3. GSM Architecture R10 IV/IV CSE

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3.
GSM Architecture

R10 IV/IV CSE


SACET

MC

Page
1


Unit
-

III
:

GSM Architecture

GSM
Architecture:

Other

2G architectures
,
Radio Interface of GSM
,
Protocols of GSM
,
Localization
,
Call Handling
,
GPRS System Architectures

GSM and Similar Architectures
:



A mobile communication standard



GSM communication─ uses
cellular networks



The GSM standard operates in the frequency ranges of 900, 1800, and 1900 MHz



Tri
-
band (operable in GSM 900/1800/1900) phones enable easy international
roaming in GSM networks



GSM─ a second generation (2G) communication standard

Three type
s of integrated services for voice and data



Teleservices



Supplementary services



Bearer services


Supplementary services

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Bearer services


Integration of teleservices, bearer services, and supplementary services


Connection

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Establishes between two TEs

s
ource and destination



The TE at destination may or may not belong to a GSM network

Connection between two terminal equipments or mobile terminals


Interfaces



A mobile terminal acts as an interface between a communications network (for
example, interface
between the GSM public land mobile network) and terminal,
TE ─ the source or destination of the service



The TE used by a caller to connect and talk (communicate) and MT for mobile
communication



Depends on the source

destination network which may be a GSM,
PSTN (public
switched telephone network), ISDN (integrated services digital network), PSPDN
(public switched public data network), or any other network carrying the data to
the end
-
point TE


Connection from caller



A caller TE transmits through interface 1
to a GSM public land mobile network



Through 2 to a PSTN network



Through 3 to a source

destination network



Through 4 to a terminal or mobile station TE



In place of the PSTN network, there may be an ISDN or PSPDN network


Connection from called TE to caller
MT

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The connected TE communicates back by transmitting through interfaces 5, 6, 7,
and 8

Sets of the Interfaces



Four sets of interfaces (1, 8), (2, 7), (3, 6), and (4, 5). There is a transceiver in
each set



The symbol U
m

(user mobile interface) conventiona
lly denotes the interface (1,
8)




Symbol
A

denotes a mobile network interface (2, 7) to a PSTN or other wired
network

Four transceivers



Transmit as well as receive in full duplex mode



Full duplex mode means simultaneous two
-
way transmission



The MT interf
ace can also be half
-
duplex transmission



Half duplex means that two
-
way transmission possible but not both ways at the
same time


Teleservices



Services offered by a mobile
-
service network to a caller (TE)



Ttelephonic
-
voice at full data rate (13.4 kbps)



Fa
x



SMS



Emergency number 112 for emergency calls



MMS [supporting GIF, JPG, WBMP, teletext, and videotext access (GIF, JPG, and
WBMP are formats of files that store pictures)]




Point
-
to
-
point
─ from a TE to another TE



A point
-
to
-
point service is implemented using cellular communication



Half data
-
rate speech or enhanced full
-
rate speech services, and these may or
may not be rendered by cellular and point
-
to
-
point access systems



A GSM smart phon
e, which connects to a GSM public land mobile network




A number of teleservices including phone, voice data (for example, recorded
message played on auto
-
answer of incoming calls), SMS, and MMS to another
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GSM or PSTN network


Phase 2 Supplementary Services



Caller line forwarding (redirection), caller line identification



Line identification to the caller



Closed user group formation



Multiparty groupings (e.g., in an enterprise)



Call holding, call waiting, and barring calls from specified numbers or groups




Re
stricted provisioning of certain services to the users



Internet and email access granted on special requests from users)



Providing information regarding call charges, remaining phone account balance,
etc


Bearer services



Transmission of data (voice signal
s are also transmitted as data) between two
user network interfaces [(1,8) and (4,5) using the intermediate interfaces [(2,7)
and (3,6)] at a mobile network



Means a set of data which is transmitted from or received by a TE i.e., the voice
-
data or data set

that has been formatted in certain specified formats



This data transmits at certain standardized rates through the interfaces



Voice
-
data─ data that is obtained after digitizing, coding, encoding, appending
error detection and correction bits, and encrypti
ng of a voice signal




Each TE has a user interface



The interface (1, 8) of a mobile station connects the MT to a GSM public land
mobile network



The interface (4, 5) of a PSTN phone connects to a PSTN network



An intermediate PSTN network acts as an interfac
e for a GSM public land mobile
network



In place of PSTN, there may be ISDN, PSPDN, or some other network




(a) transparent and uses data rates of 2.4 kbps, 4.8 kbps, or 9.6 kbps or



(b) non
-
transparent and uses lower data rates (300 bps to 9.6 kbps


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Bearer
services classification



Synchronous data transfer



Asynchronous data transfer



Synchronous data packet transfer


GSM System Architecture


Mobile communication using base station in cellular networks



A mobile station, MS, communicates with a GSM public land m
obile network
(PLMN)



In turn, may connect to a PSTN network



The PSTN connects to a source

destination network which acts as an interface
for the destination terminal, TE


GSM network architecture



Radio subsystem (RSS)



Network subsystem (NSS)



Operation subs
ystem (OSS)

Mobile Station (MS)



A mobile device or phone



Connects to GSM network



Radio transmission system used in mobile phones)



Hardware and software to transmit and receive GSM data, and a user terminal
(TE) through which the user receives and sends the

data



Transmits through the interface
Um

at a power of 1

2 W



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RSS



Consists of a number of base station controllers (BSC)



Each BSC connects to a number of base transceiver stations (BTS) which, in
turn, provide radio interfaces for mobile devices



NSS



Consists of a number of mobile services switching centres (MSC)



Each MSC of the NSS interfaces to a number of BSCs in the RSS



Home location registers (HLR)



Visitor location registers (VLR)



OSS

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Interfacing between the three subsystems in a GSM network


Mobile Communication from the MS



When a mobile station MS
x

communicates to another mobile station MS
y
, a
switching center MSC
i

establishes (switches) a connection (channel) between (i)
MS
x

interfaced to the BTS
p
, then to the BSC
q
, then to MSC
r

and (ii) M
S
y

interfaced
to the BTS
u
, BSC
V
, and MSC
w

GSM System communication



RSS and NSS for communication



MSCs must have location registries to enable the NSS to discover a path (route or
channel) between MS
x

and MS
y



The OSS facilitates the operations of MSCs


Con
nection interfaces in the RSS subsystem between BTS and the MSs

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Connection interfaces in the RSS subsystem between BSC and the BTSs


Interfaces in the RSS subsystem between MSC (in the NSS) and the BSCs


MS’s subscriber identity module (SIM)



An inserte
d card

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Provided by the GSM service provider

SIM



Uniquely identifies the user to the service



Enables the MS to connect to the GSM network



When the MS connects to the GSM subsystems, the SIM saves a temporary mobile
(dynamic) cipher key for encryption, tempo
rary mobile subscriber identity
(TMSI), and location area identification (LAI)



Information which does not change when the MS moves into another location



(i) international mobile subscriber identity (IMSI)



(ii) card serial number and type



Contains a PIN (pe
rsonal identification number)



Using the PIN, the MS is unlocked when it seeks connection to another MS



The user can use the PIN to lock or unlock the MS



Stores the PUK (PIN unblocking key) which enables the subscriber to unlock the
SIM if it is accidentall
y locked due to some reason



Stores a 128
-
bit authentication key provided by the service provider



The MS authenticates by a switching centre through an algorithm using this key
and a 128
-
bit random number dynamically sent by authentication centre



If the MS
is not authenticated, the service to that number is blocked



Also stores the international mobile subscriber identity (IMSI)



IMSI─ a unique 15 digit number allocated to each mobile user



IMSI three parts


a three digit mobile country code (MCC), a mobile net
work
code (MNC) consisting of two digits, and the mobile subscriber identity number
(MSIN) with up to 10 digits



Also stores the international mobile subscriber identity (IMSI)



IMSI─ a unique 15 digit number allocated to each mobile user



IMSI three parts


a

three digit mobile country code (MCC), a mobile network
code (MNC) consisting of two digits, and the mobile subscriber identity number
(MSIN) with up to 10 digits

Base transceiver station (BTS)

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Connects to a number of mobile stations (MSs)



Each MS establ
ishes connection through the user interface
Um

[(1,8)]



Um

is the ISDN
U

interface for mobile



The BTS to MS connection through
Um




A BTS is also connected to a BSC at through the
A
bis

interface

Main functions performed by the BTS



Formation of cells using
appropriately directed antennae



Processing of signals



Amplification of signals to acceptable strength so that they can be transmitted
without loss of data



Channel coding and decoding (for example, coding voice into bits so that it can be
transmitted at 1
3 kbps and decoding received coded signals back to voice)



Frequency hopping so that multiple channels for various mobile stations can
operate simultaneously using different channel band frequencies



Encryption and decryption of data



Paging



Adapting to th
e rate of data synchronous data transmission



The receiver clock of the transceiver at one end of an interface adapts itself
according to transmitter clock of the transceiver at the other end)



BTS to BSC interface in a GSM network


A
bis

transceiver



Transmi
ts and receives data with four multiplexed channels of 16 kbps or with a
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64 kbps channel



Usually a BTS is used to manage one cell in the GSM cellular network, but using a
sectorized antenna, a single BTS can be used to manage many cells

BSC



Manages a numbe
r of BTSs



Uses the
A
bis

interface to connect to BTSs



BSCs reserve radio frequencies for communication and manage handovers
between BTSs



A BSC along with the BTSs connected to it and the mobile stations managed
through it forms a base station system (BSS)



Also connected to an MSC in the networking and switching layer using an
interface
A


Important functions performed by the BSC



Processing of signals



Controlling signals to the connected BTSs and control of handover of signals
from one BTS to another withi
n a BSS



Control and handover of the signals from BSC to MSC



Mapping the signals of a channel─ at given instant receives signals from a BTS at
16 kbps through
A
bis

and interfaces them to an MSC at 16 kbps



Alternatively, may have to interface to a PSTN switc
hing centre at 64 kbps
through a fixed line network─ mapped by assigning a 16 kbps channel for 64
kbps signals and vice versa



Reserving radio frequencies



Frequency hopping (For example, multiple BTSs operate simultaneously by
using the different frequencie
s at a given instant



Traffic control by continuous measurement of the frequency channel spectrum
being used at a given instant



Authentication, encryption, and decryption of data



Updating location registry for the MSs



Paging

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Network subsystem (NSS)



Acts as
an interface between wireless and fixed networks



Mainly consists of switches and databases and manages functions such as
handovers between BSS’s, worldwide user localization, maintenance of user
accounts and call charges, and management of roaming



The inte
rface between the NSS components and the AuC and the OMC in the OSS



Consists of
l

mobile services switching centres (MSC),
m
and
n

home and visitor
location registers, gateway MSCs (GMSC), and inter
-
working functions (IWFs)
with the mobile switching centre
s



GMSCs and IWFs connect to the other networks (for example, PSTN, ISDN, or
PSPDN)

Basic connections and components in the NSS



Each MSC in the NSS can manage several base station systems



Every MSC has a home location register (HLR) and a visitor location r
egister
(VLR)



An MSC can connect to another MSC, GMSC, and IWF



An HLR connects to an AUC in the OSS.



A GMSC can connect to an OMC in the OSS.



GMSCs─ also used to connect to a PSTN, ISDN, or PSPDN network


Mobile services switching centre (MSC)



Consists ma
inly of high
-
performance digital ISDN switches



Connects to a number of BSCs over the
A

interface



Connect to other MSCs and to fixed
-
line networks through GMSCs



Used to manage BSCs in a geographical area

Functions performed by an MSC



Processing of signals



Establishing and terminating the connection between various mobile stations via
BSCs



The mobile stations to be connected may fall in a given MSCs own area or in the
area assigned to another MSC, in which case the communication path has to be
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via the other

MSC



Establishing and terminating the connection between an MS and a fixed line
phone via a GMSC or IWF



Monitoring of calls made to and from an MS



Call charging, multi
-
way calling, call forwarding, and other supplementary
services

Gateway Mobile Services
Switching Centre



A special node which handles connections to other fixed networks



These other networks may be ISDN, PSTN, PSPDN, or other PLMN networks



Special IWFs may be used by a GMSC to connect to public data networks such as
the X.25

Visitor Location
Register at Each MSC



A dynamic real
-
time database that stores both permanent and temporary
subscriber data which is required for communication between the MSs in the
coverage area of the MSC associated with that VLR. The VLR is an integral part of
the MSC

Home Location Register



Has the MT databases




Stores all the relevant subscriber data including mobile subscriber ISDN number
(MSISDN), details of subscription permissions such as call forwarding, roaming,
etc., subscriber’s ISMI, user’s location area, user
’s current VLR and MSC status



. Each mobile user has only one HLR record worldwide, which is updated
constantly on a real
-
time basis



Each MS must register at a specific HLR of a specific MSC



The HLR contacts AuC in the OSS for authentication



Each HLR is as
sociated to an MSC so that when an MS registered at a certain HLR
moves to another location area (LA), serviced by another MSC, the user’s home
MSC update the user’s current VLR

Interfaces in the NSS between MSC, BSCs, VLR, and OMC

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Operation subsystem (O
SS)



Administers the operation and maintenance of the entire network



Each AuC associates with an HLR in the NSS and each EIR connects to an MSC



An OMC at OSS can connect to an MSC or a GMSC in the NSS and to a BSC at RSS


Operation and Maintenance Centre



Mo
nitors and controls all other network entities through the
O

interface

OMC functions



Management of status reports



Traffic monitoring



Subscriber security management



Accounting and billing

Authentication Centre



AuC calculation of authentication parameters an
d then conveying these to the
HLR



Used by the HLR to authenticate a user



The AuC may also be a secured partitioned part of the HLR itself



Since mobile networks quite vulnerable to attacks, the GSM standard specifies
that the algorithms for key generation
should be separated out as an OSS
network entity. This entity is the AuC

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AuC database



Stores subscriber authentication keys

The equipment identity register (EIR)



Stores the international mobile equipment identity (IMEI) numbers for the entire
network



IMEI
enables the MSC in identifying the type of terminal, mobile equipment
manufacturer, and model and helps the network in locating the device in case it
is stolen or misplaced

EIR three lists



A
black list

that includes mobile stations which have been reported

stolen or are
currently locked due to some reason.



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.

GSM Radio Interface, Data bursts and Interleaving

Space Division
Multiple Access of the signals from the MSs



A BTS with
n

directed antennae─ covers mobile stations in
n

cells



Each cell defines a space



A given BTS
j

covers the
i
th

cell and the cell is presently covering
k

mobile stations,
MS1, MS2, …, MSk



k

can vary with

time ─ MS can always change its location and move into another
cell)

Uplink and downlink capacities of GSM network channels



Enhances using SDMA as this allows serving multiple users in the same
frequency but in distinct space slots


Frequency Division Mu
ltiple Access



Dividing the allotted or available bandwidth into different frequency channels
for communication by multiple sources (sets of MTs)

Radio
-
carrier channels



A set of maximum 124 radio
-
carrier channels each of 200 kHz can be used in
GSM 900 downl
ink channel (MSC to BSC, BSC to BTS, and BTS to MS)

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124 in the uplink channel (MS to BTS, BTS to BSC, and BSC to MSC)



The 124 slots in GSM 900 in the uplink frequency range

ch1: 890.1 MHz


100
kHz, ch2: 890.3 MHz


100 kHz, and so on till ch124: 914.9
MHz


100 kHz



Downlink frequency slots

ch1: 935.1 MHz


100 kHz, ch2: 935.3 MHz


100
kHz … and the last frequency is ch124: 959.9 MHz


100 kHz

Guard band



GSM 900 system permits a guard band of 50 kHz at the lowest frequency end and
a guard band of 50 k
Hz at highest frequency band



Thus Actual frequency band for the 890.1 MHz


100 kHz ch1 is 890.1 MHz


50
kHz



The guard bands guard against frequency drifts in radio carriers

Channels
allotted at a given instant to a BTS



Maximum 10



The mobile service prov
ider reserves one channel per BTS for transmission to
MS or BSC



Total number of channels assigned to a BTS is 11



A GSM system station is permitted use the ch2 to ch123 only



122 channels are available in GSM 900



Total number of reserve channels can be 32 fo
r the data transmission of mobile
service provider

BTSs



All the BTSs taken together can communicate over 90 channels (ch0, …, ch89)
available in GSM band

Data Frame in a Channel



Each channel transmits data frames of 4.615 ms (8 time
-
slots) each



The frequen
cy
-
slot for each channel is 200 kHz



A set of maximum 8 MSs (out of
l

MSs) can be assigned (by BTS
j
) a radio carrier
channel frequency for uplink



Downlink frequency is greater than the uplink frequency of a radio
-
carrier
channel by 45 MHz

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A Cell
i

formed by
SDMA with two radio
-
carrier channels ch
m

and ch
n


Up and downlink Capacities of GSM network channels



Enhances using TDMA as this allows serving multiple users in the same
frequency band but in distinct time slots


TDMA and FDMA both in GSM system



Cell
i

w
ith two radio
-
carrier channels ch
m

and ch
n

using FDMA (Up to 124
permitted)



Each MS in each channel transmitting bursts in 577

s time
-
slots using TDMA

TDMA in a radio
-
carrier channel ch
m



A set of maximum 8 MSs out of
l

MS
s

can be assigned a radio carrier

channel by a
BTS
j

using FDMA



Transmits in distinct time slots SL0, SL1, …, SL7, each of 577

s



An MS uses one of the 8 distinct time slots in a given channel

Data bursts in a data frame



A set of data bits in an SL



A set of 8 data bursts defines a data f
rame



Each frame uses different channel (radio carrier frequency)

Example of three mobile stations, MS1, MS2, and MS3



Assume B1, B2, and B3 the data bursts of MS1, MS2, and MS3, respectively)



Using the same radio
-
carrier channel ch
m



Assume B1 assigned SL0

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B
2 assigned SL1, SL4, and SL7



B3 assigned SL2 and SL6

Data frame



At an instant, a data frame can have bursts B1, B2, B3, X, B2, B3, X, B2
transmitted in 8 time slots SL0

SL7, respectively



X represents unassigned slots for access by either BTS
j

or other MSs
that are
using the same radio carrier channel

Time for data burst and frame



Since an SL = 577

s, data burst period = 577

s



Each data frame transmits in 8

577

s = 4.615 ms

Half Duplex Transmission



The transceiver of a mobile device can function in half

duplex mode when the
uplink time slot
t
u

and downlink time slot
t
d

are assigned separately by a BTS



tu



td

is constant = 3


577

s


Frequency Hopping in Data Frames



Specific frequency values result in signal fading at an instant



Do not provide expected
signal strengths



A data frame frequency channel assigned to an MS by the BTS can be changed
(hop) these select frequencies at a certain rate according to a predetermined
sequence

Frequency Hopping



This helps in ensuring better signal quality for most of th
e period



GSM hopping rates are 207.6 hop/s

Delays in Data burst during transmission



Variable delays during transmission
─ the reflected signals take different
amounts of time



Original signals ─ reconstructed using a digital signal processor (DSP)



The DSP spends computational time in processing the signals

Format of a Data Burst─ Guard space in time slot

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At the beginning and

end of every data burst of 577

s, a guard spaces of 15.25

s (equal to 4.125 bit transmission time interval) each reserved to account for
delays in the reflected signal and computational time

Format of a 577

s TDMA bst



The effective transmission time
for the data bits is, therefore, [577


(2

15.25)]
= 546.5

s



148 bits
─ transmitted in 546.5

s



Data transmission rate = (8


148) bits/4.615 ms = 256.555 kbps



Transmission by GMSK modulation and at 256.555 kbps (3.898

s/bit)


Division among 148 bits



Six bits, 3 at the head (
H
) and 3 at the tail (
T
) [called tail bits (TB)]



At H, bits─ 000



At T, bits = 000

Division among
142



26 bits in the middle of the burst are transmitted as training (
TR
) bits



The TR bits enable the receiver to (a) synchronize using
H
,
TR
, and
T

bits and (b)
select the strong components of the signals



Dire
ct path or wide reflection angle signals are the strongest ones as they travel
the least distance between the transmitter and the receiver

Division among
(142


26)/2 = 58 bits each after H and before T



Data in the burst can be of two kinds

MS data or mobi
le
-
service NSS control
data



On either side of the
TR

bits, an S bit can be placed to specify whether the source
is the MS or NSS control data



Meaningful data bits are 57 after H and 57 before T

Division among
57 bits each between H and TR, and TR and T



As
suming that only one time slot used in a data frame of 8 slots when
transmitting voice and assuming that the only data bursts are voice data bursts



Total 114 bits (57 + 57) for the user data in a data burst (timeslot)

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Total number of bits per second = 114/
4.615 bit/ms = 24.7 kbps

User and Other than user slots



12 slots for user data



User data followed by one slot for control signals data



The voice data (user data) rates
≠ 24.7 kbps but 12/13


24.7 kbps = 22.8 kbps



Additional slots required for the frequency correction and synchronization
bursts



The control data slot is replaced by an empty slot X in every alternate set of 13
frames

Traffic multiframe



Total 26 data frames

in one in which there are one control data, one empty, and
24 user data frames



Traffic multiframes transmit TCH, FACCH, and SACCH data (Next lesson)

Control channel capacity



Within a traffic multiframe one control channel



Capacity = (1

26)


24.7 kbps =
950 bps

Traffic multiframe



Transmits in 26


4.615 ms = 120 ms interval

Interleaving in a Traffic Multiframe



Interleaving means inserting in
-
between



The packets, each consisting of 456 bits in a 20 ms time slot, are interleaved in a
traffic multiframe for
voice traffic

Example



Assume two MSs, MS
i

and MS
j

multiplexed in TDMA slots



There are 57 bits after
H

and 57 bits before
T

in the data bursts



TCH/F (traffic channel full rate) transmission rate = 22.8 kbps



Therefore, there are 456 (= 8


57) bits per 20 ms

in voice traffic from two MSs



When 20 ms packets of MS
i

and MS
j

interleave, then all the 57 bit time
-
slots after
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H

in each data burst are used by MS
i

and all the 57 bits before
T

in each data
burst are used by MS
j



Interleaving distributes the effects of c
hannel characteristics variations with
time on multiple MSs


Protocol Layers in GSM

Layers defined in the open system interconnection (OSI) model



physical (layer 1)



data link (layer 2)



network (layer 3)



transport (layer 4)



session (layer 5)



presentation (l
ayer 6)



application (layer 7)


Transceiver



Receives signals



Signals processed at the different layers arranged in order from layer 1 to layer 7




Transmits the signals



Signals processed at the different layers arranged in order from layer 7 to layer 1


Each

layer additional headers (messages)



In specific formats so that these layer headers for each layer can be stripped by
the transceiver at the receiving end



Various operations can be performed on the received data


Actually used Layers



TCP/IP or GSM, a tran
sceiver need not define protocols for all 7 layers



Some layers perform the functions of neighbouring layer(s)



The MS, BTS, BSC, and MSC, for example, have just 3 layers

physical, data link,
and network



Transport and session layer functions are taken care
of by network layer
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protocols



The tasks of the presentation layer are performed by other layers



TE (user) application at either end (caller and connected ends) controls the
application layer protocols

Actually used Layers─ Examples of The Mobile station,

BTS, BSC, and MSC



Have just 3 layers

physical, data link, and network



Transport and session layer functions taken care of by network layer protocols



The tasks of the presentation layer are performed by other layers



TE (user) application at either end (cal
ler and connected ends) controls the
application layer protocols

All protocol layers between the MS and BTS


Radio protocol sublayer functions at physical layer between the MS and BTS


Data link layer sublayer LAPD
m



Controls the flow of packets to and
from the network layer and provides access
to the various services

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LAPD
m

(link access protocol
D
-
channel modified) for
Um─

data link layer
protocol between the MS and BTS



For accessing the
D
-
channel link by GSM



A modified version of the LAPD protocol for

the D
-
channel of ISDN (integrated
services digital network)



No need of appending and stripping of synchronization bits,
S

flag, and error
correction bits to and from the layer in LAPD
m

because the radio interface (
Um
)
performs these functions at the phys
ical layer itself



Communicates by wireless across the radio interface as opposed to the guided
transmission of ISDN signals in case of the LAPD

LAPD
m
(Link Access D
-
Channel protocol for mobile)

sub
-
layer Functions


Network layer



Three sub layers

call (con
nection) management (CM), mobility management
(MM), and radio resource management (RRM)

Operations in the CM sub layer


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Operations in the MM sub layer


Operations in the RRM sub layer


Interfaces of the Network sublayers


CM sub layer protocol



Supports

call establishment, maintenance, and termination



The CM sub layer also controls and supports the functioning of the SMS and
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supplementary services



The CM also supports DTMF (dual tone multiple frequency) signalling

The MM layer



Controls issues regarding m
obility management when an MS moves into another
cell (location area). The RRM manages the radio resources. The BTS implements
only RRM′ (a part of RRM) as the BSC handles the handover.

Functions of the network layer



Defines protocols for implementation o
f addressed messages received from the
data link layer



Defines addresses of the messages



Performs the following functions:



Defines protocols for implementation of addressed messages received from the
data link layer,



Defines addresses of the messages

Base
transceiver

Base station controller Signalling Protocols


Physical layer between the BTS and the BSC



A
bis

interface (of the PSTN, ISDN, or PSPDN networks)



The connection between the BTS and the BSC through a wired network (PSTN,
ISDN, or PSPDN)



Voice code
d in the 64 kbps PCM (pulse code modulation) format in a PSTN
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network



The
A
bis

interface between BTS and BSC, therefore, uses the 64 kbps PCM (or four
multiplexed 16 kbps channels) format

PCM coding techniques



Different from the 22.8 kbps TCH radio interfa
ce U
m

(between MS and BTS)



Translation between these coding formats performed by recoding the TCH bits
received from the caller MS to 64 kbps PCM and from PCM to TCH for the
receiver MS

Effect of translations



This translation and retranslation from one cod
ing format to another may affect
voice quality



Therefore, a procedure called TFO (tandem free operation) adopted at the BTSs,
BSCs, and MSCs



TFO means without performing translation and back retranslation processes
repeatedly


Data link layer protocol betw
een BTS and BSC



LAPD (link access protocol
D
-
channel) for
A
bis



The protocol prescribes the standard procedure for the
D
-
channel of ISDN
(integrated services digital network

The network layer protocol between BTS and BSC



BTSM (BTS management)


Protocols lay
ers between the BSC and the MSC


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Data link layer protocols between the BSC and MSC



MTP (message transfer protocol) and SCCP (signalling connection control
protocol).



MTP and SCCP are parts of the SS7 (signalling system No. 7) used by interface
A



The layer

protocol prescribes a standard procedure for the MTP and SCCP for
SS7 transmission and reception in a 2 Mbps CCITT PSTN/ISDN/PSPDN network

Network layer protocol at the BSC



Network layer protocol sub layers at the MSC are CM, MM, and BSSAP



BSSAP (base sub
system application part

Localization and Calling

Localization



A process by which a mobile station is identified, authenticated, and provided
service by a mobile switching centre through the base station controller and
base transceiver either at the home l
ocation of the MS or at a visiting location

Connection setup



Users want instantaneous connection setup for a call and want service on
demand even while they are on the move



The mobile service providers, on the other hand, will provide service(s) to the
use
r only after identification of MS and verification of services subscribed

Localization mechanism of the GSM



Only after identifying the mobile station (MS) of the user



Only Verifying the services subscribed

NSS (network subsystem) of GSM architecture



Period
ically updates the location of those MSs which not switched off and are not
struck off (or blocked) from the list of subscribers to the given mobile service



The SIM in a mobile station MS
i

stores location
-
area identification (LAI)


MSC sending LAI for stor
e at SIM in mobile station



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LAI



Location information which is updated by the MSC which covers the MS’s current
location area

Temporary mobile subscriber identity (TMSI)



The SIM also saves a assigned by the VLR associated to the current MS
C



The location update recorded at the VLR (visitor location register) and the LAI is
updated at the SIM card in MS
i

via the MSC, BSC, and BTS covering its current
location (interfaces
j
, 7
b
, 7
a
, and 8
a)


VLR for sending TMSI for BTS and Mobile station thro
ugh MSC and BSC









LAI updated by VLR
through MSC

For M
obile station
and BTS a TMSI

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Main functions of an HLR



Registration of information regarding IMSI (international mobile subscriber
identity)



MSISDN (mobile station international subscriber ISDN number)



Roaming restrictions



Call forwarding



Mobile subscriber ro
aming number (MSRN)



Present VLR



Present MSC

MSISDN



Internationally used code of the country followed destination area code in a
country and subscriber number



The identical coding scheme for address used in the ISDN network employing a
fixed wire or fiber l
ine)

Present VLR and MSC information



Can change when the user MS moves into another location area but the HLR
which stores this information remains the same

Main functions of VLR



Registration of information pertaining to currently associated MSs



The inform
ation about their HLR, IMSI, and MSISDN



Storing information of the MSs which are in its location area and to which the
MSC (associated with the given VLR) is currently network services



Registration of any new MS that moves into the VLR’s location area. It
copies the
information from the HLR of that MS



Deregistration of an MS, if the MS dissociates from the MSC associated with the
given VLR and moves out to another location area


Communication between a mobile station TE and another TE



The other TE could be
a mobile station TE or other TE (such as a PSTN phone)



The caller TE to be an MS communicating to the other TE via the path 1

2

3

4

5

6

7

8

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The caller TE can also be a PSTN phone




Different m
ethods and protocols are used for establishing connection and
maintaining communication in calling to and from mobile devices in a GSM
PLMN network

Various types of calls handled by a GSM network



Calls originating from a mobile TE to a PSTN destination TE

(Mobile
→ PSTN
Calls)



Calls originating from a mobile TE to a mobile destination TE (Mobile → Mobile
Calls)



Calls originating from a PSTN TE to a mobile destination TE (PSTN → Mobile
Calls)



Message exchanges between the mobile station and the base transceiver (Mob
ile
station ↔ Base transceiver message exchanges)



Refer Section 3.5.1 to 3.5.4 for additional details

GPRS

Two switching modes



Circuit Switching



Packet switching

Circuit switching



A connection first sets up

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Then the entire data transmits through the pa
th that has been set up during the
connection

Packet switching



Packets of data at any given instant can take multiple (time slots or channels or
paths or routes)



Depending on the idle slots at that instant



Receiver assembles the packets into the original
sequence in the data

General packet radio service (GPRS)



A

packet
-
oriented

service for mobile stations’ data transmission and their access
to the Internet



A speed enhanced data transmission service designed for GSM systems



Speed enhanced data transmission
─ by packetizing data and simultaneous
transmission of packets over different channels



Uses the unused slots and channels in TDMA mode of a GSM network for
packetized transmission from a mobile station



Data
-
packets of a single mobile station transmit throu
gh a number of time slots

GSM system─ a subsystem of a GPRS system



GPRS employing the GSM physical layer



Connects mobile stations for voice
-
data transmission



Connects the mobile stations to the Internet



Packet data networks at higher data rates

GPRS syste
m architecture─ Mobile to BSCs (Like GSM)


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GPRS system architecture─ BSCs to MSC (Like GSM)


GPRS deploying SGSNs (serving GPRS support nodes)



SGSN interfaces to BSCs (base station controllers) on one hand and to other
SGSNs on the other hand

GPRS GGSN (
gateway GPRS support nodes) interface



To the SGSN on one hand



A packet data network like the Internet on other hand



The BSCs also connect to the MSCs (mobile services switching centres) as in case
of the GSM system

NSS and RSS layers



Each SGSN and each MS
C in the NSS layer connects to a number of BSCs at the
RSS layer



The SGSNs use the frame relay protocol for connection to BSCs


GSS (GPRS subsystem)



Consists of the SGSNs and GGSNs



Provides GPRS connections to the Internet and other PDNs (public data
netw
orks)


GPRS system architecture ─ BSCs to SGSN at GSS

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GPRS system context



Creates and stores in the Mobile station as well as in the SGSN



Has information of the status of Mobile station, data compression flag, identifiers
for the cell and channel for the

packet data and routing area information

An EIR (GPRS equipment identity register)



Stores the equipment data through the SGSN



Helps the authentication, operation, and maintenance subsystems

GPRS Protocol Mobile station (Mobile Station) Layers



GPRS protoco
l layers similar to the GSM protocol layers



The Mobile station has four layers

physical, data link, network, and application



Session presentation and transport layer issues are taken care of by the lower
layers

BSS



Has just three layers


physical, data li
nk, and network



Transport and session layer functions taken care of by network layer protocols

The SGSN and GGSN four layers



Physical, data link, network and transport



Presentation layer functions are performed by the lower layers

Protocol layers between t
he Mobile station and BSS

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Application layer at the Mobile station



Provides end
-
to
-
end applications like voice and Internet

FR (frame relay) Physical layer for data and network



For transmission and reception of data and network information between the
BSS



and SGSN



Also implements several functions for the data logical link



Physical interface between BSS and SGSN employs a wired or fibre network


Protocol layers between the BSS and SGSN


Protocol layers between the SGSN and the GGSN

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Data link layer prot
ocol layers between SGSN and GGSN



Layer 2 (L2) protocols of the Internet or other PDN (PSTN, ISDN, and PSPDN)

Network layer protocol layers between SGSN and GGSN



IP layer 3 (L3) protocols of the Internet or other PDN

Two transport layer protocol layers at
the SGSN



TCP (or UDP) and GTP (GPRS tunnelling protocol)



TCP for X.25 protocol at layer 3



UDP for the IP protocol at layer 3

Tunnelling protocol



Uses another protocol to transmit and receive the data and information



The information for tunnelling protocol

is hidden in other protocol data

GTP (Gateway Tunneling Protocol)



Uses TCP and IP or UDP and IP



The GTP facilitates flow of packets from multiple protocols



GTP information of TID (Tunnel ID) helps in transmitting and assembling the
packets for each sessi
on of the Mobile station