GSM Introduction

05/31/13Tempus Telcosys 1
ADA CELLWORKS PVT LTD

INTRODUCTION
 The global system for mobile communications (GSM)
is a set of recommendations and specifications for a
digital cellular telephone network (known as a Public
Land Mobile Network, or PLMN). These
recommendations ensure the compatibility of equipment
from different GSM manufacturers, and interconnectivity
between different administrations, including operations
across international boundaries.

THE GSM NETWORK
 The GSM network is comprised of the following components:
 Network Elements
 The GSM network incorporates a number of network elements to
support mobile equipment. They are listed and described in the GSM
network elements section of this chapter.
 GSM subsystems
 In addition, the network includes subsystems that are not formally
recognized as network elements but are necessary for network
operation. These are described in the GSM subsystems (non-network
elements) section of this chapter.
 Standardized Interfaces
 GSM specifies standards for interfaces between network elements,
which ensure the connectivity of GSM equipment from different
manufacturers. These are listed in the Standardized interfaces section of
this chapter.

THE GSM NETWORK - CONTINUED
 Network Protocols
 For most of the network communications on these interfaces,
internationally recognized communications protocols have been used
 These are identified in the Network protocols section of this chapter.
 GSM Frequencies
 The frequency allocations for GSM 900, Extended GSM and Digital
Communications Systems are identified in the GSM frequencies section
of this chapter.

DIGITAL NETWORKS
 GSM networks are digital and can cater for
high system capacities. They are consistent
with the world wide digitization of the
telephone network, and are an extension of
the Integrated Services Digital Network
(ISDN), using a digital radio interface
between the cellular network and the mobile
subscriber equipment.

INCREASED CAPACITY
 The GSM system provides a greater subscriber capacity than
analogue systems. GSM allows 25 kHz. Per user, that is, eight
conversations per 200kHz. Channel pair (a pair comprising one
transmit channel and one receive channel). Digital channel coding
and the modulation used makes the signal resistant to interference
from the cells where the same frequencies are re-used (co-channel
interference); a Carrier to Interference Ratio (C/I) level of 9 dB is
achieved, as opposed to the 18 dB typical with analogue cellular.
This allows increased geographic reuse by permitting a reduction in
the number of cells in the reuse pattern. Since this number is directly
controlled by the amount of interference, the radio transmission
design can deliver acceptable performance.

CGI : CELL GLOBAL IDENTITY
MCC MNC LAC CI
LAI
CGI
MCC = Mobile Country Code
MNC = Mobile Network Code
LAC = Location Area Code
CI = Cell Identity

MSISDN
CC NDC SN
98 XXX 12345
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number

MSISDN
 The Mobile Subscriber ISDN (MSISDN)
number is the telephone number of the MS.
This is the number a calling party dials to
reach the subscriber. It is used by the land
network to route calls towards the MSC.

IMSI
 IMSI (International Mobile Subscriber
Identity) Network Identity Unique To A Sim.
MCC MNC MSIN
404 XX 12345..10
SIM = Subscriber Identity Module
MCC = Mobile Country Code
MNC = Mobile Network Code
MSIN = Mobile Subscriber Identity Number

IMEI
 IMEI : Serial number unique to each mobile
TAC FAC SNR SP
6 2 6 1
IMEI = International Mobile Equipment Identity
TAC = Type Approval Code
FAC = Final Assembly Code
SNR = Serial Number
SP = Spare

SUBSCRIBER IDENTIFICATION
 International Mobile Subscriber Identity (IMSI)
 Just the IMEI identifies the mobile equipment, other numbers are
used to identify the mobile subscriber. Different subscriber identities
are used in different phases of call setup. The International Mobile
Subscriber Identity (IMSI) is the primary identity of the subscriber
within the mobile network and is permanently assigned to that
subscriber.
 Temporary Mobile Subscriber Identity (TMSI)
 The GSM system can also assign a Temporary Mobile Subscriber
Identity (TMSI). After the subscriber’s IMSI has been initialized on the
system, the TMSI can be used for sending backward and forward across
the network to identify the subscriber. The system automatically
changes the TMSI at regular intervals, thus protecting the subscriber
from being identified by someone attempting to monitor the radio
channels. The TMSI is a local number and is always transmitted with
the Local numbers and is always transmitted with the Location Area
Identification (LAI) to avoid ambiguities.

SUBSCRIBER IDENTIFICATION MODULE
(SIM)
 By making a distinction between the subscriber identity and the
mobile equipment identity, a GSM PLMN can route calls and
perform billing based on the identity of the subscriber rather than
the mobile equipment being used. This can be done using a
removable Subscriber Information Module (SIM). A ”smart card” is
one possible implementation of a SIM module.
 IMSI. This is transmitted at initialization of the mobile equipment.
 TMSI This is updated periodically by the PLMN
 MSISDN This is made up of a country code, a national code and a
subscriber number.
 Location Area Identity (LAI) This identified the current location of
the subscriber.
 Subscriber Authentication Key (KI) This is used to authenticate
the SIM.

EQUIPMENT IDENTITY NUMBER
 International Mobile station Equipment Identity (IMEI)
 Each MS is identified by an International Mobile station Equipment
Identity (IMEI) number which is permanently stored in the mobile
equipment. On request, the MS sends this number over the
signalling channel to the MSC. The IMEI can be used to identify MS,s
that are reported stolen or operating incorrectly.
 Equipment Identity Register ( EIR )
 A listing of the allowed IMEI is maintained by the PLMN’s in the
Equipment Identity Register (EIR) to validate the mobile equipment.

Frequency Bands
Uplink 890 – 915 MHz 25 MHz
Downlink 935 – 960 MHz 25 MHz
100 KHz 200 KHz 100 KHz
1 43 1242 …………….
A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of
the band, this gives 124 possible carriers for the uplink and downlink. The
use of carrier 1 and 124 are optional for operators.

GSM Network Architecture
BTS
BTS
BTS
BTS
BTS
BSC
BSC
TRAU
MSC
HLR
AUC
VLR
EIR
PSTN
SMSC

MS – Mobile Station
 Mobile station provides user access to GSM network
for voice and data
 All GSM mobiles comply to GSM standards
 Subscriber data is read from a SIM card that plugs
into ME
SIM ME
MS

MS (cont..)
 Each MS has a unique number called as IMEI
number, which is stored in EIR for
authentication purposes
 Mobile camps on to the GSM network
through the BTS serving the cell
 Mobile also scans neighboring cells and
reports signal strengths
 Mobile transmits and receives voice at 13 kb/s
over the air interface

Mobile Station Output Power
 CLASS 1 20 watts Vehicle and Portable
 CLASS 2 8 watts Portable and Vehicle
 CLASS 3 5 watts Hand-Held
 CLASS 4 2 watts Hand-Held (GSM)
 CLASS 5 0.8 watts Hand-Held (DCS
1800)
 Output power determines:
 Accessibility in areas of coverage
 Talk Time and Standby time

Mobile Station Identities
 CC – Country Code
 NDC – National Destination Code
 SN – Serial Number
MSISDN : Mobile Station ISDN Number
It is the human identity used to call a Mobile
Station
CC SNNDC MSISDN
98 250 00134

IMSI (International Mobile
Subscriber Identity)
 MCC – Mobile Country Code
 MNC – Mobile Network Code
 MSIN – Mobile Subscriber Identity Number
MCC MSINMNC IMSI
3 2 or 3
Not more than 15
NMSI

IMEI (International Mobile
Equipment Identity)
 TAC – Type Approval Code
 FAC – Factory Assembly Code
 SNR – Serial Number
 SP – Spare digit (usually used to specify
software version)
TAC SPFAC IMEISNR
6 162 15

SIM ( Subscriber Identity
Module)
 Removable module inserted when the
subscriber wants to use the ME
 Two sizes: credit card size and stamp size
 SIM features and contents are personalized by
the Service Activator
 ROM – 6kb to 16 kb
 RAM – 128 bytes to 256 bytes
 EEPROM – 3kb to 8 kb
Space to insert SIM photo

Contents of SIM
 Serial Number
 IMSI, Subscriber Key Ki, Ciphering Key Kc
 Algorithms for authentication and ciphering
 Network Code
 PIN, PUK
 Charging Information
 Abbreviated Dialling
 Supplementary Features (e.g. Call barring)

SIM Security
 Two level protection
 When mobile is turned on, it will ask for user
to enter PIN (Personal Id Number)
 3 tries for PIN, after that PIN locked
 To unblock PIN, there is PUK (Pin Unblock
Key)
 10 attempts of PUK allowed
 After that SIM is blocked

BTS (Base Transceiver Station)
 BTS has a set of Transceivers (TRXs) to communicate
with mobiles in its area
 One BTS covers one or more than one cell
 The capacity of a cell depends on number of
transceivers in the cell
 BTS is connected to the BSC through Abis Interface
which is 2Mbps
 BTS transmits and receives voice at 13kbps over air
interface to the mobiles.
 BTS commands mobiles to set Tx. Power, timing
advance and Handovers

BTS

BSC – Base Station Controller
 Several BTSs are connected to the BSC
 BSC Manages channel allocation, handovers and
release of channels at connected BTSs
 BSC connects to the BTS via the Abis interface
and to the MSC on A interface
 BSC has the entire database of cell parameters
associated with the BTSs.
 No mobile data is stored in the BSC
 Less connections for MSC as intelligence is made
common to all BTSs by the BSC

BSC

TRAU – Transcoder Rate
Adaptation Unit
BTS
BSC PSTN
13 kbps 16 kbps 16 kbps 64 kbps
MSC and TRAU

TRAU (cont..)
 The MSC is based on ISDN switching. The
Fixed Network is also ISDN based.
 ISDN has speech rate of 64 kbps. Mobile
communicates at 13 kbps.
 TRAU converts the data rates between
13kbps GSM rate to 64kbps Standard ISDN
rate
 TRAU can be collocated with the BTS, BSC
or MSC or it can be a separate unit.

Location of Transcoder
 Collocated with MSC, BSC, BTS
 Separate Unit
MSC
Transco
der
BSC

MSC – Mobile Switching
Centre
BSC
BSC
BSC
BTSs PSTN
HLR
VLR

MSC (cont..)
 Exchange where calls are established, maintained and
released
 Database for all subscribers and their associated
features.
 Communicates with the BSCs on the A interface and
with PSTN on fixed line.
 MSC is weighted on the number of subscribers it can
support. E.g. an MSC of 1 lac subscribers means one
MSC is enough till subscriber base increases upto 1 lac,
beyond which another MSC is required.

Multiple MSCs
 When there is more capacity, there are more than
one MSCs.
 All MSCs have to communicate with one another
and to the outside world.
 Very complicated to connect each MSC to each
other and each MSC to PSTN
 So there is a concept of GMSC (Gateway MSC)
BSC
BSC
MSC
MSC
GMSC PSTN

HLR – Home Location Register
 MSC has all subscriber database stored
in HLR
 HLR has all permanent subscriber
database
 HLR has a database which describes the
subscriber’s profile i.e. basic features
and supplementary services
 MSC communicates with the HLR to get
data for subscribers on call

VLR – Visiting Location Register
 A subscription when activated is registered in
VLR
 VLR has all the subscriber numbers which are
active.
 VLR has a temporary database of all active
subscribers (on/off, location information)
MSC VLRVLR
HLR

VLR (cont..)
 MSC communicates with HLR for subscribers
coming from different MSCs. If the subscriber is
found valid, then it registers the subscriber in
the VLR
MSC MSCVLRVLR
HLR
VLR

AUC – Authentication Centre
 Authentication is a process by which a SIM is
verified
 Secret data and the verification process
algorithm are stored in AUC
 AUC is the element which carries out the
verification of the SIM
 AUC is associated with the HLR
MS MSC HLR AUC

EIR (Equipment Identity Register)
 EIR is the Mobile Equipment Database
which has a series of IMEIs
 MSC asks the Mobile to send its IMEI
 MSC then checks the validity of IMEI with
the EIR
 All IMEIs are stored in EIR with relevant
classifications
EIR
MSC

Classification of IMEIs
White list: This contains the IMEI of
type approved mobiles
Black List: List of IMEIs which should be
barred because either they are stolen or
are not functioning properly
Grey list: List of IMEIs which are to be
evaluated before they are put in black list

Billing Centre (BC)
 BC Generates the billing statement for each
subscriber
 BC may be directly connected to the MSC or
through a mediation device
 MSC sends CDRs (Call Detail Records) to the BC
 According to the template of pulse rates and
units set, BC creates a bill according to the
destination called and the call duration

Billing Centre (BC) (cont..)
CDRs
Templates for unit costs

OMC – Operations and
Maintenance Centre
 Also called the NOC (Network Operations
centre)
 It is the central monitoring and remote
maintenance centre for all network elements
 OMC has links to BSCs and MSCs

OMC
OMC System
BSC
BSC
BSC
BTSs
BTSs
BTSs
OMC Terminals

GSM Channels

GSM Channels
 Physical Channel
 One time slot on one carrier is called physical
channel.
 Logical Channel
 Information carried by physical channels is called
logical Channels.
 Logical channels are mapped on physical
channels.

Logical Channels
 Traffic channels: Used for speech and data
 Full Rate(TCH/F)
 Half Rate(TCH/H)
 Control channels: Used for signaling .i.e.
setting up a radio connection, call or controlling
an MS during conversation
 BCH(Broadcast channels)
 CCCH(common control channels)
 DCCH(dedicated control channels)

Traffic Channels(TCH)
TCH/F
(full Rate)
TCH/H
(half Rate)
Traffic Channels(TCH)

Control Channels(CCH)
CCH(Control Channel)
BCH CCCH DCCH
CCH RACH CBCH SDCCH ACCHSynch.
Chanels
SACCHFACCH
PCH/
AGCHFCCHSCH

BCH(Broadcast Channels)
 BCCH(Broadcast Control Channels)
 Downlink Only.
 Broadcast information of the serving cell (System
Information).
 Transmitted on timeslot zero of BCCH carrier.
 Reads only by idle mobile at least once every 30
secs.

BCH(Broadcast Channels) cont’d
 SCH(Synchronisation Channels)
 Downlink Only
 Carries information for frame synchronisation.
 Contains frame number and BSIC(Base Station
Identity Code).

BCH(Broadcast Channels) cont’d
 FCCH(Frequency Correction Channels)
 Downlink Only.
 Enable MS to synchronies to the frequency.

CCCH(Common Control
Channel)
 RACH(Random Access Channel)
 Uplink only.
 Used by the MS when making its first access to
the Network.
 The reason for access could be initiation of a call
or a page response.

CCCH(Common Control Channel)
cont’d
 AGCH(Assess Grant Channel)
 Downlink only.
 Used for acknowledgement of the access attempt
sent on RACH.
 Used by the network to assign a signaling cannel
upon successful decoding of access bursts.

CCCH(Common Control Channel)
cont’d
 PCH(Paging Channel)
 Downlink only.
 The network will page the MS ,if there is a
incoming call or a short Message.
 It contains the MS identity number, the IMSI or
TMSI.

DCCH(Dedicated Control
Channel)
 SDCCH (Stand-alone Dedicated Control
Channel)
 Uplink and Downlink.
 Used for call setup, authentication, ciphering
location update and SMS.

Channel) cont’d
 SACCH(Slow Associated Control Channel)
 Downlink and Uplink.
 Used to transfer signal while MS have ongoing
conversation on traffic or while SDCCH is being
used.
 On the forward link, the SACCH is used to send
slow but regularly changing control information to
each mobile on that ARFCN, such as power
control instructions and specific timing advance
instructions

 SACCH(Slow Associated Control Channel)
cont’d
 The reverse SACCH carries information about
the received signal strength and quality of the
TCH, as well as BCH measurement results
from neighboring cells.

Channel) cont’d
 FACCH(Fast Associated Control Channel)
 Downlink and uplink.
 Associate with TCH only.
 It is used to send fast message like hand over
message.
 Work by stealing traffic bursts.

Mapping on Physical
Channels
 The Logical channels are mapped on the
physical channels.
 The TDMA frames are grouped together into
multi-frame.
 26 TDMA multi-frame for Traffic.
 51 TDMA multi-frame for control signal.

Channel Combination
 Combined
 All the controlling signals are in the time slot 0 of
the Multi-frame.
 Non Combined
 Dedicated controlling signals are in time slot 1 of
the Multi-frame.

Combined
 Cell with single carrier.
 Timeslot 0 :BCCH+CCCH+SDCCH.
 Timeslot 1-7 :TCH/FACCH+SACCH.

Non Combined
 Cell with Two carrier
 Timeslot 0 (of carrier 1) BCCH+CCCH.
 Timeslot 1 (of carrier1) SDCCH+SACCH.
 Timeslot 2-7 & 0-7(of both carriers)
TCH/FACCH+SACCH.

BROADCAST MESSAGES
 System information 5 and 6 sent on the SACCH
immediately after Handover or whenever nothing else is
being sent.
 Downlink SACCH is used for system information
messages while uplink SACCH is used for measurement
reports.
 System Information types 7 and 8 (optional) are an
extension to type 4 and broadcast on the BCCH.

SYSTEM INFORMATION

SYSTEM INFORMATION 1
 When frequency hopping is used in cell MS needs to
know which frequency band to use and what frequency
within the band it should use in hopping algorithm.
 Cell channel description
Cell Allocation Number(CANO)-Informs the band
number of the frequency channels used.
00-Band 0(current GSM band)
Cell Allocation ARFCN(CA ARFCN):- ARFCN’s
used for hopping.It is coded in a bitmap of 124 bits.

SYTEM INFORMATION 1
124 123 122 121
024 023 022 021 020 019 018 017
016 015 014 013 012 011 010 009
008 007 006 005 004 003 002 001

 RACH Control Parameters
Access Control Class(ACC) :-Bitmap with 16 bits.
All MS spread out on class 0 –9 . Priority groups use
class 11-15. A bit set to 1 barred access for that class.
Bit 10 is used to tell the MS if emergency call is allowed
or not.
0 – All MS can make emergency call.
1 - MS with class 11-15 only can
make emergency calls.
Cell barred for access(CB):-
0- Yes
1- No

Re-establishment allowed(RE):-
0- Yes
1- No
Max_retransmissions(MAXRET):-Number of times
the MS attempts to access the Network [1,2,4 or 7].
Tx-integer(TX):- Number of slots to spread access
retransmissions when a MS attempts to
access the system.
Emergency call allowed:- Yes/No.

 System Information Type 2 message consists of
the Double BA list which defines the BCCH
frequencies used in the neighboring cells.
 The Double BA list provides the MS with
different frequencies on which to measure,
depending on whether the MS is in idle or active
mode.
 In active mode, the MS should measure on a
reduced number of frequencies in order to
improve the accuracy of measurements.

 In Idle mode,the MS should measure on larger
number of frequencies, so that the time required
for the MS to access the network after power on
is reduced.
 The MS is also informed which PLMN’s it may
use.
 As well as System Information Type 2,it is also
possible to have System Information Type 2 Bis
and System information Type 2 Ater, depending
on the size of the BA List.
 System Information Type 2 Bis/Ter are optional.

 Neighbor Cell Description:-
BA Indicator(BA IND):- Allows to differentiate
measurement results related to different list of BCCH
frequencies sent to MS.
BCCH Allocation number(BANO):-
Band 0 is used.
 PLMN Permitted(NCCPERM):-This the PLMN
color codes permitted and tells the MS which network
color codes(NCC) on the BCCH carriers it is allowed to
monitor when it is in this cell.
.

Access Control Class(ACC) :-Bitmap with 16 bits.
All MS spread out on class 0 –9 . Priority groups
use class 11-15. A bit set to 1 barred access for
that class. Bit 10 is used to tell the MS if emergency
call is allowed or not.
0 – All MS can make emergency call.
1 - MS with class 11-15 only can
make emergency calls.
Cell barred for access(CB):-
0- Yes
1- No

Re-establishment allowed(RE):-
0- Yes
1- No
Max_retransmissions(MAXRET):-Number of times
the MS attempts to access the Network [1,2,4 or 7].
Tx-integer(TX):- Number of slots to spread access
retransmissions when a MS attempts to
access the system.
Emergency call allowed:- Yes/No.

BCCH ARFCN Number(BAIND):- ARFCN’s used for in
a Bitmap of 124 bits
124 123 122 121
024 023 022 021 020 019 018 017
016 015 014 013 012 011 010 009
008 007 006 005 004 003 002 001

 The System Information Type 3 contains information on
the identity of the current LA and cell identity, because a
change means that the MS must update the network.
 System Information 3 also as Control Channel
Description parameters used to calculate the Paging
group.
 When the MS is in idle mode it decides which cells to lock
to. Information needed by the MS for cell selection is also
broadcast in the Type 3 information.

8 7 6 5 4 3 2 1
1 1 1 1
LAC
LOCATION AREA IDENTITTY(LAI)
MCC DIG 1MCC DIG 2
MCC DIG 1
MNC DIG 1MNC DIG 2
CI
CI
CELL IDENTITY
LAC

 Control Channel Description
Attach / Detach(ATT):-
0 = Allowed
1 = Not Allowed
bs_agblk:-Number of block reserved for AGCH [0-7]
Ba_pmfrms:-Number of 51 frame multi-frames
between transmission of paging messages to MS of
the same group
T3212:- Periodic location update timer .
[1-255 deci hours].

cch_conf Physical channels combined No. of CCH
0 1 timeslot(0) No 9
1 1 timeslot(0) Yes 3
2 2 timeslot(0,2) No 18
4 3 timeslot(0,2,4) No 27
6 4 timeslot(0,2,4,6) No 36

 Cell options
DTX:-Whether Discontinuous Transmission
used or not.
PWRC:-Power control on the downlink.
0 = Not used.
1 = Used.
Radio link timeout(RLINKT):-
Radio link time-out is the time before an MS
disconnects due to failure in decoding SACCH
message. Sets the timer T100 in the MS.

 Cell Selection Parameters
Rxlev_access_min:- Minimum received signal level
at the MS for which it is permitted to access the
system.
0-63 = -100 dBm to –47 dBm.
Mx_txpwr_cch:- Maximum power the MS will use
when accessing the system.
Cell_reselect_hysteresis:- Used for cell reselection.
 RACH Control Parameters.

 Location Area Identification.
 Cell Selection Parameters
Rxlev_access_min:- Minimum received signal level
at the MS for which it is permitted to access the
system.
0-63 = -100 dBm to –47 dBm.
Mx_txpwr_cch:- Maximum power the MS will use
when accessing the system.
Cell_reselect_hysteresis:- Used for cell reselection.

max_retransmissions(MAXRET)
tx_integer(TX)
Cell barred for access(CB).
Re-establishment allowed(RE)
Emergency Call Allowed
Access Control Class (ACC)

 CBCH Description(Optional) :
CHN:- This is the channel number for CBCH. It is
controlled internally in BSC.
TSC:- Training Sequence Code. Base Station Color
Code(BCC) part of BSIC is used.
CBCHNO:- Absolute RF channel number of CBCH.
MAC:- Mobile Allocation in the cell, describes the
frequencies to be used in the hopping sequence if
frequency hopping is used.

Hopping Channel(H):-Informs if CBCH Channel is
hopping or single.
ARFCN:- If H=0;
MAIO:- If H=1, informs the MS where to
start hopping.
Values [0-63].
HSN:- If H=1, informs the MS in what order
the hopping should take place. Values[0 –63].
HSN=0 Cyclic Hopping.
MA:-Indicates which RF Channels are used
for hopping. ARFCN numbers coded in
bitmap.

 Sent on the SACCH on the downlink to the MS in
dedicated mode.
 On SAACH, the MS also receives information about the
BCCH carrier in each neighboring cell. This may differ
from those sent in System information type 2.
 It is also possible to have system Information Type 5 Bis
and System Information Type 5Ter, depending on the
size of the BA list.

 Neighbor Cell Description:-
BA-IND:-Used by the Network to discriminate
measurements results related to different lists of
BCCH carriers sent by the MS(Type 2 or 5).
Values 0 or 1(different from type 2).
BCCH
Allocation number:-00-Band 0(current GSM band).

BCCH ARFCN:-Neighboring cells ARFCN’s. Sent as a
bitmap.
0-Not used
1-Used.
124 123 122 121
024 023 022 021 020 019 018 017
016 015 014 013 012 011 010 009
008 007 006 005 004 003 002 001

 Ms in dedicated mode needs to know if the LA has
changed.If so, it must perform location updating when
the call is released.
 MS may change between cells with different Radio link
timeout and DTX.
 Cell Identity.
 Location Area Identification.
 PLMN permitted.

 Cell options:
DTX
PWRC
Radio Link timeout.

SYSTEM INFORMATION 7/8
 System Information Types 7 and 8 contain Cell
Reselect parameters. Their function is to
supplement System Information Type 4.

GSM Interfaces
 (Um) Air interface - MS to BTS
 A bis interface - BTS to BSC
 A Interface - BSC to MSC
 B Interface - MSC to VLR
 C interface - MSC to HLR

MSC
BSC
VLRHLR
AUC
EIR
GMSC
MS
A Interface
A bis Interface
Air Interface
B Interface
C Interface
F Interface
D Interface
H Interface
To other
Networks

GSM Interfaces
 The interfaces between MSC and MS is called A,
Abis and Um interfaces.
 On these interfaces only three layers are
defined.They are not corresponding to the OSI
(Open System Interconnection) model.

A Interface
 A interface between the BSC and the MSC
 The A interface provides two distinct types of
information, signalling and traffic, between the
MSC and the BSC.
 The speech is transcoded in the TRC and the SS7
(Signalling system) signalling is transparently
connected through the TRC or on a separate link
to the BSC.

Abis Interface
 The A-bis interface responsible for transmitting traffic
and signalling information between the BSC and the
BTS.
 The transmission protocol used for sending signalling
information on the A-bis interface is Link Access
Protocol on the D Channel (LAPD)

(Um) Air Interface
 This is the interface between the mobile station and the
Base station.
 The Air interface uses the Time Division Multiple Access
(TDMA) technique to transmit and receive traffic and
signalling information between the BTS and MS.
 The TDMA technique is used to divide each carrier into
eight time slots.These time slots are then assigned to
specific users,allowing up to eight conversations to be
handled Simultaneously by the same carrier.

7 56 34 12 0
1 2 43 5 76
Down Link
Up Link 0
Time Slot
• This interface is the radio interface between the
mobile station and the network and uses layer
Three messages.
• On Layer three messages we have the division
of message types into CM (communication
Management), MM (Mobility Management), and
RR (Radio Resource Management).

Connection Management
(CM)
There are three entities within CM:
 Call Control(CC) – Which handles the procedures
concerning call control. e.g. setup,Change of bearer
service.
 Supplementary Service (SS) – Which handles such as call
bearing, call waiting , call forwarding etc.
 Short Message Service (SMS) – Enables the MS to handle
short message transfer to and from the network.

Mobility Management (MM)
 Mobility management handles functions for
authentication, location updating, identification and
others concerning the mobility of the mobile station.

Radio Resource Management
(RR)
 It contains the functions concerning the radio link. Here
we find the capability to establish,maintain and release
the radio connection between the network and the
mobile station, which includes the handover procedure.

B Interface
 The B interface between the MSC and the VLR uses the
MAP/TCAP protocol.
 Most MSCs are associated with a VLR, making the B
interface "internal".
 Whenever the MSC needs access to data regarding a MS
located in its area, it interrogates the VLR using the
MAP/B protocol over the B interface.

C Interface
 The C interface is between the HLR and a MSC.
 Each call originating outside of GSM (i.e., a MS
terminating call from the PSTN) has to go through a
Gateway to obtain the routing information required to
complete the call, and the MAP/TCAP protocol over the
C interface is used for this purpose.
 Also, the MSC may optionally forward billing information
to the HLR after call clearing.

D Interface
 The D interface is between the VLR and HLR.
 It uses the MAP/TCAP protocol to exchange the data
related to the location of the MS and to the management
of the subscriber.

E Interface
 The E interface interconnects two MSCs.
 The E interface exchanges data related to handover
between the anchor and relay MSCs using the
-MAP/TCAP+ISUP/TUP protocol.

F Interface
 The F interface connects the MSC to the EIR.
 It uses the MAP/TCAP protocol to verify the status of the
IMEI that the MSC has retrieved from the MS.

G Interface
 The G interface interconnects two VLRs of different
MSCs.
 It uses the MAP/G protocol to transfer subscriber
information, during e.g. a location update procedure.

Topics for discussion
 Speech Encoding
 Data Encoding
 Interleaving for Voice,Control and Data
signals

Speech Encoding
 We shall start with a raw voice signal fed into
the microphone, travel through the various
stages involving vocoding, channel coding etc
till it reaches the final burst format on the Air
Interface.

Speech Encoding ckt
Voice
Encoding
Channel
coding
interleaving
RF Modulation
Raw
Voice
signal

Speech Encoding ckt
 The voice is sampled at the rate of 50 samples
per second.
 This results in 20 msec blocks of speech
 Each of this 20 msec block is passed on to the
13Kbps vocoder.
 There are 260 information bits from the
output of the vocoder for every 20 msec input
i.e.; 13Kbps *20msec = 260 bits.

Voice Encoding ckt
Vocoder I/p
20 msec speech
blocks
13Kbps Vocoder Vocoder O/p
260 bits

Channel coding
 Channel Coding is done to protect the logical
channels from transmission errors introduced
by the radio path.
 The coding schemes depend on the type of
the logical channels, hence the coding can
differ from speech, control and data .

Channel Coding for speech
Class class 1b class 2
1a
50 3 132 4 tail
Bits parity bits
Convolutional coder
½ coder, k=5
456 bits=378 bits from Convolution coder + 78 class 2 bits
260 bits

Channel coding for Speech
 The 260 bits of speech info from the vocoder is
broken down into three parts.
 Class 1a- 50 bits , these represent the filter
coefficients of the speech and are the most
important for proper detection of the speech at
the receiver and hence are given maximum
protection. 3 additional parity bits are derived
from the class 1a bits for cyclic redundancy check
(CRC).

Channel coding for Speech
cont’d
 Class 1b - 132 bits are not parity checked but
are fed into the convolutional coder along
with 4 tail bits which are used to set the
registers in the receiver to a known state for
decoding purpose.
 Class 1b- 78 bits, these are not so important
and are not protected but are combined with
the output of the convolution coder.

Convolutional coder CC
 The Convolutional coder is a series of shift
registers implemented using logic gates, where
for every one input bit we get 2 output bits.
Hence it is called ½ coder.
 Here k=5 is the constraint length, it means there
are 5 shift register and each bit has memory
depth of 4 , meaning it can influence the output
of up to four next successive bits. This is useful
during reception as bits can be derived even if a
few consecutive bits are lost due to errors or
corruption.

½ Convolutional coder
R1 R2 R3 R5R4
+
+
C0
output
C1
output
0110..
Input bits
+ EX-OR
R=register

Convolutional coder cont’d
 The output of the CC* is now 378 bits.
(50+3+132+4)*2=378
The total number of bits now is 378+78=456 bits.
*Note : The bit rate from the vocoder was 13Kbps
for the 20 msec speech block, but after CC the bit
rate increases to 22.8Kbps.
456 bits *20msecs=22.8Kbps
* CC = Convolutional Coder.

Control Channel Coding
184 bits
Control data
184 40 4 tail
Fire coded parity bits
½ Convolutional Coder
456 bits output

Control Channel Coding
 The control information is received in blocks of
184 bits.
 These bits are first protected with a cyclic code
called as Fire code, which is useful in correction
and detection of burst errors.
 40 Parity bits are added, along with 4 tail bits.
 These 228 bits are given to the CC whose output
is again 456 bits at a bitrate of 22.8Kbps.
 The control channels include the RACH, PCH,
AGCH etc.

Data Channel Coding
240 bits 4 tail
Data bits
½ Convolutional Coder
Output= 488 bits
After Puncturing
Output=456 bits

Data Channel Coding
 The data bits are received in blocks of 240 bits.
These are directly convolution coded after
adding 4 tail bits.
 The output of the CC is now 488 bits, which
actually increases the bitrate to 24.4 Kbps.
 To keep the bitrate constant on the air interface
we need to puncture the output of the CC.
Hence, we have a final bitrate of 22.8 Kbps again
.

Channel Coding cont’d
 The above explanation was given keeping in
view a full rate Traffic, Control, or Data
channel.
 For Half rate or Lesser rates the same
principle of channel coding holds good, with
slight differences in the encoding process.

Interleaving
 Having encoded the logical channel
information, the next step is to build its bit
stream into bursts that can be transmitted
within the TDMA frame structure. This is the
stage where the interleaving process is
carried out.
 Interleaving spreads the content of one
information block across several TDMA
timeslots or bursts.

Interleaving cont’d
 The following interleaving depths are used :
 Speech – 8 blocks
 Control – 4 blocks
 Data – 22 blocks
 The interleaving process for a speech block is
shown wherein which a 456 bit speech block is
divided into 8 blocks of 57 bits each and each of
these odd and even 57 bit blocks are interleaved
diagonally on to alternate bursts on the TDMA
frame.

Speech Interleaving
8* 57 bits each = 456 bits
Of Speech block N
57
Even
Of N-1
57
Even
Of N
Speech block
N-1
57
odd
Of N-1
57
odd
Of N
The speech is spread over 8 such normal bursts
Each normal burst consists of two blocks of 57 bit speech
from different 20msec blocks (say N, N-1) along with
26 bit training sequence T and 2 flag F plus 6 start stop bits .
T+FT+FT+F
456 bit speech data

Control Data Interleaving
114 114 114 114
456 bits control data
The control data is spread over 4 blocks using rectangular
interleaving instead of diagonal interleaving as in
speech the receiver will have to wait for at least
2 multiframes before being able to decode the control
message
TDMA
Burst blocks

Data Interleaving
114 114 114 114
Burst 1 Burst 22Burst 2 Burst 3 Burst 4 Burst 19
First 6
bits
First 6
bits
Last 6
bits
Last 6
bits
456 bit data block

Data Interleaving cont’d
 Here the data block of 456 bits is divided into 4
blocks of 114 bits each.
 The first 6 bits from each of the 114 bit blocks is
inserted in to each frame, the second 6 bits from
each of the 114 bits into the next frame and so on
spreading each 114 block over 19 TDMA bursts
while the entire 456 bits is spread over 22 TDMA
bursts.
 Thus the data interleaving is said to have a depth
of 22 bursts.

Data Interleaving cont’d
 The reason why data is spread over such along
period of time is that if data burst is corrupted or
lost, only a small part of it is lost which can be
reproduced at the receiver.
 This wide interleaving depth does produce a
time delay during transmission but that is
acceptable since it does not affect the data
signal quality at the receiver, unlike speech
where delay could result in bad quality of signal
to the subscriber.
 *Note – The interleaving used in data is diagonal
interleaving.

Before Deinterleaving
3 successive bursts corrupted
After Deinterleaving
The corrupted bursts are spread over a length equal to the
interleaving depth so that the effect of the errors is
minimized.
Interleaving Advantage

Air Interface Bitrate
 The information which is now coded and
interleaved at 22.8 Kbps now has to be
transmitted over the Air interface to the BTS.
 The information burst is not sent directly , but is
sent in ciphered form within a burst envelope.
This ciphering is done using ciphering keys and
algorithms known both by the mobile and the
BSS.

cont’d
 The Kc is the ciphering key and A5 algorithm
are applied to the information(speech or
data) which increases the bitrate to a final
rate of 33.8 Kbps from/to each mobile.
 If we assume all 8 timeslots of the cell to be
occupied then the bitrate of the Air interface
comes to 33.8 * 8= 270.4 Kbps/channel.

cont’d
A5 Algorithm
Kc Information
Block 22.8 Kbps
Sent on Air interface
Ciphered information burst
33.8 Kbps

cont’d
1 2 3 4 5 6 7 8
Mobile
Tx’s at
33.8 Kbps
Cell rx’s 8*33.8
KBps = 270.4 Kbps
Per TDMA frame
Cell coverage area
TDMA Fn TDMA Fn+1

Decoding and Deinterleaving at
the Receiver
 At the receiver the reverse process of
Deinterleaving and decoding have to take place
respectively, so as to recover the information
from the signal.
 After Deinterleaving the signal will be decoded
which is the reverse process of the Convolutional
coding, using Viterbi decoders.
 The decoder can recover lost or corrupted data
up to 4 successive bits, because the memory
depth of the CC is 4(for k=5).

Channelization
 Frequency band has several application
segments
 Certain blocks of the Band are reserved for
certain applications by regulating authorities
 Technologies have decided their frequency
bands
 E.g. AMPS/DAMPS: 824-894 MHz

Channelization methods
Channelization can be done primarily by three
methods:
 FDMA (Frequency Division Multiple Access)
 TDMA (Time Division Multiple Access)
 CDMA (Code Division Multiple Access)

FDMA
 E.g. AMPS band is divided into 30 KHz channels
(1666 Freq. channels)
 Television Channels (Star, Zee, Sony,..)
Frequency
Time
Power

TDMA
 E.g. AMPS has 3 timeslots on each 30 KHz
channel
Frequency
Time
Power

CDMA
 Frequency channel is divided into code
channels
 E.g. in IS-95 CDMA, 1.228 MHz channel is
divided into 64 Code Channels
 Each user has a particular code
 Codes are orthogonal to each other, do not
interfere with each other

Duplex Access Methods
 Frequency Division Duplex (FDD)
 Transmit on one frequency and receive on
another frequency
F1 F2 Frequency
Amplitude
Time
Tx Rx

Time Division Duplex
 Time division duplex
 Tx and Rx is on the same frequency but on
different times
F1 Frequency
Amplitude
Time
Tx
Rx

GSM Air Interface
 Separate Bands for Uplink and Downlink
 Downlink: 935-960Mhz (EGSM: 925-960MHz)
 Uplink: 890-915 MHz (EGSM: 880-915 MHz)
• TDMA and TDMA Multiplex
– 124 Frequency Channels (ARFCN) for
GSM900
– 1 to 124 fro current band
– 975 to 1023 for E-GSM
– 200kHz Channels
– 8 Mobiles share ARFCN by TDMA

GSM Air interface (1800)
 1800: Downlink: 1805-1880 MHz
 1800: Uplink: 1710-1785 MHx
 374 ARFCNs
 Separation of 95 MHz
 ARFCNs are numbered from 512 to 885
inclusive

The GSM Burst
3 357 261 571 8.25
Tail Bits
Data
Control
Bit
Midamble
Control
Bit
Data
Tail Bits
Guard
Period

Speech Coder
 RPE/LTP coder (Regular
Pulse excitation/Long term
Prediction)
 Converts 64 kbps speech
to 13 kbps
 At the end we get 13kbps
speech i.e. 260 bits in 20
ms
20 ms blocks
Speech Coder
Bits Ordered
50 very
important
bits
132
important
bits
78 other
bits

Error Correction
Type 1a 50 3(CRC)Type 1b 132 Type II 78
Reordering
25 66366 25 4 Type II 78
Type 1a
Type 1b Type 1b
Type 1a
Tail
Half rate convolutional code
378 Type II 78
456 bits from 20 ms of speech

Diagonal Interleaving
 Traffic channel (TCH) bursts carry two 57 bit blocks
(114)
 Each 120 ms of speech = 456*6 = 2736 bits
2736/114 = 24 bursts i.3. 24 frames
Multiframe has 26 frames in 120ms.
There are 2 spare frames .. 1 SACCH, 1 Idle
456 bits from 20ms of speech 456 bits from 20ms of speech
57 57575757575757 57 57575757575757
57 57 57 5757 5757 5757 5757 5757 5757 57

Convolutional Coding and
Interleaving
 Bits to be Tx ed: HELLO
 Convolutionally encoded: HHEELLLLOO
 Interleaved: EE HH LL LL OO
 Bits Rx ed: EE HH LL LL OO
 De-Interleaved: HHEELLLLOO
 Viterbi Decoded: HELLO

Speech Coding Process
20 ms
Speech Coder
260 bits 13 kbps
50 1a 132 1b 78 II
Channel Coder
456 bits 22.8 kbps
Transceiver (BTS)
Transcoder Handler
260 bits
456 bits
16 kbps
TRAU frame
260 + 60 = 320 bits
Abis
13 kbps

TRAU frame
 260 bits info + 60 TRAU bits = 320 bits/20ms =
TRAU frame
 60 bits contain frame Information data which
indicates speech, data, O&M, full rate/half
rate
 60 bits = 35 synchronization + 21 control + 4
timing

Midamble or Training Bits
 8 midamble patterns (Colour codes) of 26 bits (BSIC)
 RACH and SCH have longer 41 and 64 bit Midambles
 Equalizer estimates channel impulse response from
midamble
 Mathematically construct inverse filter
 Uses inverse to decode bits
3 357 261 571 8.25
Tail Bits
Data
Control Bit
Midamble
Control Bit
Data
Tail Bits
Guard
Period

Downlink and Uplink
 Uplink lags downlink by 3 timeslots
 Uplink and downlink use same timeslot number
 Uplink and downlink use same channel number
(ARFCN)
 Uplink and downlink use different bands (45
MHz apart for GSM 900)

Measurements made by MS and
BTS
 RxQual
0 < 0.2% 1 0.2 – 0.4 %
3 0.4 – 0.8 % 4 0.8 – 0.16 %
5 1.6 – 3.2 % 6 3.2 – 6.4 %
7 6.4 – 12.8 %
Uplink RXLEV (-48 to -110 dbm)
Uplink RXQUAL (0-7)
Uplink RXLEV (-48 to -110 dbm)
Uplink RXQUAL (0-7)

Mobile Power Control
 Mobile is commanded to change its Transmit
Power
 Change in Power is proportionate to the Path
Loss
 Change in Power is done in steps of 2 dbs
Path Loss
Power Command

Timing Advance
 TDMA approach requires signals to arrive at
BTS at the correct time
 A mobile at 30 km will be late by 100micro
seconds
 Timing advance is in the range of 0-62
 One unit is 550m
 So maximum cell size is 63*0.55 = ~35 kms

Concepts of Channels in GSM
 A company vehicle is used for several purposes in a
day
 Similarly in GSM, the timeslots are used for different
purposes at different times

Frames and Multiframes
0 654321 7
3 Data 1Midamble1 Data 3 8.25 bits
156.25 bits 576.92 micro sec
4.615 ms
Time
Slot
Frame
0 50 0 25
Control Channel
Multiframe
Traffic Channel
Multiframe

GSM Operations
 Location Update
 Mobile
Originated Call
 Mobile
Terminated Call
 Handover
 Security
Procedures
 Cell Barring
 DTX
 Cell Broadcast
 Short Message
Service
 Emergency calls
 Supplementary
Services
 Roaming

Mobile Turn On
 Mobile Searches for Broadcast
Channels (BCH)
 Synchronizes Frequency and Timing
 Decodes BCH sub-channels (BCCH)
 Checks if Network Allowed by SIM
 Location Update
 Authentication

Location Area
Location Area 1Location Area 1
Location
Area 2
Location
Area 2
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BSC
BSC
BSC
MSC

Location Area Identity
 Location area is the area covered by one
or more BTSs where a mobile can move
freely without updating the system
 One Location area can be covered by one
or more BSCs, but ony one MSC.
MCC LACMNC

Importance of Location Area
 Reduce Paging load
 Resource Planning
Smaller Location Areas – Location update
increases
Larger Location Areas – Paging load increases

What is Location Update?
 MSC should know the location of the
Mobile for paging
 Mobile is continuously changing
location area
 Mobile when changes Location Area
informs the MSC about its new LA
 Process of informing MSC about new
Location area is Location Update

Types of Location Updates
1. Normal Location
Update
2. IMSI Attach
3. Periodic Location
Update
Hi,
I am in Location area
xxx

IMSI Attach
 Mobile turns off and sends an IMSI Detach
to MSC
 Mobile turns on again and compares LAI
 If same, sends an IMSI attach to MSC
Is the received
LAI same as
before
If same,
Sends
IMSI
attach

Normal Location Update
 Mobile Turns on Power
 Reads the new LAI
 If different, does a Location Update
Is the received
LAI same as
before
If different,
does
Location
Update

Periodic Location Update
 The periodic location Update time is set
from OMC/MSC
 After the periodic location update timer
expires, the mobile has to do a location
update

What happens at Location
Update?
 Mobile changes location area
 Reads the new Location Area from
BCCH
 Sends a RACH (request for channel)
 Gets a SDCCH after AGCH
 Sends its IMSI and new and old LAI in a
Location Update request to MSC on
SDCCH

What happens at location
update cont..
….. . .
 MSC starts Authentication
 If successful, Updates the new Location area
for the Mobile in the VLR
 Sends a confirmation to the Mobile
 Mobile leaves SDCCH, and comes to idle mode

Mobile Originated Call
Channel Request
Immediate Assign
Service Request
Call Proceeding
Set Up
Ciphering
Authentication
Alerting
Assignment
Connection

Mobile Terminated Call
Paging
Channel Request
Immediate Assign
Set Up
Ciphering
Authentication
Paging Response
Assignment
Call Confirmed
Alerting
Connection

Security Features
 Authentication
 Process to verify Authenticity of SIM
 Mobile is asked to perform an operation
using identity unique to SIM
• Ciphering
–Process of coding speech for
secrecy
–The speech bits are EXORed with
bit stream unique to MS

Security Features (TMSI
Reallocation)
GSM
Infrastructure
Mobile
Location Update
TMSI Allocation
Call Setup
TMSI Reallocation
TMSI- Temporary Mobile Subscriber Identity

Security Features
(Identity Check)
EIR
Sends IMEI
Identity Check
White listed /Grey Listed/ Black
Listed mobiles

Handover
Cell 1 Cell 2
Handover is a GSM feature by which the
control/communication of a Mobile is transferred
from one cell to another if certain criteria’s are
met. It is a network initiated process.

Criteria for Handover
 Receive Quality (RXQUAL) on uplink and
downlink
 Receive Signal Strength (RXLEV) on uplink
and downlink
 Distance (Timing Advance)
 Interference Level
 Power Budget

Handover Decision
 BSC process the measurements reported by Mobile
and the BTS
BTS
BTS
BTS
BTS
BTS
BTS
Mobile has measurements of six neighbors

Handover Decision (cont..)
 BSS performs averaging function on these
measurements every SACCH frame (480ms)
 Handover Decision algorithm is activated after a
set number of SACCH frame periods by
comparison against thresholds

Types of Handovers
 INTRA-CELL HANDOVERS
 INTER-CELL HANDOVERS
 INTRA-BSC HANDOVERS
 INTER-BSC HANDOVERS
 INTER-MSC HANDOVERS

INTRA-CELL HANDOVER
C0
C1
Handover between timeslots of same frequency
Handover between different frequencies of the same cell
(to reduce interference)
MSC is not aware about this

Inter-cell Handover
Handover between cells of the same BTS
BTS
Cell 1 Cell 2

Inter-cell Handover (cont..)
 MSC is told about HO
 BTS -> BSC -> MSC
 Why MSC is informed?
 In case of change of LA, MSC may need LAC for
paging. As MS is busy, a link already exists. So, MSC
can send a tone in case of call waiting, and does not
need to page again.
 This is needed also for billing and call tracing

INTRA-BSC Handover
MSC BSC
BTS
BTS
This HO takes place if the cell to which handover
is to be done belongs to the same BSC

Inter BSC Handover
MSC
BSC BTS
BTSBSC
The MSC is completely involved in this Handover

Inter MSC Handover
BSC
BSC
MSC
MSC
BTS
BTS
GMSC/
PSTN/
Backbone
In this case the handover takes place through the
interconnecting element which can be GMSC or
PSTN or private Backbone between the MSCs

Cell Barring
BTS
Cell Barring is a GSM feature by which certain
mobiles could be barred access to certain cells
Cell barring is activated/deactivated at BTS level
Cell barring is done for mobile categories and
priorities

Cell Barring
 Every mobile has an access class
 The access class is stored in the SIM
 Classes 0-9 are termed normal calsses
 Classes 11-15 are emergency classes
• Every cell has a set parameter which
defines which access classes are
barred for the particular cell. This
parameter is broadcasted on the
BCCH

What is DTX?
 DTX (Discontinous Transmission)
 Each direction of Transmission is only 50%
 Transmitter is switched ON for useful
information frames
Need for DTX
•To increase battery life
•To reduce the average interference
level
DTX is done by DTX handlers which

VAD (Voice Activity
Detector)
 Senses for speech in 20ms blocks
 Removes stationary noise
 VAD is an energy detector
 Compares Energy of filtered speech threshold
 It determines which 20ms blocks contain
speech and it only forwards those frames

Evaluation of Background Noise
 Background noise is always present with
speech
 DTX cuts off this noise with speech
 Gives an uncomfortable feeling to the listener
 VAD takes care of this by inserting comfort
noise at the receiving end when speech
discontinues.

Emergency Calls
 GSM specs define 112 as an emergency
number
 ‘112’ is accessible with or without SIM
 Without SIM it is sent on the best
channel
 Mobile on sensing ‘112’ sets the
establishment cause to emergency call in
the RACH
 Routing of this call be done to a desired
location defined in the switch

Cell (Re)selection
 Cell reselection is done using C1 path loss
criterion.
 The purpose is to ensure that the MS is
camped on to the cell with the best
transmission quality.
 The MS will camp on to the cell with the
highest C1 value if C1 > 0.

The following parameters are used to
calculate the C1 criterion
 The received signal at the MS side.
 Rxlev_access_min - broadcast on the BCCH -
The minimum received level at the MS
required for access to the network.
 Ms_txpwr_max_cch - the maximum power
that an MS may use when initially accessing
the network.
 The maximum power of the MS

C1 = A - Max(B,0)
 A = Received level Average -
Rxlev_access_min.
 B = MS_txpwr_max_cch - maximum output
power of the MS

Cell Reselect Hysteresis
 Cell reselection on the border of two location areas result
in a location update. When an MS moves on the border
of two location areas lots of location updates take place.
To avoid these location updates, the reselect hysteresis
is introduced.
 A location update is performed only if:
 The C1 value of the new location area is higher than
the C1 value in the current location area and
 The received signal strengths have at least a
difference of the reselect hysteresis.

Cellular concept

Why to use the cellular
concept ?
 Solves the problem of Spectral congestion
and user capacity by means of frequency
reuse.
 Offers high capacity in a limited spectrum
allocation.
 Offers system level approach, using low
power transmitters instead of a single, high
power transmitter (large cell) to cover larger
area.

 A portion of the total channels available is
allocated to each base station.
 Neighboring base stations are assigned
different groups channels, in order to
minimize interference.

Cell shape

1-Omni-directional cell-site (Omni-directional
antenna).
2-Rhombus-shaped sectors (Directive antenna).
3-Hexagonal shaped sectors (Directive
antenna).

Cell size
Large cell : (up to 70km in diameter)
It exists where :
1-Radio waves are unobstructed.
2-Transmission power can cover the area.
3-low subscriber density.
Small cell : (up to 2km in diameter)
It exists where :
1-Radio waves are obstructed.
2-Low transmission power to decrease interference.
3-High subscriber density.

Types of cells
1-Macro-cells 2-Micro-cells.
3-Pico-cells. 4-Umbrella-cells.

What is a cluster ?
 A cluster is a group of
cells.
 No channels are
reused within a
cluster.
 It is the unit of
design.

Cluster size
 Definition : It is The number of cells per
cluster
N = i^2 + ij + j^2
Where :
i = 0, 1, 2….& j = 0,1,2…. etc.
N = 1 , 3 , 4 ,7, 9 , 12 ,……

Types of clusters
1-N=7 omni frequency plan (2-directional).
2-N=7 trapezoidal frequency plan
(1-directional).
3-N=9 omni frequency plan.
4-Tricellular plans
a) N=3 tricellular plan (3/9).
b) N=4 tricellular plan (4/12).

Channel assignment
strategies
 Considerations :
1) Max. capacity.
2) Min interference.
3) Perfect handover.
 Types of assignment strategies :
1) Fixed :
 Each cell has permanent predetermined set of voice
channels.
 New calls served by unused channels of this cell.
 Borrowing strategy if all channels are occupied.
 High probabiltity that call is Blocked if channels are
occupied.( disadv.)

2) Dynamic :
 Channels are not allocated to different cells
permanently.
 Each new call BTS requests new channel from
MSC.
 MSC allocate a channel, by using an algorithm
that takes into account:
1- Frequency is not already in use.
2- Min. reuse distance to avoid co-channel
interference.

 Adv. of dynamic assignment strategy :
1) Increase channel utilization
( Increase trunking efficiency ).
2) Decrease probability of a blocked call.

Frequency reuse
Concept

Reuse cluster

Co-channel Reuse ratio
(Q) :
 R : cell radius.
 D : reuse distance.
 Q = D/R. =
sqrt(3N).
Where :
N : cluster size

Handover

Definition : procedure that allows MS to
change the cell or time-slot to keep as good
link as possible during all the call.

Types of handover
 IntraCell : bet. 2 channels of same cell.
 InterCell : bet. 2 channels of 2 different cell &
same BTS.
 InterBTS (intra BSC) : 2 cells of different BTS
Same BSC.
 InterBSC : bet. 2 cells of different BSC’s & same
MSC.

Measurements before
handover
1- Measurements from MS to BSC :
a) Strength of BTS signal.
b) Quality of BTS signal.
c) Signal strength of 6 neighbor BTS’s.
2-Measurements from BTS to BSC :
a) Strength of MS signal.
b) Quality of MS signal.
c) Distance between serving BTS & MS.

Different causes of handover
Better cell HOEmergency HO
Level Quality
PBGT
Traffic causes
InterferenceDistance
Different causes of
Handover

Basic handover
algorithms
a)“Min. acceptable performance” algorithm:
MS power is increased when quality deceases
till handover is the only way.
b) “Power budget “ algorithm:
Prefer direct handover when quality deceases
without increasing MS power first .

Handover priority
1) UL quality cause (or interference).
2) DL quality cause (or interference).
3) UL level cause.
4) DL level cause.
5) Distance cause.
6) Better cell cause.

Interference

Sources of interference
include:
1) Another mobile in the same cell.
2) A call in progress in the neighboring
cell.
3) Other BTS’s operating in the same
frequency band.

Interference effects :
 In voice channel causes crosstalk
 In control channels it leads missed and
blocked calls due to errors in the digital
signaling.

Main types of
interference :
1) Co-channel interference.
2) Adjacent channel interference.

1) Co-channel interference
 Source : Near cell using same frequency.
It is a function of reuse distance(D/R).
 General rule :
io = No. of co-channel interfering cells.
S = Signal power from a desired BS.
Ii = interference power caused by the ith
interfering co-channel cell BS.

 Another form :
C/I = 10 log {(1/n)(D/R)*m}
Where :
m = propagation constant
(dep’s on nature of environment)
n = number of co-channel interferers.
Can be minimized by :
Choosing minimum reuse distance
= (2.5….3)(2R).

2) Adjacent channel
interference
 Source : A cell using a frequency adjacent to the
one in another cell due to imperfect reciever’s
filter.

Can be minimized by :
1-careful filtering
2-careful channel assignments
3-Directional antenna.
 General rule : ACI= -10 Log[(d1/d2)*m] – Adj ch
isolation.
Where :
d1: distance between MS & proper BTs d2:
dist. Bet MS & adj BTS causing
interference.
Adj ch isolation = Filter isolation = -
26db.

Traffic engineering
theory

Why do we need to
know traffic?
 The amount of traffic during peak hours allows
us to dimension our wireless system for a certain
GOS.
 GOS : probability of having a call blocked
during busy hour (block rate).

Traffic intensity (E)
 Erlang : A unit of traffic intensity measure.
 1 Erlang = 1 circuit in use for 1 hour.
 T ( in Erlangs) = [No. of calls per hour*average
call holding time(sec.)] / [3600]

Typical traffic profile

Traffic tables
Erlang B
Table
Blocked calls are not
held
Erlang C
Table
Blocked calls are held in
the queue indefinitely
Poisson
Table
Blocked calls are held in
the queue for a time =
the mean holding time05/31/13Tempus Telcosys 239

Erlang – B table
 P(N;T) = [ (T^N)*exp(-T) ] / N!
N GOS
1%
GOS
2%
2 0.153 0.223
4 0.869 1.093
10 4.46 5.084
20 12.0 13.182
40 29.0 30.99705/31/13Tempus Telcosys 240

Trunking
 Sharing channel among several users.
 Trunking efficiency (nT) : Measures the number
of subscribers that each channel in every cell can
accommodate.
nT = (traffic in Erlangs / no. of channels)*100.

Trunking efficiency
in presence of one
operator :
N = 7 , 312 one direction
voice channels
No. of channels / cell = 312 /
7 = 44 ch./cell.
From Erlang-B table @GOS
2%,this’s equivalent to 35
Erlangs
nT = 35 / 44 = 79.55.
Trunking efficiency
in presence of two
operators :
N = 7 , 312 / 2 = 156 one
direction voice channel for
each operator.
No. of channels / cell = 156 /
7 = 22 ch./cell.
From Erlang-B table
@GOS 2%,this’s
equivalent to 15 Erlangs.
nT = 15 / 22 = 68.18.

System capacity

 S : total duplex channels available for use = k*N
Where:
N : cluster size.
k : No. of channels / cell.
 C : total No. of duplex channels in system;
C = M*k*N.
Where :
M : No. of times the cluster is repeated.

Improving system
capacity
 Cell splitting.
 Sectoring.

Cell splitting

Sectoring
 We use directional antennas instead of being
omnidirectional

What does sectoring
mean?
 We can now assign frequency sets to sectors
and decrease the re-use distance to fulfill :
1) More freq reuse.
2) Higher system capacity.
3) Improve S/I ratio ( better signal quality ).
 How S/I ratio is improved?
-e.g. In 120 degree sectoring there’s only
2 interferers instead of 6 incase of omnidirectional N=7
cluster.

Directional frequency reuse
 Here we use 7/21
pattern for frequency
allocation.

Comparison
between various
types of clusters

N = 7 omni frequency
plan :
 n = 6 , m = 4.
 D / R = 4.583.
 1) Co-channel
interference ratio :
C / I = 18.6 dB.
 2) Adjacent channel
interference :
ACI = -26 dB @ d1= d2.

N = 7 trapezoidal
frequency plan
 n = 2 , m = 4.
 D / R = 6.245.
 1) Co-channel interference
ratio :
C / I = 28.8.
interference : disappears
because the channels are
assigned alternatively to the
cells.

 Trunking efficiency :
 312 one direction voice channels
N = 7
 312 / 7 = 44.57 ~ 44 ch./cell.
 From Erlang-B table @ GOS = 2%
T = 35 E.
 nT = 35 / 44 = 79.55 %.

N = 9 omni frequency plan
 n = 4 , m = 4.
 D / R = sqrt ( 3 * 9 ) = 5.2.
 1) Co-channel
interference :
C / I = 22.6 dB.
interference :
ACI = -38 dB @ d2 = 2
(d1).

 312 one direction voice channels
N = 9
 312 / 9 = 34.67 ~ 34 ch./cell.
 From Erlang-B table @ GOS = 2%
T = 25.529 E.
 nT = 25.529 / 34 = 75.085 %.
Conclusion : nT 7 > nT 9
But C/I 7 > C/I 9
ACI 7 > ACI 9

4 / 12 cell pattern
 n = 1 , m = 4.
 D / R = sqrt (3* 4) = 3.732.
 C / I = 22.87 dB.
 No. of channels/cell
= 312 / 12 = 26 ch./cell.
 From Erlang-B table @
GOS = 2 %.
 T = 18.4 E/cell.
 nT = 18.4 / 26= 70.77%.

3 / 9 cell pattern
 n = 1 , m = 4.
 D / R = sqrt (3* 3) = 3.
 C / I = 19.1 dB.
 No. of channels/cell
=312 / 9 = 34 ch./cell.
 From Erlang-B table @ GOS
= 2 %.
 T = 25.5 E/cell.
 nT = 25.5 / 24 = 75 %.

120 degree cell sectoring
 n = 2 , m = 4.
 D / R = sqrt(3 * 7) = 4.583.
 Co-channel interference :
C / I = 23.436 + 6dB(due to
isolation) = 29.436 dB.
 No. of channels/cell = 312 / 21 =
14.857.
 From Erlang-B @ GOS=2% T=
8.2003.
 nT = 8.2003 / 14.857
=56.216%.

 References :
 Motorola CP02
 NOKIA SYSTRA

If any Query
Contact
9903867731
TempusTelcosys@gmail.com

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