GSM Logical Channels

Channels
• Physical Channels
– Associated with frequency bands, time slots, codes
– Physical channels transfer bits from one network
element to another
• Logical Channels
– Distinguished by the nature of carried information
and the way to assemble bits into data units
– Three types
• one-to-one: traffic channels between a BTS and a MS
• one-to-many: synchronization signals from BTS to MSs in a
cell
• many-to-one: from MSs to the same BTS
11/18/2021 2

Bursts and Frames
2047
3
2
1
0
Hyper frame = 2048 Super frames = 2,715,648 TDMA frames
3 Hours 28 Minutes 53 Seconds and 760 milliseconds
50
0
25
0
1 superframe = 1326 TDMA frames (6.12 seconds)
= 51 (26-frame) multi frames or 26 (51-frame) multi frames
51 Multiframe
Approx 235 mS
26 Multiframe
120 mS
25
24
2
1
0 50
49
48
1
0
7
6
5
4
3
2
1
0
TDMA frame
4.615 mS
11/18/2021 3

Organisation of Speech & Data
24 25
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
BP 7
BP 6
BP 5
BP 4
BP 3
BP 2
BP 1
BP 0
8.25
3
57
1
26
1
57
3
Frames 0-11 : TCH Frames 12 :
SACCH
Frames 13-24 :
TCH
Frames 25 :
Unused
26 – frame
multiframe
Duration: 120 ms
TDMA frame
Duration: 60/13
ms
=4.615 ms
Tail
bits
Data bits Stealing
bit
Training
sequence
Stealing
bit
Data bits Tail
bits
Guard
space
Normal burst
Duration 15/26 ms
11/18/2021 4

Bursts and Frames
• There are two types of multiframe
– 26 TDMA-frame multiframe is used to carry
TCH, SACCH and FACCH
– 51 TDMA-frame multiframe is used to carry
BCCH, CCH, SDCCH and SACCH
11/18/2021 5

Burst
• The information contained in one time slot is a
burst
• Five types of burst
– Normal Burst (NB)
• To carry information on traffic and control channels
– Frequency Correction Burst (FB)
• To synchronize the frequency of the mobile
– Synchronization Burst (SB)
• To synchronize the frames of the mobile
– Access Burst (AB)
• For random and handover access
– Dummy Burst
• For padding the frame
11/18/2021 6

Bursts and Frames
0 1 2 3 4 5 6 7
GP
8.25
TB
3
Encrypted bits 57
flag
1
Training sequence 26flag
1
Encrypted bits 57
TB
3
TB
3
Fixed bits 142 GP
8.25
TB
3
GP
8.25
TB
3
TB
3
Encrypted bits 39 Synchronization sequence 64 Encrypted bits 39
TB
3
Synchronization sequence
41
Encrypted bits 36 TB
3
GP 68.25
GP
8.25
TB
3
TB
3
Mixed bits 58 Training sequence 26 Mixed bits 58
Normal Burst (NB)
Frequency correction
burst (FB)
Synchronization
burst (SB)
Access burst (AB)
Dummy burst (DB)
TB: Tail bits
GP: Guard period
1 TDMA frame = 8 timeslots
1 timeslot = 156.25 bit durations (15/26 =~ 0.577 ms)
(1 bit duration 48/13 =~ 3.69 micro sec)
11/18/2021 7

Normal Burst
T3 Coded Data
57
Training
Sequence
26
Coded
Data 57
GP
8.25
S1 S1 T3
148 bits = 0.54612 msec
This burst is used to carry information on TCH and on the control
channelsBCCH, PCH, AGCH, SDCCH, ASCCH and FACCH.
Note: GP (Guard Period)
11/18/2021 8

Random Access Burst
T8 Synchronization
sequence 41
Coded Data
36
Guard Period
68.25
T3
88 bits = 0.32472 msec
68.25 bits = 0.252 msec which is equivalent to 75.5 km
propagation delay.
That is, the max. allowed distance between mobile station
and BTS is 37.75 km. Based on other system parameters,
the max cell size is 35 km in GSM.
11/18/2021 9

Traffic
channels
(TCH)
Signaling
channel
TCH/F: Full-rate Traffic Channel
TCH/H: Half-rate Traffic Channel
FCCH: Frequency correction
SCH: Synchronization
BCCH: Broadcast control
PCH: Paging
AGCH: Access grant
RACH: Random access
SDCCH: Stand-alone dedicated control
SACCH: Slow associated control
FACCH: Fast associated control
Two-way
Base-to-
mobile
Two-way
Logical Channel List
BCH
CCCH
DCCH
11/18/2021 10

20
higher GSM frame structures
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
time
1 2 3 4 5 6 7 8
GSM TDMA frame
4.615 ms
GSM - TDMA/FDMA
GSM time-slot (normal burst)
546.5 µs
577 µs
tail user data Training
S
guard
space S user data tail
guard
space
3 bits 57 bits 26 bits 57 bits
1 1 3
S: indicates data or control
?
11/18/2021

LOGICAL CHANNELS
One or more logical channels can be transmitted on a
physical channel. There are different types of logical
channels. The type of logical channel is determined by the
function of the information transmitted over it.
The following types of logical channels exist:
Traffic channels
Broadcast channels
Common control channels
Dedicated control channels
Note that the first channel type carries speech and data, and
the other types control information (signaling).
11/18/2021 22

Traffic Channels
Full Rate Traffic Channel
(TCH/FS)
Traffic Channel Multiframes are composed of only 26 TDMA frames. On each
multiframe, there are 24 frames for Traffic Channels, 1 frame for a SACCH, and
the last frame is Idle.
Half Rate Traffic Channel
(TCH/HS)
When using Half-Rate Speech Encoding (TCH/HS), the speech encoding bit
rate is 5.6 kb/s, so one time slot can handle two half-rate channels. In this
case, one channel will transmit every other TDMA frame, and the other
channel would be transmitted on the other frames. The final frame (25), which
is normally used as an Idle frame, is now used as a SACCH for the second
half-rate channel.
11/18/2021 23

GSM-Frame Structure
11/18/2021 24

Here is the math:
As you remember from the Introduction to TDMA Tutorial. A
single time slot has a data throughput of 24.7 kilobits per second
(kb/s).
1. Calculate bits per TCH Multiframe: 114 bits per burst, 24 of the
26 frames used for voice/data.
114 bits × 24 frames = 2736 bits
2. Calculate bits per microsecond (ms): 2736 bits per TCH
multiframe, multiframe duration = 120ms
2736 bits ÷ 120ms = 22.8 bits/msec
3. Convert microseconds (ms) to seconds:
22.8 bits/sec × 1000 = 22,800 bits per second (22.8 kb/s)
11/18/2021 25

TRAFFIC CHANNELS
The traffic channels are used to send speech or data services. There
are two types of traffic channels. They are distinguished by their
transmission rates.
The following traffic channels are provided:
TCH/F (Traffic CHannel Full rate)
The TCH/F carries information at a gross bit rate of 22.8 kbit/s (after
channel coding). The net (or effective) bit rate at the TCH/F is for
speech 13 kbit/s and for data 12, 6 or 3.6 kbit/s (before channel
coding). The transmission rates of the data services allow services
which are compatible to the existing, respectively, 9.6, 4.8 and 2.4
kbit/s PSTN and ISDN services.
TCH/H (Traffic Channel Half rate)
The TCH/H carries information at a gross bit rate of 11.4 kbit/s. The net
bit rate at the TCH/H is for speech 5.6 kbit/s and for data 6 or 3.6
kbit/s.
11/18/2021 27

TCH/EFR (Enhanced Full rate)
The EFR provides a voice coding algorithm
offering improved speech quality. The algorithm
is fully compatible with a BSM speech quality.
The algorithm is fully compatible with a GSM 13
kbit/s speech channel. The main benefit will be
improved voice quality which offers prospects to
compete with PSTN networks.
A TCH/F or a TCH/H may also be used to send
signaling information (for example call
forwarding and short messages).
11/18/2021 28

Signaling Channels
Signaling channel
Broadcast
Channels (BCH
Cell Broadcast
Channel (CBCH)
Synchronization
Channel (SCH)
Frequency
Correction Channel
(FCCH)
Broadcast Control
Channel (BCCH)
Common Control
Channels (CCCH)
Paging Channel
(PCH)
Random Access
Channel (RACH)
Access Grant
Channel (AGCH)
Standalone
Dedicated Control
Channel (SDCCH)
Associated Control
Channel (ACCH)
Fast Associated
Control Channel
(FACCH)
Slow Associated
Control Channel
(SACCH)
11/18/2021 29

Broadcast Channels (BCH)
• To help the MH (Mobile Handset) measures
– to turn to a BTS
– to listen for the cell information
• to start roaming, waiting for calls to arrive, making calls
– Because BTSs are not synchronized with each
other, every time a MH decides to camp to
another cell, its FCCH, SCH, and BCCH must be
read.
11/18/2021 31

BROADCAST CHANNELS
• The information distributed over the
broadcast channels helps the mobile
stations to orient themselves in the mobile
radio network.
• The broadcast channels are point-to-
multipoint channels which are only
defined for the downlink direction (BTS to
the mobile station).
11/18/2021 32

BCCH (Broadcast Control
Channel)
• It inform the Mobile about system
confirmation parameter
• LAI Location Area Identity,
• Cell -Identity
• Neighboring Cell Identity.
• MS select best cell to attach BCCH on
full power, never Frequency hopped.
The BCCH is also known as beacon.
11/18/2021 33

LAI = MCC + MNC + LAC
MCC - Mobile Country Code. For India 404/405 USA 310/316 It is
assigned by the ITU-T
MNC - Mobile Network Code. . It is assigned by the government (DOT) of each
country West bengal 74, Bihar-75,Orissa-76
LAC - Location Area Code . It is assigned by an operator
• Maximum length of LAC is 16 bits.
• Thus there can be maximum of 216 =65,536 Location Areas defined in one GSM
PLMN.
MCC MNC LAC
3 digits 2 digits 2 octets max
LAI
( 16 bits )
Location Related Identities
11/18/2021 34

CELL GLOBAL IDENTITY (CGI)
The CGI is used for cell identification within a location area.
This is done by adding a Cell Identity (CI) to the components of a LAI. CI has a
length of 16 bits.
CGI consists of:CGI = MCC + MNC + LAC + CI
11/18/2021 35

BASE STATION
IDENTITY CODE (BSIC)
NCC = Network Color Code (3 bits 0-7), identifies the
PLMN. Note that it does not uniquely identify the
operator. NCC is primarily used to distinguish between
operators on each side of a border.
BCC = Base Station Color Code (3 bits 0-7), identifies
the Base
Station to help distinguish between BTS using the same
BCCH frequencies.
113
77
76
79
66
63
FOR BSNL NCC=7
7-0 ,7-1,……..7-7.
12 NON HOPING
FREQ X 7=96 NO
OF COMBINATION .
BCCH+BSIC
COMBINATION
SHOULD NOT BE
SAME IN NEAR BY.
11/18/2021 36

Base Station Identification Code
(BSIC).
• BSIC allows for cell changes across cells
that are not necessarily neighbors in a
geographic sense. So an MS could do a
cell reselection across cells with the same
frequency but different BSIC. Such a
possibility arises if the immediate
neighbors are hidden (no line of sight)
whereas a far away cell can provide better
service due to clear LOS radio path.
BSIC+ARFCN is unique in a particular area.
11/18/2021 37

After the subscriber has switched on his mobile station and typed in his PIN code, the
mobile station searches for a network
•But how does it log on to the network the subscriber is registered with?
•For this purpose, the BTS sends out the Frequency Correction Channel (FCCH) at short
regular intervals, to help the mobile station find a frequency for downlink reception and
adjust its frequency oscillator for the uplink transmission
•To do so, it picks out the strongest received signal
•The Synchronization Channel (SCH) then helps the mobile station to synchronize itself to
timeslot 0 sent out by the BTS
•This means the mobile station must adjust to the rhythm given bythe BTS
•The SCH contains the TDMA frame number as well as the Base Station Identity Code,
containing basic information about the network operator that canbe compared with the
info stored on the SIM card
•After this step, the mobile is able to decide whether it has chosen the proper network. If
not, it starts the same procedure again trying with the second strongest FCCH received
•While the mobile station uses the FCCH to adjust its frequency, and the SCH for
synchronization and network identification, the Broadcast Control Channel (BCCH),
which is also sent by the BTS, supplies the mobile station with additional information
about the selected cell, for example for ciphering
•For some Value Added Services, for example location-dependent services, additional
information has to be transmitted from the BTS to the mobile
•The Cell Broadcast Channel CBCH is used for this purpose to transmit geographical
parameters, for example Gauss-Krueger-Coordinates of the BTS, to the mobile
•The FCCH, SCH, BCCH and CBCH are Broadcast Channels, and exist only in the
downlink
•They are the first logical channels belonging to the Common Channels.
11/18/2021 39

Frequency Correction
Channel (FCCH)
• Provide MH with the frequency
reference of the system
– To enable the Mobile Handset (MH) to
synchronize with the frequency
• Transmission properties
– Transmit on the down-link
– Point to multi-point.
11/18/2021 40

HSN,MAIO,FREQ PLANNING.
BSNL got 31 no of radio carrier.
Fixed F1 to F15
Hopping Freq F16 to F31.
CYCLIC HSN=0, MAIO=1,MAIO2
RANDOM HSN=1.
HSN=63.
MAIO Sequence (Mobile Allocation Index
Offset)
F16,F17,F18,F19..F31 MAIO=1
F17,F18,F19..F31,F16 MAIO=2
F18,F19..F31,F16, F17MAIO=3
41
F5
F9
F1
F16
F8
F12
F4/
F19
F6
F10
F2/
F17
F7
F11
F3/
F18
4/12
11/18/2021

FCCH (Frequency Correction
Channel)
To communicate with the BTS the mobile
station must tune to the BTS. The FCCH
transmits a constant frequency shift of the
radio frequency carrier that can be used by
the mobile station for frequency correction.
11/18/2021 42

Synchronization Channel
(SCH)
• MH synchronize with the structure within the
locative cell
– MH can receive information from the proper time slots on the TDMA
structure
• To ensure a GSM BTS is chose
– The Base Station Identity Code (BSIC) can only be decoded by a
GSM BTS
– Transmit on down-link
– Point to multi-point.
11/18/2021 43

SCH (Synchronization
CHannel)
The SCH is used to time synchronize the
mobile stations. The data on this channel
carries the TDMA frame number and the
BSIC (Base Station Identity Code).
11/18/2021 44

1.The A-bisinterface connects the Base Transceiver Station
(BTS) with the Base Station Controller (BSC)
2. In the PCM30 configuration, the data at this interface is
transmitted via cable or via microwave transmission at a
bit rate of 2 Mbit/s
3. A cable connection is more resistent to interference, but
a network operator must lease it from a fixed network
operator
4. The microwave links can be operated independently,
and are easily configured by the network operator, but
they are more sensitive to interference
•4 types of information can
be transmitted over the A-
bis interface:
–user information
–synchronization data
–signaling information
–and data for the operation
and maintenance of the
BTS, known as O&M
alarms
11/18/2021 45

Broadcast Control Channel
(BCCH)
• BTS broadcast cell information to MH
– LAI ( Location Area Identity), to start
roaming, waiting for calls to arrive, making
calls
– maximum output power allowed in the cell
– information about BCCH carriers for the
neighboring cells
• MH will perform measurement to BTS
– Point to multi-point
11/18/2021 46

CBCH (Cell Broadcast
CHannel)
• The CBCH is used for the transmission of
generally accessible information (Short
Message Service messages) in a cell,
which can be polled by the mobile station.
11/18/2021 47

• The mobile station has now adjusted its frequency and synchronized its
TDMAs, and has picked out the best cell available•But before it can be
reached by other subscribers, and before itcan initiate calls, a Location
Update and authentication procedure are necessary•Only after that is the
mobile station logged on to the network and has radio coverage•It can now
be reached by other mobile stations, or initiate a call. For this purpose,
Common Control Channels are required•Common Control Channels are
"point-to-multipoint" channels, which exist either only in the uplink, or only in
the downlink•When a subscriber is called, the Paging Channel (PCH) is
broadcast in the downlink by all base stations within a Location Area, so
that the mobile station concerned canreact•To initiate a call, the mobile
station sends out a Random AccessChannel (RACH), which carries its
identification and request, for example for registration, to thenetwork•This
channel only exists in the uplink. In return, the network sends the Access
Grant Channel (AGCH) in the downlink direction, to assign resources to the
mobile station, by granting it a Stand-Alone Dedicated Control Channel,
SDCCH. The PCH, RACH and AGCH form the group ofthe Common
Control Channels belonging also to the Common Channels.
11/18/2021 49

COMMON CONTROL CHANNELS CONT..
PCH (Paging CHannel)
The PCH is used in the downlink direction for paging the mobile
stations. ----used for paging when Incoming call.
AGCH (Access Grant CHannel)
The AGCH is also used in the downlink direction. A logical channel for
a connection is allocated via the AGCH if the mobile station has
requested such a channel via the RACH.
RACH (Random Access CHannel)
The RACH is used in the uplink direction by the mobile stations for
requesting a channel for a connection. It is an access channel that
uses the slotted Aloha access scheme.
The Following figure gives an overview of the different types of logical
channels.
11/18/2021 50

Paging Channel (PCH)
• Used by BTS to page particular MH in the
cell
– MH actively listen to PCH to check contact
info within certain time
– Contact could be incoming call or short
message
• Contact info on PCH include
– IMSI (MH’s identity number), or
– TMSI (temporary number)
– point to point
11/18/2021 51

Random Access Channel
(RACH)
• Used by MH to request a dedicated
channel for call setup
– Shared by any MH attempts to access the
network
– Channel request message contains the
reason for the access attempt
– Transmit on up-link
– Point to pint.
11/18/2021 52

Access Grant Channel (AGCH)
• The network assigns a signaling
channel via AGCH
– A Stand alone Dedicated Control Channel
(SDCCH) is assigned
– Point to point
11/18/2021 53

Dedicated Control Channels (DCCH)
• DCCH are used for transferring nonuser
information between the network and the MH
– Messages on DCCH Including
• channel maintenance
• mobility management
• radio resource management
• Four kinds of DCCH
– Stand alone Dedicated Control Channel (SDCCH)
– Cell Broadcast Channel (CBCH)
– Slow Associated Control Channel (SACCH)
– Fast Associated Control Channel (FACCH)
11/18/2021 54

Dedicated
control channels
are full-duplex,
point-to-point
channels. They
are used for
signaling
between the BTS
and a certain
mobile station
11/18/2021 55

Slow Associated Control
Channel (SACCH)
• Carries control and measurement parameters
along with routine data necessary to maintain a
radio link between the MH and the BTS, which is
always allocated to a TCH or SDCCH.
– On the uplink, MS sends averaged measurements (signal strength and
quality) of current and neighboring BCCH
– On downlink, MS receives information about transmitting power to use and
an instruction with time advance/retard
– Bidirectional channel, transmit on both up and down link
– Point to point
In 26 Frame Traffic Multi frame 13th frame (frame no. 12)
is used for SACCH.
Note that the SACCH is only used for non-urgent
procedures.
11/18/2021 56

FACCH (Fast Associated Control Channel)
• The FACCH is used as a main signaling link for
the transmission of signaling data (for example
handover commands).
• It is also required for every call set-up and
release.
• During the call the FACCH data is transmitted
over the allocated TCH instead of traffic data; this
is marked by a flag called a stealing flag.
• The process of stealing a TCH for FACCH data is
called pre-emption.
- Used for Urgent Signalling; like handover
during a call. Appears on demand.
11/18/2021 57

Stand alone Dedicated
Control Channel (SDCCH)
• Transfer signaling information between
the BTS and the MH
• Typically used for location updating prior
to use of a traffic channel
– Bidirectional channel, transmit on both up
and down-link
– Point to point.
11/18/2021 58

Cell Broadcast Channel
(CBCH)
• To carry Short Message Service Cell
Broadcast (SMSCB)
• Use the same physical channel as
SDCCH
– Point to multi-point
11/18/2021 59

SDCCH (Stand-alone
Dedicated Control Channel)
• The SDCCH is a duplex, point-to-point channel which
is used for signaling in higher layers.
• It carries all signaling between the BTS and the mobile
station when no TCH is allocated.
• The SDCCHs are used for service requests (for
example Short Message Service), location updates,
subscriber authentication, ciphering initiation,
equipment validation and assignment to a TCH.
The net SDCCH bit rate is about 0.8 kbit/s.(160 x 5
=800bits)
11/18/2021 60

Control Channel UP
LIN
K
DOWN
LINK
FUNCTION
Broadcast Control Channel
(BCCH) ▼ These parameters include the Location Area Code (LAC), the Mobile
Network Code (MNC), the frequencies of neighboring cells, and access
parameters
Frequency Correction
Channel (FCCH) ▼ This channel contains frequency correction bursts
Synchronization Channel
(SCH) ▼ This channel is used by the MS to learn the Base Station Information
Code (BSIC) as well as the TDMA frame number (FN). This lets the MS
know what TDMA frame they are on within the hyperframe.
Cell Broadcast Channel
(CBCH) ▼ Broadcast such as weather, traffic, sports, stocks, etc. Messages can
be of any nature depending on what service is provided. The CBCH is
nt allocated a slot for itself, it is assigned to an SDCCH
Paging Channel (PCH)
▼ incoming traffic. voice call, SMS, or some other form of traffic.
Random Access Channel
(RACH) ▲ MS to request an initial dedicated channel from the BTS
Access Grant Channel
(AGCH) ▼ This channel is used by a BTS to notify the MS of the assignments of an
initial SDCCH for initial signaling
Standalone Dedicated
Control Channel (SDDCH) ▲ ▼ This channel is used for signaling and call setup between the MS and
the BTS.
Fast Associated Control
Channel (FACCH) ▲ ▼ This channel is used for control requirements such as handoffs. There
is no TS and frame allocation dedicated to a FAACH. The FAACH is a
burst-stealing channel, it steals a Timeslot from a Traffic Channel (TCH).
Slow Associated Control
Channel (SACCH) ▲ ▼ This channel is a continuous stream channel that is used for control
and supervisory signals associated with the traffic channels.
11/18/2021 61

62
Channel Combination Type
Several logic channels combine
together in some way to form
some specific types of channel
to transmit user data or
signaling information. They are
called combined channels. One
combined channel can be
mapped to a physical channel.
11/18/2021

CHANNEL COMBINATIONS
The channel combinations are:
1. TCH/F + FACCH/F + SACCH/F
2. TCH/H + FACCH/H + SACCH/H
3. (TCH/F + FACCH/F + SACCH/F) or (TCH/H +
FACCH/H + SACCH/H)
4. FCCH + SCH + CCCH + BCCH
5. FCCH + SCH + CCCH + BCCH + SDCCH/4 +
SACCH/4
6. BCCH + CCCH (CCCH = PCH + RACH + AGCH)
7. SDCCH/8 + SACCH/8
The extensions “/4” and “/8” in the above mentioned terms mean, respectively, that
four and eight logical channels are mapped onto one physical channel (timeslot).
11/18/2021 63

Control Channel
4. FCCH + SCH + CCCH + BCCH
UPLINK
DOWNLINK
6. CCCH + BCCH
11/18/2021 64

Downlink
The SACCH that is associated with each SDCCH is only
transmitted every other Multiframe. Each SACCH only gets half
of the transmit time as the SDCCH that it is associated with. So,
in one Multiframe, SACCH0 and SACCH1 would be transmitted,
and in the next Multiframe, SACCH2 and SACCH3 would be
transmitted. Uplink
FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)
The two sequential Multiframe would look like this:
11/18/2021 65

Downlink
Uplink
You will also notice that the downlink and uplink Multiframes
do not align with each other. This is done so that if the BTS sends an
information request to the MS, it does not have to wait an entire Multiframes
to receive the needed information. The uplink is transmitted 15 TDMA frames
behind the downlink. For example, the BTS might send an authentication
request to the MS on SDCCH0 (downlink) which corresponds to TDMA frames
22-25. The MS then has enough time to process the request and reply on
SDCCH0 (uplink) which immediately follows it on TDMA frames 37-40.
11/18/2021 66

SDCCH/8(0-7) + SACCH/C8(0 - 7)
Once again, the SACCH that is associated with an SDCCH is only transmitted every
other multiframe. Two consecutive Multiframes would look like this:
Downlink
Uplink
SKGOCHHAYAT,SDE(COMP)
11/18/2021 67

68
• Small capacity cell with only 1 TRX
TN0:
FCCH+SCH+CCCH+BCCH+SDCCH/4(0,_,3)+SACCH
/C4(0,_,3);
TN1-7: TCH/F+FACCH/F+SACCH/TF
• The medium-size cell with 4 TRXs
1TN0 group: FCCH+SCH+BCCH+CCCH;
2 SDCCH/8(0,_,7)+SACCH/C8(0,_,7);
29 TCH/F+FACCH/F+SACCH/TF
Channel Assignment inside cells
11/18/2021

69
• Large-size cell with 12 TRXs
1 TN0 group: FCCH+SCH+BCCH+CCCH;
1 TN2 group, 1 TN4 group and 1 TN6 group:
BCCH+CCCH;
5 SDCCH/8(0,_,7)+SACCH/C8(0,_,7);
87 TCH/F+FACCH/F+SACCH/TF
Channel Assignment inside cells
11/18/2021

medium-sized cell might be set up with 4 TRX (ARFCNs)
11/18/2021 70

Modes Mobile Station ( MS )
• MS Switched Off - can not receive incoming paging messages.
- can not make outgoing call.
- No dedicated channel is associated with it.
• MS Switched On - can receive incoming paging messages.
And in Idle Mode - can make outgoing call.
- No dedicated channel is associated with it.
• MS in Dedicated Mode -MS is in conversation and a
dedicated traffic channel TCH is
associated with it.
11/18/2021 71

Technical Terms Used w.r.t. MM (Mobility Management )
• IMSI – International Mobile Subscriber Identity
• IMSISDN – Mobile Subscriber ISDN number
• IMEI – International Mobile Equipment Identity
• TMSI – Temporary Mobile Subscriber Identity
• MSRN – Mobile Subscriber Roaming Number
• LAI – Location Area Identity
• A3 – Authentication Algorithm
• A5 – Ciphering Algorithm
• A8 – Kc Generation Algorithm
11/18/2021 72

GSM Network Identities
1. MSISDN – Mobile Subscriber ISDN number
Subscriber Related Identities -
CC NDC SN
Max. 15 digits
CC – Country Code.
NDC – Network Destination Code.
SN – Subscriber Number.
Eg 91-98100-12345
11/18/2021 73

2. IMSI – International Mobile Subscriber Identity.
3. TMSI – Temporary Mobile Subscriber Identity
Subscriber Related Identities -
MCC MNC MSIN
Max. 15 digits
MCC – Mobile Country Code.
MNC – Mobile Network Code.
MSIN – Mobile Subscriber Identification Number.
GSM Network Identities
Equipment Related Identities -
1. IMEI – International Mobile Equipment Identity
11/18/2021 74

1. MSRN – Mobile Subscriber Roaming Number
It is a temporary number used for routing the call to MS.
Format : MSRN = CC + NDC + SN
CC = Country Code
NDC = Network Destination Code
For example for Patna MSC in IMPCS network MSRN looks like 91 98750 00100.
SN = Subscriber Number
11/18/2021 75

PSTN GMSC
HLR
MSC
VLR
MSISDN
1
MSISDN
2
IMSI
3
IMSI MSRN
4
MSRN
5
MSRN 6
MSRN
7
MSRN
11/18/2021 76

INTERNATIONAL MOBILE
EQUIPMENT IDENTITY (IMEI)
IMEI = TAC + FAC + SNR + spare
TAC = Type Approval Code, determined by a central GSM
body.
FAC = Final Assembly Code, identifies the manufacturer.
SNR = Serial Number, an individual serial number of six digits
uniquely identifies all equipment within each TAC and FAC.
Spare = A spare bit for future use. When transmitted by the MS
this digit should always be zero.
IMEI has the total length of 15 digits.
11/18/2021 78

MSRN
MSRN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
NOTE: In this case, SN should not be interpreted as the actual subscriber
identity. It represents the address to the serving MSC.
11/18/2021 79

MS Initialization / Network Attachment
1. MS Powered on.
- FCCH present in time slot T0 & has a fixed pattern of 142 0’s.
- MS scans entire 124 carriers and monitors RF levels.
- several readings are taken to get accurate estimate
the channel power.
- MS then searches for Frequency Correction burst (FCCH) on the channel with
strongest RF level.
2. Frequency Synchronization
- After tuning to FCCH MS synchronizes in time with SCH .
11/18/2021 80

- SCH occurs in next frame in same time slot as FCCH.
- The occurrence of SCH is eight burst period ( BP’s ) later than FCCH.
3. Synchronization in Time
- After successful synchronization the MS will read the TDMA frame number and Base
Station Identity Code BSIC .
- SCH contains precise timing information and current frame number to which MS is
synchronized.
- BSIC consists of - PLMN colour code ( 3 bits )
- BS colour code ( 3 bits ).
11/18/2021 81

• MS decodes information on BCCH and gets Location Area Identity ( LAI ).
• If the information stored on SIM does not match with the information MS thinks that it
has moved to new location and sends LA update request to BSS.
• MS location is determined by cell identification of strongest BCCH signal received by
MS.
• MS regularly measures the received signal strength of BCCH at least once every 6 sec
( Superframe Duration).
• MS stores at least six strongest BCCH and their cell identification in SIM.
11/18/2021 82

Network Attachment Process
MS
. Cell should be of selected PLMN.
Switch -on
Measure strongest
BCCH channels
PLMN Selection
Cell Selection
Location Update
Idle-Mode
. Cell should not be “ barred “.
. Radio path loss between BTS and
the MS must be below a threshold set
by the operator. ( Register its presence
to the network. )
Limited Service Mode
11/18/2021 83

MS BTS MSC
BSC
RIL3 - CM
LAPD m
Layer 1
RIL3 - MM
RIL3 - RR
Layer 1
LAPD m LAPD
Um A bis A link BCDEFG
MTP
SCCP
BSSMAP
DTAP
MM
TCAP
DTAP
CM
U
P
SCCP
M
A
P
Layer 1
Layer 1
GSM Signaling Protocols Overview
LAPD MTP
I
S
SCCP
BSSMAP
BTSM
RIL 3-
RR
BTSM
RR
11/18/2021 84

What is Mobility Management?
• Subscriber Data management at MSC/VLR
• Subscriber data from HLR are retrieved by
MM at the time of first location up-dating of a
subscriber.
• Dynamic data changes by subscriber are
also managed by MM at MSC/VLR.
• Operator initiated data changes for a
subscriber are also managed by MM.
11/18/2021 85

Security in GSM
• Security services
– access control/authentication
• user  SIM (Subscriber Identity Module): secret PIN (personal
identification number)
• SIM  network: challenge response method
– confidentiality
• voice and signaling encrypted on the wireless link (after successful
authentication)
– anonymity
• temporary identity TMSI
(Temporary Mobile Subscriber Identity)
• newly assigned at each new location update (LUP)
• encrypted transmission
• 3 algorithms specified in GSM
– A3 for authentication (“secret”, open interface)
– A5 for encryption (standardized)
– A8 for key generation (“secret”, open interface)
“secret”:
• A3 and A8
available via the
Internet
• network providers
can use stronger
mechanisms
11/18/2021 86

GSM - authentication
A3
RAND
Ki
128 bit 128 bit
SRES* 32 bit
A3
RAND Ki
128 bit 128 bit
SRES 32 bit
SRES* =? SRES SRES
RAND
SRES
32 bit
mobile network SIM
AC
MSC
SIM
Ki: individual subscriber authentication key SRES: signed response
11/18/2021 87

GSM - key generation and encryption
A8
RAND
Ki
128 bit 128 bit
Kc
64 bit
A8
RAND Ki
128 bit 128 bit
SRES
RAND
encrypted
data
mobile network (BTS) MS with SIM
AC
BTS
SIM
A5
Kc
64 bit
A5
MS
data data
cipher
key
11/18/2021 88

Services Provided to Upper Layers
• MM handover procedures provide smooth
transition from one radio link to other.
• MM sub-layer provides basic services to
upper layer CM sub-layer.
Subscriber Authentication and Confidentiality
management
• MM procedures ensure authenticity of a
subscriber.
• MM procedures ensure data confidentiality at
radio interface.
11/18/2021 89

Subscriber Data at MSC/VLR
• MS status data:
• Data related to identification and numbering ( IMSI,
MSISDN,LMSI and TMSI ).
• Data related to MS type.
• Hand-over data.
• Authentication and Ciphering Data.
• Data related to supplementary services.
11/18/2021 90

MM Procedures
• TMSI Reallocation Procedure.
• IMSI Detach Procedure.
• Ciphering Procedure.
• Abort Procedure.
MM Common Procedures :
• Authentication Procedure.
• Identification Procedure.
MM Specific Procedures :
• Location Updating Procedure.
• IMSI Attach Procedure.
11/18/2021 91

• During the lifetime of a MM specific procedure if a MM
connection establishment is requested by a CM entity,
the request will either be rejected or delayed until the
running of MM specific procedure is terminated.
• Any MM common procedure Procedure ( except
IMSI detach) may be initiated during MM specific
procedure.
• A MM specific procedure can only be started if no
other MM specific procedure is running.
• The MS side should wait for release of RR connection
used for a MM specific procedure before a new MM
specific procedure or MM connection establishment is
started.
MM Procedures
11/18/2021 92

• Location area consist of one or more than one cell which may be served by one or
more BSC s.
• All the cells in a Location area are served by single VLR.
2. Location Area Identity (LAI ) -
• Each location area in a GSM PLMN is identified by a Location Area Identity ( LAI ).
11/18/2021 93

HLR
SS 7
BSC
MSC 2
VLR 1
VLR 2
BSC
BSC
BSC
LA 1
LA 2
LA 3
BTS
BTS
BTS
BTS
BTS
BTS
Location Area
MSC 1
11/18/2021 94

95
SDE RTTC BHUBANESWAR
11/18/2021 95

TAX
CA
TAX
ND
GMSC
MSC
GMSC
MSC/VLR
ND Mobile moves to CA and Powers on MS
CA
ND
HLR
HLR
ND
MS
9810098111
CA
ND
( Update Location/
Authentication )
De register
Mobile to
Old VLR
ACK from
old
MSC/VLR
VLR
LOCATION UPDATION
VLR
VPLMN
HPLMN
Location Update
( TMSI, LAI )
LU Accept
( New TMSI)
Update
Location
Update
Location
LU Accept
(Subs data)
LU Accept
Update
Location
LU Accept
LU Accept
New VLR records
subscriber data.
• Listens
BCCH/FCCH
11/18/2021 97

LOCATION UPDATE SCENARIO - I
MS BSS
Old
MSC/
VLR
HLR
/AUC
New
MSC/
VLR
1
2
Channel Request
RACH
AGCH
SDCCH Assignment
Location Update Request ( TMSI , LAI 0 )
SDCCH
3
Request IMSI
Send IMSI
4
Request For Service
( TMSI )
11/18/2021 98

MS BSS
Old
MSC/
VLR
HLR/
AUC
New
MSC/
VLR
5
6
Get Authentication Parameters
Authentication Response ( SRES )
SDCCH
7
( IMSI )
( RAND,Kc,SRES )
Authentication Parameters
SDCCH
Authenticate MS ( RAND )
Cipher Radio Channel
( Kc )
Cipher Up Link Channel
SDCCH
Authentication and Ciphering
LOCATION UPDATE SCENARIO - II
11/18/2021 99

8
9
BSS Ciphers Down link Channel .
SDCCH
Ciphering Complete
MS BSS
Old
MSC/
VLR
HLR/
AUC
New
MSC/
VLR
LOCATION UPDATE SCENARIO - III
Authentication and Ciphering
Up Link Ciphered
11/18/2021 100

LOCATION UPDATE SCENARIO - IV
10
Update Location
Update HLR/VLR & TMSI Reallocation
De Register Mobile
Mobile De-registered
11
MS BSS
Old
MSC/
VLR
HLR/
AUC
New
MSC/
VLR
Location Updated
Location Updated Accept TMSI
SDCCH
12
13
11/18/2021 101

SDCCH
Location Updated Complete
Clear Signaling Connection
Clear Complete
Release Radio
Signaling Channel
SDCCH
14
16
15
MS BSS
Old
MSC/
VLR
HLR/
AUC
New
MSC/
VLR
LOCATION UPDATE SCENARIO - V
Update HLR/VLR & TMSI Reallocation
11/18/2021 102

Connection Management enables different CM sub-layer entities to
communicate with their pier entity using different MM connections.
Connection Management
What is Connection Management ?
Different CM sub-layer entities use different MM connections.
Different MM connections are identified by Protocol Discriminator PD and,
additionally by Transaction Identifier TI.
11/18/2021 103

MS BTS MSC
BSC
RIL3 - CM
LAPD m
Layer 1
RIL3 - MM
RIL3 - RR
Layer 1
LAPD m LAPD
Um A bis A link BCDEFG
MTP
SCCP
BSSMAP
DTAP
MM
TCAP
DTAP
CM
U
P
SCCP
M
A
P
Layer 1
Layer 1
GSM Signaling Protocols Overview
LAPD MTP
I
S
SCCP
BSSMAP
BTSM
RIL 3-
RR
BTSM
RR
11/18/2021 104

MS
RIL3 - CM
LAPD m
Layer 1
RIL3 - RR
CC SMS SS
RIL3 - MM
CC = Call Connection
SMS = Short Message Service
SS = Supplementary Services
• There are different entities of CM sub-
layer.
• For each entity different MM connection
will be established over same RR
connection.
11/18/2021 105

• Several MM connections may be active at same time.
• All simultaneous MM connections from a given MS use the same RR
connection.
• An MM connection requires an RR connection.
• Mobility Management ( MM) sub-layer provides connection management services
to different entities of upper Connection Management ( CM ) sub-layer.
• MM connection is established and released on request from a CM entity.
11/18/2021 106

Phases Of Connection Management
• MM connection establishment initiated by MS.
MM Connection Establishment -
• MM connection establishment initiated by n/w.
MM Connection Information Transfer Phase -
• Sending CM messages.
• Receiving CM messages.
MM Connection Release -
• Release associated RR connection.
11/18/2021 107

TIME
MS N/W
CM Service Request
CM Service Reject
CM CONNECTION ESTABLISHMENT INITIATED BY MS
CM Service Accept
1. IMSI unknown in VLR
2. Illegal ME
3. Network Failure
4. Congestion
5. Service Option not supported.
6. Requested service option not subscribed.
7. Service option temporarily out of order.
CM Service Reject Causes :
11/18/2021 108

MS N/W
CM
MM
RR
MM
CM
RR
CC SMS SS CC SMS SS
CM connection establishment initiated by the Network
( In case no RR connection Exists )
CM Service Reject
CM Service Reject
11/18/2021 109

Zone Codes
• Zone codes provide mechanism for LAC based subscription.
• A set of LACs form a Zone Code.
• Subscribers can be given access to a set of Zone codes.
• If a subscriber updates location in a Zone Code other than his subscribed ones,
it is equivalent to sending IMSI Detach.
• For the above event MM will mark Regional Barring flag for the subscriber.
Example:
• In a case where MSC covers several cities Zone Codes may be used to restrict
the subscribers of one city from using n/w services in other cities.( Regional
Service )
11/18/2021 110

Mobile To Land Call Set-up Scenario
Phases of Mobile To Land Call
• SETUP PHASE
( Mobile Originated Call- MO Call )
• RINGING PHASE
• CONVERSATION PHASE
• RELEASE PHASE
11/18/2021 111

- REQUEST FOR SERVICE ( CHANNEL ) BY MS
- CM SERVICE REQUEST
- AUTHENTICATION
- CIPHERING
- SETUP MESSAGE
- EQUIPMENT VALIDATION ( EIR CHECK )
- VALIDATION AT VLR
- ASSIGNMENT
• SETUP PHASE
11/18/2021 112

MS
MSC/
VLR
BSS
1
2
3
Channel Request
Request For Service
MOBILE TO LAND CALL - I
RACH
SDCCH Assignment
AGCH
CM Service Accept
SDCCH
4
CM Service Request
11/18/2021 113

• CM SERVICE REQUEST
- The Call Control (CC) entity of MS initiates the establishment of
CC connection by requesting MM sub-layer of the mobile to
establish MM sub-layer connection.
- Upon establishment of MM connection CM SERVICE ACCEPT
message is received from n/w.
• SETUP MESSAGE
- CC entity sends a setup message to its peer entity.
11/18/2021 114

MS
MSC/
VLR
BSS
5
6
Call Setup Request
7
SDCCH
SDCCH
Call Setup
MOBILE TO LAND CALL - II
Assign Trunk and radio
Assign TCH
Radio Assignment Complete
Call Proceeding
SDCCH
SDCCH
Trunk and Radio Assignment Complete
11/18/2021 115

MSC/VLR
Call Setup message
Is the requested basic service
provisioned for the subscriber ?
Yes
No
Release Complete
message to MS
Preliminary Digit Analysis
Is the number of digits less than the
minimum expected?
Yes
Release Complete
message to MS
No 1. Check of ODB( BAOC/BOIC/ BOIC Ex HC )
2. Subscriber call barring checks ( BAOC/BOIC et
ASSIGNM ENT
11/18/2021 116

• ASSIGNMENT
- A BSSMAP message ‘Assignment Command ’ is sent by MSC to
BSS.
- The message contains a free CIC on A-Interface for terrestrial
channel allocated by MSC.
- A free TCH is allocated by the BSS and MS is informed to attach
to this TCH.
- On receiving a response from BSS MSC switches the speech
path towards the calling MS.
Note : Mobile has not connected user
connection as yet.
11/18/2021 117

MS
MSC/
VLR
BSS
8
9
10
11
Call Setup
MOBILE TO LAND CALL - III
GMSC PSTN
HLR
Network Setup ( Dialed Digits )
FACCH
Network Alerting
Connect
Connect Acknowledge
FACCH
Alerting
Connect
FACCH
Start Billing
11/18/2021 118

MS
MSC/
VLR
BSS
12
13
14
15
Call Release
MOBILE TO LAND CALL - IV
GMSC PSTN
HLR
Disconnect
FACCH
Network Release
Release Complete
Release Complete
FACCH
Release
FACCH
Clear Command
Channel Release
11/18/2021 119

Land To Mobile Call Set-up Scenario
Phases of Land To Mobile Call
• Routing Analysis
• Paging
• Authentication
• Call-Setup
• Ciphering
• Call Release
( Mobile Terminating - MT Call )
11/18/2021 120

Architecture For Mobile Terminated Call
GMSC- B BSS-B
MS
VLR-B
MSC-B
VISITING PLMN
A- Interface
Air Interface
HLR-B
IAM ( ISUP )
11/18/2021 121

LAND TO MOBILE CALL - I
MSC/VLR GMSC PSTN
HLR
1
2
Incoming Call
3
4
( MSISDN )
Get Route
( MSISDN )
Get Route
( IMSI )
Routing Information
( MSRN )
Routing Information
( MSRN )
( MSRN )
Incoming Call
5
Routing Analysis
11/18/2021 122

MS MSC
BSS
6
7
8
Perform Page
(TMSI)
Page
PCH
Channel Request
( RACH )
SDCCH
Paging Response
( TMSI, LAI )
9
Access Granted
( AGCH )
Page Response
Paging
LAND TO MOBILE CALL - II
11/18/2021 123

MS
MSC/
VLR
BSS
13
14
15
Connect ( off-hook )
FACCH
Call Setup
LAND TO MOBILE CALL - IV
GMSC PSTN
HLR
FACCH
Network Alerting
Mobile Alerting
Connect
Connect Acknowledge
Start Billing
11/18/2021 124

MS
MSC/
VLR
BSS
16
17
18
Disconnect
Release
19
FACCH
FACCH
Call Release
LAND TO MOBILE CALL - V
GMSC PSTN
HLR
Network Release
FACCH
Release Complete
Release Complete
Clear Command
Clear Channel
Clear Complete
FACCH
Stop Billing
11/18/2021 125

Mobile To Mobile Call Set-up Scenario
Phases of Mobile To Mobile Call
• Request for service.
• Authentication
• Call-Setup
• Ciphering
• Call Release
• Routing Analysis
• Paging
• Authentication
• Ciphering
• Call-Setup
Mobile Originating
Mobile Terminating
• Call-Release
11/18/2021 126

TAX
ND
GMSC
MSC/VLR
1. ND PSTN Subscriber Dials ND Mobile Subscriber in ND.
ND
LE TANDEM
HLR
ND
MS
9810098100
ND
SRI
PRN to
MSC/VLR
ND
ACK from
MSC/VLR
ND
(MSRN)
MSRN
MSISDN
MSISDN
MSISDN
Page(TMSI)
11/18/2021 127

TAX
ND
TAX
CA
GMSC
MSC/VLR
GMSC
MSC/VLR
2. ND PSTN Subscriber Dials CA Mobile Subscriber in CA.
ND
CA
LE
HLR
HLR
CA
MS
9881098300
ND
CA
SRI
PRN to
MSC/VLR
CA
ACK from
MSC/VLR
CA
(MSRN)
MSRN
MSISDN
Page(TMSI)
TANDEM
MSISDN
MSISDN
MSISDN
11/18/2021 128

TAX
CA
TAX
ND
GMSC
MSC/VLR
GMSC
MSC/VLR
3. ND PSTN Subscriber Dials ND Mobile Subscriber in CA.
CA
ND
LE TANDEM
HLR
HLR
ND
MS
9810098200
CA
ND
SRI
PRN to
MSC/VLR
CA
ACK from
MSC/VLR
CA
(MSRN)
MSRN
MSRN
MSRN
MSRN
MSISDN
MSISDN
MSISDN
Page(TMSI)
11/18/2021 129

TAX
CA
TAX
ND
GMSC
MSC/VLR
GMSC
MSC/VLR
4. CA PSTN Subscriber Dials ND Mobile Subscriber in CA.
CA
ND
LE
HLR
HLR
ND
MS
9810098400
CA
ND
SRI
PRN to
MSC/VLR
CA
ACK from
MSC/VLR
CA
(MSRN)
MSRN
MSISDN
Page(TMSI)
TANDEM
MSISDN
MSISDN
MSISDN
MSRN
MSRN
MSRN
11/18/2021 130

TAX
CA
TAX
ND
GMSC
MSC/VLR
GMSC
MSC/VLR
4.A CA PSTN Subscriber Dials ND Mobile Subscriber in CA
CA
ND
LE
HLR
HLR
ND
MS
9810098410
CA
ND
PRN to
MSC/VLR
CA
ACK from
MSC/VLR
CA
(MSRN)
MSRN
Page(TMSI)
TANDEM
MSISDN
MSISDN
Roaming agreement with Optimal Routing
SRI
11/18/2021 131

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