GSM Channels - Physical and Logical

7/29/2019 GSM Channels - Physical and Logical

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6/13/2010 GSM - Channels - Physical and Logical -

Mohit Kumar - http://mohitkumar.net

GSM CHANNELS

By Mohit Kumar

http://mohitkumar.net


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GSM?

Global system for mobile communication

2nd Generation mobile telephony

Duplexing schemes FDD/TDD

Multiple access TDMA


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Nomenclature

MS (Mobile Station) = MT (Mobile Terminal ) +TE (TerminalEquipment)

BSS (Base Station Subsystem) = BTS (Base Transceiver Station)+BSC (Base Station Controller)

NSS (Network Switching Subsystem)

MSC (Mobile Switching Center): telephony switching function andauthentication of user


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ChannelsPhysical Channels

Associated with frequency bands, time slots, codes Physical channels transfer bits from one network element to

anotherLogical ChannelsDistinguished by the nature of carried information and the way to assemble

bits into data units

Three types one-to-one: traffic channels between a BTS and a MS one-to-many: synchronization signals from BTS to MSs in a cell many-to-one: from MSs to the same BTS


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PHYSICAL CHANNELS


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GSM frequency bands Original GSM radio band is 900 MHz (GSM 900).

Another band that was later assigned termed as DCS was 1800 MHz(DCS 1800).

Can be classified further as

P-GSM, Standard or Primary GSM-900 Band

E-GSM, Extended GSM-900 Band (includes Standard GSM-900band)

R-GSM, Railway GSM-900 Band (includes Standard andExtended GSM-900 band)

T-GSM or TETRA-GSM (TETRA = TErrestrial Trunked RAdio,formerly Trans-European Trunked Radio)


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FDD

Each GSM band is divided into two parts UPLINK and DOWNLINK


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Radio channel GSM radio channel is 0.2 MHz wide. Each channel has a fixed ID

number, called Absolute Radio Frequency Channel Number (ARFCN)

Example: GSM 900

ARFCN 0 represents the 0.2 MHz channel from 890 to 89.2 MHz(usually called 890 MHz channel)

ARFCN 1 to 124 represent 890 + ARFCN * 0.2 MHz channels

Note: ARFCN 0 is reserved as a guard band between GSM bandand its neighboring band.


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GSM represents an ARFCN with a 10-bit number (0 to 1023).

When the network assigns a channel to an MS (mobile station) itidentifies this number


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Radio channels. Each ARFCN channel is a duplex channel and consists of an up and a

down links.

When we say ARFCN 1 we mean uplink 890.2 MHz and its downlink935.2 MHz channels as a duplex.

The uplink and downlink of all ARFCNs maintain a frequency distanceequal to band gap + unidirectional bandwidth.

Eg.-GSM 900 has band gap = 20 + 25 = 45 MHz


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TDMA in GSM Each frequency channel or ARFCN (200 kHz bandwidth) is shared by

multiple users and/or control signal functions one at a time.

For a voice channel every 8th time-slot belongs to the same user.


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TDMA Frame


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TDD One uplink slot and a downlink slot of a duplex GSM frequency (an

ARFCN) forms a pair to provide one voice connection.

These slots carry voice traffic bursts. This is called Traffic Channel(TCH) according to GSM terminology. The slots are like physical carrier, good for any type of data. A duplex

pair of them becomes TCH when they are used (or designated) forvoice traffic.

The timing of uplink and downlink slots maintains a 3-slot distance inorder to ensure that a cell-phone does not require transmission andreception operations simultaneously.

Avoids a number of complexities including the requirement of high peakpower, processor speed and large memory. This also helps simplifytransceiver circuit.


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GSM LOGICALCHANNELS.


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Logical channels One or more logical channels can be mapped to one physical channel.


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Traffic channels Full rate

It transmits full ratespeech (13Kbps).

A full rate TCH occupiesone physical channel

Half rate

It transmits half ratespeech(6.5 Kbps).

Two half rate TCHs canshare one physical

channel to double thecapacity


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6/13/2010 GSM - Channels - Physical and Logical -Mohit Kumar - http://mohitkumar.net

Control channels

Can also be termed as signaling channels

These channels are used for non-traffic partthat is used to support the traffic.

Can be divided into three parts

Broadcast channels (BCH)

Common Control Channels (CCCH)

Dedicated Control Channels(DCCH)


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Broadcast Channels Helps the MSs to orient themselves in the mobile radio network.

Point to multi point channels ( BTS to MSs)

Downlink only


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FCCH The logical Channel contains no information (continuous zeros)

pure carrier is transmitted so that the mobile can tune itself and

synchronise with the frequency.

FCCH will be transmitted in the same beacon carrier as BCCH.

DOWNLINK


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BCCH

Through the BCCH logical channel MS is informed about thesystem configuration parameters like

Local Area Identity

Cell Identity

Identity of BCCH carrier frequencies of Neighbor cells. Max trans power of MS to access

Configuration of control channels

BCCH is also known as beacon. Each cell/sector will have one

carrier frequency on which BCCH will be broadcasted.

DOWNLINK


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SCH

This logical channel transmits information about TDMA framestructure in a cell /sector (frame number)

Base Station Identity Code which can be decoded only if the BTSbelongs to the network for which the MS has subscribed

SCH will also be transmitted in the same carrier of FCCH andBCCH.

DOWNLINK


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Common Control Channels support common procedures required to establish a dedicated link with

the network.

are point-to-multipoint channels that are primarily intended to carrysignaling information for access handling functions.


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PCH PAGING CHANNEL

Used by BTS to page particular MS in the cell MS actively listen to PCH to check contact info within certain time

Contact could be incoming call or short message

Contact info on PCH include IMSI (MSs identity number), or

TMSI (temporary number)

Transmission properties Transmit on down-link

point to point

DOWNLINK


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RACH REQUEST ACCESS

CHANNEL

Used by MH to request a dedicated channel for call setup

Shared by any MS attempts to access the network

Channel request message contains the reason for the accessattempt

Transmission properties

Transmit on up-link

Point to pint

UPLINK


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AGCH ACCESS GRANT

CHANNEL The network assigns a signaling channel via AGCH

A Stand alone Dedicated Control Channel (SDCCH) is assigned


Transmit on down-link

Point to point

DOWNLINK


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DEDICATED CONTROL

CHANNEL -DCCH

DCCH are used for transferring nonuser information between thenetwork and the MS

Messages on DCCH Including

channel maintenance

mobility management

radio resource management


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Stand alone Dedicated Control

Channel (SDCCH)

Transfer signaling information between the BTS and the MS

Typically used for location updating prior to use of a traffic channel


Bidirectional channel, transmit on both up and down-link

Point to point.


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Types of SDCCH

SDCCH/4 sdcch is combined with the

bcch channel

the bcch and sdcch remainactive for almost half of the

time in a multiframe

Transmits only on TS0

SDCCH/8 takes up an entire TCHand keeps trasmitting

on that[alongwithSACCH].

the TCH it is to occupy,can be defined

SDCCH uses 1/8 of time slots.


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Cell Broadcast Channel (CBCH)

To carry Short Message Service Cell Broadcast (SMSCB)

Use the same physical channel as SDCCH


Transmit on down-link

Point to multi-point


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Fast Associated Control Channel

(FACCH) An FACCH is used over a TCH where it steals time slots from a TCH

a 20 ms segment of speech is stolen to carry handover signaling

information Appears on demand


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Slow Associated Control

Channel (SACCH) Carries control and measurement parameters along with routine data

necessary to maintain a radio link between the MS and the BTS On the uplink, MS sends averaged measurements (signal strength and

quality) of current and neighboring BCCH On downlink, MS receives information about transmitting power to use and

an instruction with time advance/retard.


Bidirectional channel, transmit on both up and down link Point to point


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Channel Combinations Several logical channels can be mapped onto

physical channels There are many combinations that can be

used in case of GSM logical channels


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Combination I

TCH/F + FACCH/F + SACCH.

This combination is used for full rate traffic.

TCH/F FACCH/F SACCH


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TCH/F + FACCH/F + SACCH.


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Combination II

TCH/H + FACCH/H + SACCH.

This combination is used for half rate traffic

when only one channel is needed.


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Combination III

2 TCH/H + 2 FACCH/H + 2 SACCH

This combination is used for half rate traffic.


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Combination II & III


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Combination IV

FCCH + SCH + BCCH + CCCH.

This is the standard combination for mediumand large cells.


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Combination IV


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GSM - Channels - Physical and Logical -


Combination V

FCCH + SCH + BCCH + CCCH +

4 SDCCH + 4 SACCH.

This is the typical C0T0 combination for small cells, which allows the

BTS to trade unnecessary CCCH capacity for a pool of 4 SDCCHs.


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Combination V


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Combination VI

BCCH + CCCH.

This combination is used to provide additionalCCCH capacity in large cells.


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Combination VI


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Combination VII8 SDCCH + 8 SACCH.

This combination is used to provide additionalSDCCH capacity in medium and large cells.


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Combination VII


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GSM Bursts

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GSM Channels - Physical and Logical