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Mobile Communications

Lec 3: GSM Air Interface (Continued)

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Um Interface

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Five different burst types are defined in GSM standard;

Normal Burst: This burst is used to carry information on traffic and control channels. For TCH it contains114 encrypted b its, and includes a guard time of 8.25 bit duration (30.46 us). The stealing flag is used to

signal the presence of FACCH, rather than user data.

Frequency Correction Burst: This burst is used for frequency synchronization of the MS. It consists of

zeroes only. This burst is only used in one of the logical channels i.e. Frequency Correction Channel (FCH).

Synchronization Burst: This burst is used for time synchronization of the MS. It contains a long trainingsequence and carries the information of the TDMA Frame Number (FN) and Base Station Identity Code

(BSIC). This burst is only used in one of the logical channels i.e. Synchronization Channel (SCH).

GSM Burst Formats

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Access Burst: This burst is used for random access and handover access. It is characterized by a long guard

period (68.25 bit duration or 252 ms), to cater for burst transmission from an MS that does not know the

timing advance at the first access (or at handover). This allows for a cell radius of 35 km. The access burstis used on the Random Access Channel (RACH) and for access at handover.

Dummy Burst: This burst is transmitted on radio frequency when no other type of burst is to be sent. This

means that the base station always transmits on the frequency carrying the system information, thusmaking it possible for the MSs to perform power measurements on the BTS in order to determine which

BTS to use for initial access or which to use for handover. In order to achieve this, a dummy page and a

dummy burst is defined in the GSM recommendations. CCCH is replaced by the dummy page, when there

is no paging message to transmit. This dummy page is a page to a non-existing MS. In the other TSs not

being used, a dummy burst with a pre-defined set of fixed bits is transmitted.

GSM Burst Formats

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Logical Channels page 49

The logical channels can be separated into two categories. They are traffic channels and signaling channels.

There are two forms of Traffic Channels:

Full rate TCH (TCH/F) - this channel carries information at a gross rate of 22.8 kbit/s.

Half rate TCH (TCH/H) - this channel carries information at a gross rate of 11.4 kbit/s.

Signaling channels are subdivided into three categories:

Broadcast Channels (BCH)

Common Control Channels (CCCH)

Dedicated Control Channels (DCCH)

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Broadcast Control Channels

Frequency Correction CHannel (FCCH)

On FCCH, bursts only containing zeroes are transmitted. This serves two purposes. First to make sure that this isthe BCCH carrier, and second to allow the MS to synchronize to the frequency. FCCH is transmitted downlinkonly.

Synchronization CHannel (SCH)

The MS needs to synchronize to the time-structure within this particular cell, and also ensure that the chosenBTS is a GSM base station. By listening to the SCH, the MS receives information about the frame number in thiscell and about Base Station Identity Code (BSIC) of the chosen BTS. SCH is transmitted downlink only.

Broadcast Control CHannel (BCCH)

The MS must receive some general information concerning the cell in order to start roaming, waiting for calls toarrive or making calls. The needed information is broadcast on the Broadcast Control CHannel (BCCH) andincludes the Location Area Identity (LAI), maximum output power allowed in the cell and the BCCH carriers forthe neighboring cells on which the MS performs measurements. BCCH is transmitted on the downlink only.

Using FCCH, SCH, and BCCH the MS tunes to a BTS and synchronized with the frame structure in that cell. TheBTSs are not synchronized to each other. Therefore, every time the MS camps on another cell, it must listen toFCCH, SCH and BCCH in the new cell

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Common Control Channels

Paging CHannel (PCH)

At certain time intervals the MS listens to the PCH to check if the network wants to make contact with the MS.

The reason why the network may want to contact the MS could be an incoming call or an incoming short

message. The information on PCH is a paging message, including the MSs identity number (IMSI) or a

temporary number (TMSI). PCH is transmitted downlink only.

Random Access CHannel (RACH)The MS listens to the PCH to determine when it is being paged. When the MS is paged, it replies on the RACH

requesting a signaling channel. RACH can also be used if the MS wants to contact the network. For example,

when setting up a mobile originating call. RACH is transmitted uplink only.

Access Grant CHannel (AGCH)

The networks assigns a signaling channel (Stand-alone Dedicated Control CHannel (SDCCH)) to the MS. This

assignment is performed on the AGCH. AGCH is transmitted downlink only.

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Dedicated Control Channels

Stand alone Dedicated Control CHannel (SDCCH)

The MS as well as the BTS switches over to the assigned SDCCH. The call set-up procedure is performed on the

SDCCH, as well as the textual message transmission (short message and cell broadcast) in idle mode. SDCCH is

transmitted both uplink and downlink. When call set-up is performed, the MS is told to switch to a TCH.

Slow Associated Control CHannel (SACCH)

The SACCH is associated with SDCCH or TCH (i.e. sent on the same physical channel). On the uplink, the MS

sends averaged measurements on its own BTS (signal strength and quality) and neighboring BTSs (signal

strength). On the downlink, the MS receives information concerning the transmitting power to use and

instructions on the timing advance. SACCH is transmitted both uplink and downlink.

Fast Associated Control CHannel (FACCH)

If a handover is required the FACCH is used. FACCH works in stealing mode meaning that one 20 ms segment of

speech is exchanged for signaling information necessary for the handover. Under normal conditions the

subscriber does not notice the speech interruption because the speech coder repeats the previous speech

block.

Cell Broadcast CHannel (CBCH)

CBCH is only used downlink to carry Short Message Service Cell Broadcast (SMSCB) and uses the same physical

channel as SDCCH.

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Only certain combinations of logical channels are permitted according to the GSM recommendations. The list

below shows the way in which logical channels can be combined on to Basic Physical Channels (BPC).

(i) TCH/F + FACCH/F + SACCH/F

(ii) TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)

(iii) TCH/H(0) + FACCH/H(0) + SACCH/TH(0) + TCH/H(1)

(iv) FCCH + SCH + BCCH + CCCH

(v) FCCH + SCH + BCCH + CCCH + SDCCH/4(0...3) + SACCH/C4(0...3)

(vi) BCCH + CCCH

(vii) SDCCH/8(0...7) + SACCH/C8(0...7)

(viii) TCH/F + FACCH/F + SACCH/M

(ix) TCH/F + SACCH/M

(x) TCH/FD + SACCH/MD

Where CCCH = PCH + AGCH + RACH

Logical Channel Combinations

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Control Channel Multiframe

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BCCH/CCCH Multiframe

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DCCH/8 Multiframe

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Traffic Channel Multiframe

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SACCH Multiframe

SACCH Multiframe cosists of 4 TCH Multiframes and the duration is 480msec.

SACCH Multframe carries Measurement Reports in the Uplink and System Information Messages in the Downlink.

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The Um interface is the interface between the MS and the BTS. For signalling on the Um Interface three

layers are required;

In each layer, there are entities responsible for specific signaling procedures. Peer entities are entities

belonging to the same layer in different nodes. These peer entities communicate with each other by

sending and receiving messages according to a specific protocol.

Each layer communicates with its peer layer in the other node. Each layer utilizes the services provided by

the layer below and offers services to the layer above.

Introduction to Signaling on the Um Interface

165

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Hierarchical Structure of Um Interface

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Layer 3 Signaling

Layer 3 provides the Mobile Network Signaling (MNS) service to the user application. Layer 3 signaling is used for;

Functions to establish, maintain and terminate circuit switched connections across a GSM PLMN and other networksto which the PLMN is connected.

Supporting functions for supplementary services and short

message service control.

Functions for mobility management and radio resource

management.

Layer 3 consists of three sublayers;

Connection Management (CM)

Mobility Management (MM)

Radio Resource management (RR)

In the MS, entities from all three sublayers are present.

On the network side there is a distribution of the signaling functions

between different network equipment. The RR functions residemainly in the BSC, although some RR functions may reside in

the MSC. In the BTS, most of the RR messages are handled as

transparent messages. However, some of them must be

interpreted by the BTS.

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MOBILE STATION ISDN NUMBER (MSISDN)

These are the digits dialed when calling a mobile subscriber.

INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)

The IMSI is a unique identity allocated to each subscriber tovallow correct identification over the radio path

and through the network and is used for all signaling in the PLMN. All network related subscriber information is

connected to the IMSI. The IMSI is stored in the SIM, as well as in the HLR and in the serving VLR. The IMSI

consists of three different parts:

IMSI = MCC + MNC + MSIN

where

MCC = Mobile Country Code

MNC = Mobile Network Code

MSIN = Mobile Subscriber Identification Number

According to the GSM specifications, IMSI has a maximum length of 15 digits.

Overview of various Identifiers in GSM Network

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TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)

The TMSI is a temporary number used instead of IMSI to identify an MS. The TMSI is used for the subscribers

confidentiality on the air interface. The TMSI has only local significance (that is, within the MSC/VLR area) and

is changed at certain events or time intervals. The TMSI structure can be chosen by each operator but should

not consist of more than four octets.

INTERNATIONAL MOBILE EQUIPMENT IDENTITY (IMEI)

The IMEI is used for equipment identification and uniquely identifies a MS as a piece or assembly of

equipment. IMEI has the total length of 15 digits. It consists of the following:

IMEI = TAC + FAC + SNR + spare

where

TAC = Type Approval Code, determined by a central GSM body.

FAC = Final Assembly Code, identifies the manufacturer.

SNR = Serial Number, an individual serial number of six digits uniquely identifies all equipment within each TAC

and FAC.

Spare = A spare bit for future use.


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LOCATION AREA IDENTITY (LAI)

The LAI is used for paging, to indicate to the MSC in which Location Area (LA) the MS is currently situated and

also for location updating of mobile subscribers. The LAI contains the following:

LAI = MCC + MNC + LAC

where

MCC = Mobile Country Code, same as IMSI MCC

MNC = Mobile Network Code, same as IMSI MNC

LAC = Location Area Code, the length of LAC is 16 bits, enabling 65,536 different

location areas to be defined in one PLMN.

CELL GLOBAL IDENTITY (CGI)

The CGI is used for cell identification within a location area. This is done by adding a Cell Identity (CI) to thecomponents of a LAI. CI has a length of 16 bits. CGI consists of:

CGI = MCC + MNC + LAC + CI


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BASE STATION IDENTITY CODE (BSIC)

BSIC allows a mobile station to distinguish between different neighboring base stations. BSIC consists of:

BSIC = NCC + BCC

Where

NCC = Network Color Code (3 bits), identifies the PLMN. Note that it does not uniquely identify the operator.

NCC is primarily used to distinguish between operators on each side of a border.

BCC = Base Station Color Code (3 bits), identifies the Base Station to help distinguish between BTS using the

same BCCH frequencies.

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