GSM Channels

Lecture 12

GSM Control Channels

Three main types of control channels Broadcast ch (BCH) Common control ch (CCCH) Dedicated control ch (DCCH)

Each control ch comprises several logical chs which are distributed in time to provide nec GSM control functions

GSM Control Channels BCH and CCCH forward control chs are implemented only

on certain ARFCNs and are allocated TSs in a specific fashion

Normally allocated TS0 and are broadcast only during certain frames within repetitive 51 control ch MF sequence

TS1 through TS7 carry regular TCH traffic So ARFCNs designated as BCH can still carry traffic on

seven out of eight TSs GSM specs defines 34 ARFCNs as standard BCH For each BCH frame 51 is an idle frame However, reverse CCCH channels still able to receive MS

transmissions during TS0 of any frame (incl idle frame) DCCH data may be sent during any TS and any frame and

entire frames are specifically dedicated to certain DCCH transmissions

GSM Control Channels (BCH) BCH is defined by three

separate channels which are given access to TS0 during various frames of the 51 fame sequence. Three types of BCH are

Broadcast Control Channel (BCCH)— a forward control channel used to broadcast information such as

cell and network identity, and

operating characteristics of the cell (current control channel structure, channel availability, and congestion)

Also broadcast a list of channels currently in use within the cell

Frame 2 through 5 in a control MF (4 out of every 51 frames) contain BCCH data

GSM Control Channels

GSM Control Channels (BCH) Frequency correction

channel (FCCH) A special data burst

which occupies TS0 for the very first GSM frame (frame 0) and is repeated every 10th frame within a control channel MF

Allows each MS to synchronize its internal frequency standard (local oscillator) to the exact frequency of the BS

GSM Control Channels (BCH) Synchronization

Channel (SCH) broadcast in TS0 of the

frame immediately after FCCH frame

Allows each mobile to frame synchronize with BS

FN (0 to 2,715,647) is sent with BSIC during the SCH burst

BSIC assigned to each BTS

An MS may be 35 km away from a serving BS

SCH transmitted once every 10th frames within the control channel MF

GSM Control Channels (CCCHs) Common Control Channels

(CCCHs) On the broadcast (BCH) ARFCN,

the CCCHs occupy TS0 of every frame not otherwise used by BCH or the Idle frame.

Consists of three different channels

Paging channel (PCH), forward link

Random access channel (RACH) reverse link, and

Access grant channel (AGCH), forward link

Most commonly used channels used to

page specific subscribers assign signalling channels to specific

users and receive mobile requests for service

GSM Control Channels (CCCHs)

Paging Channel (PCH) Provides paging signals from the BS to all

mobiles in the cell Notifies a subscriber of a Mobile Terminating

Call Transmits TMSI of the target subscriber, along

with a request for ack from mobile unit on RACH

Alternatively may be used to provide cell broadcast and Text messages to all subscribers,

as part of SMS feature

GSM Control Channels (CCCHs)

Random Access Channel (RACH) A reverse link channel used by a

subscriber unit to ack a page from the PCH

Also used by mobiles to originate a call All mobiles must request access or

respond to a PCH alert within TS0 BTS responds to a RACH channel using

Access Grant Channel to assign a stand-alone dedicated control channel (SDCCH) for signalling during a call

GSM Control Channels (CCCHs) Access Grant Channel (AGCH)

Used by BS to provide forward link communication to the mobile

Carries data which instructs mobile to to operate in a particular physical channel (TS and ARFCN) with a particular dedicated control channel (SDCCH)

Final CCCH message sent by BS before a subscriber is moved off the control channel

Used by BS to respond to a RACH sent by a mobile station in a previous CCCH frame

GSM Control Channels (DCCHs) Dedicated Control Channels (DCCHs)

Three types of dedicated control channels in GSM Bidirectional like TCHs and have the same format

and function on both the forward and reverse link Like TCHs, DCCHs may exist in any time slot and on

any ARFCN except TS0 of the BCH ARFCN Stand-alone dedicated control channels (SDCCHs) used for

providing signalling services required by the users Slow-and Fast- Associated Control Channel (SACCHs and

FACCHs) used for supervisory data transmission between the MS and the BS during a call

GSM Control Channels (DCCHs) Stand-alone Dedicated Control Channels

(SDCCHs) Carries signalling data following the connection of the

mobile with the BS, and just before a TCH is issued by BS

Ensures that the MS and the BS remain connected while the BS and MSC verify the subscriber unit and allocate resources for the mobile

Can be thought of as an intermediate and temporary channel that holds the traffic while waiting for BS to allocate a TCH

May be assigned their own physical channel or may occupy TS0 of the BCH if there is low demand for BCH or CCCH traffic

GSM Control Channels (DCCHs)

Slow Associated Control Channel (SACCH) Always associated with TCH or a SDCCH and maps onto

the same physical channel So each ARFCN systematically carries SACCH data for all

of its current users On the forward link, SACCH used to send slow but

regularly changing control information to the mobile, such as transmit power level instruction and specific TA instructions for each user on the ARFCN

Reverse SACCH carries information about the received signal strength and quality of the TCH, as well as BCH measurement results from neighbouring cells

SACCH transmitted during 13th frame (and 26th frame when half-rate traffic is used) of every speech/dedicated control channel MF

Within this frame,all eight TSs are dedicated to providing SACCH data to each of the eight users on the ARFCN

GSM Control Channels (DCCHs)

Fast Associated Control Channels (FACCHs) Carries urgent message, and contains essentially

the same type of information as the SDCCH Assigned whenever a SDCCH has not been dedicated

for a particular user and there is an urgent message (such as a handoff request)

Gains access to a TS by “stealing” frames from the TCH to which it is assigned

Done by setting two special bits, in a TCH forward channel burst

If the stealing bits are set, the TS is known to contain FACCH data, not a TCH, for that frame

Example of a GSM Call When the MS is turned on it will listen to BCH carrier

of a BTS having highest received power. Then BTS will use FCCH in order to syn to the carrier

frequency Then the MS listen to the SCH to get info on the

TDMA frame structure The MS will then listen to the BCCH to get info such

as location area, Max allowed O/P power & neighboring cells

The MS will periodically listen to the PCH to determine if someone is trying to call it.

If the MS hears a page it will use the RACH to ask for access to the system in order to respond to the incoming call

Example of a GSM Call On the other hand, to originate a call , the user first

dials the intended digit combination and presses the “send” button on the GSM phone

The mobile transmits a burst of RACH data using the same ARFCN as the BS to which it is locked

The BS responds with an AGCH message on the CCCH which assigns the mobile to a new channel for SDCCH connection

The subscriber unit, which is monitoring TS0 of the BCH, would receive its ARFCN and TS assignment from the AGCH and would immediately tune to the new ARFCN and TS

This new ARFCN and TS assignment is physically the SDCCH (not the TCH)

Example of a GSM Call Once tuned to the SDCCH, the subscriber unit

first waits for the SACCH frame to be transmitted, which informs the mobile of any TA and transmitter power command

The BS is able to determine the proper TA and signal level from mobile’s earlier RACH transmission and sends the proper value over the SACCH for the mobile to process

Upon receiving and processing the TA information in the SAACH, the subscriber is now able to transmit normal burst messages as required for speech traffic

Example of a GSM Call The SDCCH sends message between the mobile

unit and the BS, taking care of authentication and user validation, while the PSTN connects the dialled party to the MSC and the MSC switches the speech path to the serving BS

After a few seconds, the mobile unit is commanded by the BS via SDCCH to return to a new ARFCN and new TS for TCH assignment

Once returned to TCH, speech data is transferred on both forward and reverse links, the call is successfully underway and the SDCCH is vacated

Example of a GSM Call For calls from PSTN, BS broadcasts a PCH message

during TS0 with in an appropriate frame on the BCH The mobile station, locked onto that same ARFCN,

detects its page and replies with an RACH message acknowledging receipt of the page

BS then uses the AGCH on the CCCH to assign the mobile unit to a new physical channel for connection to SDCCH and SACCH while the network and the serving BS are connected

Once the subscriber estb TA and authentication on the SDCCH, the BS issues a new physical ch assignment over the SDCCH, and TCH assignment is made

Rest of the procedure is the same as described for Mobile originating call

Signal Processing in GSM

Channel Coding Interleaving Authentication & Ciphering GMSK Modulation Identifiers

Channel Coding For channel coding 260 bits of data in a TRAU frame

separated into 182 class-1 bits (very important) and 78 class-2 bits (less important)

Channel coding protects the two classes with different priorities

After channel coding original data packet of 260 bits (user data) or 184 bits (signaling data) extended to a data block of length 456 bits

Data block then mapped on various bursts for the actual transmission

Channel Coding for User Data

Channel Coding for Signalling Data

Interleaving

Packets of 456 bits spread over a larger time period in separate TSs

Spreading depends on application the bits represent Signalling & data traffic are spread more than voice traffic Goal - to minimize the impact of Air-interface peculiarities that

account for rapid, short-term changes of the quality of the transmission channel

A particular channel may be corrupted for a very short period of time and all the data sent during that time are lost

That could lead to loss of complete data packets of n times 114 bits

Interleaving does not prevent loss of bits If there is a loss, the same number of bits are lost However, in interleaving, the lost bits are part of several

different packets bits These few bits can be recovered by error-correction

mechanisms

Interleaving