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Chapter 4:
Telecommunication Systems
1600
Mobile phone subscribers
worldwide
approx. 1.7 bn
2009:
>4 bn!
1400
Subscribers [million]
1200
GSM total
1000
TDMA total
CDMA total
PDC total
800
Analogue total
W-CDMA
600
Total wireless
Prediction (1998)
400
200
0
1996
1997
1998
1999
2000
2001
2002
2003
2004 year
CT0/1
AMPS
NMT
CT2
IMT-FT
DECT
IS-136
TDMA
D-AMPS
TDMA
FDMA
Development of mobile
telecommunication systems
GSM
PDC
EDGE
GPRS
IMT-SC
IS-136HS
UWC-136
IMT-DS
UTRA FDD / W-CDMA
HSPA
IMT-TC
CDMA
UTRA TDD / TD-CDMA
IMT-TC
TD-SCDMA
1G
IS-95
cdmaOne
cdma2000 1X
2G
2.5G
IMT-MC
cdma2000 1X EV-DO
1X EV-DV
(3X)
3G
What is GSM?
The Global System for Mobile
Communications is a digital cellular
communications system.
It was developed in order to create a common
European mobile telephone standard but it has
been rapidly accepted worldwide.
GSM was designed to compatible with ISDN
(integrated services digital network) services.
It signifies an extremely successful technology
and bearer for mobile communication system.
GSM today Covers 71% of all the digital wireless
market.
What is GSM?
People use it not only in business but also in
everyday personal life.
Its principle use it is for wireless telephony and
messaging through SMS.
It also supports facsimile and data
communication.
Due to its innovative technologies and strengths
GSM rapidly became truly global.
Many of the new standardization initiative came
from outside Europe.
What is GSM?
Depending on locally available frequency bands,
different air interfaces were defined.
These are 900MHz, 1800MHz, and 1900MHz.
However, architecture, protocols, signaling and
roaming are identical in all networks independent
of the operating frequency bands.
GSM uses a combination of FDMA (frequency
division multiple access) and TDMA ( Time
division multiple access).
Why GSM?
GSM uses radio frequencies more effectively than
the older system.
The data transmission services and the quality of
the speech are better than in analog system.
There are two kinds of advanced security services
available on the radio path : user identity and data
confidentiality.
New services and ISDN compatibility are offered.
It makes international roaming possible.
The big uniform market hardens the competitions
and lowers the prices.
Later on it also leads to lower system costs.
History of GSM
GSM is based on a set of standards, formulated in
the early 1980s.
In 1982 the conference of European posts and
Telegraphs formed a study group called the
Groupe Special Mobile (GSM) to study and
develop a pan-European mobile system, which
was later introduce as Global System for Mobile
Communication.
GSM Services


GSM offers
 several types of connections
 voice connections, data connections, short
message service
 multi-service options (combination of basic
services)
Three service domains
 Bearer Services
 Tele Services
 Supplementary Services
GSM Services (cont…)
TE=Terminal
MT=Mobile Termination
PLMIN=public land mobile network
PSTN=public switched telephone network
ISDN=integrated services digital network
bearer services
MS
TE
MT
R, S
GSM-PLMN
Um
transit
network
(PSTN, ISDN)
source/
destination
network
tele services
Fig: Bearer and Tele services referene model
TE
(U, S, R)
GSM Services: Tele Services


Telecommunication services that enable voice
communication via mobile phones.
All these basic services have to obey cellular
functions, security measurements etc.
Telephony
Facsimile group 3
Emergency call
Teletex
Short message Services (SMS)
Fax mail
Voice mail
Electronic mail
GSM Services: Bearer Services
A bearer service is used for transporting user data.
Some of the bearer services are listed below:
Asynchronous and Synchronous data, 3009600 bps.
Alternate speech and data, 300-9600 bps.
Asynchronous PAD (packet-switched access,
300-9600 bps.
Synchronous dedicated packet data access,
2400-9600 bps.
GSM Services:
Supplementary Services
Call Forwarding- the subscriber can forward
incoming calls to another number if the called
mobile is busy, unreachable or if there is no reply.
Call Barring-There are different types of call
barring services:
Barring of all outgoing calls
Barring of outgoing international calls
Barring of all incoming calls
Barring of incoming calls when roaming.
Call hold-puts an active call on hold.
GSM Services:
Supplementary Services
Call Waiting
Closed user group, CUG-it corresponds to a group
of users with limited possibilities of calling,
locking of the mobile terminal (Incoming and
outgoing calls)
GSM Architecture
The coverage area of a cellular system is
partitioned into a number of smaller area or
cells with each cell served by a Base Station
(BS) for radio coverage.
The base station are connected through fixed
links to a mobile switching center (MSC),
which is a local switching exchange with
additional features to handle mobility
management requirements of a cellular system.
GSM Architecture
MSCs also interconnect with the public switched
telephone network (PSTN) because the majority
of calls in a cellular mobile system either originate
form or terminate at fixed network terminals.
In the next slide figure a typical cellular system
architecture/ GSM network architecture.
GSM: elements and interfaces
radio cell
MS
BSS
MS
Um
radio cell
MS
BTS
RSS
BTS
Abis
BSC
BSC
A
MSC
NSS
MSC
VLR
signaling
VLR
GMSC
HLR
IWF
O
OSS
EIR
AUC
OMC
ISDN, PSTN
PDN
GSM Architecture
GSM system consist of three subsystems:
Radio Subsystem (RSS)
Network and Switching Subsystem (NSS)
Operation Subsystem (OSS)
GSM Architecture:
Radio Subsystem (RSS)
Mobile Station:
Mobile station consist of two units:
Mobile hand set is one of the most complicated
GSM device. It provides user the access to the
network.
Subscribe identity module (SIM) is a
removable module goes into the mobile
handset. Each SIM has unique number called
international mobile subscriber identity (IMSI).
It has built in micro-computer & memory into
it.
GSM Architecture:
Radio Subsystem (RSS)
Base station subsystem (BSS):
A GSM network comprises many BSS, each
controlled by a base station controller (BSC).
The BSS performs all function necessary to
maintain radio connections to an MS,
coding/decoding of voice, and rate adaptation
to/from the wireless network part.
Besides a BSC, the BSS contains several BTS.
GSM Architecture:
Radio Subsystem (RSS)
Base transceiver station (BTS):
BTS has set of transceiver to talk to MS.
One BTS covers one or more than one cell.
Capacity of BTS depends on no of transceivers.
BTS is connected to BSC via A’bis interface.
Transmission rate on A’bis is 2 Mbps.
Interface between MS and BTS is called Um.
Transmission rate on Um interface is 13 Kbps.
Each transmission has 8 TDMA channels to carry
voice & signaling.
GSM Architecture:
Radio Subsystem (RSS)
Base station controller (BSC):
BSC controls several BTSs.
BSC manages channel allocation & handover of
called from one BTS to another BTS.
BSC is connected to MSC via A’ interface.
Transmission rate on A I/F is 2 Mbps.
Interface between BSC & BTS is called A’bis I/F.
BSC has database for all of its BTS’s parameters.
BSC provides path from MS to MSC.
GSM Architecture:
Network and Switching Subsystem
(NSS)
Mobile services switching center (MSC):
MSC is hear of the entire network connecting
fixed line network to mobile network.
MSC manages all call related functions and billing
information.
MSC is connected to HLR & VLR for subscriber
identification & routing incoming calls.
MSC capacity is in terms of no of subscribers.
MSC is connected to BSC at one end and fixed
line network on other end.
Call Detail Record (CDR) is generated for each &
every call in the MSC.
GSM Architecture:
Network and Switching Subsystem
(NSS)
Home location register (HLR):
All subscribers data is stored in HLR.
It has permanent data base of all the registered
subscribers.
GSM Architecture:
Network and Switching Subsystem
(NSS)
Visitor location register (VLR):
Active subscriber is registered in VLR.
It is a temporary data base of all the active
subscribers.
HLR validates subscriber before registration.
MSC ask VLR before routing incoming call.
GSM Architecture:
Operation Subsystem (OSS)
Operation and maintenance center (OMC):
All the network elements are connected to OMC.
OMC monitors health of all network elements &
carries out maintenance operation, if required.
OMC is linked to BTSs via parent BSC.
OMC keeps records of all the faults occurred.
OMC can also generate Traffic analysis reports.
GSM Architecture:
Operation Subsystem (OSS)
Authentication center (AuC):
Authentication is a process to verify the subscriber
SIM.
Secret data & verification algorithm are stored in
to the AUC.
AUC & HLR combined to authenticate the
subscribers.
Subscriber authentication can be done on every
call if required.
GSM Architecture:
Operation Subsystem (OSS)
Equipment identity register (EIR):
The equipment identity register stores the
international mobile equipment identity (IMEI)
numbers for the entire network.
IMEI enables the MSC in denitrifying the type of
terminal, mobile equipment manufacturer, and
model and helps the network in locating the
device in case it is stolen or misplaced.
The EIR contains three different types of lists:
A Black list: includes mobile stations which
have been reported stolen or are currently
locked due to some reason.
GSM Architecture:
Operation Subsystem (OSS)
Equipment identity register (EIR):
A White list: which records all MSs that are
valid and operating.
A Grey list: including all those MSs that may
not be functioning properly.
According to category the MS can make calls or
can be stopped from making calls.
Radio Interface
The radio interface is the interface between the
mobile stations and the fixed infrastructure.
It is one of the most important interfaces of the
GSM system.
One of the main objectives of GSM is roaming.
Therefore, in order to obtain a complete
compatibility between mobile stations and
networks of different manufactures and operators,
the radio interface must be completely defined.
Radio Interface
The spectrum efficiency depends on the radio
interface and the transmission, more particularly
in aspects as the capacity of the system and the
techniques used in order to decrease the interface
and to improve the frequency reuse scheme.
The specification of the radio interface has then an
important influence on the spectrum efficiency.
Radio Interface
Frequency allocation:
Two frequency band of 25 MHz each one, have
been allocated for the GSM system:
The band 890-915 MHz has been allocated for
the uplink direction (transmitting from the
mobile station to the base station)
The band 935-960 MHz has been allocated for
the down link direction (transmitting from the
base station to the mobile station)
But not all the countries can use the whole GSM
frequency band.
Radio Interface
Multiple Access Scheme:
The multiple access scheme defines how different
simultaneous communications, between different
mobile stations situated in different cells, share the
GSM radio spectrum.
A mix of Frequency Division Multiple Access
(FDMA) and Time Division Multiple Access
(TDMA) combined with frequency hopping, has
been adopted as the multiple access scheme for
GSM.
Radio Interface
FDMA:
Using FDMA, a frequency is assigned to a user.
So the larger the number of users in a FDMA
system the larger the number of available
frequencies must be.
The limited available radio spectrum and the fact
that a user will not free its assigned frequency
until he does not need it anymore, explain why the
number of users in a FDMA system can be
‘quickly” limited.
Radio Interface
TDMA:
On the other hand, TDMA allows several users to
share the same channel.
Each of the users, sharing the common channel, is
assigned their own burst within a group of bursts
called a frame.
Usually TDMA is used with a FDMA structure.
GSM - TDMA/FDMA
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
higher GSM frame structures
time
GSM TDMA frame
1
2
3
4
5
6
7
8
4.615 ms
GSM time-slot (normal burst)
guard
space
tail
3 bits
user data
S Training S
user data
57 bits
1 26 bits 1
57 bits
guard
tail space
3
546.5 µs
577 µs
Radio Interface
Data is transmitted in small portion is called
bursts.
Each carrier frequency is then divided in time
using a TDMA scheme.
This scheme splits the radio channel, with a width
of 200KHz, into 8 burst.
A burst is the unit of time in TDMA system, and
its lasts approximately 0.577 ms.
A TDMA frame is form with 8 bursts and lasts
consequently, 4.615 ms.
Each of the eight burst, that form a TDMA frame,
are then assigned to a single user.
Radio Interface:
Channel Structure
A channel corresponds to the recurrence of one
burst every frame.
It is defined by its frequency and the position of its
corresponding burst within a TDMA frame.
GSM in TDMA: Each carrier consists of eight time
slots.
In GSM there are Two types of Channels:
Physical Channel:
Logical Channel:
Radio Interface:
Channel Structure
Physical Channel:
A physical channel is a single slot on a single
frequency.
Thus there are eight physical channels per
frequency pair of TDMA frame.
The information within the physical channel is
termed a burst.
Radio Interface:
Channel Structure
Logical Channel:
A logical channel is the content within a burst,
e.g. Speech.
Signalling or measurement, the way in which
we organize these channels is partly dependent
upon the application.
But is dependent on whether the information is
sent uplink or downlink or bi-direction.
Radio Interface:
Channel Structure
Logical Channel:
There are two type of logical channel:
1. The Traffic Channels used to transport speech and
data information.
2. The Control Channels used for network
management message and some channel maintenance
tasks.
Radio Interface:
Channel Structure
1. Traffic Channels (TCH):
Full-rate traffic channels (TCH/F) are defined
using a group of 26 TDMA frame called a 26multiframe.
The 26-multiframe lasts consequently 120 ms.
In this 26-multiframe structure the traffic
channels for the downlink and uplink are
separated by 2 bursts.
As a consequence, the mobiles will not need to
transmit and receive at the same time which
simplifies considerably the electronics of the
system.
Radio Interface:
Channel Structure
1. Traffic Channels (TCH):
The frame that form the 26-multiframe structure
have different functions:
24 frames are reserved to traffic.
1 frame is used for the slow associated control
channel (SACCH).
The last frame is unused. This idle frame
allows the mobile station to perform other
functions, such as measuring the signal
strength of neighboring cells.
Radio Interface:
Channel Structure
1. Traffic Channels (TCH):
Half-rate traffic channels (TCH/H) which double
the capacity of the system, are also grouped in a
26-multifram but the internal structure is
different.
Radio Interface:
Channel Structure
2. Control Channels:
according to their functions, four different
classes of control channels are defined:
2.1 Broadcast channels (BCH).
2.2 Common control channels (CCCH).
2.3 Dedicated control channels (DCCH).
2.4 Associated control channels (ACCH).
Radio Interface:
Channel Structure
2.1 Broadcast channels (BCH):
the broadcast channels are used, by the base
station to provide the mobile station with the
sufficient information.
It needs to synchronize with the network.
There are different types of BCH can be
distinguished:
FCCH (Frequency Correction Channel):
it carries no real information.
All bits are set to zero, which generates a pure
sine wave in the modulator.
Radio Interface:
Channel Structure
2.1 Broadcast channels (BCH):
FCCH (Frequency Correction Channel):
It allows the mobile to tune its synthesizer
roughly and indicates that on this frequency
broadcast information is transmitted.
SCH (Synchronization Channel):
The SCH transmits the used for handover and
the TDMA frame number which is used for
ciphering.
BCCH (Broadcast Control Channel):
The BCCH contains cell specific information
like cell ID, used frequency hopping sequences,
adjacent cells etc.
Radio Interface:
Channel Structure
2.2 Common control channels (CCCH):
The CCCH channels help to establish the calls
from the mobile station or the network.
There different types of CCCH can be defined:
PCH (Paging Channel (Downlink)):
This channel transmits the paging request for a
mobile in case of an incoming call.
Radio Interface:
Channel Structure
2.2 Common control channels (CCCH):
AGCH (Access Grant Channel (Downlink)):
On this channel the mobile gets initial time
advance and the information which signaling
channel should be used.
RACH (Random Access Channel (Uplink)):
Only on this the mobile can access a cell.
It contains an identifier of the mobile.
Radio Interface:
Channel Structure
2.3 Dedicated control channels (DCCH):
The DCCH channels are used for message
exchange between several mobiles or a mobile
and the network.
Two different types of DCCH can be defined:
SDCCH (Stand alone dedicated control
channel):
This channel is bi-directional and is used for cell
set up procedures such as authentication and it
assigns a particular traffic channel to the mobile.
Radio Interface:
Channel Structure
2.3 Dedicated control channels (DCCH):
SACCH (Slow associated control channel):
A SACCH is associated with every SDCCH and
every TCH too.
During the call setup and a call in progress.
The system has to know, if a handover is
required the information pertaining to this are
transmitted on this particular channel.
It is also used to control the power of the MS
and to maintain the correct timing alignment of
a mobile moving within the cell.
Radio Interface:
Channel Structure
2.4 Associated control channels (ACCH):
The Fast Associated control channels (FACCH)
replace all or part of a traffic cannel when
urgent signaling information must be
transmitted.
The FACCH channels carry the same
information as the SDCCH channels.
Radio Interface:
Frame hierarchy
In next slide figure, the pattern of 26 slots
occurs in all TDMA frames with traffic channel
(TCH).
The combination of these frames is called
traffic multiframe.
the logical combination of 26 frames to a
multiframe with a duration of 120 ms.
This type of multiframe is used for traffic
channel (TCH), slow associated dedicated
control channel (SACCH), Fast Associated
control channels (FACCH).
Radio Interface:
GSM hierarchy of frames
hyperframe
0
1
2
2045 2046 2047 3 h 28 min 53.76 s
...
superframe
0
1
0
2
...
1
48
...
49
24
50
6.12 s
25
multiframe
0
1
...
0
1
24
2
120 ms
25
...
48
49
50
235.4 ms
frame
0
1
...
6
7
4.615 ms
slot
burst
577 µs
Radio Interface:
Frame hierarchy
TDMA frames containing data for the other
logical channels are combined to a control
multiframe.
Control multiframe consist of 51 TDMA frames
and have a duration of 235.4 ms.
This logical frame hierarchy continues, combining
26 multiframe with 51 frames or 51 multiframe
with 26 frames to form a superframe.
Radio Interface:
Frame hierarchy
2,048 super frames build a hyperframe with a
duration of almost 3.5 hours.
Altogether, 2,715,648 TDMA frames form
hyperframe.
GSM Protocols & Interfaces
Next slide figure shows the signaling protocols
between the MS and BTS, between the BTS and
BSC, between the BSC and the MSC.
These protocols between some interfaces
presented.
MS and BTS between Um interface used:
The air interface is used for exchanges
between a MS and a BSS.
It is used for transmitting signaling further.
GSM protocol layers for signaling
Um
Abis
MS
A
BTS
BSC
CM
CM
MM
MM
Layer 3
RR
RR’
Layer 2
MSC
BTSM
RR’
BTSM
LAPDm
LAPDm
LAPD
LAPD
radio
radio
PCM
PCM
BSSAP
BSSAP
SS7
SS7
PCM
PCM
Layer 1
16/64 kbit/s
64 kbit/s /
2.048 Mbit/s
GSM Protocols & Interfaces
BTS and BSC between Abis interface used:
This is a BSS internal interface linking the
BSC and a BTS, and it has not been
standardized.
The Abis interface allows control of the radio
equipment and radio frequency allocation in
the BTS.
GSM Protocols & Interfaces
BSC and MSC between A interface used:
The A interface linking the BSC and MSC.
The A interface manages the allocation of
suitable radio resources to the MSs and
mobile management.
GSM Protocols
In GSM, there are different types of protocols
used in different layers.
These layers protocols function are describe
bellow:
Mobility Management (MM):
The MM layer is in charge of maintain the
location data, in addition to the authentication
and ciphering procedures.
GSM Protocols
Communication Management (CM):
The CM layer consists of setting up call at the
users request.
Its functions are : call control, which manages
the supplementary services configuration,
short message services which provides pointto-point short message services.
GSM Protocols
Radio Resource (RR):
The RR management layer is in charge of
establishing and maintaining a stable
uninterrupted communication path between the
MSC and MS over which signaling and user data
can be covered.
Handovers are part of the RR layer responsibility.
Most of the functions are controlled by the BSC,
BTS and MS though some are performed by the
MSC.
GSM Protocols
Radio Resource’ (RR’):
The RR’ layer is the part of the RR
functionality which is managed by the BTS.
Base Transceiver Station Management
(BTSM) :
The BTSM is responsible for transferring the
RR information to the BSC.
GSM Protocols
Link access protocol for the ISDN D-channel
(LAPD) :
This is the ISDN LAPD protocol providing
error-free transmission between the BSC and
MSC.
LAPDm:
The layer two protocols are provided for by
LAPDm over the air-interface.
This protocol is a modified version of the
LAPD protocol.
GSM Protocols
LAPDm:
The main modification are due to the tight
synchronization required in TDMA and bit
error protection mechanism required over the
air-interface.
GSM Protocols
Base Station System Application Part
(BSSAP) :
The BSSAP is split into two parts the Base
station system management application part
(BSSMAP) and the Direct transfer
application part (DTAP).
The message exchanges are handled by SS7.
Messages which are not transparent to the
BSC are carried by the BSSMAP, which
supports all of the procedures between the
MSC and the BSS that require interpretation
and processing of information related to
single calls and resource management.
GSM Protocols
Signaling Connection Control Part (SCCP):
The SCCP from SS7.
Message Transport Part (MTP):
The MTP of SS7.
What is SS7?
Signaling system No. 7 (SS7) is used for
signaling between an MSC and a BSC.
This protocol also transfer all management
information between MSCs, HLR, VLRs, AuC,
EIR and MOC.
Localization
The localization is a process by which a mobile
station is identified, authenticated and provided
service by a mobile switching center through the
base station controller and base Tran receiver either
at the home location of the MS or at a visiting
location.
Mobile service providers, on the other hand will
provide services to the user only after identifying
the mobile station (MS) of the user and verifying
the services subscribed to by the user or the
services presently allowed to that MS.
Localization
Localization mechanism of the GSM system fulfils
both the requirement.
GSM distinguishes explicitly between the user and
the equipment.
It also distinguishes between the subscriber identity
and the telephone number.
GSM deals with many addresses and identifiers.
Localization
Mobile Subscriber ISDN Number (MSISDN):
The MSISDN number is the real telephone number
as is known to the external world.
MSISDN number is public information.
This is a number published and known to everybody.
In GSM a mobile station can have multiple MSISDN
number.
When a subscriber send a Fax and Data.
He/she is assigned a total of 3 numbers: one for
voice call, one for fax call and another for data call.
Localization
The MSISDN categories follow the international ISDN
(integrated system data network) numbering plan as
following:
Country code (CC): 1 to 3 decimal digits of country
code
National destination code (NDC): typically 2 to 3
decimal digit,
Subscriber number (SN): maximum 10 decimal digit.
In India a MSISDN number looks like 919845062050.
In this number 91 is CC and 98 is NDC and 45062050 is
the SN.
Localization
International Mobile Subscriber Identity
(IMSI):
When registered with a GSM operator each
subscriber is assigned a unique identifier.
The IMSIO is stored in the SIM card and secured
by the operator.
A mobile station can only be operated when it has a
valid IMSI.
The IMSI consists of several parts.
Localization
International Mobile Subscriber Identity
(IMSI):
These are:
3 decimal digits of mobile country code (MCC).
For Indian MCC is 404.
2 decimal digit of mobile network code (MNC).
This uniquely identifies a mobile operator within
a country. For Airtel in Delhi this code is 10.
Maximum 10 decimal digits of mobile
subscriber identification number (MSIN). This is
a unique number of the subscriber within the
home network.
Localization
Temporary Mobile Subscriber Identity (TMSI):
This a temporary identifier assigned by a serving
VLR.
It is used in place of the IMSI for identification and
addressing of the mobile station.
TMSI is assigned during the presence of the mobile
station in a VLR.
Thus, it is difficult to determine the identity of the
subscriber by listening to the radio channel.
Localization
Temporary Mobile Subscriber Identity (TMSI):
The TMSI is never stored in the HLR.
However, it is stored in the SIM card.
Together with the current location are, a TMSI
allows a subscriber to be identify uniquely.
Localization
Mobile Station Roaming Number (MSRN):
When a subscriber is roaming in another network
a temporary ISDN number is assigned to the
subscriber.
This ISDN number is assigned by the local VLR
in charge of the mobile station.
The MSRN has the same structure as the
MSISDN.
Calling
There are different methods and protocols are used
for establishing connection and maintaining
communication in calling to and from mobile
devices in a GSM network.
The various types of calls handled by a GSM
network are:
Mobile originated call (MOC)
Mobile Terminated call (MTC)
Calling :
Mobile originated call (MOC)
Initially when the user enters the called number and
presses the send key.
The MS establishes a signaling connection to the
BSS on a radio channel.
This may involve authentication and ciphering.
Once this has been established the call setup
procedures will take place according to the
sequence show in the next slide figure.
Calling:
Mobile originated call (MOC)
VLR
3 4
6
PSTN
5
GMSC
7
MSC
8
2 9
MS
1
10
BSS
Calling :
Mobile originated call (MOC)
The MS sends the dialed number indicating service
requested to the MSC (via BSS).
The MSC checks from the VLR if the MS is
allowed the requested service. If so, MSC asks the
BSS to allocate necessary resource for the call.
If the call is allowed, the MSC routes the call to the
GMSC (Gateway MSC).
The GMSC routes the call to the local exchange of
called user via public switched telephone network
(PSTN).
Calling :
Mobile originated call (MOC)
The PSTN alert (applies ringing) the called
terminal.
Answer back (ring back tone) from the called
terminal to PSTN.
Answer back signal is routed back to the MS
through the serving MSC which also completes the
speech path to the MS.
Calling:
Mobile Terminated call (MTC)
The sequence shown in next slide figure relates to a
call originating in the PSTN and terminating at an
MS in a GSM network.
The PSTN user dials the MSISDN of the called
user in GSM.
Local route of PSTN the call to the GMSC of the
called GSM user.
The GMSC uses the dialed MSISDN to determine
the serving HLR for the GSM user and interrogates
it to attain the required routing number.
Calling :
Mobile Terminated call (MTC)
HLR
4
5
3 6
calling
station 1
PSTN
2
GMSC
10
7
VLR
8 9
14 15
MSC
10 13
16
10
BSS
BSS
BSS
11
11
11
11 12
17
MS
Calling:
Mobile Terminated call (MTC)
The HLR requests the current serving VLR for the
called MS for a MSRN (MS roaming number) so
that the call can be routed to the correct MSC.
The VLR passes the MSRN to the HLR.
The HLR passes the MSRN to the GMSC.
Using the MSRN the GMSC routes the call to the
serving MSC.
The MSC interrogates the VLR for the current
location area identity (LAI) for the MS.
The VLR provides the current location (LAI) for
the MS.
Calling:
Mobile Terminated call (MTC)
The MSC pages the MS via the appropriate BSS.
The MS responds to the page and set up the
necessary signaling links.
When the BSS has established the necessary radio
links, the MSC is in formed and the call is
delivered to the MS.
When the MS answers the call, the connection is
completed to the calling PSTN user.
MTC/MOC
MS
MTC
BTS
MS
MOC
BTS
paging request
channel request
channel request
immediate assignment
immediate assignment
paging response
service request
authentication request
authentication request
authentication response
authentication response
ciphering command
ciphering command
ciphering complete
ciphering complete
setup
setup
call confirmed
call confirmed
assignment command
assignment command
assignment complete
assignment complete
alerting
alerting
connect
connect
connect acknowledge
connect acknowledge
data/speech exchange
data/speech exchange
Handover
Cellular systems require handover procedures, as
single cells do not cover the whole service area,
but, e.g. only up to 35 km around each antenna.
The smaller the cell size and the faster the
movement of a mobile station through the cells, the
more handovers of ongoing calls are required.
However a handover should not cause a cut-off
also called call drop.
Handover
There are two basic reasons for a handover :
The mobile station moves out of the range of a
BTS or a certain antenna of a BTS respectively.
Thus, the received signal level becomes lower
continuously until it falls underneath the minimal
requirement for communication.
The wired infrastructure (MSC,BSC) may decide
that the traffic in one cell is too high and shift some
MS to other cells with a lower load. Thus handover
may be due to load balancing.
Handover
In the next slide figure shows four possible
handover scenarios in GSM.
Intra-cell handover:
Within a cell, narrow-band interference could make
transmission at a certain frequency impossible.
The BSC could then decide a change the carrier
frequency (Scenario 1).
In short, handover of channels in the same cell.
4 types of handover
1
MS
BTS
2
3
4
MS
MS
MS
BTS
BTS
BTS
BSC
BSC
BSC
MSC
MSC
Handover
Inter-cell, intra-BSC handover:
This is a typical handover scenario.
This mobile station moves from one cell to another,
but stays within the control of the same BSC.
The BSC then performs a handover, assigns a new
radio channel in the new radio channel in the new
cell and releases the old one (Scenario 2).
In short, handover of cells controlled by the same
BSC.
Handover
Inter-BSC, intra-MSC handover:
As a BSC only controls a limited number of cells,
GSM also has to perform handovers between cells
controlled by different BSCs.
This handover then has to be controlled by the
MSC (scenario 3).
In short, handover of cells belonging to the same
MSC but controlled by different BSCs.
Handover
Inter MSC handover:
Finally, a handover could be required between two
cells belonging to different MSCs.
Now both MSCs perform the handover together
(Scenario 4).
In short, handover of cells controlled by different
MSCs.
Handover
In order to provide all information necessary for a
handover due to a weak link, MS and BTS both
perform periodic measurements of the downlink
and uplink quality respectively.
The measurement reports are sent by the MS about
every half-second and contain the quality of the
current link used for transmission as well as the
quality of certain channels in neighboring cells.
Handover
Next slide figure show the typical behavior of the
received signal level while an MS moves away
from one BTS (BTSold) closer to another one
(BTSnew).
In this case the handover decision does not depend
on the actual value of the received signal level, but
on the average value.
Therefore, the BSC collects all values bit error rate
and signal levels from uplink and downlink from
BTS and MS and calculates average values.
Handover decision
receive level
BTSold
receive level
BTSold
HO_MARGIN
MS
MS
BTSold
BTSnew
Handover
These values are then compared to thresholds, i.e..,
the handover margin (HO_MARGIN), which
includes some hysterics to avoid a ping-pong
effect.
Still even with the HO_MARGIN, the ping-pong
effect may occur in GSM- a value which is too
high could cause a cut-off.
Handover
Next slide figure shows the typical signal flow
during an inter-BSC, intra-MSC handover.
The MS sends its periodic measurements reports,
the BTSold forwards these reports to the BSCold
together with its own measurements.
based on these values and e.g., on current traffic
conditions, the BSCold may decide to perform a
handover and sends the message HO_required to
the MSC.
Handover procedure
MS
BTSold
BSCold
measurement
measurement
report
result
MSC
HO decision
HO required
BSCnew
BTSnew
HO request
resource allocation
ch. activation
HO command
HO command
HO command
HO request ack ch. activation ack
HO access
Link establishment
clear command clear command
clear complete
clear complete
HO complete
HO complete
Handover
The task of the MSC then comprises the request of
the resources needed for the handover from the
new BSC, BSCnew.
This BSCnew checks if enough resources are
available and activates a physical channel at the
BTSnew to prepare for the arrival of the MS.
The BTSnew acknowledges the successful channel
activation, BSCnew acknowledges the handover
request.
The MS now breaks its old radio link and accesses
the new BTS.
Handover
The next steps include the establishment of the link.
Basically, the MS has then finished the handover, but it
is furthermore important to release the resources at the
old BSC and BTS and to signal the successful
handover using the handover and clear complete
messages as show in figure.
Future handover scenarios would include seamless
handover between different systems, e.g. from GSM to
DECT (digital enhanced cordless telecommunication)
or satellite-based services without interruption.
Security
GSM offers several security services using
confidential information stored in the AuC and in
the individual SIM.
As stated above, the SIM stores personal, secret
data and is protected with a PIN (Personal identity
number) against unauthorized use.
Security
The security services offered by GSM are
explained in the following:
Access control and authentication:
The first step includes the authentication of a valid
user for the SIM.
The user needs a secret PIN to access the SIM.
The next step is the subscriber authentication.
Security
Confidentiality:
All user-related data is encrypted.
After authentication, BTS and MS apply encryption
to voice, data and signaling.
This confidentiality exists only between MS and
BTS, but it does not exist end-to-end or within the
whole fixed GSM/telephone network.
Security
Anonymity:
To provide user anonymity, all data is encrypted
before transmission, and user identifiers which
would reveal an identity are not used over the air.
Instead, GSM transmits a temporary identifier
(TMSI-temporary mobile subscriber identity),
which is newly assigned by the VLR after each
location update.
Additionally, the VLR can change the TMSI at any
time.
Authentication
The operation and maintenance subsystem of the GSM
network has an AuC for authenticating an MS.
The AuC first authenticates the subscriber MS and only
then does the MSC provide the switching service.
Authentication algorithms like A3,A5,A8 use a random
number sent by the AuC during the connection setup and
an authentication key which is already saved in the SIM.
Authentication algorithms used can differ for different
mobile service providers.
Authentication
For authentication, the VLR sends the random value
RAND to the SIM.
Both sides, network and subscriber module, perform the
same operation with RAND and the key ki, called A3.
The MS sends back the SRES generated by the SIM, the
VLR can now compare both values.
If they are the same, the VLR accepts the subscriber,
otherwise the subscriber is rejected.
In the next slide show figure for subscriber authentication.
GSM - authentication
SIM
mobile network
Ki
RAND
128 bit
AC
RAND
128 bit
RAND
Ki
128 bit
128 bit
A3
A3
SIM
SRES* 32 bit
MSC
SRES* =? SRES
SRES
SRES
32 bit
Ki: individual subscriber authentication key
32 bit
SRES
SRES: signed response
Encryption
The BTS and the MS have to perform ciphering
before call initiation or before connecting for
receiving a call.
The MS uses a cipher (encryption key) for
encryption.
The cipher is a result of performing mathematical
operation on: (A) the cipher key saved in the SIM,
and (B) the cipher number received from the BTS
when the call setup is initiated.
Encryption
The BTS transmits the cipher number before a
call is set up or transmitted.
The encryption algorithm is identical for all
mobile service providers.
The random numbers used in authentication and
ciphering processes are also known as challenge
to the mobile station to generate the results of
the algorithms and only if these results are
correct, do the BTS and other units grant access
to the challenged MS.
Encryption
After authentication, MS and BSS can start using
encryption by applying the cipher key Kc.
Kc is generated using the individual key ki and a
random value by applying the algorithm A8.
MS and BTS can now encrypt and decrypt data
using the algorithm A5 and the cipher key Kc.
GSM - key generation and encryption
MS with SIM
mobile network (BTS)
Ki
AC
RAND
128 bit
RAND
128 bit
RAND
128 bit
A8
cipher
key
BSS
Ki
128 bit
SIM
A8
Kc
64 bit
Kc
64 bit
data
A5
encrypted
data
SRES
data
MS
A5
New data services
The GSM system provides data rates of
TCH/13.4, TCH/HS11.4, TCH/12.8, TCH/F14.4,
TCH/F4.8, TCH/F9.6.
These rates are good for transmission of voicedata but too low for high-speed data transfer.
Speed enhancement is required for a GSM system
to be able to provide data services such as transfer
of large files and internet access.
New data services
New data services such as General packet radio
service (GPRS) and high-speed circuit switched
data (HSCSD) use different coding and
multiplexing techniques to provide high transfer
speeds to GSM users.
The three major approaches to enhance
transmission speed are as follows:
Combining several slots in a packet-switched
network. GPRS is an example this type of speed
enhancement.
New data services
Combining several slots in a circuit-switched
network. For example, HSCSD is an
improvement on GSM as it combines several
time-slots for high-speed transmission of circuitswitched data.
Use of other technology such as digital
enhanced cordless telecommunication system
(DECT) which is used for short range
communication.
HSCSD
High-speed Circuit Switched Data (HSCSD) is an
innovation to use multiple time slots at the same
time.
HSCSD is a 2.5G, GSM phase 2 standard defined
by the ETSI- European telecommunications
standards institute.
It is an enhancement of circuit-switched data
(CSD), which is the original data transmission
mechanism in GSM system.
HSCSD
Large parts of GSM transmission capacity were
used up by error correction codes in the original
CSD transmission.
HSCSD, however, offers various levels of error
correction that can be used in accordance with the
quality of the radio link.
As a result, so where CSD could transmit at only
9.6 kbps, the HSCSD data rates go up to 14.4 kbps.
HSCSD can also use multiple time-slots at the
same time.
HSCSD
Several GSM traffic channels can join to transmit
data at high speed.
In transmission of normal voice-data traffic,
HSCSD given smaller latency to data as compared
to GPRS.
HSCSD offers better quality of service than GPRS
due to the dedicated circuit-switched
communication channels.
However, HSCSD is less bandwidth efficient than
GPRS.
What is GPRS?
 In early 2000, only a small portion of GSM
subscribers used data services, because existing
GSM systems do not support easy access, high data
rate and attractive prices.
 GSM operators must offer better services to
simulate the demand.
 The solution is the General Packet Radio Service
(GPRS).
 GPRS reuses the existing GSM infrastructure to
provide end-to-end packet-switched services.
What is GPRS?
 Existing GSM networks use circuit-switched
technology to transfer information between
users.
 However, GPRS uses packet switching which
means there is no dedicated circuit assigned to
the GPRS mobile phone.
 Once the data has been sent, the resource can
be re-allocated to other users for more efficient
use of the network.
What is GPRS?
 By allowing information to be delivered more
quickly and efficiently GPRS is relatively
inexpensive mobile data service compared to Sort
Message Services (SMS) and Circuit-Switched
Data.
Key Features of GPRS
 GPRS have service and network features that make
it an attractive mobile data communication service.
 Some of the key services features are as follows:
Bandwidth on demand for point-to-point
transmission.
Negotiated quality of services (QOS).
Point-to-point and point-to-multipoint service.
Multicast and group call services.
Key Features of GPRS
Value added services like broadcast information
service (e.g. traffic report, stock prices)
Design for easy internet access and WebBrowsing
GPRS architecture and
interfaces
 GPRS technology brings many changes to the
existing GSM network.
 Most of the changes are improvements made by
adding new blocks rather than by modifying
existing resources.
 A simplified view of this new hybrid network
shows the elements introduced by GPRS.
 In the next slide figure show a GPRS architecture
and different types of interfaces used in that.
GPRS architecture and
interfaces
SGSN
Gn
BSS
MS
SGSN
PDN
GGSN
PCU
Um
Gb
Gn
HLR/
GR
MSC
VLR
EIR
Gi
GPRS architecture and
interfaces
 Gateway GPRS support node (GGSN):
 It is similar to the GSM gateway mobile
switching center (GMSC) and provides a
gateway between the GPRS network and the
public packet data network (PDN) or other
GPRS networks.
 The GGSN provides authentication and location
management functions, connects to the home
location register (HLR) by means of the Gc
interface and counts the number of packets
transmitted for accurate subscriber billing.
GPRS architecture and
interfaces
 Serving GPRS support node (SGSN):
 It is like the GSM mobile switching center and
visitor location register (MSC/VLR), controls
the connection between the network and the
mobile station (MS).
 The SGSN provides session management and
GPRS mobility management functions such as
handovers and paging.
 It attaches to the HLR via the Gr.
GPRS architecture and
interfaces
 Packet Control Unit (PCU):
 Which include converting packet data into a
format that can be transferred over the air
interface, managing radio resources and
implementing quality of Service (QoS)
measurements.
 The signaling links between the GPRS nodes is
defined by the GPRS specifications.
GPRS architecture and
interfaces
 New physical interfaces include the Gb
interface, which connects the SGSN to the PCU
and is usually located in the base station
subsystem (BSS).
 The Gn interface which connects the GGSN and
SGSN, and Gc, Gr and Gs interfaces, which
carry SS7 base protocols.
GPRS architecture and interfaces:
GPRS mobile phone operation states
 Mobile phones go through different states of
communication.
 For example, when a GSM phone comes onto a
network, the phone enters an idle state in which
it uses very few network resources.
 When the user makes a call request or receives a
call, however, the phone goes into the dedicated
state in which it is assigned a continuous
resource until the connection is terminated.
GPRS architecture and interfaces:
GPRS mobile phone operation states
 GPRS mobile phones will also have defined
states, which are described bellow:
 GPRS Idle: it is the state in which the mobile
phone comes onto the GSM network.
 The phone receives circuit switched paging and
behaves as a GSM phone.
 Although it does not interact with the GPRS
network in this state, it still possesses GPRS
functionality.
GPRS architecture and interfaces:
GPRS mobile phone operation states
 GPRS Ready: it is the state achieved when the
GPRS mobile attached itself to the network.
 In this state the mobile phone can activate a
packet data protocol (PDP) context, which
allows the phone to establish a packet transfer
session with external data networks to transmit
and receive data packets.
 Once a PDP context is activated resource block
are assigned to the session until data transfer
causes for a specified period and the mobile
phone moves into the standby state.
GPRS architecture and interfaces:
GPRS mobile phone operation states
 GPRS Standby: it is a state in which the
mobile is state in which the mobile is connected
to the GPRS network, but no data transmission
occurs.
 If a data packet for the mobile arrives, the
network will page the mobile, which in turn
activates a PDP context session to the bring the
mobile back to the ready state.
GPRS architecture and interfaces:
GPRS/GSM Mobile Classes
 European telecommunications standards
institute (ETSI) define three different classes of
mobiles for the hybrid GPRS/GSM network:
 Class A :
 Class A mobiles can attach to the GPRS and
GSM network simultaneously.
 They can receive GSM voice/data/SMS calls
and GPRS data calls.
GPRS architecture and interfaces:
GPRS/GSM Mobile Classes
 For this to happen the mobiles must monitor
both the GSM and GPRS networks for
incoming calls.
 Class A mobiles also can make and receive
GPRS and GSM call simultaneously.
 Operational requirements of this class include
an additional receiver in the mobile phone for
neighbor cell measurements.
GPRS architecture and interfaces:
GPRS/GSM Mobile Classes
 Class B:
 This class is similar to class A with the
exception that class B mobile phones will not
support simultaneous traffic.
 If a GPRS call is ON, the phone cannot receive
GSM calls and vice versa.
GPRS architecture and interfaces:
GPRS/GSM Mobile Classes
 Class C:
 This class of mobile phones will have both GSM
and GPRS functionality but will attach to only one
network at a time.
 Thus, if the phone is attached to the GPRS
network, it will be remove form the GSM network
and will not be able to make or receive GSM calls.
 Conversely, if it is attached to the GSM network, it
will not be able to make or receive GPRS calls.
 Today most manufacturers are building Class B
phones.
GPRS architecture and interfaces:
The GPRS Attach Procedure
 A GPRS attach is a GPRS mobility
management (GMM) process that is always
initiated by the mobile phone.
 Depending on the settings of the mobile phone,
the GPRS attach may be performed every time
the phone is powered on or it may be initiated
manually by the user.
 This request for a GPRS attach is made to the
SGSN in a process that is transparent to the
BSS.
GPRS architecture and interfaces:
The GPRS Attach Procedure
 First the mobile notifies the SGSN of its identity
as an International Mobile subscriber Identity
(IMSI) or packet temporary mobile subscriber
identity (P-TMSI).
 Then it sends its old routing area identification
(RAI), class mark, and desired attach type.
 The latter indicates to the SGSN whether the
mobile wants to attach as a GPRS device, a
GSM device, or both.
 The SGSN will attach the mobile and inform the
HLR if there has been a change in the RAI.
GPRS architecture and interfaces:
The GPRS Attach Procedure
 If the desired attach type is both GPRS and
GSM, the SGSN will also update the location
with the VLR, provided that the Gs interface
exists.
 For this to occur, the mobile has a activate a
communication session using PDP context.
GPRS architecture and interfaces:
PDP Context Activation
 Packet Data Protocol context activates a packet
communication session with the SGSN.
 During the activation procedure, the mobile
phone either provide a static IP address or
requests a temporary one from the network.
 It also specifies the access point name (APN)
with which, it wants to communicate- for
example, an Inter net address or an Internet
service provider.
GPRS architecture and interfaces:
PDP Context Activation
 The mobile request a desired quality of service
(QoS) and a network service access point
identifier (NSAPI).
 Because a GPRS mobile can establish multiple
PDP context session for different application,
the NSAPI is used to identity the data packets
for a specific application.
 Upon receiving information from the mobile,
the SGSN determines which GGSN is
connected to the APN and forwards the request.
 The SGSN also provides a negotiated QoS
based on the user’s subscription information and
the availability of services.
GPRS architecture and interfaces:
PDP Context Activation
 If the mobile phone has a static IP address, the
GGSN directly connects the mobile to the
desired access point.
 Otherwise, it obtains a temporary IP address
from the APN.
 The GGSN also provides some transaction
identifiers for data communication between
GGSN and SGSN.
 Once the communication and activation
procedure at the GGSN is successful, the
appropriate data transfer information is
forwarded to the mobile.
GPRS protocol layer architecture
 The GPRS data and signaling transmission
plane consists of standard protocols such as IP
and some new, GPRS-specific protocols.
 There are different types of protocols under Gn,
Gb, Um interfaces.
 Next slide figure show a GPRS transmission
plane protocol reference model.
GPRS protocol layer architecture
MS
BSS
Um
SGSN
Gb
Gn GGSN
apps.
IP/X.25
IP/X.25
SNDCP
LLC
RLC
MAC
RLC
MAC
radio
radio
BSSGP
FR
GTP
LLC
GTP
UDP/TCP
UDP/TCP
BSSGP
IP
IP
FR
L1/L2
L1/L2
SNDCP
Gi
GPRS protocol layer architecture :
Gn interface Protocols
 GPRS tunneling Protocols (GTP):
 GTP receives IP datagram packets from the external
network and tunnels them across the GPRS support
nodes.
 Because there will be multiple GGSN and SGSN
interfaces, the GTP provides for every packet a tunnel
identifier (TID) that identifies the destination and
transaction to which the packet/datagram belongs.
 Transactions are identified using logical identifiers as
well as the International Mobile subscriber Identity
(IMSI).
GPRS protocol layer architecture :
Gn interface Protocols
 TCP/UDP:
 It is consists of the transmission control
protocol (TCP), which is used to transfer PDUs
(protocol data units) across the Gn interface
with reliability.
 The user datagram protocol (UDP) is used
across the Gn interface to carry the GTP-PDUs
for all signaling information and user data that
do not require reliability.
GPRS protocol layer architecture :
Gn interface Protocols
 Internet protocol (IP):
 It is used to route user data and signaling
information across the Gn interface.
 The IP datagram size will be limited to the
physical layer-maximum transmission unit
(MTU) capabilities.
 An IP datagram can be as large as 65,535 octets,
but if the physical layer MTU is less than this,
fragmentation must be done.
GPRS protocol layer architecture :
Gn interface Protocols
 The source gateway support node (GGSN or
SGSN) has to first decide the MTU size and
then carry out the fragmentation.
 The IP addressing used will route the data
across the Gn interface, including any
intermediate GSNs (Gateway support nodes), to
the GSN address at the final destination.
GPRS protocol layer architecture :
Gb interface Protocols
 Sub network dependent Convergence
protocol (SNDCP):
 It is used between the SGSN and the mobile
phone.
 This protocol converts the network layer PDUs
on the Gn interface into a format suitable for the
underlying GPRS network architecture.
GPRS protocol layer architecture :
Gb interface Protocols
 SNDCP performs a number of functions:
 Multiplexing of N-PDUs from one or several
network layer entities onto the appropriate LLC
(Logical link control) connection.
 Buffering of N-PDUs from the acknowledged
service.
 Compression and decompression of the protocol
information and user data
 Negotiation of the control parameters between
SNDCP entities.
GPRS protocol layer architecture :
Gb interface Protocols
 Logical link control (LLC):
 This protocols provides a highly reliable,
ciphered logical link between the SGSN and the
mobile phone.
 The LLC uses both acknowledged and
unacknowledged modes of frame transmission
depending buffering and information length
based on the negotiated QoS delay class.
GPRS protocol layer architecture :
Gb interface Protocols
 Base station system GPRS protocol (BSSGP):
 It routes information between the SGSN and the
BSS.
 This protocol conveys QoS information but
does not carry out any form of error correction.
 Its primary function is to provide radio-related
information for use by the radio link control
(RLC) and medium access control (MAC)
function on the air interface.
GPRS protocol layer architecture :
Gb interface Protocols
 The LLC layer uses the services of the BSSGP from
transfer.
 The relay function at the BSS transfers LLC frames
between the RLC/MAC layer and the BSSGP layer.
 The BSSGP sends information to the network services
layers to determine the transfer destination:
BSSGP virtual connection identifier (BVCI):
it is sent to the network services layer for routing
signaling and data information to the correct peer
function entities.
Each BVCI between two peer entities is unique.
GPRS protocol layer architecture :
Gb interface Protocols
Link selection parameter (LSP):
It is used in conjunction with the BVCI to aid
in selecting a physical link for the load-sharing
process.
Network service entity identifier (NSEI):
Used at the BSS and the SGSN provides the
network management functionality required for
operation of the Gb interface.
The NSEI together with the BVCI uniquely
identifies a BSSGP virtual connection.
GPRS protocol layer architecture :
Gb interface Protocols
 Network Service (NS):
 This layer uses frame relay across the Gb
interface and could be a point-to-point
connection between the SGSN and the BSS or a
frame relay network.
 The NS layer uses a DLCI (data link connection
identifier) look-up table to indicate the routing
path between the SGSN and the BSS.
GPRS protocol layer architecture :
Gb interface Protocols
 The initial value of the DLCI field is derived
from the BVCI, NSEI and LSP supplied by the
BSSGP layer.
 This value changes as the frame passes through
the frame relay network and reaches its final
destination.
GPRS protocol layer architecture :
Um interface Protocols
 Radio link control (RLC):
 It is responsible for a number of functions:
Transferring LLC-PDUs between the LLC
layer and the MAC function
Segmentation of LLC-PDUs into RLC data
blocks and re-assembly of RLC data blocks to
fit into TDMA frame blocks
Segmentation and re-assembly of RLC/MAC
control messages into RLC/MAC control block
GPRS protocol layer architecture :
Um interface Protocols
Backward error correction for selective
transmission of RLC data blocks.
 The RLC segmentation function is a process of
taking one or more LLC-PDUs and dividing
them into smaller RLC blocks.
 The LLC-PDUs are known collectively as a
temporary block flow (TBF) and are allocated
the resources of one or more packet data
channels (PDCH).
GPRS protocol layer architecture :
Um interface Protocols
 The TBF is temporary and is maintained only
for the duration of data transfer.
 Each TBF is assigned a temporary flow identity
(TFI) by the network.
 The RLC data blocks consist of an RLC header,
an RLC data unit and spare bits.
 The RLC data block along with a MAC header
may be encoded using one of four defined
coding schemes.
 The coding scheme is critical in deciding the
segmentation process.
GPRS protocol layer architecture :
Um interface Protocols
 Medium access control (MAC):
 It controls the access signaling across the air
interface, including the management of shared
transmission resources.
 MAC achieves these functionalities by placing a
header in front of the RLC header in the
RLC/MAC data and control blocks.
 The MAC header contains several elements
some of which are direction-specific referring to
the downlink or uplink.
GPRS protocol layer architecture :
Um interface Protocols
 The key parameters of MAC header are:
 Uplink status flag (USF) is sent in all downlink
RLC/MAC blocks and indicates owner or use of
the next uplink radio block on the same
timeslot.
 Relative reserved block period (RRBP)
identifiers a single uplink block in which the
mobile phone will transmit control information.
GPRS protocol layer architecture :
Um interface Protocols
 Payload type (PT) the type of data contained in
the remainder of the RLC/MAC block.
 Countdown value (CV) is sent by the mobile to
allow the network to calculate the number of
RLC data blocks remaining in the current uplink
TBF.