Total GSM Concept

TEMPUS TELCOSYS
(P)LIMITED
Telecom Tutorials
5/31/2013www.TempusTelcosys.com
1

GSM INTRODUCTION
2

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.
3

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.
4

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.
5

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.
6

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.
7

CGI : CELL GLOBAL IDENTITY
8
MCC MNC LAC CI
LAI
CGI
MCC = Mobile Country Code
MNC = Mobile Network Code
LAC = Location Area Code
CI = Cell Identity

MSISDN
9
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.
10

IMSI
 IMSI (International Mobile Subscriber
Identity) Network Identity Unique To A
Sim.
11
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
12
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.
13

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.
14

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.
15

FREQUENCY BANDS
Uplink 890 – 915 MHz 25 MHz
16
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 ARCHITECTURE5/31/2013www.TempusTelcosys.com
17
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
18
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
19

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
20

MOBILE STATION IDENTITIES
 CC – Country Code
 NDC – National Destination Code
 SN – Serial Number
21
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
22
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)
23
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
24
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)
25

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
26

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
27

BTS
28

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
29

BSC
30

TRAU – TRANSCODER RATE
ADAPTATION UNIT
31
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.
32

LOCATION OF TRANSCODER
 Collocated with MSC, BSC, BTS
 Separate Unit
33
MSC
Transco
der
BSC

MSC – MOBILE SWITCHING CENTRE
34
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.
35

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)
36
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
37

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)
38
MSC VLR
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
39
MSC MSCVLR
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
40
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
41
EIR
MSC

CLASSIFICATION OF IMEIS
42
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
43

BILLING CENTRE (BC) (CONT..)
44
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
45

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
46

OMC
47
OMC System
BSC
BSC
BSC
BTSs
BTSs
BTSs
OMC Terminals

48
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.
49

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)
50

TRAFFIC CHANNELS(TCH)
51
TCH/F
(full Rate)
TCH/H
(half Rate)
Traffic Channels(TCH)

CONTROL CHANNELS(CCH)
52
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.
53

BCH(BROADCAST CHANNELS) CONT’D
 SCH(Synchronisation Channels)
 Downlink Only
 Carries information for frame synchronisation.
 Contains frame number and BSIC(Base Station
Identity Code).
54

BCH(BROADCAST CHANNELS) CONT’D
 FCCH(Frequency Correction Channels)
 Downlink Only.
 Enable MS to synchronies to the frequency.
55

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.
56

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.
57

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.
58

DCCH(DEDICATED CONTROL CHANNEL)
 SDCCH (Stand-alone Dedicated Control Channel)
 Uplink and Downlink.
 Used for call setup, authentication, ciphering location
update and SMS.
59

DCCH(DEDICATED CONTROL 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
60

 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.
61

DCCH(DEDICATED CONTROL 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.
62

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.
63

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.
64

COMBINED
 Cell with single carrier.
 Timeslot 0 :BCCH+CCCH+SDCCH.
 Timeslot 1-7 :TCH/FACCH+SACCH.
65

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.
66

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.
67

SYSTEM INFORMATION
68

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.
69

SYTEM INFORMATION 1
70
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
71

 RACH Control Parameters 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.
72

 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.
73

 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.
74

 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.
.
75

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
76

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.
77

BCCH ARFCN Number(BAIND):- ARFCN’s used for in
a Bitmap of 124 bits
78
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.
79

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
80

 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].
81

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
82

 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.
83

 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.
84

 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.
85

max_retransmissions(MAXRET)
tx_integer(TX) Cell
barred for access(CB). Re-
establishment allowed(RE) Emergency Call
Allowed
Access Control Class (ACC)
86

 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.
87

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.
88

 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.
89

 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).
90

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
91

 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.
92

 Cell options:
DTX
PWRC Radio
Link timeout.
93

SYSTEM INFORMATION 7/8
 System Information Types 7 and 8 contain Cell Reselect
parameters. Their function is to supplement System
Information Type 4.
94

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
95

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
96

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.
97

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.
98

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)
99

(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.
100

7 56 34 12 0
1 2 43 5 76
Down Link
Up Link 0
Time Slot
101
• 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.
102

MOBILITY MANAGEMENT (MM)
 Mobility management handles functions for
authentication, location updating, identification and
others concerning the mobility of the mobile station.
103

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.
104

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.
105

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.
106

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.
107

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.
108

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.
109

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.
110

ENCODING AND INTERLEAVING
OF INFORMATION SIGNAL
IN
GSM
111

TOPICS FOR DISCUSSION
 Speech Encoding
 Data Encoding
 Interleaving for Voice,Control and Data signals
112

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.
113

SPEECH ENCODING CKT
114
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.
115

VOICE ENCODING CKT
116
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 .
117

CHANNEL CODING FOR SPEECH
118
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).
119

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.
120

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.
121

½ CONVOLUTIONAL CODER
122
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.
123

CONTROL CHANNEL CODING
124
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.
125

DATA CHANNEL CODING
126
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 .
127

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.
128

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.
129

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.
130

SPEECH INTERLEAVING
131
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
132
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
133
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.
134

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.
135

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.
136
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.
137

AIR INTERFACE BITRATE 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.
138

139
A5 Algorithm
Kc Information
Block 22.8 Kbps
Sent on Air interface
Ciphered information burst
33.8 Kbps

140
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).
141

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
142

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)
143

FDMA
 E.g. AMPS band is divided into 30 KHz
channels (1666 Freq. channels)
 Television Channels (Star, Zee, Sony,..)
144
Frequency
Time
Power

TDMA
 E.g. AMPS has 3 timeslots on each 30 KHz
channel
145
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
146

DUPLEX ACCESS METHODS
 Frequency Division Duplex (FDD)
 Transmit on one frequency and receive on
another frequency
147
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
148
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)
149
• 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
150

THE GSM BURST
151
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
152
20 ms blocks
Speech Coder
Bits Ordered
50 very
important
bits
132
important
bits
78 other
bits

ERROR CORRECTION
153
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
154
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
155

SPEECH CODING PROCESS
156
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
157

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
158
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)
159

MEASUREMENTS MADE BY MS AND BTS
160
 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
161
 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
162

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
163

FRAMES AND MULTIFRAMES
164
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
165

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
166

LOCATION AREA
167
Location Area 1
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.
168
MCC LACMNC

IMPORTANCE OF LOCATION AREA
 Reduce Paging load
 Resource Planning
Smaller Location Areas – Location update increases
Larger Location Areas – Paging load increases
169

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
170

TYPES OF LOCATION UPDATES
1. Normal Location
Update
2. IMSI Attach
3. Periodic Location
Update
171
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
172
Is the received
LAI same as
before

NORMAL LOCATION UPDATE
 Mobile Turns on Power
 Reads the new LAI
 If different, does a Location Update
173
Is the received
LAI same as
before

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
174

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
175

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
176

MOBILE ORIGINATED CALL
177
Channel Request
Immediate Assign
Service Request
Call Proceeding
Set Up
Ciphering
Authentication
Alerting
Assignment
Connection

MOBILE TERMINATED CALL 5/31/2013www.TempusTelcosys.com
178
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
179
• Ciphering
–Process of coding speech for secrecy
–The speech bits are EXORed with bit
stream unique to MS

SECURITY FEATURES (TMSI REALLOCATION)
180
GSM
Infrastructure
Mobile
Location Update
TMSI Allocation
Call Setup
TMSI Reallocation
TMSI- Temporary Mobile Subscriber Identity

SECURITY FEATURES
(IDENTITY CHECK)
181
EIR
Sends IMEI
Identity Check
White listed /Grey Listed/ Black
Listed mobiles

HANDOVER 5/31/2013www.TempusTelcosys.com
182
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
183

HANDOVER DECISION
 BSC process the measurements reported by Mobile
and the BTS
184
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
185

TYPES OF HANDOVERS
 INTRA-CELL HANDOVERS
 INTER-CELL HANDOVERS
 INTRA-BSC HANDOVERS
 INTER-BSC HANDOVERS
 INTER-MSC HANDOVERS
186

INTRA-CELL HANDOVER
187
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
188
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
189

INTRA-BSC HANDOVER
190
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
191
MSC
BSC BTS
BTSBSC
The MSC is completely involved in this Handover

INTER MSC HANDOVER
192
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
193
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
194
• 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
195
Need for DTX
•To increase battery life
•To reduce the average interference level
DTX is done by DTX handlers which have
the following functions.

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
196

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.
197

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
198

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.
199

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
200

C1 = A - MAX(B,0)
 A = Received level Average - Rxlev_access_min.
 B = MS_txpwr_max_cch - maximum output
power of the MS
201

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.
202

CELLULAR CONCEPT
203

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.
204

 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.
205

CELL SHAPE
206

207

1-Omni-directional cell-site (Omni-directional
antenna).
2-Rhombus-shaped sectors (Directive
antenna).
3-Hexagonal shaped sectors (Directive
antenna).
208

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.
209

TYPES OF CELLS
1-Macro-cells 2-Micro-cells.
3-Pico-cells. 4-Umbrella-cells.
210

WHAT IS A CLUSTER ?
 A cluster is a group
of cells.
 No channels are
reused within a
cluster.
 It is the unit of
design.
211

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 ,……
212

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).
213

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.)
214

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.
215

 Adv. of dynamic assignment strategy :
1) Increase channel utilization
( Increase trunking efficiency ).
2) Decrease probability of a blocked call.
216

FREQUENCY REUSE
CONCEPT
217

218

REUSE CLUSTER
219

CO-CHANNEL REUSE
RATIO (Q) :
 R : cell radius.
 D : reuse distance.
 Q = D/R. =
sqrt(3N).
Where :
N : cluster size
220

HANDOVER
221

Definition : procedure that allows MS to
change the cell or time-slot to keep as
good link as possible during all the call.
222

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.
223

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.
224

DIFFERENT CAUSES OF HANDOVER
225
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
.
226

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.
227

INTERFERENCE
228

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.
229

INTERFERENCE EFFECTS :
 In voice channel causes crosstalk
 In control channels it leads missed and blocked
calls due to errors in the digital signaling.
230

MAIN TYPES OF
INTERFERENCE :
1) Co-channel interference.
2) Adjacent channel interference.
231

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.
232

 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).
233

2) ADJACENT CHANNEL
INTERFERENCE
 Source : A cell using a frequency adjacent to
the one in another cell due to imperfect
reciever’s filter.
234

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.
235

TRAFFIC ENGINEERING THEORY
236

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).
237

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]
238

TYPICAL TRAFFIC PROFILE
239

TRAFFIC TABLES
Blocked calls are not
held
Erlang B
Table
Blocked calls are held in
the queue indefinitely
Erlang C
Table
Blocked calls are held in
the queue for a time =
the mean holding time
Poisson
Table
240

ERLANG – B TABLE
 P(N;T) = [ (T^N)*exp(-T) ] / N!
GOS
2%
GOS
1%
N
0.2230.1532
1.0930.8694
5.0844.4610
13.18212.020
30.99729.040
241

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.
242

 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.
243

SYSTEM CAPACITY
244

 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.
245

IMPROVING SYSTEM
CAPACITY
 Cell splitting.
 Sectoring.
246

CELL SPLITTING
247

SECTORING
 We use directional antennas instead of being
omnidirectional
248

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.
249

250

251

DIRECTIONAL FREQUENCY
REUSE
 Here we use 7/21
pattern for
frequency allocation.
252

COMPARISON BETWEEN
VARIOUS TYPES OF
CLUSTERS
253

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.
254

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.
255

 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 %.
256

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).
257

 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
258

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%.
259

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 %.
260

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%.
261

Total GSM Concept

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