defines the
procedures and protocol by which network elements in the public
switched telephone network (PSTN) exchange information over
a digital signaling network to effect wireless (cellular) and wire
line call setup, routing and control.

Section 6 – SS7

The SS7 network and protocol are used for:

• basic call setup, management, and tear down
• wireless services such as personal communications services
(PCS), wireless roaming, and mobile subscriber authentication
• local number portability (LNP)
• toll-free (800/888) and toll (900) wireline services
• enhanced call features such as call forwarding, calling party
name/number display, and three-way calling
• efficient and secure worldwide telecommunications

Section 6 – SS7

Signaling Types

There are two types of Signaling :

1. Channel Associated Signaling (CAS)
2. Common Channel Signaling (CCS7)

Channel Associated Signaling: signaling is always sent on the same
connection as that of speech.The Signaling is associated with speech.

Section 6 – SS7

Common Channel Signaling: signaling network is separated from the
speech network.Every signaling information will have a label which
indicates to which speech connection this signaling information belongs
to.The signaling channel has no specific position (timeslot).The same
signaling channel carries information for all speech circuits as and when
required basis.

Section 6 – SS7

Advantage Of CCS7 Over CAS

• A dedicated signaling link required for each speech channel in
CAS e.g. 3 channels in 3 PCMs : CCS 7 uses only 1 channel for
a number of PCMs
• CAS is slow, so longer call setup times : CCS 7 - 64kbps fast &
efficient.
• In CAS, no possibility of signaling during the “talking phase” :
CCS 7 signaling is independent of speech.
• CAS supports limited set of signals : CCS 7 supports signal units
of variable length max. 279 octets - so much more signaling info
can be exchanged than is possible with CAS.

Section 6 – SS7

• Usage of messages instead of pre-defined bit patterns enables to
transfer call related signaling info (call establishment) as well as non call
related call info ( location update , handover , short messages etc.)
• CCS 7 - modular ; easy introduction of new & advanced services.

Section 6 – SS7

SS7 Signaling Link Types

Section 6 – SS7

C7 Signaling Network

STP

SL(SLC)

SP 1 2 3 4 5 1 3 3 SP
(SPC) 6 0 1
(SPC)

CIC SLS SL(SLC) •SP: Signaling Point
•SPC: Signaling Point Code
•STP: Signaling Transfer
Point
•SL: Signaling Link
•SLC: Signaling Link Code
•SLS: Signaling Link Set
•CIC: Circuit Identity code

Section 6 – SS7

signaling Network Components

• Signaling Points
• logically separate entities from a signaling network point of
view.

• Origination Point Code
• A signaling point at which a message is generated, i.e. the
location of the source User Part function, is the originating
point of that message.

Section 6 – SS7

• Destination Point Code
• A signaling point to which a message is destined, i.e. the
location of the receiving User Part function, is the destination
point of that message.

• Signal Transfer Point
• A signaling point at which a message is received on one
signaling link and is transferred to another link, i.e. neither
the location of the source nor the receiving User Part
function, is a Signal Transfer Point (STP).

For a particular signaling relation, the two signaling points thus function as
originating and destination points for the messages exchanged in the
two directions between them.

Section 6 – SS7

• Signaling Links
• The common channel carrying signaling information is called
Signaling link.
• Link Set
• A number of signaling links that directly interconnect two
signaling points constitute a signaling link-set.
• Signaling Routes
• The pre-determined path, consisting of a succession of SPs/
STPs and the interconnecting signaling links, that a message
takes through the signaling network between the origination
point and the destination point is the signaling route for that
signaling relation

Section 6 – SS7

• Signaling Modes
• The term “signaling mode” refers to the association between the
path taken by a signaling message and the signaling relation to
which the message refers.

Section 6 – SS7

CCS 7 Architecture
1

Layers 4 to 7 TCAP ISUP TUP
Level 4 : User Parts
SCCP
Layer 3
Signaling Network Level 3

MTP
Layer 2 Signaling link Level 2

Layer 1 Signaling data link Level 1

Section 6 – SS7

Message Transfer Part (MTP)

• Function:
• to provide a reliable transfer and delivery of signaling
information across the signaling network and to have the
ability to react and take necessary actions in response to
system and network failures to ensure that reliable transfer is
maintained.
• Includes the functions of layers 1 to 3 of the OSI reference
model.
• User functions in CCS 7 MTP terms are:
– the ISDN User Part (ISUP)
– the Telephone User Part (TUP)

Section 6 – SS7

the signaling Connection Control Part (SCCP)
– the Data User Part (DUP)

• The SCCP also has Users. These are:
– the ISDN User Part (ISUP)
– Transaction Capabilities (TC)
– Operations Maintenance and Administration Part (OMAP)

Section 6 – SS7

Functions of MTP

f

Message Network Level 3
handling management

signaling link Level 2

signaling data
link Level 1

Section 6 – SS7

Signaling Data Link (MTP Level 1 )
• Defines the physical, electrical and functional characteristics and the
physical interface towards the transmission medium (PCM30)
• signaling Data Link is a bi-directional transmission path for signaling
consisting of two data channels operating together in opposite
directions at the same data rate.
• Digital : 64 kbps channels. For PCM30 HDB3 coding is used

- Minimum allowed bit rate for telephone call control application :
4.8kbps

Section 6 – SS7

Signaling Link Functions (MTP Level 2)

• Together with signaling data link, the signaling link functions provide a
signaling link for the reliable transfer of signaling messages between
two adjacent signaling points.

• Messages are transferred over signaling link in variable length
messages called signal Units which contain additional information to
guarantee a secure transmission.

Section 6 – SS7

Functions:

• Delimitation of signaling units by means of Flags.
• Flag limitation prevention by bit stuffing.
• Error detection by means of Check bits included in each
signaling unit.
• Error control by re-transmission and signaling unit sequence
control by means of sequence numbers and continuous
ACKs
• Signaling link failure detection by signaling unit error rate
monitoring and signaling link recovery by special procedures.

Section 6 – SS7

Signaling Network Functions (MTP Level 3)

• Level 3 in principle defines those transport functions and
procedures that are common to and independent of the
operation of individual signaling links.
These functions fall into two major categories:

Signaling message handling functions – These transfer the
message to the proper signaling link or User Part.The main
functions are:-
• Message discrimination function
• Message distribution function
• Message routing function

Section 6 – SS7

signaling network management functions – These control the current
message routing and configuration of the signaling network facilities
and in the case of signaling network failures, control the
reconfigurations and other actions to preserve or restore the normal
message transfer capability. Contains signaling link management,
traffic management and route management.The main functions are:-

• Signaling link management
• Signaling traffic management
• Signaling route management

Section 6 – SS7

MTP User functions (Level 4)

• User Parts defines the functions and procedures of the signaling
system that are particular to a certain type of user of the system.
The following entities are defined as User Parts in CCS 7.
• Telephone User Part (TUP)
• The TUP Recommendations define the international
telephone call control signaling functions for use over CCS 7.

• Data User Part (DUP)
• The Data User Part defines the protocol to control
interexchange circuits used on data calls, and data call
facility registration and cancellation.

Section 6 – SS7

• ISDN User Part (ISUP)
• The ISUP encompasses signaling functions required to
provide switched services and user facilities for voice and
non-voice applications in the ISDN.

• Signaling Connection Control Part (SCCP)
• The SCCP provides additional functions to the Message
Transfer Part to provide connectionless and connection-
oriented network services to transfer circuit-related, and non-
circuit-related signaling information.

• Key Enhancements by SCCP

Section 6 – SS7

• Enhanced Addressing Capability
• upto 255 users can be addressed by the use of Subsystem
Numbers (SSN)
• SCCP provides a routing function which allows signaling
messages to be routed to a signaling point based on, for
example, dialled digits. This capability involves a translation
function which translates the global title (e.g. dialled digits)
into a signaling point code and a sub-system number.
• Connectionless and Connection-Oriented Services
• Class 0 : basic connectionless service
• Class 1 : sequenced connectionless service
• Class 2 : basic connection-oriented service
• Class 3 : flow control connection-oriented service

Section 6 – SS7

TCAP

• TCAP provides services for non-circuit related services.TCAP
receives messages from SCCP and routes it to the user.TCAP
makes it possible to have several transactions running
simultaneously.

• TCAP consists of component sub-layer and the transaction sub-
layer.The component layer provides information exchange
between two layers by the means of dialogues. A dialogue will
contain several components like action , response etc.The
transaction identifier gives each transaction a unique identity
which is also known as transaction identifier.

Section 6 – SS7

• TCAP acts as a secretary to a manager who has several engineers
reporting to it. The secretary handles all the transactions from the
manager and sends it across the appropriate engineer and also keeps
track of each transactions by having identified files for each engineers
transaction.

Section 6 – SS7

Global Title

Global title is the address of the Signaling Point which does not clearly
mention the destination address for routing. It is translated by SCCP to
get the destination address.e.g. the dialled digits.On an incoming
call,GMSC uses the Global title to determine the destination.

A MAP message entering or originating from an exchange must either be a
terminating message or a message to be routed to another exchange.

Section 6 – SS7

By analyzing the global title(GT) of the called address,the SCCP will either
route the message to another node with the help of global title routing
case (GTRC) or terminate the message in the node.

In the terminating node the message will be distributed to the correct user
with the help of the subsystem number (SSN).

Section 6 – SS7

Organization of Signaling Information

• Signal Unit : - A group of bits forming a separately transferable entity
used to convey information on a signaling link.

• Are of variable length; maximum length : 280 bytes (including 272
signaling information bytes)

• Three types of signal units, differentiated by the length indicator field
contained in each.

Section 6 – SS7

• {length limitation is imposed to control the delays one message can
cause to others due to their emission time}

• Fill-in signal unit (FISU) ; LI = 0
• Link status signal unit (LSSU) ; LI = 1or 2
• Message signal unit (MSU) ; LI = 3 to 63

Section 6 – SS7

Signal Units
• MSU:
• convey the signaling information between the user parts
(level 4) of the adjacent signaling points. E.g. IAM , ACM ,
REL.
• LSSU:
• a signal unit which contains status information about the
signaling link.
• FISU :
• a signal unit containing only error control and delimitation
information which is transmitted when there are no MSUs or
LSSUs to be transmitted.

This is done to allow for a consistent error monitoring so
that faulty links can be quickly detected and removed
from service even when traffic is low.

Section 6 – SS7

Signal Units
1

F CK SIF SIO LI FIB FSN BIB BSN F
8 16 8n,n>=2 8 2 6 1 7 1 7 8

MSU

F CK SF LI FIB FSN BIB BSN F
8 8 or 16 2 6
16 1 7 1 7 8

LSSU

F CK LI FIB FSN BIB BSN F
8 16
2 6 1 1
7
7 8 FISU

Section 6 – SS7

SU Delimitation / Flag imitation Prevention

• Signal Unit Delimitation :
• A unique pattern on the signaling data link is used to delimit a signal
unit :- 0111 1110.

01111110 Main part of Message 01111110

•Flag imitation Prevention :
>> to ensure that no false flags are produced in the
signal units, only five consecutive one’s are allowed inside
the signal unit. If more than five one’s occur consecutively,
a zero is inserted after the fifth one and is removed again in
the receiving signal terminal. This is called “bit stuffing”.

Section 6 – SS7

Error Detection

• Error Detection :
-each signal unit has standard CCITT 16 bit cyclic redundancy
check (CRC) checksum to enable the receiving terminal to
check that all bits have been received correctly.
• CK generated by transmitting SP on all fields except the
Flag.
• Receiving SP calculates CK and compares with CK in the
signal unit.
• Mismatch interpreted as error in received signal unit & error
correction procedures are invoked.

Section 6 – SS7

Error Correction

• Two forms of error correction methods are used :
• Basic method
• Preventive cyclic re-transmission (PCR)

• Basic Method:
• re-transmission occurs only when transmitting SP is informed by
receiving SP about the signal units received in error
• is a positive / negative ACK re-transmission error correction system

Section 6 – SS7

• For sequence control, each signaling unit is assigned forward &
backward sequence numbers and forward & backward indicator bits.
• Sequence Numbering is performed independently at the two SPs
interconnecting the link.

The sequence numbers are 7 bits long, meaning that at most 127
messages can be transmitted without receiving a positive ACK.

Section 6 – SS7

Positive Acknowledgment
1

FSN=125,FIB=BIB=1 Correctly received
MSU saved in RTB
FSN=126,FIB=BIB=1
MSU saved in RTB Correctly received
BSN=126,FIB=BIB=1
Both MSU deleted fm RTB MSU with positive ack,FSN=34
FSN=35,FIB=BIB=1
MSU,BSN remains 126

Section 6 – SS7

Negative Acknowledgment

• Errored MSU is discarded and not delivered to level 3 of MTP
• SP sends a negative ack in the next SU
• BSN retains the FSN of last correctly received MSU
• BIB is inverted

• All messages with FSN > received BSN sent one by one by
fetching from RTB
• FIB value inverted in all retransmitted messages

• Until all messages in the RTB are retransmitted, no fresh MSUs
are sent.

Section 6 – SS7

Preventive Cyclic Re-transmission

• Preventive Cyclic Retransmission:
• Retransmission takes place for signal units whose correct reception
is not confirmed by the receiving SP
• is a positive ACK cyclic re-transmission forward error correction
system.
• A copy of the transmitted MSU is retained at the transmitting
terminal unit until a positive ACK for that MSU is received.

Section 6 – SS7

• Re transmission Rules :
• when there are no new MSUs to be sent, all MSUs not
positively acknowledged are retransmitted cyclically.
• If new signal units are available, the retransmission cycle (if
any) is interrupted and the signal units transmitted with first
priority.
• Under normal conditions, with no MSUs to be transmitted or
cyclically re-transmitted, FISUs are sent continuously.

Section 6 – SS7

Basic Versus PCR

• In both methods, only errored MSUs and LSSUs are corrected.
• Errors in FISUs are detected but not corrected

• Both methods are designed to avoid out of sequence and duplicated
messages when error correction takes place.

• PCR method is used when the propagation delay is large (satellite
transmission).

Section 6 – SS7

• With large propagation delays, the basic method becomes
inappropriate because NACK system causes message delays to be
too long for the erroneous MSUs
• CCITT recommendations : PCR should be used when one
way propagation delay exceeds 15ms.

• Drawback of PCR : inefficient bandwidth utilization
• I.e. the maximum load level a link can be engineered for is
much less with PCR.

Section 6 – SS7

Error Rate Monitoring

• Level 2 functions detect a failure in the following circumstances:
High error rate on the signaling units.
Excessive re-alignment period.
Excessive ACK delay.
Signaling terminal failure.
Reception of continuous FISUs.
• Two types of signaling error rate monitor is provided
signaling Unit Error Rate Monitor (SUERM).
Alignment Error Rate Monitor(AERM).

Section 6 – SS7

Signaling Unit Error Rate Monitor

• Is used while a signaling link is In Service. It provides the criteria for taking a
signaling link OOS due to excessive error rate.

• Is based on a signaling unit error count (including FISUs) , incremented &
decremented using the “leaky bucket” algorithm.

Section 6 – SS7

• For each errored signaling unit , the count is incremented by
one and for each 256 signaling units received (whether
errored or not), a positive count is decremented by one (a
zero count is left at zero). When the count reaches 64, an
excessive error rate indication is sent to Level 3 and the
signaling link is put OOS.

• The error rate on signaling units should not exceed
• 64 consecutive erroneous signaling units or
• 1 erroneous signaling unit out of every 256 on an
average.

Section 6 – SS7

Alignment Error Rate Monitor

• Is used while a signaling link is in the proving state of the initial
alignment procedure.

• Provides a criteria for rejecting a signaling link for service during the
initial alignment due to an excessive error rate.

Section 6 – SS7

• The Alignment error rate monitor is a linear counter which is started
at zero at the start of the proving period and the count is
incremented by one for each LSSU unit received in error. A proving
period is aborted if the threshold for the alignment error rate monitor
count is exceeded before the proving period timer expires.

Parameter Value

Tin 5

Tie 1

M 5

Section 6 – SS7

Message Label types (SIF)
M T P m a n a g e m e n t m e s s a g e s : L a b e l ty p e A
O r i g in a t i n g D e s t i n a tio n
M a n a g e m e n t in f o r m a t i o n S LC
p o in t c o d e p o in t c o d e

T U P m e s s a g e s : L a b e l ty p e B
C i r c u it ID c o d e O r i g in a t i n g D e s t i n a tio n
S i g n a ll i n g i n f o r m a t i o n
S LS p o in t c o d e p o in t c o d e

IS U P m e s s a g e s : L a b e l t y p e C
C i r c u it O r i g in a t i n g D e s t i n a tio n
S i g n a ll i n g i n f o r m a t i o n S LS
ID c o d e p o in t c o d e p o in t c o d e
S C C P m e s s a g e s : L a b e l ty p e D
O r i g in a t i n g D e s t i n a tio n
S ig n a l lin g in f o r m a t io n S LS
p o in t c o d e p o in t c o d e

R o u tin g la b e l
T 1 1 5 6 1 1 0 -9 3 /d 0 6

F IG U R E 7 /Q .7 0 0
S S N o. 7 m e ssa g e la b e l ty p e s

Section 6 – SS7

Message Label
• CIC
• identity of the physical circuit that carries the call for which
the signaling information is meant.
• SLS
• signaling link selection is used for load sharing between
signaling links.
• SLC
• signaling link code identifies the signaling link connecting the
origination and destination SPs
For implementation of level 3 functions, the required fields are :
Service Information Octet (SIO)
Routing Label

Section 6 – SS7

Service Information Octet
• Includes :-
• service indicator (SI- 4-bits)
• sub service indicator or network indicator (NI- 2-bits)

• The SI will determine the “User”, e.g. TUP, SCCP, ISUP and the NI will determine which network is
concerned, e.g. international or national.

• Subservice Field Codes (NI)

D C B A Spare
0 0 International network
0 1 Spare (for international use only)
1 0 National network
1 1 Reserved for national use

Section 6 – SS7

Service Indicator Codes

D C B A
0 0 0 0 Signaling network management messages
0 0 0 1 Signaling network testing and maintenance messages
0 0 1 0 Spare
0 0 1 1 SCCP
0 1 0 0 Telephone User Part
0 1 0 1 ISDN User Part
0 1 1 0 Data User Part (call and circuit-related messages)
0 1 1 1 Data User Part(facility registration & cancellation messages)
1 0 0 0 Reserved for MTP Testing User Part
1 0 0 1 Broadband ISDN User Part
1 0 1 0 Satellite ISDN User Part
1 0 1 1 )
to
1 1 1 1 ) Spare

Section 6 – SS7

Routing Label

• 32 bits , consists of :
• Origination Point Code - 14 bits
• Destination Point Code - 14 bits
• Signaling link selection - 4 bits

SLS Originating Point Code Destination Point Code

• The NI, together with 14-bit point code, allows for four
signaling networks each with up to 16,384 point codes.

Section 6 – SS7

Signaling Message Handling

• Discrimination :
• discrimination function compares the DPC in the routing label with
the point code of own SP
• If DPC = own SP ; message meant for this SP
• If DPC <> own SP ; further processing performed by routing
function

• Distribution :
• distribution function examines Service Indicator to deliver the
message to the desired user part

Section 6 – SS7

• Routing :
• routing function determines the signaling link on which the message
is to be sent
• concerned with OG signaling messages
• routing table is examined along with DPC in the message to
determine the OG SLS available to route the message.

Section 6 – SS7

Signaling Link Management

• Controls the links connected to the SP to maintain certain minimum
capability of carrying signaling traffic under normal operation & in the
event of failures
» Link activation
• process of making a signaling link ready to carry signaling
traffic
» Link restoration
• procedure to bring a previously failed link back into service

Section 6 – SS7

Flow Control

• CCS 7, in common with other transport mechanisms, needs to
limit the input of data when congestion onset is detected. The
nature of CCS 7 will lead to SP/STP overload congestion being
spread through the signaling network if no action is taken. This
will result in impaired signaling performance and message loss.
In addition to signaling network congestion within a node,
congestion will also require action to prevent signaling
performance from deteriorating. There is thus a need for flow
control within the signaling system to maintain the required
signaling performance.

Section 6 – SS7

Exercise

Q1. Name the two different kind of signaling types and compare the two.

Q2. Name the users of the TCAP.

Q3. How many types of connections occur in SCCP?

Q4. Out of following, which is used for monitoring the status of link MSU,
LSSU, FISU

Section 6 – SS7

Q5. How many consecutive 1s are allowed in signaling units and why?

Section 7 – Dimensioning

Dimensioning


Objective


• Mapping on the air interface
• Microwave planning concepts
• signaling link dimensioning and load sharing
• Routing strategies
• Erlang B, Erlang C
• Numbering plan used in mobile networks
• GPRS concepts


Introduction

In a traditional telephony - signaling means the passing of
information from one point to another for setting up and
supervision of telephone calls.
• subscriber – exchange signaling (signaling between subscriber
and the local exchange)
• inter-exchange signaling (signaling between exchanges).

With the development of the CCITT Signaling System No. 7 the
capabilities have been enhanced to be able to handle non-call
related data. End user data can be transferred, as with the Short
Message Service.


Abis Mapping

Besides the traffic channels, the Abis interface also carries the required
signaling information in 64 Kbit/s channels. One signaling channel is
normally provided for each transceiver within a BTS for controlling upto
8 subscribers per carrier frequency.


Sig TRX 2

0 1 2 3

4 5 6 7 TRX 2

BSC
Sig TRX 1
TRX 1

0 1 2 3

4 5 6 7

TS 0


TS Arrangement on PCM Link :

1 Sector occupies 2TS for TCH (64 Kbps)
1TS for signaling

Total number of Time slot in one PCM 32
Out of which 1 is used as FAS and other for internal signaling.

TS available for carrying the information 30

Therefore total number of TRXs that can be cater on one PCM
= 30/3 = 10


Example:
Assuming that network has BTSs of 2 TRX in each sector, then max
number of BTSs that can share the 1PCm link is:

1 Sector occupy 5TS
Therefore, one BTS occupy 15TS

Hence, totoal number of BTSs are = 30/15
=2


TS BTS 1 BTS 2
0 PCM Management Information
1 TRX 1
2 TRX 1
3 TRX1
4 TRX1
5 TRX 2
6 TRX 2
7 TRX 2
8 TRX 2
9 TRX 3
10 TRX 3
11 TRX 3
12 TRX 3
13 TRX 4
14 TRX 4
15 TRX 4
16 TRX 4
17 TRX 5
18 TRX 5
19 TRX 5
20 TRX 5
21 TRX 6
22 TRX 6
23 TRX 6
24 TRX 6
25 Signalling BTS1, Sector1
31 Control Ring


Microwave Links

A Telecom Network has two main constituent

3. Access Network and
4. Connectivity which is the backbone connectivity.

Optical fiber is most popular for high–capacity routes in Network
however microwave radio used in lower capacity routes, in
difficult terrain, in private and military communication where
the advantage of flexibility, security and speed of installation
offered by radio are particularly valuable.


Cellular Network Application

BTS
MSC BSC

BTS


Microwave Hop: It is a bi-directional transmission system
containing 2 DMR one at each end of connecting elements.

The information could be on 2MB or higher interface. The
microwave frequency bands and the radio channel spacing in
these bands have been all standardized by CCIR.

Some typical frequency bands are 2, 4, 6,7,8, 11 & 14 GHz. Above
11GHz rain attenuation becomes a greater problem and hence
restrict to short haul (shorter hop length). Each band is further
divided into several blocks of channels which is a pair of
frequencies, f & f’ for transmission and reception.


Propagation

Microwave beam passes through the part of the atmosphere, which
is in close proximity of surface of the earth. Radio waves, like
light waves are also electromagnetic waves, though of lesser
frequency, also have the properties of light waves like
attenuation, refraction, diffraction, scattering and polarization.
While designing the system and engineering link, the effect of all
these are to be taken into consideration.

The loss between the transmitting and receiving antenna with


transmission medium as vacuum is termed as Free Space Loss.

Lfs = 92.4 + 20 log d + 20 log f

d = distance in Kms
f = frequency in Ghz


Refraction K-factor

It is the scaling factor that helps to quantify the curvature of the
radio beam

K = effective earth radius / true earth radius
True earth radius = 6370 km

The angle of curvature by refraction is denoted by the k-factor,
defined as the ratio of the effective earth radius (radius of earth
which allow the beam to draw as a straight line) to the true earth
radius.


Path Clearance Process

• Microwave Link is based on LOS
• Microwave Path curvature is based on Refraction (K)
• Microwave Path should also have Fresnel Zone clearance to
avoid diffraction

Fresnel Zone: The area around the line of sight path which results
into a reflection of 180° (half wave length) at the receiver is
termed as First Fresnel Zone. The area which results in 2 and 3
half wave lengths are Second Fresnel Zone.


Fn = 17.3 Sqrt ( nd1d2/f D)
Fn = Radius of Fresnel Zone (center point at path)
d1 = distance from one end of path to reflection point (km)
d2 = distance from other end of path to reflection point (km)
D = d1 + d2
f = frequency (GHz)
n = number of Fresnel Zone


Path Profile

Linear Method
• Microwave beam is drawn as a straight line
• The effective earth curvature height (h) is calculated for a
desired k-factor

h= (d1d2) / 12.75 k
• Fresnel Zone clearance is then calculated for the same k value

Earth Bulge = Effective earth curvature height + Fresnel Zone
clearance


Countermeasures

Flat Fading:
• Link Overbuilding (Antenna gains, improved receiver
performance, power)
• Shorten distance between sites
• Path diversity
Selective Fading:
• Space diversity
• Frequency diversity
Equipment Reliability:
Hot- Standby arrangement


Space Diversity


Frequency Diversity

Tx 1 Rx 1

Tx 2 Rx 2


Over Reach Interference

f1 f2 f1
f1’ f2’ f1’


Signaling Planning Objective

The main planning objectives are:
• Reliability - disturbances in the signaling should be avoided.
• Robustness - a fault in one part of the network should not affect other
parts.
• Simple Network Architecture - the structure of the network should be
easy to understand.
• Short Delay Times - to cater for high quality of service.


Signaling Link Dimensioning
Purpose: to dimension the correct amount of hardware to meet
the requirements.
• Over dimension > inefficiency
• Under dimension > congestion
• Input data: - subscriber data
- network data
- GoS
- equipment limitations


Simplicity is achieved by introducing hierarchical levels. Hierarchical
networks are flexible and allow fast expansion of the PLMN. Hierarchical
networks are also easy to operate and manage.

Major part of signaling network delay is induced in intermediate nodes and
not so much on the links (in a properly dimensioned network).
Hierarchical network structures are therefore also to be preferred from
his point of view.


Definition of Traffic

BHCA x MHT
A=
3600

Where: A is the traffic expressed in Erlang (E)
BHCA = Busy Hour Call Attempts
MHT is the average holding time (s)
3600 is the number of seconds per hour


When designing the network, redundancy is of major importance. There
are cases though when separation of the connections on different routes
is not plausible. One should then at least consider hardware
redundancy.


Traffic Link Redundancy

80% of the traffic saved if one link goes down

2 separated routes 3 separated routes

eg 10E per link then:
80*(10+10+10)/2=16E
80*(10+10)=16E
The redundancy factor becomes 1.6 and 1.2 respectively


C7 Signaling Concept in the GSM Network

Maximum signaling load per signaling link
30 % under normal conditions
60 % under overload conditions
64 kbit/s = 8000 octets/s (1 octet = 8 bits)
Normal load = 0.3 x 64 = 19.2 kbit/s or
0.3 x 8000 = 2400 octets/s
Overload = 0.6 x 64 = 38.4 kbit/s or
0.6 x 8000 = 4800 octets/s


A widely used dimension rule, based on No. 7 signaling link
dimensioning for plain PSTN with TUP, is to allow 30% load on
links in normal operation and 60% in failure situations.

In GSM networks 20% load in normal operation is often used. With
MAP MSUs instead of TUP the same signaling volume is
generated by fewer and longer MSUs that implies a more bursty
load requiring more margin to achieve the same quality.


Signaling Volumes
Signaling is required not only for setting up of call connections, but also for
finding and upgrading the present location of the subscriber. Enhanced
security including both authentication and equipment identity control require
No. 7 signaling.

Estimates of the signaling generated by different events in the network can be
used to calculate the approximate signaling load.


Signaling Calculation Model:

The main input parameters are:
• Traffic per subscriber
• Mean Call holding Time
• Percentage MT traffic
• Location Updates per subscriber and hour
• Inter MSC handovers per call
• IMSI attach per subscriber and hour
• Number of authentication triplets fetched at a time
• short messages per subscriber and hour


signaling Volume Example

Model 1 Model 2

Traffic per sub 0.030E 0.025E
Mean holding time 100s 120s

MT Percentage 33% 25%

Location Updates new VLR / 1.1 0.45
sub&hour
Inter MSC Ho/call 0.10 0.05

SM / sub&hour 0.5 0.1

MSC - HLR kb/s per ksub 1.55 0.65

MSC - MSC kb/s per ksub 0.35 0.15
MSC -EIR kb/s per ksub 0.20 0.10


There is a different possibilities for the operator to influence the signaling
volumes per subscriber:

• Placing of MSC borders as well as LA borders impact the mobility
experienced in the network. (it reduces the Location Area update
signaling)
• Parameter settings in the AUC for use of selective authentication
• Parameter settings in the EIR for IMEI check


C7 Routing Strategies

In order to meet the need for extended services, i.e.
communication with databases without speech connections, the
SCCP is used. SCCP maintains connection oriented (CO),
connectionless (CL) network services, circuit related and non-
circuit related signaling.

• Connection-oriented signaling: used when many messages to
transfer between two signaling points (SP) and when messages
are so long that segmenting is needed.


• Connectionless signaling is used for MAP. In connectionless
signaling all message signaling units contain all information
required to route each message unit to the correct destination.
• Circuit related signaling is signaling related to a specific speech
or data connection
• Non circuit related signaling is signaling not connected to any
speech or data connection, i.e. roaming signaling in mobile
application.

SCCP make possible routing of the message on a higher level
(Global Title Translation (GTT), SCCP rerouting), i.e. handle the
logical signaling connection, and MTP is responsible for
transporting the message through the network in a reliable
manner.


SCCP Routing


The SSN indicates the subsystem so the message is distributed to the right
software in the terminating node. SSN points out MAP HLR, MAP VLR,
MAP MSC/GMSC, BSSAP, MAP EIR, MAP AUC, MAP SC, and ISUP.


MTP Routing
The routing procedure as well as the load sharing between link sets and
within link sets is done using:
• Network Indicator (NI),
• Destination Point Code,
• an Originating Point Code (OPC) and
• a four bit signaling Link Selection code (SLS).

NI identifies a No.7 Network. DPC and OPC are the signaling Point
Code (SPC) that uniquely defines a signaling Point (SP) in the No.7
signaling network.


MTP signaling route could either be one signaling link set or load sharing
over signaling link sets.


Signaling route alternatives with different priorities can be defined
and the routing alternative with lower priority will not be set into
action until the alternative with the higher priority is totally
blocked.

Signaling routing in the GSM can be understand by the example of
the network having three HLRs in three different zones along
with STPs.


Routing principles for No. 7 signaling:

• Western MSC load-share signaling to HLRs over Western STP
to East HLR and East STP to East HLR. Second choice, if both
link sets are out of order, signaling is routed over Central STP to
East HLR.
• Similar is the case for other two HLRs.
• HLRs are connected to all three STP. Routing of signaling
depends on destined MSC group:
• signaling towards western MSCs is routed in load-share over W
E and E E. Second choice, if both link sets are out of order,
signaling is routed over C E.


• signaling towards central MSCs routed in load-share over W E and C E.
Second choice, if both link sets are out of order, signaling is routed over
E Tr.
• signaling towards eastern MSCs routed in load-share over C E and E E.
Second choice, if both link sets are out of order, signaling is routed over
C E.


Signaling Load Sharing

For load sharing both between link-sets and between the links on
the link-sets the signaling Link Selection code is used. This is a
four-bit code that is set by the MTP user. Which bit to be used as
the load sharing bit for load sharing between the link sets is set
by the LSHB-parameter (Load sharing Bit) in the exchange data.

If all links get the same number of SLS codes they will all carry the
same load, i.e. the load is evenly distributed. If all the links do
not get the same number of SLS codes then the load will not be
evenly distributed.


The maximum load on the link set is limited by the signaling links carrying
most of the signaling load


C is the maximum load in normal operation for one link. For example, if we
allow 30% maximum load on each 64kb/s link and we have 8 signaling
links in a link-set. Then, assuming that we do not load share with
another link-set (i.e. four bit load share within the link-set) the capacity
of link set is 8*30%*64kb/s=153.6 Kbps.


MTP Changeover in case of link failure


Traffic Models
Two commonly used models are Erlang B and Erlang C:


Erlang B

This is a loss model, in that blocked calls are simply lost rather
than being held in some form of queuing system.

It assumes that call arrivals follow a Poisson process, that the
number of users is much greater than the number of channels.

From the Erlang-B table, 7 channels and a GoS of 0.02 (2%)
corresponds to A= 2.9354 Erl of offered traffic.


Therefore, carried traffic = A (1- GoS)
= 2.9354 (1- 0.02)
= 2.87669Erl

Channel Utilization: This is the ratio of carried traffic to
number of channels

Therefore,
Channel Utilization = 2.87669/7
= 0.41 or 41%


Erlang C

Calls that cannot be handled are put in a queue until a channel becomes
available. The queuing delay is a function of the offered packet traffic,
the maximum number of links available and the mean holding time of
each call. The Erlang C formulas are used to determine the probability
of a delay occurring, the probability of such a delay being larger than a
certain time and also the mean delay.


Example: As compared with circuit switched traffic with a blocking
probability of 2% 17.5 Erlangs corresponds to 22 Erlang in C
table.

This suggests that there is a gain in trunking efficiency offered by
tolerating a 10 ms delay in transmission.

Mean delay depends on the mean holding time, which in turn is
proportional to the packet size. Packet size can be reduce in
order to reduce the holding time but it increase the signaling
overheads.


Processor Load


Definitions
The processor load is the proportion of time that the processor executes
instructions having real time requirements. It is normally expressed in
percentage of its full capacity.

It has following components:
Idle load: This component depends on the functionality and to some
extent on the size of the exchange. The idle load is not dependent on
the traffic or other external activities but varies from processor to
processor.

Continued…..


Usage load: This component is caused by operation and
maintenance activities such as data dumps, commands, traffic
measurements and printout of statistics.

Traffic load: This component is used for traffic handling.

Loadability: The loadability is the upper limit for the allowed
processor load. It depends on the processor but also on the job
lengths and delay requirements.

Continued…..


Load per call: This is the amount of execution time that the
processor has to spend in setting up and disconnecting a call.
Load per call is normally expressed in milliseconds (ms), but is
sometimes expressed as the number of ASA (assembler)
instructions necessary to fulfill the task.

Traffic peak margin: Is sometimes referred to as Safety margin.
The traffic peak margin is normally 20-35% of the available
traffic load. This is needed to allow for unpredictable traffic
peaks.


Capacity

Traffic capacity, (e g 2,500 Erlang), tells how many simultaneous
calls a unit can handle. One Erlang corresponds to one busy
line. If a subscriber calls 25 mErlang during busy hour, he is in
average calling 25/1000 of the hour (=25*60*60/1000 = 90
seconds).

Erlang can be limited by for example the group switch, available
speech trunks, transcoders etc. But this does not give any idea
about the processor loading as well as nor about non call
activities.

Continued…..


Call capacity, (e g 100,000 BHCA), tells how many call attempts a
unit can handle during busy hour. This figure is a better measure
of processor capacity but still, this measure does not take into
account non-call related activities.

Subscriber capacity, (e g 60,000 subscribers), tells how many
subscribers that can be served by a unit. This figure is strongly
depending on subscriber behavior.

Continued…..


Addressing capacity, (e g 1020 TRXs), tells how many HW or SW
devices that can be connected / defined. This is also known as system
limits. Here, no considerations to real-time processing needs or amount
of traffic are made.


Traffic Load Distribution


In the default traffic load distribution for a GMSC/MSC/HLR the call part
takes about 70% of the capacity of the traffic load, the location updating
part about 25%, the SMS part 3% and supplementary services
approximately 2%.

If one look into the traffic part (70% of traffic load) the actual basic load part
is 53% of the usage load, a gate way load part is 7.5%, a charging part
5%, a handover part 3% and a part used for authentication about 1%


Numbering Plan

The MSISDN is a number which uniquely identifies a mobile telephone
subscription in the public switched telephone network numbering plan. These
are the digits dialed when calling a mobile subscriber.

In GSM 900/GSM 1800, the MSISDN consists of the following:
MSISDN = CC + NDC + SN


CC = Country Code
NDC = National Destination Code


International Country National Subscriber
Prefix Code Destination Code Number
0091 98 113 23448

The digits ‘113’ identify the GSM 900/GSM 1800 PLMN area
code.

The digits ‘23448’ define the five digits, which identify the
mobile subscriber.


A NDC is allocated to each PLMN. In some countries, more than
one NDC may be required for each PLMN.

The international MSISDN number may be of variable length.The
maximum length is 15 digits, prefixes not included.

Example: Singapore PSTN subscriber is calling to an Indian
GSM PLMN subscriber

Continued…..


International Mobile Subscriber Identity (IMSI)

The IMSI is the information which uniquely identifies a sub in a GSM
PLMN. It is used in all the signaling in the PLMN.

It will be stored in the in the Subscriber Identity Module (SIM), as well as in
the HLR and in the serving VLR.

It consists of three different parts


IMSI = MCC + MNC + MSIN

MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits)
MSIN= Mobile Station Identification Number

All network related subscriber information is connected to the IMSI.


In GSM 1900, the MSISDN consists of the following:
MSISDN = CC + NPA + SN
CC = Country Code
NPA = Number Planning Area


The NPA is allocated to each GSM 1900 PLMN. The length of MSISDN
depends on the structure and operating plan of each operator. The
maximum length is 15 digits, prefixes not included.


Examples:
xyz = operator code
abcde = Subscriber number
STD code = PSTN area code (11 for delhi)

• Call from PSTN to PLMN
Local Call 98 xyz abcde
Outside area call 0 98 xyz abcde

• Call from PLMN to PSTN
Local Call 0+STD code+SN
Outside area call 0+STD code+SN


GPRS Core Network Planning


Circuit Vs Packet Data

Circuit Switched Service:
• 2G system (primarily voice and data on circuit switched air interface)
• Call charging based on channel holding time.
• Maximum number of users per TDMA channel is 8
• Suitable for constant bit rate applications
• Resource allocation is done such that UL and DL are paired.


Packet Switched Service:

• Several users can share the same channel.
• Charges based on channel usage (actual usage of byte
transferred).
• Well suited for bursty traffic.
• Resource allocation done independently on UL and DL (good for
applications with asymmetrical bit rate)
• Dynamic allocation of resources
• Can multiplex traffic (voice, data, video).


S peech traffic leaves s ome capacity for
10
9
packet data
8
TCH 14
7
6
5 12
4
3 GSM
2 10

1 capacity
0
8

Offered GPRS Traffic
6

14 4

12

10 2

8
TCH 0
6

4

2

0

Circuit Switched Traffic

2


GPRS System feature

• Variable quality of service.
• Independent packet routing.
• Protocol transparent (encapsulation & tunneling)
• Slotted ALOHA for random access procedure
• Provides IP connectivity to mobile subscriber.
• Build on existing GSM infrastructure with added nodes for supporting
packets.
 Serving GPRS Support Node (SGSN)
 Gateway GPRS Support Node (GGSN)


Conceptual View on GPRS

GSM Voice

Access
Point

GPRS Core Internet
BTS BSC
Corporate Intranet

Shared GSM and GPRS GPRS Infrastructure IP World
Infrastructure


Air Interface - Mobile Terminal

• Type C GPRS only
(or manually switched between GPRS and speech modes)

• Type B GPRS and Speech (not at same time)
(Automatically switches between GPRS and speech modes)

BSC
• Type A GPRS and Speech at the same time

BTS


GPRS Attach / Detach

• Attach
 Performed when the MS indicates its presence to PLMN for the purpose
of using GPRS service
 Carried out between MS and SGSN
 MS identifies itself with its GSM identity
 GPRS subscription necessary for successful attach
• Detach
 Performed when the MS indicates to the PLMN that it no longer be
using GPRS services
 MS identifies itself with its GSM identity


System Architecture

HLR
BTS Gc
Gr

BTS BSC SGSN GGSN

Gb Gn
BTS
Gi

Data
Networks
Um Abis


SGSN

• Responsible for delivery of packets to mobile subscribers in its
service area.
• Mobility Management
• Logical link management, authentication
• GPRS user- related data needed by SGSN to perform routing
and transfer functionality stored in GPRS Register eg current
cell, current VLR, user profile including IMSI and its address in
PDN.
• Interface point between core and Radio networks


GGSN

• Acts as an interface between GPRS network and external PDNs
• Mainly responsible for packet routing, transfer and mobility
management
 Converts packets from SGSN into appropriate PDP format and
sends them out to corresponding PDN
 PDP addresses of incoming data packets from PDN are
converted to IMSI of the destination user and sent to the
responsible SGSN.
 Tunneling


GPRS and GS M Res ource s haring
Circuit
Switched
TRX 1 CCCH TS TS TS TS TS TS TS
Territory

TRX 2 TS TS TS TS TS TS TS TS Packet
Switched
Territory
Additional Default Dedicated
GPRS GPRS GPRS
Capacity Capacity Capacity

Territory border moves
Dynamically based on Circuit
Switched traffic load

• Circuit Switched traffic has priority
• In each cell Circuit Switched & Packet Switched territories are defined
• Territories consist of consecutive timeslots


Capacity Management

• Dedicated GPRS Capacity
TCHs reserved exclusively for GPRS use.

• Default GPRS Capacity
TCHs always allocated to the GPRS when circuit switched load
permits.
Keeps GPRS timeslots consecutive (important for multislot
operation)

PDP Context Activation - 1
Accessing the HLR
• (1) MS sends "Activate PDP
Context Request" to SGSN
– Access Point Name
HLR
– PDP Type (IP)
2. – PDP Address (empty ==
BTS BSC
dynamic)
SS7
1. – QoS & other options
APN=
"Intranet.Ltd.com" SGSN
DNS (2) SGSN checks against HLR
Access Point Name
Dynamic / static IP address
GPRS Access
Backbone Point QoS
IP Network Intranet

GGSN

Internet

• Access Point Name = Reference to an
external packet data network the user
wants to connect to


Finding the GGSN

• (1) SGSN gets the GGSN IP address
from DNS
– APN maps to the GGSN IP
address
(2) SGSN sends "Create PDP Context
Request" to GGSN
BTS BSC PDP Type (IP)
PDP Address (if empty=> dynamic
address)
Access Point Name
SGSN QoS
1. DNS

GPRS Access
Backbone Point
2. IP Network Intranet

GGSN

• DNS = Domain Name System = mechanism to
map logical names to IP addresses


Access Point Selection

• Access Point Name refers to the
external network the subscriber
wants to use
–Physical/logical interface in
GGSN
BSC
• Access Point configuration in
BTS
GGSN defines where to connect
the user
• If dynamic address, allocated by
SGSN
GGSN
DNS
APN="Intranet.Ltd.com"
GPRS
Backbone
IP Network Intranet

GGSN

Internet


Context Activated

• (1) GGSN sends "Create PDP
Context Response" back to SGSN
• (2) SGSN sends "Activate PDP
Context Accept" to the MS
• SGSN now ready to route user
BTS BSC traffic between MS and GGSN

2.
SGSN

GPRS
Backbone
IP Network
GGSN Intranet
1.

Internet


Exercise

Q1. How many PCMs are required for one BTS with 2,1,2 and
other with 3,2,1 configuration?

Q2. Calculate the free space loss for 20Km distance at 15GHz
frequency?

Q3. Calculate the 2nd Fresnel Zone for total distance of 20Km at a
distance of 10Km from one end. Frequency used is 15GHz.

Q4. What precaution has to be taken to avoid the over reach
problem in the microwave links?

Section 8 – Optimisation

Optimisation


Objective


• signaling delay in the network
• Effect on the network while introducing the new releases
• Impacts of subscriber behavior
• TCP/IP concepts


Introduction

The goal of optimization is to ensure the network is operating at
optimum efficiency and within the defined quality of service
constraints. This is achieved by implementing corrective action
and procedures to rectify network problems identified though
analysis of performance management monitoring parameters.

Vendors are continually seeking ways of maximizing revenue
generation with minimum additional investment. One way of
achieving this is to identify areas where the network is not
operating at peek efficiency and making adjustments for
improvement.


Optimization is a Cyclic Process


Signaling Delay

The signaling network delay depends on a variety of parameters,
among others: bit error rate, signaling link propagation and
processing time, average link load, mean MSU length on link,
mean MSU length of transmitted signal, number of signaling
links in signaling path, number of STPs in signaling link path,
buffering and queuing times in STP etc.
Key parameters that are varied are mean MSU-length, mean
signaling link load, and number of STPs and signaling links in
path.


Typical values used for calculating the delay:

Bit Error Rate on link 8.3x10-4
Mean MSU lengths a) 23 oct
b) 74 oct
STP delay 20ms

Signaling link propagation 10ms
and processing


For a constant bit error rate of 8.3x10-4 and basic error correction, the
waiting times (Tw) on the outgoing side are shown in table below for
mean MSU length 23 octets and for mean MSU lengths of 74 octets.


STP Delay (TSTP): In CCITT Blue Book, a cross STP delay of 20ms is
estimated for 0.2 link load.

Propagation and Processing Time (TL): This includes transmission
time on link and processing time of message. The overall main part of TL
is the transmission time. For ground-installed links for which basic error
correction is used, TL should be less than 15ms.


Signaling Network Delay Example: Consider two cases

3. the signal passes one intermediate STP before reaching its
destination

2. the signal passes two intermediate STPs before reaching its
destination


Signaling Network Delay with one intermediate STP.

The signaling link delay, SLD is derived from:
SLD = 2x(TW + TL) + TSTP

Signaling Network Delay with two intermediate STPs.

The signaling link delay, SLD is derived from:
SLD = 3x(TW + TL) + 2xTSTP


It is to be mentioned that dependence between the MSU lengths and the
delay times is not necessarily linear.


Impacts On Capacity

• When introducing a new release
New releases typically mean a drop of 10-15% of system
capacity. The BSC decrease is often less than for MSC. The
reason is that new BSC releases often contain more O&M
improvements than traffical ones.
• Subscriber Behavior
The call type affects the capacity required per call, e.g., the load
per call is different depending on type of call. Load per call is
defined as the execution time of a call. This is the time
necessary to execute the program code for a call in the CP
(Central Processor). By a call is meant a call setup, call release
and information sent in connection with the call.


Call attempts have the highest impact on capacity. One call setup
plus clear consumes about 25 ms execution time. SMS point-to-
point takes about 2/3 of call execution in the BSC (2/3 of 25 ms).
Most SMS/ptp are mobile terminated, and need paging as well.

Registrations take roughly 1/3 of call execution in the BSC. Due to
the big number of them, the total CP load from registrations is
often higher than for calls.


• Network Configuration
The number of BSCs per MSC can have a major impact on the
system capacity due to the shift of intra-BSC handovers to the
inter-BSC handovers, which will increase in case of a higher
number of BSCs. An increase of the number of inter-BSC
handovers with a factor of 10 will take 7% more of the capacity.

A MSC configuration with stand alone HLR will increase the
capacity of the MSC with 15% compared to a MSC with
integrated HLR (worth mention that this 15% figure has been
derived from comparing the total MSC/HLR capacity with the
maximum capacity of a MSC without HLR).


The BSC covering areas should generally be chosen so that the
boundaries as far as possible are located in areas with low
handover intensity. The reason is that high handover frequency
decreases MSC and BSC capacity. Consequently, boundaries
through city kernels and areas close to highways should, if
possible, be avoided.

The value that the periodic location update is set to affects the
capacity. The period can be set between 6 and 1530 minutes in
steps of 6 minutes. The minimum period sustainable by the
system depends on the number of subscribers and their traffic
behavior.


The number of periodic location updates has a significant impact
on the MSC capacity, therefore it is advisable to set the periodic
location update timer very carefully. Most operators choose a
short period for the forced registration, caused by the fear of
loosing track of the subscribers. In case of system recovery after
a large restart the periodic location update rate will impact the
recovery time severely. Therefore the recommendation is to use
120 minutes for the timer value. It is worth mention that the
positive effect on the MSC may impact the BSC performance
negatively due to a higher number of pagings.


Number of Location Areas (LAs) has impact on BSC load. If there are
many cells per area, the local page attempts will be quite heavy. If
increasing the number of LAs, the paging load will go down. On the
other hand: If high movability for mobiles, the load from location updates
will increase. When finding the optimal point, also load in MSC must be
looked into.


• Adding New Applications
The following table presents the CP capacity impacts on an
average node

AUC (Authentication Center) -0.4%
FNR (Flexible Numbering) -2.5%
SCF (Service Control Function) -2.0%
(Based on 10% IN calls)
SSF (Service Switching Function) -10%
(Based on 10% IN calls)
PRA (Primary Rate Access 30B + D) -19%
(Based on 10k BHCA PRA traffic)


Capacity Gains

• IMEI Check on Location Update
It is possible to switch off the IMEI check function for location
update, which increases the capacity with 2%.

• Usage of Toll Ticket
Output only those call data records that are needed, where
possible accounting should be used instead. For instance
switching off the Land to Land call data record increases the
capacity with 3.2%.


• TMSI Reallocation
Switching off the TMSI reallocation at location update, change of LAI, intra-
MSC function will result in 2% more capacity.

• Authentication at Location Update
Switching off authentication at location update, change of LAI, intra-MSC
will result in an increase of the capacity with 1%.


• Selective IMEI Check
It is advisable to use the selective IMEI check for all access types,
which results in a gain of capacity of 4%. To be able to decrease
the system recovery time it is recommended to switch off IMEI
checking for the access type location update.

• Selective Authentication
The usage of selective authentication for all access types is
strongly recommended from a capacity point of view. In case of
the activation of selective authentication instead of
authentication for each access, the increase of capacity is equal
to 6.2%.


Conclusion

A better network and cell planning will result in some cases in more
capacity, when less location updates and handovers are
needed. Moreover the number of small nodes in a network may
decrease the overall network capacity, since they may introduce
more inter-MSC handovers, more new registrations and a higher
amount of transit traffic compared to a network with several big
nodes. Furthermore the split of GMSC and MSC allows a better
maintainable network and more capacity in the separate entities,
also the usage of different processors for each entity will be
possible. Stand-alone HLR will also increase the total capacity in
the network.


GPRS TCP/IP Strategies

Datagram: It is a technical term for a packet of data and composed of
many components. The most basic is:

Header To: 129.23.88.12
From: 136.24.87.23
010001010100101010100100101111010100101010010101010010101001010
100101010101001010101001011100001111101001001000101010001000000
011110010010100100010101001010101001001011110101001010100101010
100101010010101001010101010010101010010111000111110100100100010
101000100000001111001001010010001010100101010100100101111010100
Data 101010010101010010101001010100101010101001010101001011100001111
1010010010001010100010000000111100100101001000100


IP Datagram Components

Version IHL Type of Service Total Length
Identification Flags Fragmentation Offset
Time to Live Protocol Header Checksum
Source Address
Destination Address
Options (and padding)

Data


What’s in a Datagram

• Version: Version of IP (example: IPv4, IPv6)
• IP Header Length: The datagram’s header size in 32 bit words.
• Type of Service: Indicates “priority” of the packet. This is determined
by the type of data in the packet. (QoS - Quality of Service)
• Total length: Size of the IP packet (in bytes).
• Identification: An integer number identifying the datagram.


• Flags: A 3-bit field of which the low-order 2 bits control
fragmentation. One bit specifies whether the packet can be
fragmented; the second bit specifies whether the packet is the
last fragment in a series of fragmented packets.
• Fragmentation Offset: A sequence number for the bytes in
this packet when reassembling.
• Time-to-live: A counter that discards the datagram when it
reaches a limited. This prevents the packet from looping
endlessly on the network.
• Protocol: Indicates which upper-layer protocol receives
incoming packets after IP processing is complete.


• Header Checksum: Helps ensure IP header integrity.
• Source Address: Specifies the sending node.
• Destination Address: Specifies the receiving node.
• Options: Allows IP to support various options, such as security.
• Data: Information payload.


• TCP/IP is the Packet Data
technology used by the Internet.
• GPRS will also be using the
TCP/IP standard.


Application
Presentation WWW, e-mail, data services
(OSI Reference Model)
TCP/IP 7-Layer Stack

Session
Transport TCP

Network IP

Link Network Interface Card

Physical Fiber cable, Microwave link


TCP Characteristics

• Concerned only with the origin
and destination on the network.

• Adapts to congestion

• Provides virtual connection


IP Addressing

• For example:
• 150.215.17.9 (Octets 0-255)
• In binary form, it looks like:
10010110.11010111.00010001.00001001
• “IP number” is like an address

136.20.2.1 136.20.2.2 136.20.2.3


• An IP address consists of two parts
• Identifies the network
• Identifies the node or host
• These two parts specifies the class where the node belongs..


Address Classes

• There are 5 different address classes.
• The first byte of the first octet determines the class of the address.
• Class A addresses start with 0.
• Class B addresses start with 10.
• Class C addresses start with 110.
• Class D addresses start with 1110.
• Class E addresses start with 1111


5 Classes of IP Address

1

125 Class A: 1-126
127: Reserved (loopback)
Quantity of
Domains
(Networks)
in each
Class 63 Class B: 128-191

31 Class C: 192-223
15
15


Finding an IP’s Network Address
• When a node receives a packet, it needs to determine the Network
Address of the network where the destination node belongs.
• This is done by using the network subnet mask.
• Subtracting the subnet mask to an IP address results in the identification
of the network and node sections of an the IP address

10010110.11010111.00010001.00001001
150.215.017.009
- 11111111.11111111.00000000.00000000
255.255.000.000
10010110.11010111.00000000.00000000
150.215.000.000


Transmission Methods

• Transmission is the supporting layer under TCP/IP.

• Types of transmission
• Frame Relay
• ATM (Asynchronous Transfer Mode)


ATM

Asynchronous Transfer Mode - A high speed, low delay, multiplexing and
switching technology that can support any type of traffic including voice,
data, and video applications. ATM is ideally suited to applications that
cannot tolerate time delay, as well as for transporting frame relay and IP
traffic that are characterized as “bursty”.


Other Packet-Based Networks

• X.25 --- A popular standard for packet-switching networks.

• CLNP --- (Connection-Less Network Protocol) derived from IP.

GSM ARCHITECTURE

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