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Mobile Communication SystemMobile Communica

tion System

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GSMThe Global System for Mobile

Communications

OMC

MSC

ISC

GMSC

BSC

BTS

BTS

BSC

BTS

PLMN &Internet

PSTN

ISDN

PDN

MS

MS

MS

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Outline

General Description Addresses and Identifiers

System Architecture

BSS: The radio network

SMSS: The mobile switching network

OMSS: The management network

Mobile subscribers

Examples

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3

BSC

Radio resource management: channelallocation, BTS measures processing, BTSand MS power control, handoff...

Interfaces management: with the MSC

(gathers the traffic towards the MSC) andwith the BTSs.

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Power Control

Design issues making it desirable to include dynamicpower control in a cellular system

Received power must be sufficiently above the backgroundnoise for effective communication

Desirable to minimize power in the transmitted signal fromthe mobile

Reduce co-channel interference, alleviate health concerns, savebattery power

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Types of Power Control

Open-loop power controlDepends solely on mobile unit

No feedback from BS

Not as accurate as closed-loop, but can react quicker to

fluctuations in signal strength

Closed-loop power control

Adjusts signal strength in reverse channel based on metric

of performanceBS makes power adjustment decision and communicates to

mobile on control channel

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Base Station Controller (BSC)

Architecture of the Base Station ControllerSwitch Matrix

act as a concentrator where many different low capacity connectionsto BTSs become reduced to a smaller number of connectionstowards the MSC.

Terminal Control Elements of the Abis-Interface

A-Interface Terminal Control Elements

Database

including information such as carrier frequency, frequency hoppinglists, power reduction levels, receiving levels for cell bordercalculation, are stored in the BSC.

Central Module

Connection to the OMC

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Mobile Communication SystemMobile Communication System

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Functions of a BSC

One BSC may control up to 40BTS (kept in database)

switch calls from MSC to

correct BTS and converselyProtocol and coding

conversion for traffic (voice)& signaling (GSM-specific toISDN-specific)

Manage mobility of MS(handoff between differentBTS)

Enforce power control

BSC

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TRAU

Transcoding Rate and Adaptation Unit (TRAU)Performs coding between the 64 kpbs PCM coding used in the

backbone network and the lower-rate coding used for the MobileStation

When the traffic is not voice but data such as fax or email, theTRAU enables its rate adaptation unit function to givecompatibility between the BSS and MSC data rates.

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TRAU

Possible sites for the TRAU in the signal chain.

M bil C i i SM bil C i

i S

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Transcoder

The transcoder is responsible for transcoding the voicechannel coding between the coding used in the differentnetwork. (PLMN, PSTN, Internet).

Specifically, GSM uses a regular pulse excited-long termprediction coder (RPE-LPC) for voice data between the mobile

device and the BSS, but pulse code modulation (A-law or -lawstandardized in ITU G.711) upstream of the BSS. RPE-LPCcoding results in a data rate for voice of 13 kbit/s where standardPCM coding results in 64 kbit/s.

In fixed network voice over IP, the narrowband codecs (e.g.G.723.1/5.3 or 6.3 kbps and wideband codecs (e.g. G.722, datarates from 4k to 64kbps) are widely used.

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M bil C i ti S tM bil C i

ti S t

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Voice Coding Algorithms

Algorithm Rate (kb/s) MOS /5 MIPS

PCM (G.711) 64 4.3 .01

ADPCM (G.721) 32 4.1 2

LD-CELP (G.728) 16 4 19

RPE-LTP (GSM) 13 3.47 6

Skyphone-MPLP 9.6 3.4 11

VSELP (IS-54) 8 3.45 13.5

CELP (IS-95) 4.8 3.2 16

MIPS- Million Instructions Per Second

M bil C i ti S tM bil C i

ti S t

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GSM Radio Interface

Multiple access and multiplex Total frequency spectrum 50 MHz. 50 MHz = 2 X 25MHz 2 x (124 x 200KHz ) = 248 FDMA channels.

Normally 1 ~ 15 FDMA channels in each cell.

Each FDMA channel with 8 x 0.577ms TDMA subscribers.

Slow FH for severe multipath problem cell.

Traffic Channel (TCH): TCH/F 13Kbps for voice and 2.4, 4.8 and 9.6Kbps for data

TCH/H 6.5Kbps for voice and 2.4, 4.8Kbps for data.

Control Channel (CCH): BCCH, CCCH( PCH,RACH and

AGCH), SDCCH, SCH, FCCH and ACCH Speech coding

RPE-LTP (Regular Pulse Excitation Long-Term Prediction)


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Radio Channels

Two 25 MHz cellular bands set

reverse link

the 890-915 MHz band was for subscriber-to-base transmissions

forward link

the 935-960 MHz band was for base-to-subscriber transmissions.

The available forward and reverse frequency bands are divided into200 kHz wide channels called ARFCNs .Absolute Radio Frequency Channel Numbers.

Thus 124 bearer frequency pairs possible, suggested to use only 122 tokeep additional guard top and bottom

In practice, due to power control and shadowing, adjacent channelscannot be used within the same cell

The ARFCN denotes a forward and reverse channel pair

which is separated in frequency by 45 MHz

Each channel is time shared between as many as eight subscribersusing TDMA, Each of the eight subscribers uses the same ARFCNand occupies a unique timeslot (TS) per frame.


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Radio Channels by Frequency and Time


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Radio Channels

Radio transmissions on both the forward and reverse linkare made at a channel data rate of 270.833 kbps

Using GMSK modulation with a normalized bandwidthexpansion factor of 0.3.

The signaling bit duration is 3.692s The effective channel transmission rate per user is 33.854

kbps

( 270.833 / 8 = 33.854 ) .


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Radio Interface Specifications

Parameter Specifications Parameter Specifications

Reverse ChannelFrequency

890-915 MHz Users per Frame (FullRate)

8

Forward ChannelFrequency

935-960 MHz Time Slot Period 576.9

ARFCN Number 0 to 124 and 975 to1023 Bit Period 3.692

Tx/Rx FrequencySpacing

45 MHz Modulation 0.3 GMSK

Tx/Rx Time SlotSpacing

3 Time slots ARFCN ChannelSpacing

200 kHz

Modulation Data Rate 270.833333 kbps Interleaving (max.delay)

40 ms

Frame Period 4.615 ms Voice Coder Bit Rate 13.4 kbps


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Um: FDM & TDM

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8 periodic time slots0.577ms each TDM frame composed of 8 timeslots equals to 4.615ms

Every time slot a so called burst - succession of 148bit istransmitted

Between the bursts a security buffer of 8.25bit/burst is putin between


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Um: FDM & TDM

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Through FDM/TDM hybrid in GSM 992 channels available

In DCS1800 more channels: 75MHz band split into 200kHzchannels allows a total of 374 carriers


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Um: Fequency Hopping

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Not all channels in a given cell are of equal quality and multipath reception and adjacent channels may disruptcommunication

Thus frequency hopping is introduced

avoid frequency-selective fading, co-channel interference


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Frame Structure for GSM

Each user transmits a burst of data during the time slotassigned to it.

These data bursts may have one of five specific formats,as defined in GSM.

Normal burstFrequency Correction Channel (FCCH) burst

Synchronization Channel (SCH) burst

Random Access Channel (RACH) burst

Dummy Burst to fill in inactive bursts in Broadcast ControlChannel (BCCH, direction from BTS to MS) to have most poweron this channel (helpful, when MS needs to find BCCH)


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FCCH(Frequency Correction Channel) Burst

It is a downlink-only control channel in the GSM Umradio interface.

The FCCH burst is an all-zero sequence that produces afixed tone in the GMSK modulator output. This tone

enables the Mobile to lock its local oscillator to the BSclock.

FCCH is responsible for first part of MS tuning (synchronisation ofmobile device to BTS signal)

Immediately after follows a SCH burst

Used for transmission of frequency reference andfrequency correction burst of 148 bits length.

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SCH(Synchronization Channel) Burst

It is a downlink only control channel. The purpose of theSCH is to allow the mobile station to quickly identify anearby cell (or BTS) and synchronize to that BTS'sTDMA structures. Each radio burst on the SCH contains:

the current frame clock of the serving BTS,

the base station identity code (or BSIC)

an extended Training Sequence that is easily detected with amatched filter.

Used to provide the synchronization training sequencesburst of 64 bits length to the MS.

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yy

RACH(Random Access Channel) Burst

Itis used in mobile phones or other wireless device on aTDMA-based network when it needs to synchronize itstransmission with the base station.

RACH is a shared channel that is used by wireless access

terminals to access the network, especially for initialaccess and bursty data transmission.

A key feature of a RACH is that messages are notscheduled . There is no certainty, that only a singledevice makes a connection attempt at one time, andcollisions can result.

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Bursts in GSM

Normal

3 tail bits 58 bits of encrypted data 26 trainingbits

58 bits of encrypted data 3 tail bits 8.25 bits gapperiod

FCCH burst

3 tail bits 142 fixed bits of all zeroes 3 tail bits 8.25 bits gapperiod

SCH burst

3 tail bits 39 bits of encrypteddata

64 bits of training 39 bits of encrypteddata

3 tail bits 8.25 bits gapperiod

RACH burst

8 tail bits 41 bits of synchronization 36 bits of encrypteddata 3 tailbits 68.25 bit extended gap period

Dummy burst

3 tail bits 58 mixed bits 26 trainingbits

58 mixed bits 3 tail bits 8.25 bits gapperiod


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yy

Normal Burst

The normal burst is used to carry data and most signaling.

It has a total length of 156.25 bits, made up of two 57 bit information bits, a 26bit training sequence used for equalization, 1 stealing bit for each informationblock (used for FACCH), 3 tail bits at each start and end position, and an 8.25bit guard sequence. The 156.25 bits are transmitted in 0.577 ms.

Training sequence bits - Used for equalization. Bits which get the base station

and mobile in "tune" with each other.

Stealing bits - Whereby a bit is stolen from message bits, just temporarily, tomake way for the Fast Associated Channel. It runs in a blank and burst mode. Ittransmits during handoff or when the slow associated channel can't sendinformation quickly enough

Tail bits - The tail bits clear the code that has gone before, setting everythingback to 0 or a null state.

Guard bits - Empty time spaces separating data packets to make sure one burstdoes not run into another.

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TDMA Frame

Multi-frame

Tail bit3

CodedData 57

Stealingflag 1

Midamble26

Stealingflag 1

CodedData 57

Tailbit 3

Guard Period8.25

576.92 s (156.25 bits)Time slot

Hierarchy of Frames in GSM


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Hierarchy of Frames in GSM

Multi-frame

Super-frame

Hyper- frame


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Physical Channel & Logical Channel

Physical channel

The combination of a TS number and an ARFCN constitutes a physicalchannel for both the forward and reverse link.

Each physical channel in a GSM system can be mapped intodifferent logical channels at different times.

Each specific time slot or frame may be dedicated to eitherhandling

traffic data

(user data such as speech, fax, or teletext data),

signaling data

(required by the internal workings of the GSM system),

or control channel data

(from the MSC, base station, or mobile user).


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GSM Channel Types

Traffic channels (TCHs)

Traffic channels carry digitally encoded user speech or user dataand have identical functions and formats on both the forward andreverse link.

There are six different types of TCHs provided in GSM.

Control channels (CCHs)Control channels carry signaling and synchronizing commands

between the base station and the mobile station.

an even larger number of CCHs.

Certain types of control channels are defined for just the forwardor reverse link.


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GSM Channel Types


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GSM Traffic Channels (TCHs)

GSM traffic channels may be either full-rate or half-rate. When transmitted as full-rate,

user data is contained within one TS per frame.

When transmitted as half-rate,

user data is mapped onto the same time slot, but is sent inalternate frames.

Two half-rate channel users would share the same time slot, butwould alternately transmit during every other frame.


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Full-Rate TCH

The following full rate speech and data channels aresupported:

Full rate traffic channel for 14.4 kbps user data (TCH/F14.4);

Full rate traffic channel for 9.6 kbps user data (TCH/F9.6);

Full rate traffic channel for 4.8 kbps user data (TCH/F4.8);Full rate traffic channel for 2.4 kbps user data (TCH/F2.4);

With additional forward error correction coding appliedby the GSM standard, the data is sent at 22.8 kbps.


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Half-Rate TCH

The following half-rate speech and data channels aresupported:

Half rate traffic channel for 4.8 kbps user data (TCH/H4.8);

Half rate traffic channel for 2.4 kbps user data (TCH/H2.4).

With additional forward error correction coding appliedby the GSM standard, the data is sent at 11.4 kbps.


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GSM Channel Types


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GSM Control Channels (CCH)

Beside the traffic channels are a group of control channelsdefined

They handle system information, connection setup andconnection control

Broadcast Control Channel (BCCH) group handles beaconsignaling, synchronization of MS with the serving BTS,timing advance adjustment, it comprises of

BCCH Broadcast Control Channel

FCCHFrequency Control Channel

SCHSynchronization Channel


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BCCH

BCCH is responsible for

Sending out of beacon on one frequency per cell (by BTS)

Contains 16bit Location Area (LA) code

MUST BE on Time Slot #0, following time slots might used by TCH

BCCH provides:Details of the control channel configuration

Parameters to be used in the cell

Random access backoff values

Maximum power an MS may accessMinimum received power at MS (RXLEV_ACCESS_MIN)

List of carriers used in the cell

Needed if frequency hopping is applied

List of BCCH carriers and BSIC of neighboring cells 2013/1/2836


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CCH

Next group Common Control Channel (CCCH) it consists of

Random Acces Channel (RACH)

Access Grant Channel (AGCH)

Paging Channel (PCH)

Third group is the Dedicated/Associated Control Channel(DCCH)/ (ACCH)

Stand-alone Dedicated Control Channel SDCCH

Fast/Slow Associated Control Channel SACCH/FACCH

FACCH used when several signalling information needs to betransmitted

Call setup, Handover

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Control Channel Multiframe

0

F

1

S

2

B

3

B

4

B

5

B

6

C

7

C

8

C

9

C

10

F

11

S

12

C

13

C

14

C

20

F

21

S

22

C

39

C

40

F

41

S

42

C

49

C

50

I

0

R

1

R

2

R

3

R

4

R

5

R

6

R

46

R

47

R

48

R

49

R

50

R

Forward link for TS0 ( BS MS )

Reverse link for TS0 ( MS BS )

F: FCCH burst (BCH) B: BCCH burst (BCH) I: Idle

S: SCH burst (BCH) C: PCH/AGCH burst (CCCH)

R: Reverse RACH burst (CCCH)

Control Multiframe = 51 TDMA Frames

235 ms ( 4.615 x 51 = 235.365 )


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Control Channels (CCH)

Name Abbreviation Task

Broadcast common control

channel (DL)

BCCH To transmit system information (as cell ornetwork identity)

Frequency correction

channel (DL)

FCCH The lighthouse of a BTS

Synchronization channel (DL) SCH PLMN/base station identifier of a BTS plussynchronization information (frame number)

Paging channel (DL) PCH Carries the PAG_REQ message

Access grant channel (DL) AGCH SDCCH channel assignment (the AGCH carriesIMM_ASS_CMD)

Random access channel (UL) RACH Communication request from MS to BTS

Standalone dedicated

control channel

SDCCH Exchange of signaling information between MSand BTS when no TCH is active

Slow associated control

channel

SACCH Transmission of signaling data during aconnection (one SACCH TS every 120 ms)

Fast associated control

Channel

FACCH Transmission of signaling data during aconnection (used only if necessary)


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BCCHs

Broadcast Control Channel (BCCH)The BCCH is a forward control channel that is used to

broadcast information such as cell and networkidentity, and operating characteristics of the cell

current control channel structure, channel availability,and congestion

The BCCH also broadcasts a list of channels that arecurrently in use within the cell.

Frequency Correction Channel (FCCH)

Synchronization Channel (SCH)


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BCCHs

Broadcast Control Channel (BCCH)


The FCCH is a special data burst which occupies TS 0for the very first GSM frame (frame 0) and is repeated

every ten frames within a control channel multiframe.The FCCH allows each subscriber unit to synchronize

its internal frequency standard (local oscillator) to theexact frequency of the base station.



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BCCHs

Broadcast Control Channel (BCCH)



SCH is broadcast in TS 0 of the frame immediately following the FCCH frameand is used to identify the serving base station while allowing each mobile toframe synchronize with the base station.

Theframe number (FN) is sent withthe base station identity code (BSIC)during the SCH burst.

FN: ranges from 0 to 2,715,647

A mobile is often necessary to adjust the timing of a particular mobile usersuch that the received signal at the base station is synchronized with the basestation clock.

The BS issues coarsetiming advancement commands to the mobile stationsover the SCH, as well.

The SCH is transmitted once every ten frames within the control channelmultiframe.


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Common Control Channels (CCCHs)

On the broadcast (BCH) ARFCN, the common control channels

occupy TS 0 of every GSM frame

that is not otherwise used by the BCH or the Idle frame.

CCCH consists of three different channels:


Random Access Channel (RACH)


CCCHs are the most commonly used control channels and are usedto

page specific subscribers,

assign signaling channels to specific users, and

receive mobile requests for service.


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CCCH s


The PCH provides paging signals from the base station to allmobiles in the cell, and notifies a specific mobile of an incomingcall which originates from the PSTN.

The PCH transmits the IMSI of the target subscriber, along with

a request for acknowledgment from the mobile unit on theRACH.

Alternatively, the PCH may be used to provide cell broadcastASCII text messages to all subscribers, as part of the SMSfeature of GSM.




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CCCH s



The RACH is a reverse link channel used by a subscriber unit toacknowledge a page from the PCH, and is also used by mobilesto originate a call.

In establishing service, the GSM base station must respond to theRACH transmission

by allocating a channel and assigning a stand-alone dedicatedcontrol channel (SDCCH) for signaling during a call.

This connection is confirmed by the base station over the AGCH.



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CCCH s




The AGCH is used by the base station to provide forward linkcommunication to the mobile,

and carries data which instructs the mobile to operate in a particularphysical channel (time slot and ARFCN) with a particular dedicatedcontrol channel.

The AGCH is the final CCCH message sent by the base stationbefore a subscriber is moved off the control channel.

The AGCH is used by the base station to respond to a RACHsent by a mobile station in a previous CCCH frame.


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Dedicated Control Channels (DCCHs)

Stand-alone Dedicated Control Channels (SDCCHs)

used for providing signaling services required by the users

Slow Associated Control Channel (SACCH)

Fast Associated Control Channels (FACCHs)

used for supervisory data transmissions between the mobilestation and the base station during a call.


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Stand alone Dedicated Control Channels

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Stand-alone Dedicated Control Channels(SDCCHs)

The SDCCH carries signaling data

following the connection of the mobile with the base station,

before a TCH assignment is issued by the base station.

The SDCCH ensures that the mobile station and the base

station remain connectedwhile the base station and MSC verify the subscriber unit and

allocate resources for the mobile.


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Slow Associated Control Channel (SACCH)

The SACCH carries general information between theMS and BTS.

On the forward link, the SACCH is used to send slowbut regularly changing control information to themobile,

such as transmit power level instructions and specific timingadvance instructions for each user on the ARFCN.

The reverse SACCH carries information about

1. Received signal strength2. Quality of the TCH

3. BCH measurement results from neighboring cells.


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(Normal)Speech Multiframe = 26 TDMA frames

120 ms ( 4.615 x 26 = 119.99 )

Speech Dedicated Control Channel Multiframe

T0 T1 T2 T10 T11 T12 S T13 T14 T15 T22 T23 T24 I/S

TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

1 TDMA frame

4.615 ms

156.25 bits

576.92 s

Tn : nthTCH frame

S : Slow Associated Control Channel frame (SACCH)

I : Idle frame

TSn : nthTime Slot


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Fast Associated Control Channels (FACCHs)

FACCH carries urgent messages,

contains essentially the same type of information as theSDCCH.

A FACCH is assigned whenever a SDCCH has not beendedicated for a particular user and there is an urgentmessage (such as a handoff request).

The FACCH gains access to a time slot by"stealing frames from the traffic channel to which it isassigned.

This is done by setting two special bits, called stealing bits,in a TCH forward channel burst.

If the stealing bits are set, the time slot is known to containFACCH data, not a TCH, for that frame.


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Originate a Call using Logical Channels

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Service request a SDCCH isrequired

Same authorization andencryption procedure has to bedone

Signaling of desired serviceand assignment of properpayload channel

Signaling of call signal tothe subscriber at the MS

Call setup if called partyanswers


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Receive a Call

Receive a call in a mobilephone network

The called device/user has tobe looked up in a givenlocation area (paging)

To be able to answer the MShas to request a channel

It gets assigned a controlchannel by the BSCimmediately if cell is notcongested

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109-GSM System 2 (1)

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