fundamentals: signalling in gsm the osi reference...

Fundamentals: Signalling at the Air-Interface

Rohde & Schwarz Training Center V 1.0

Fundamentals: Signalling in GSM

Signalling is the language used for communication between machines or computers.The language understood by the machines and used for information transfer isdescribed by protocols. The chapter below gives a short overview of the signallingprocedures used in GSM. However, it only describes signalling at the air interfaceand not the whole of the signalling in the backbone network. Tackling the theory first,a description of the OSI reference model will make signalling easier to understand.

The OSI Reference Model

A system as complex as GSM requires a lot of planning and organization both at thedefinition and implementation phase. A structure for a generic data communicationnetwork has been developed by the International Standards Organization ISO and isreferred to as the open system interconnection (OSI) model.The OSI reference model provides for a number of layers, each layer communicatingexclusively, and according to well-defined rules, with the layers immediately aboveand below it. Communication is, therefore bi-directional except for the lowest, orphysical layer, where information passes only to the layer above.Tasks can, therefore, be assigned to specific layers and transactions modularized.

General "rules" for the OSI model:The layers operate independently of each other and this also applies to any twoconsecutive layers in the hierarchy. Each layer can be thought of as providing aservice to the layer immediately above and receiving a service from the layerimmediately below.Each layer directly communicates only with the layer immediately above byexchanging primitives. The latter are instructions to the protocol layer in question andhelp to transfer information.Each layer also communicates with its corresponding layer at the remote end. This iscalled peer-to-peer communication.Layers 4 to 7 are required only in the terminal equipment. In our example, where acall is made by the mobile, layers 4 to 7 are incorporated by the equipment involved(session and transport layer), the presentation of language (presentation layer) andthe call content (application layer).The functionality of layers 1 to 3 is available between two network nodes, e.g.between the mobile station and the network.

The GSM specifications follow the stipulations for the bottom three layers of the OSImodel.



Figure: OSI reference model as applied to GSM

In the lowest layer (Layer 1), the physical characteristics of the transmission mediumare specified. In the context of GSM radio links, this definition not only includes theRF carrier frequency and GMSK but also correct timing of burst transmissionsnecessary because of the use of TDMA, time-division multiple access. This layer alsoincorporates methods for correct bit transmission. It adds redundant bits for errorcorrection through convolutional coding and spreads data transmission byinterleaving.The second GSM layer (Layer 2), referred to as the "data link layer", consists of anintelligent entity responsible for the secure transmission of data messages betweenthe mobile and the base station. To do this, the transmit side structures the datamessages from the higher layer to match the physical constraints of the Layer 1medium. It requests an acknowledgement of the sent data from the receiver so thatany packet that was not received can be sent again. At the receive side of Layer 2,messages are reconstructed from the received frames and the acknowledgement isformulated. The third GSM layer (Layer 3), also referred to as the "network layer", isresponsible for the management of an established connection and of the associatedactivities in the radio network. In GSM, these tasks are further subdivided into thefollowing sublayers:

- Call control management CC- Mobility management MM- Radio resource management RR

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical LayerModulation

Channel CodingForward Error Correction

Segmentation / Concatenation

Acknowledgement

Call ControlMobility Management

Radio Ressource Management

OSI-Modell Realisation in GSM

Tasks of theuser

Taks of the fixed network

Tasks of the GSM-network

1

7

6

5

4

3

2



Implementation of OSI Layers 1 to 3 at theGSM Air Interface:

Fig. 8: Block diagram of GSM mobile station

As can be seen from the greatly simplified functional block diagram of the GSMtransmitter and receiver, this segregation of functions not only provides a useful basisfor apportioning the design effort, but also ensures that the measurement interfaceswithin the system are clearly defined.

T r a n s m i t t e r R e c e iv e r

R F -M o d u la t io n

R F -D e m o d u la t io n

Equalisation

Error detectionDeinterleaving

Channel codingerror correctionInterleaving

Frame buildingAcknowledge request

Frame concatenationAcknowledgement

Signalling Signalling

User data User data

Speech Speech

O S I L a y e r 3n e tw o r k

O S I L a y e r 2D a t a - T r a n s f e r

O S I L a y e r 1P h y s i c a l L a y e r



Implementation of Layer 1 at the GSM Air Interface:

As has been said already, Layer 1 of the OSI reference model in GSM is the physicallayer. A few aspects have already been described in the section dealing with the airinterface: the burst types required for data transmission, the physical and logicalchannels used and GMSK (Gaussian minimum shift keying). A few other featuresrequired to support data transmission are described in the following.

Channel Coding in GSM [GSM05.03]

As the propagation conditions in mobile radio channels are highly variable, bit errorrates (BER) as high as 10-3 to 10-1 can occur. To obtain high-quality and highlycompressed speech and data communication, the BER has to be reduced to a rate of10-5 to 10-6 through the use of appropriate error correction methods. This is thepurpose of GSM channel coding. GSM channel coding is a combination of severalmethods each appropriately modified for use in the various logical channels.This section gives an overview of these methods and of derived components, theBCCH being used as an example to illustrate the special forms of application.

Components of GSM Channel Coding

GSM channel coding comprises block coding for error detection, convolutional codingfor error correction and interleaving to eliminate error bursts. Channel coding isperformed in this order at the data source and in the reverse order at the data sink.The Fig. below gives an overview of channel coding and its components. The terms"outer" and "inner" error control indicate the position of the components in thetransmission chain. The following sections describe each of the components andoutline their hardware implementations.

Fig.: GSM channel coding components

Blockcoding

Con-volutionalcoding

Interlea-ving

Deinter-leaving

Con-volutionaldecoding

Paritycheck

Inner error control

Outer error control



Block Coding

Block coding involves calculating a certain number of parity bits from a data-bit blockand then appending them to the data block. At the data sink, i.e. the receive side,errors in the received code word can be detected with these parity bits.GSM uses two types of block code - CRC, (cyclic redundancy check) and a FIREcode (short, cyclic, binary code). The cyclic codes of this kind are also called (n,k)codes, n representing the number of code symbols (bits) and k the number of datasymbols (bits). The number of check bits is therefore n-k.These check bits, and so the code word, are produced by a generating polynomial. Apolynomial is used to represent words, the power of each term in the polynomialcorresponding to a bit position and the coefficient of each term to a bit (Dm). A dataword with k bits is, therefore, represented as:

011

1 ... DxDxDxD kk

kk +⋅++⋅+⋅

−

−

.

To calculate the check bits, the data word D(x) is multiplied by xn-k and then dividedby the generating polynomial G(x) which is of degree (n-k). The remainder R(x) is thecheck word comprising the check bits:

⋅=

−

)()(Re)(

xGxDxstxR

kn

.

The code word (C(x) is now obtained by appending the check word to the data word.

)()()( xRxDxxC kn+⋅=

−

If there are no transmission errors, the code word C(x) is divisible by G(x). Theprobability is, therefore, high that errors will be detected.The division (and multiplication) of polynomials is easiest to implement with afeedback shift register. The type of feedback is determined by the generatingpolynomial G(x), the arithmetical operation (multiplication/division) by the feedbackdirection. A combination of multiplication and division by a shift register is alsopossible. A shift register of this type is shown in the Fig. below. The significantfeature is that the data word D(x) is first multiplied by xn-k and then divided by thegenerating polynomial G(x) of degree n-k.

Input Output

Fig.: Coder performing automatic multiplication by xn-k



When the shift register performs division, the remainder left by division remains in theregister after the dividend has been completely shifted through. If this remainder, i.e.the check word, is appended to the shifted data word and the code word obtained inthis way is shifted through an identical shift register, only 0s will remain in the registerbecause in the case of an error-free transmission a code word without errors must bedivisible without a remainder. The check word is appended by the shift registershown in the Fig. above. The data word is not only shifted into the register but alsoapplied to the output. The gate is blocked after k clocks and the register content isshifted to the output.The block codes could also be used for correcting the detected errors but thiscapability is not used by GSM.

Forward Error Correction, FEC, or Convolutional Coding:

With FEC (forward error correction), redundant bits are inserted into data packets(bursts) at the transmit end to ensure that most of the information is not lost if there isa burst error. A convolutional coder is used. A convolutional coder "remembers" thelast n bits sent and adds each input bit to the stored n bits. The words obtained at theoutput are usually longer than 1 bit. Error correction is based on the fact that aprevious state, i.e. a word, or a bit sequence, can only assume one of two new statesdepending on whether 0 or 1 has been entered. If a word arrives at the receiver in astate that cannot be reached from a state obtainable from one of the two inputcombinations, a transmission error has occurred and needs to be corrected. Thisprocedure is equivalent to tracing a path through a trellis diagram which is familiarfrom coding theory.

Convolutional coding in GSM supports error correction by generating the necessaryredundancy. The amount of redundancy is determined by the ratio of the data blocklength to the code block length, i.e. the coding rate r, of the convolutional coder.Like cyclic block codes, convolutional codes, too, can be represented by polynomialsand realized by means of shift registers. The shift register has k memory locationswhich are read as defined by ν generating polynomials. The Fig. below shows a shiftregister with the constraint length K=5 used in GSM and the ν=2 generatingpolynomials. GSM defines seven different generating polynomials G0 to G6 forconvolutional coding (DSM05.03, Ann. B). In the Fig. below, the followingpolynomials are used:

43

43

1110

DDDGDDG+++=

++=

Fig.: Block diagram of a convolutional coder



After the data bits have been clocked into the register, the outputs of the flip-flopscorresponding to each of the generating polynomials are added mod 2. Each data bitis, therefore, XORed with (K-1) preceding bits; the convolutional coder is also said tohave a "memory" of (K-1) bits. Each data bit is, therefore mapped onto ν=2 code bitswith each clock cycle. The convolutional coder has a rate of r = 1/ν = 1/2 in this case.While K=5 is selected for all channels in GSM, the rate r varies depending on thenumber of generating polynomials used (GSM05.03, Sec. 3 and 4). The Figure abovealso shows why the term "convolution" is used to describe this code – something thatis hardly ever made clear. Each generating polynomial and the data word aremultiplied together each time the data word is shifted and the results are added. Thisis the mathematical operation of "convolution".Error correction is performed during decoding, generally using the Viterbi algorithm.The ability to correct errors increases with increasing K and decreasing r, but a smallr, i.e. high redundancy, reduces transmission rates.

Interleaving

Interleaving means that the information to be transmitted is spread or distributed overseveral bursts in such a way that contiguous information is split up and transmitted inseveral bursts. The original bursts can be regenerated with the aid of FEC even ifone of the bursts is lost. For instance, the information in 4 bursts is divided into 2 x 4blocks, one block containing the even bits and the other the odd bits. Two evenblocks or two odd block are then always combined to form a burst.

The result obtained when the convolutional code is decoded strongly depends on thefrequency of occurrence and the distribution of bit errors. Extended periods of fadingin the mobile channel cause successive bit errors, referred to as "error bursts". Theseerror bursts have a particularly adverse effect on decoding.To avoid error bursts, an attempt is made to spread the bit errors over several codewords. This is achieved by interleaving several code words. This method is alsocalled diagonal interleaving (see Fig. below). Another kind of interleaving is blockinterleaving. Blocks of code words are written row-by-row into a matrix and then readcolumn-by-column as shown in the Fig. below. With both methods, consecutive bitsof a code word are never transmitted consecutively, and conversely, when the bitsare de-interleaved at the receive end, error bursts are spread over several codewords.As several code words are interleaved, the decoder has to "wait" a certain time untilall bits of a particular code word arrive. This delay, i.e. the "measure for spreadingover time" is referred to as the "interleaving depth". The greater the interleavingdepth, the more code words are available for spreading the error bursts and thegreater the probability that errored bits can be corrected. However, the greater theinterleaving depth, the greater the transmission delay.The different types of interleaving and interleaving rules used for the variouschannels are specified by GSM05.03, sec 3 and 4).



Spreading

Interleaving

Fig.: Diagonal interleaving

Write

Read

Fig.: Block interleaving


Rohde & Schwarz Training Center V 1.0Fig.: Error correction coding at Layer 1 of the GSM air interface

Speech frame

260 Bits

Signalling

184Bits

Class 1 cyclic code267 Bits

FIRE code229 Bits

Convolutional Coding

456 Bits

Reorganization, SegregationInsertion of Stealing Flags

456 Bits in 8 subblocks

Block diagonalinterleave

Block rectangularinterleave

Intra-Burstinterleave

Ciphering

To Modulator



Implementation of Layer 2 at the GSM air interface:

The GSM Layer 2 is the data transmission layer, i.e. it is responsible for a reliabletransmission of the data packets of Layer 3. To perform this task, it uses the physicaltransmission services of Layer 1.The LAPDm protocol, the link access protocol for channel D, is used in this case.This protocol used for the data transmission layer is familiar from ISDN, but is used ina slightly modified form at the GSM air interface. The "m" in the protocol designationstands for "modified".The reason for this modification is the low data transmission rate at the air interface.Signaling blocks have, therefore, been omitted whenever possible. In contrast to theLAPD protocol used for ISDN, the LAPDm protocol does not contain start and endflags for synchronization or a terminal end point identifier for the terminal equipmentin question. Since point-to-point is used at the air interface, terminal identification isnot required. The LAPDm protocol does not require an FCS (frame check sequence),used for identifying transmission errors, either. In GSM, error control is performed bythe physical layer.

LAPDm Frame FormatsDifferent frames are used by the LAPDm protocol for transmitting different kinds ofdata, either data associated with higher layers or Layer 2 signalling data. Three frameformats are defined. A length of 23 octets is specified for the frames, i.e. each frameconsists of 23*8 = 184 bits. We can now see how this relates to Layer 1, where asequence of max. 184 bits is specified for signalling before channel coding.

The general frame format is as follows:

Fig.: General frame format of the LAPDm protocol

The individual fields can be further subdivided. The following structures are obtained.

Adress Field

Control Field

Frame Length

Information Field

1octet

1octet

1octet

N201octets



Address Field:

Structure of address field:

EA bit:

This bit marks the end or the beginning of the address field. If the bit is 0, anotheraddress octet will follow. If the bit is 1, this is the last octet of the address field.Remember that an LAPDm address field is always 1 octet long. This means that thevalue of this bit at the GSM air interface is always 1.

C/R bit, command/response bit:

This bit indicates whether the present frame is a command or a response and theinitiator of the command. The following values indicate the direction:

Link protocol discriminator LPD:

The GSM link protocol discriminator always has the binary code 00 with a singleexception: messages of the cell broadcast channel CBCH are assigned the LPDcode 01.

LPD SAPI C/R EA1

Bit 8 7 6 5 4 3 2 1

Command: C/R = 0

Response: C/R = 0

Command: C/R = 1

Response: C/R = 1



Service access point identifier SAPI:

The service access point is the interface between two protocol layers. It defines the"handover point" where data to be transmitted are received, or received data handedon to Layer 3.In GSM, the following two decimal values are currently defined for SAPI at the airinterface.

Decimal value of SAPI Explanation0 Radio resource RR

Mobility management MMCall control CC

3 Short message service SMSSupplementary services SS

Control Field:

The Layer 2 control field specifies the frame type and contains the transmit andreceive sequence number for I-frames. In the LAPDm protocol, the length of thecontrol field is always limited to one octet. No general structure can be shown for thecontrol field as the structure is different for different frame types. The frame types andcontrol field structures defined by GSM are described in detail in the sequel.

Frame Length Field

The frame length specifies the total length of the information field. Values between 0and N201 are permissible. Structure of frame length field:

EL bit:

This bit indicates whether this octet is the last one in the frame length field or not.EL = 1 indicates the end of the frame length field. With GSM, a frame for the airinterface is always 1 octet long, i.e. this bit is always set to 1. There is an option forlonger frame formats in the future.

M bit:If a message is longer than one frame, the M bit indicates that another informationframe will follow. In this case M = 1. If M is 0, the frame is the last frame in themessage.

Length M EL

Bit 8 7 6 5 4 3 2 1



Frame Formats for Layer 2 of the GSM Air Interface:The general frame format is sub-classified using 3 further frame types: Bbis format,B format and A format. What distinguishes the frames are the logic channels they areused for and the type of information they transmit. The following frames are defined:

A Frame:

The A frame can be used on all DCCHs (dedicated control channels) on the uplinkand the downlink. An A frame is sent when it is not necessary to send signallinginformation after a connection has been established. This means that the A frame isa filler frame used as pseudo signalling. The structure of the A frame, particularly thatof the control field, is identical to that of the B frame, but the information field inframe A contains only filler bits instead of Layer 3 signalling information. This frameformat is sent, say, directly after channel setup, provided it is not necessary to sendLayer 3 signalling information.

B Frame:

The B frame is used for "real" signalling. Its information field contains the Layer 3information. The B frame is used in all signalling channels, ACCHs (associatedcontrol channels) and DCCHs (dedicated control channels). The constant N201defines the length of the information field, which is different for the different channeltypes although the whole frame is still 23 octets long.

Structure of the A Frame and B Frame Control Field for Different Frame Types:

As mentioned already, the control field distinguishes between the frame types usedfor the different applications. As shown in the Fig. below, several frame types areused.

P, F and P/F bit, polling and final bit:

This bit is contained in all control frames. It is called a polling bit in command framesand a final bit in response frames. In frames which may be used for a command or aresponse, either the polling bit or the final bit can be used. This bit indicates that thetransmit side expects a response from the receive side although this is notcompulsory for the frame type.P bit = 1 means that the transmitter of the command frame is waiting for a responsefrom the receiver. The response must be given as F bit = 1.



Fig.: Structure of A and B frame control field, different frame types

Information Frame, I-frame:The information frame is used to transmit Layer 3 signalling data. It is the transportframe for this kind of information. Each control field contains a 3-bit counter for thesent frames N(S) and the received frames (N(R). The N(S) counter indicates thenumber of frames marked by the transmitter as sent and the N(R) counter thenumber of frames the receiver indicates as received, i.e. this counter practically actsas a confirmation field for correct information transmission. Note that, due to the 3-bitcoding of the N(R) count, not more than 8 I-frames can be sent without aconfirmation.

Bit 7 6 5 4 3 2 12

0

N(R) N(S) 0P

Information-Frame

I-Frame

N(R) 0 1P/F 00

Supervisory-Frames

RR-Frame

N(R) 0 1P/F 01

N(R) 1 1P/F 00

RNR-Frame

REJ-Frame

0 1 1P 111 1

Unnumbered-Frames

0 1 1F 110 0

0 0 1P 100 0

0 0 1P 101 0

0 0 1F 101 1

SABM-Frame

DM-Frame

UI-Frame

DISC-Frame

UA-Frame



Supervisory Frames:

The supervisory frames group contains three different types of frame: RR, RNR andREJ. These frames are used for Layer 2 peer-to-peer signalling.

Receive ready frame RR:RR frames are used to acknowledge I-frames. They contain the N(R) field, i.e. thereceiver tells the transmitter the number of the last frame which has been confirmedas received. If the transmitter has sent more frames than the receiver confirms, thetransmitter knows that all the frames after this number are not confirmed andtherefore have to be sent again. Polling is also performed with the RR frame. Thismeans that each of two communicating parties checks at regular intervals whetherthe other can still be reached (i.e. mutual polling).

Receive not ready frame RNR:With this message, the receiver indicates that it cannot receive more I-frames atpresent. This means that the transmitter has to stop sending I-frames until a higherlayer signals a call clear-down or until data transmission can be continued.

Reject frame REJ:Unlike RNR which signals an overload, this message tells the transmit end that thereceived I-frame was faulty and has been rejected. The counter N(R) again indicatesthe number of the frame from which data must be retransmitted.

Unnumbered Frames:

The unnumbered frames are another group of signalling frames used peer-to-peer atLayer 2. From the structure of the control field we see that the field does not containa sequence number, i.e. that it does not confirm frames, and so is referred to as anunnumbered frame. The following frame types are available at the GSM air interface.

Set asynchronous balanced mode frame SABM:This frame initiates a Layer 2 connection, i.e. one end requests the other to changeto the "balanced mode" where both ends can exchange information on a peer-to-peerbasis. This frame can be thought of as setting up a peer-to-peer connection and isactive until a Layer 2 connection has been made.

Disconnected mode frame DM:This frame indicates that the Layer 2 link cannot be maintained. The transmit endthen indicates that it will immediately terminate the connection and not wait for anyconfirmation from the receive end. The DM frame is one way of terminating a Layer 2connection.

Unnumbered information frame UI:This is another frame for transmitting Layer 3 information. However, unlike thenormally used I-frame, this frame does not contain a transmit and receive sequencenumber.



The UI frame is, therefore, used when Layer 3 data are not referenced andtransmitted in the non-acknowledge mode. The receive end need not send an RRmessage to confirm reception of an UI frame.

Disconnect frame DISC:This frame too terminates a Layer 2 connection. The transmit end uses the DISCframe to inform the receive end that it intends to terminate the Layer 2 connection.The transmitter however waits until it receives a UA frame as acknowledgement.

Unnumbered acknowledgement frame UA:The UA frame is the receive end’s response to an SABM or DISC frame. It is used toconfirm the successful setup of a Layer 2 connection and also its termination.

Bbis Frame:

Unlike the other two frames, the Bbis format does not use a header. The reason forthis is that it is only used on the broadcast control channel BCCH and the commoncontrol channels AGCH (access grant channel) and PCH (paging channel) and that itis for all intents and purposes generally valid. This makes detailed addressing of aspecific receiver with a protocol header superfluous. CCCHs (common controlchannels) use the point-to-multipoint mode for information transmission. This,therefore, simplifies the general structure of a Bbis frame:

Fig.: Structure of Bbis frame

Information Field

(Layer 3 Messages)

N201 octets

(here N201 = 23)



Implementing Layer 3 at the GSM Air Interface:

As described already, Layer 3 data are embedded in the protocol information ofLayer 2. The general structure of a layer 3 message comprises three fields: typeidentifier, message type and data field (see Figure below).

Fig.: Structure of a Layer 3 message

Type Identifier:The 8-bit type identifier contains the protocol discriminator which divides theinformation into different groups. Structure of the type identifier:

Fig.: Structure of type identifier

Protocol Discriminator PD:The 4-bit protocol discriminator divides the Layer 3 messages into different groupspermitting different users to be addressed within this layer. Each message isassigned to one and only one of the groups. With the aid of the PD the followingmessage groups are distinguished:

Protocol discriminator User group06 Radio resource management

(RR)05 Mobility management (MM)03 Call control (CC)

Supplementary services (SS)Short message service (SMS)

Layer 2 Header

Typ Identifier

Message Type

Data Field

1octet

1octet

Protocol DiscriminatorTI-Value

Protocol DiscriminatorSkip Indicator „0000“

TI-FlagCC-Messages

MM / RR-Messages

Typ Identifier



The three most important groups in Layer 3 are the RR, MM and CC functions:

Radio Resource Management RR:The radio resource management is responsible for the management and organizationof radio connections at the air interface. This includes, for instance, the organizationof physical and logical channels at the air interface and functions for setting up andmaintaining a radio link. In contrast to the two other message groups, RRmanagement in the network is mainly performed by the base station subsystem BSS,i.e. by the base station (BTS) and the base station controller (BSC). Some messagesare, however, evaluated by the MSC (mobile switching center). The message typedecides which messages these are. It then provides a "tunnel" through the BSS tothe MSC.

Mobility Management MM:The main task of mobility management is to obtain and store information about thelocation of subscribers in the network. Examples are the identification and evaluationof the cell identity at the mobile end or the storage of the location area in the VLR(visitor location register).The MM uses the channels provided by the radio resource for transparent datatransmission between the mobile station and the MSC. An example is the locationupdate. First, a radio link is established, then data are sent from the MS to the MSC.The data are acknowledged and the radio link is terminated. This illustrates thehierarchy within the 3 message groups: MM is an application layer for the radiosource management.

Call Control CC:With the aid of the call control function, connections are set up and maintained. Thisnot only applies to the air interface; a connection is managed through to the ISDNterminal at the subscriber end. Call control also uses the services of the RR radioresource management.

Skip Indicator:The protocol discriminator of an MM or RR message is followed by the 4-bit skipindicator. This indicator has no specific function in GSM at present and ispermanently set to 0000. Any other coding should be ignored at the receiver end.

Transaction Identifier TI:If the Layer 3 message is from the call control management group, the skip indicatoris replaced by the transaction identifier TI. It consists of the TI flag and the TI value.Before an explanation is given, an example will illustrate why the TI is needed: Withcall control, it is possible for a user to set up two or more transactions. Let us assumea mobile originated call (MoC) is in progress. The user goes to hold and calls anothersubscriber. The CC function now has to manage a second connection. Thetransaction identifier makes it possible to distinguish between several simultaneoustransactions.The TI flag differentiates between the side initiating the call (TI flag = 0) and the sidethat is responding to the transaction (TI flag = 1). In the case of a mobile originatedcall, the TI flag of the call control message is, therefore, always set to 0. The TI flagfor the response messages from the NSS (network subsystem) is always set to 1.The call-initiating end also assigns a TI value between 0 and 6.



Message Type:

The message type indicates what the message is for, i.e. a 1-octet designatorcontains the message name and the command. GSM specification 04.08 defines allavailable message types, briefly describes what they are for and specifies theparameters they contain. The further structure of the information or data fielddepends on the message type. Each message is normally assigned a compulsorydata field consisting of mandatory information elements IEs and another data fieldcontaining optional information elements. Both fields can have a fixed or variablelength, depending on the message type. If the parameters following the IEI(information element identifier) field do not have a fixed length, the actual length ofthe parameter field is indicated in a length field.

fundamentals: signalling in gsm the osi reference...

Documents