FACCH Call Setup

FACCH Call Set-up

dn9814074Issue 9-0 en

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2003353Nokia BSC/TCSM S11.5 ProductDocumentation

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FACCH Call Set-up

Contents

Contents 3

List of tables 4

List of figures 5

Summary of changes 7

1 Overview of FACCH Call Set-up 9

2 Procedures of FACCH Call Set-up 112.1 Channel required procedure 112.2 Channel reservation procedure 142.3 Channel activation procedure 152.4 Immediate assignment procedure 172.5 Mode modify procedure 19

3 User interface of FACCH Call Set-up 25



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Contents

List of tables



FACCH Call Set-up

List of figures

Figure 1. Channel required procedure 11

Figure 2. CHANNEL REQUEST message, only one octet long 12

Figure 3. NECI parameter is set to OFF 12

Figure 4. NECI parameter is set to ON 13

Figure 5. Channel activation procedure 15

Figure 6. Immediate assignment procedure 17

Figure 7. Mode modify procedure 20



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List of figures



FACCH Call Set-up

Summary of changes

Summary of changes

Changes between document issues are cumulative. Therefore, the latest documentissue contains all changes made to previous issues.

Changes made between issues 9 and 8

A note has been added to section Channel Request Message about HR/FR data.

Specification references have been updated.

Multirate configuration information has been omitted from the document.

The document has been revised throughout to comply with the latestdocumentation standards.


The document has been revised throughout to comply with the latestdocumentation standards. No changes have been made in the document due tonew functionality.


No major changes, only a note about AMR and FACCH Call Set-up added inchapter Introduction to FACCH Call Set-up.



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Summary of changes



FACCH Call Set-up

1 Overview of FACCH Call Set-up

FACCH Call Set-up is an optional functionality.

When an MS requests a dedicated resource by sending a random access, the BSCtries to allocate an SDCCH for the MS. If there are no SDCCHs available at thatmoment, the BSC tries to allocate a TCH instead of rejecting the resourcerequest. If there are free TCHs, FACCH Call Set-up is used to assign the MS fromthe CCCH to the TCH - instead of the SDCCH - with the immediate assignmentprocedure. The FACCH is used for call set-up signalling instead of the SDCCH.The TCH is used for signalling and thus no assignment procedure is needed in theradio interface. Instead, only the mode modify procedure is required to changethe channel mode from signalling to, for example, speech or data.

Note

The BTS must support AMR, otherwise AMR with FACCH Call Set-up does notwork.

Related topics

. Procedures of FACCH Call Set-up

. User interface of FACCH Call Set-up



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Overview of FACCH Call Set-up



FACCH Call Set-up

2 Procedures of FACCH Call Set-up

The procedures of FACCH Call Set-up are described in more detail in thefollowing sections. For an overview, see Overview of FACCH Call Set-up.

Note

The causes presented in this document are BSSMAP causes. For moreinformation on the corresponding DX causes, see Call Related DX Causes inBSC.

2.1 Channel required procedure

The channel required procedure offers the MS the means to start theestablishment of a mobile originating call or to answer the PAGING REQUESTmessage.

The channel required procedure is presented in the following figure.

Figure 1. Channel required procedure

MS BTS BSC MSC

a)CHANNEL REQUEST

b)CHANNEL REQUIRED



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Procedures of FACCH Call Set-up

CHANNEL REQUEST message

The MS requests for a dedicated signalling channel SDCCH by sending theCHANNEL REQUEST message (a) to the BTS on the RACH. The message issent in random mode and consists of two different information elements that arecoded as follows:

Figure 2. CHANNEL REQUEST message, only one octet long

The establishment cause information field indicates the reason for requesting theestablishment of a connection. This field has a variable length (from 3 bits up to 6bits). The random reference is an unformatted field with variable length (from 5bits down to 2 bits). The establishment cause information field length is 3 whenthe MS is GSM phase 1 level or when the MS is GSM phase 2 level and the newestablishment causes support (NECI) parameter is set to OFF.

The following establishment causes are supported: (the random reference field isfilled with 'x').

MESSAGE MEANING OF ESTABLISHMENT CAUSE

8 .... 1

000xxxxx Other procedures (SDCCH).

100xxxxx Answer to paging. (SDCCH or TCH/F)

101xxxxx Emergency call. (SDCCH or TCH/F)

110xxxxx Call re-establishment. (SDCCH or TCH/F)

111xxxxx Originating call. (SDCCH or TCH/F)

Figure 3. NECI parameter is set to OFF

MESSAGE MEANING OF ESTABLISHMENT CAUSE

8 .... 1

0000xxxx Location updating (SDCCH needed).

0001xxxx Other procedures which can be completed

with an SDCCH.

Bit

Octet 1

8 7 6 5 4 3 2 1

ESTABLISHMENTCAUSE

RANDOMREFERENCE



FACCH Call Set-up

100xxxxx Answer to paging. (SDCCH or TCH/F)

101xxxxx Emergency call. (SDCCH or TCH/F)

110xxxxx Call re-establishment (SDCCH or TCH/F)

111xxxxx Originating call (SDCCH or TCH/F)

0010xxxx Answer to paging. Dual rate MS and TCH/F

is requested. (TCH/F or SDCCH)

0011xxxx Answer to paging. Dual rate MS and TCH/H

or TCH/F is requested. (TCH/H, TCH/F or SDCCH)

0100xxxx Originating speech call from dual rate MS

when TCH/H is sufficient. (TCH/H, TCH/F or SDCCH)

0101xxxx Originating data call from dual rate MS

when TCH/H is sufficient. (TCH/H, TCH/F or SDCCH)

011010xx Call re-establishment; TCH/H was in use

and the network sets NECI bit to 1.

(TCH/H, TCH /F or SDCCH)

Figure 4. NECI parameter is set to ON

Note

HR data call is not supported in the BSC.

FR is the preferred rate for data call request and re-establishment call request. If acall request based on the establishment cause can be either data or speech, counterTCH_FAST_CALL_CH_RATE_NOT_DET is incremented and FR is set as thepreferred rate.

The network uses the random reference and the MS access slot number to identifyand to address the MS.

The MS starts the GSM timer T3120 and waits for its access acceptance, which isthe IMMEDIATE ASSIGNMENT message on the AGCH.



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If the MS is not granted access within the time limit defined by the GSM timerT3120, the MS makes a second attempt and sends another CHANNELREQUEST message with a new random reference on the next time slot. A newvalue of the GSM timer T3120 is computed and used. The MS repeats thisprocedure either until it is granted access or until the number of repetitionsreaches the maximum.

CHANNEL REQUIRED message

After receiving the CHANNEL REQUIRED message (b) from the BTS, the BSCstarts to allocate a relevant channel.

The CHANNEL REQUIRED message includes the following data:

. message discriminator: common channel management

. message type: channel required

. channel number: uplink CCCH (RACH)

. request reference: establishment cause, random reference received in thechannel request and the frame number on which it was received

. access delay: delay of the access burst as measured by the BTS.

2.2 Channel reservation procedure

After receiving the CHANNEL REQUIRED message, the BSC begins to searchfor an SDCCH channel. If there are no SDCCHs available, the BSC starts tosearch for a TCH channel. If there is a TCH channel available in the cell fromwhich the original access was granted, the BSC reserves it and the immediateassignment procedure starts.

If all the SDCCH and TCH channels are reserved in the cell in question, the BSCsends an IMMEDIATE ASSIGNMENT REJECT message to the MS.

The BSC can allocate TCH or SDCCH channels as follows:

. TCH/F or TCH/H channel due to the CHANNEL REQUIRED message, ifthere are no SDCCHs available and FACCH Call Set-up is allowed

. SDCCH channel due to the CHANNEL REQUIRED message, if there areSDCCHs available and FACCH Call Set-up is allowed

. SDCCH channel due to the CHANNEL REQUIRED message, if FACCHCall Set-up is not allowed.



FACCH Call Set-up

The number of TCH and SDCCH channels depends on the radio configuration.

2.3 Channel activation procedure

After a new TCH has been successfully reserved, the BSC activates the channelby sending the CHANNEL ACTIVATION message to the BTS. The GSM timerT9103 supervises the channel activation procedure. The channel mode for theTCH channel is signalling.

The channel activation procedure is presented in the following figure.

Figure 5. Channel activation procedure

CHANNEL ACTIVATION message

The BSC sends the CHANNEL ACTIVATION message (a) to the BTS in orderto activate the new TCH channel.

The CHANNEL ACTIVATION message includes the following data:

. message discriminator: dedicated channel management

. message type: channel activation

. channel number

. activation type

. channel mode: DTX control and channel type (signalling, TCH/F or TCH/H, no resources required)

. BS power: maximum BS power level authorised in the cell (optional)

. MS power: maximum MS power level authorised in the cell (optional)

MS BTS BSC MSC

a)CHANNEL ACTIVATION

b)CHANNEL ACTIVATION ACK



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. timing advance to be used by the MS in subsequent communications(optional)

. SACCH information (optional)

SACCH information is used in dual band cell. For more details, see Dual BandNetwork Operation. Other optional elements are always present.

CHANNEL ACTIVATION ACKNOWLEDGE message

The BTS acknowledges a successful activation with the CHANNELACTIVATION ACKNOWLEDGE message (b). The BTS starts the transmissionand reception on the associated SACCH, using the power levels and the timingadvance received in the CHANNEL ACTIVATION message.

Correspondingly, the BTS acknowledges a failure with the CHANNELACTIVATION NACK message. The possible causes for a failure are listedbelow. For more information, see 3GPP TS 48.058.

. radio resource not available

. radio channel already activated or allocated

. protocol error and subclause, such as mandatory information error

. O&M intervention when the channel cannot be used for O&M reasons

. encryption algorithm not implemented

. equipment failure

. service or option not available

. not implemented unspecified

. mandatory IE error

. general IE error.

If the BSC does not receive the CHANNEL ACTIVATION ACKNOWLEDGEor CHANNEL ACTIVATION NACK message within the time limit of the GSMtimer T9103, or if it receives the CHANNEL ACTIVATION NACK message, itreleases the allocated channel and sends an IMMEDIATE ASSIGNMENTREJECT message to the MS.



FACCH Call Set-up

2.4 Immediate assignment procedure

The IMMEDIATE ASSIGNMENT message facilitates the assignment of the MSonto a TCH channel. Correspondingly, the IMMEDIATE ASSIGNMENTREJECT message facilitates the refusal of the access: if the TCH channelreservation or activation fails, the BSC sends the IMMEDIATE ASSIGNMENTREJECT message to the MS. The GSM timer T3101 supervises the immediateassignment procedure.

The figure below shows the immediate assignment procedure.

Figure 6. Immediate assignment procedure

IMMEDIATE ASSIGN COMMAND message

After a successful TCH channel activation, the BSC sends the IMMEDIATEASSIGN COMMAND message (a) to the BTS. This message contains theIMMEDIATE ASSIGNMENT message, which the BTS sends to the MS.

The IMMEDIATE ASSIGN COMMAND message includes the following data:

. message header

. message type

. channel number

. immediate assignment information.

The immediate assignment information contains the complete IMMEDIATEASSIGNMENT message:

MS BTS BSC MSC

a)IMMEDIATE ASSIGN COMMAND

b)IMMEDIATE ASSIGN

SABM c)d)ESTABLISH INDICATION

e)UA



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. message header

. page mode: normal paging

. channel description of the TCH allocated, the associated SACCH andfrequency or hopping sequence

. request reference as sent by the MS in the channel request

. initial timing advance

. mobile allocation if frequency hopping is used

. rest octet only for phase 2.

IMMEDIATE ASSIGNMENT message, SABM, UA frame

After receiving the IMMEDIATE ASSIGNMENT message, the MS tunes to theassigned TCH and starts to establish the signalling link across the network. TheMS sends the layer 2 SABM (Set Asynchronous Balanced Mode) to the BTS onthe TCH. The SABM (c) contains a layer 3 service request message. The MSspecifies to the network the required service type with the service requestmessage.

The service request message contains one of the following:

1. CM SERVICE REQUEST - for mobile originating calls and for a mobileoriginating SMS (Short Message Service) - contains:

. header

. CM service types

. ciphering key sequence number

. MS classmark 2

. mobile identity.

2. PAGING RESPONSE contains:

. header

. ciphering key sequence number

. MS classmark 2

. mobile identity.

3. CM RE-ESTABLISHMENT REQUEST

4. Emergency setup.

The BTS acknowledges the SABM by sending the UA frame (e) to the MS. TheMS expects this acknowledgement within time T; otherwise it acts as described in3GPP TS 44.018.



FACCH Call Set-up

ESTABLISH INDICATION message

The BTS forwards the MS service request to the BSC in the ESTABLISHINDICATION message (d), which includes the following data:

. message discriminator: radio link layer management

. message type: establishment indication

. channel number: TCH + SACCH

. link identifier: main signalling channel TCH

. L3 Information - complete L3 service request as received from the MS.

Having received the ESTABLISH INDICATION message, the BSC starts themobile measurement (power control and handover) algorithm.

2.5 Mode modify procedure

The mode modify procedure changes the channel mode from signalling to speechor data. The procedure is presented in the figure below.



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Figure 7. Mode modify procedure

ASSIGNMENT REQUEST message

The MSC sends an ASSIGNMENT REQUEST message (a) to the BSC for theassignment of channels on the A interface and the Radio interface.

The BSC expects to receive the following fields in the ASSIGNMENTREQUEST message:

. message type

. channel type: speech, TCH/F or TCH/H, speech algorithm

. L3 header information

. priority (optional)

. circuit identity code indicating the channel to be used on the A interface

. downlink DTX flag (optional)

MS BTS BSC MSC

a)DT1(ASSIGNMENT REQUEST)

b)MODE MODIFY

c)MODE MODIFY ACKNOWLEDGE

d)DATA REQUEST(CHANNEL MODE MODIFY)

e)CHANNEL MODE MODIFY

f)CHANNEL MODE MODIFY ACK

g)DATA INDICATION(CHANNEL MODE MODIFY ACK)

h)DT1(ASSIGNMENT COMPLETE)



FACCH Call Set-up

. radio channel identity (optional)

. interference band to be used (optional).

The BSS responds to the ASSIGNMENT REQUEST message by sending theMSC the ASSIGNMENT COMPLETE or ASSIGNMENT FAILURE message.On receiving an ASSIGNMENT FAILURE message, the MSC can either issue aCLEAR COMMAND or retry.

The MSC runs a supervisory timer for the assignment procedure: if the MSC failsto receive a response from the BSS within the defined time limit, it clears the callwith a CLEAR COMMAND message. The BSS releases the MS connection withthe channel release procedure.

If the BSC identifies any of the following situations on analysing theASSIGNMENT REQUEST message, it rejects the request by sending theASSIGNMENT FAILURE message to the MSC, without taking any actiontowards the BTS:

1. If the BSC is not able to use the A channel indicated in theCIC (Circuit Identification Code) because it is used foranother call, the BSC rejects the request with the causerequested terrestrial circuit already allocated.

2. If the BSC is not able to use the A channel indicated in theCIC because the channel has been marked as faulty, the BSCrejects the request with the cause requested terrestrial resourceunavailable.

3. If the CIC is blocked, the BSC rejects the request with thecause requested terrestrial resource unavailable. The BSCalso sends a BLOCK message to the MSC. For moreinformation on reasons which can cause a circuit to beblocked by the BSC, see 3GPP TS 48.008.

4. If there is no circuit pool on the A interface which is able tosupport the requested TCH type, the BSC rejects the requestwith the cause requested transcoding/rate adaption notavailable.

5. If there is a circuit pool on the A interface which is able tosupport the requested TCH type but the pool implied by theCIC is totally contradictory with the TCH type requirement,the BSC rejects the request with the cause circuit poolmismatch.



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6. If the circuit pool implied by the CIC is at least partiallycompliant with the requested TCH type, changing of thecircuit pool may be required. Reasons for the change includethe actual traffic channel resource situation of the cell and theattempt to allocate the resources in the most optimal way. Inthis case the BSC rejects the request with the cause switchcircuit pool.

7. If the requested speech coder versions are not supported, theBSC rejects the request with the cause requested speechversion unavailable.

8 If the radio interface data rate requirement is totallycontradictory with the requested TCH channel rate, the BSCrejects the request with the cause invalid message contents.

If the MSC receives an ASSIGNMENT FAILURE message with a causeindicating a CIC problem, the MSC may choose to send another ASSIGNMENTREQUEST for the same call using a different circuit.

MODE MODIFY message

If the ASSIGNMENT REQUEST message is valid, the BSC proceeds to performthe mode modify procedure.

The BSC first sends the MODE MODIFY message (b) to the BTS to change thechannel mode from signalling to speech or data. At this point the BSC connectsthe Abis channel to the A channel and the speech path is switched through.

Synchronization with the transcoder starts as described in 3GPP TS 48.060.

The MODE MODIFY message includes the following data:

. message discriminator: dedicated channel management

. message type: mode modify

. channel number

. channel mode: DTX control and channel type (speech, TCH/F or TCH/H,speech algorithm)



FACCH Call Set-up

When the BSC receives the MODE MODIFYACKNOWLEDGE message (c) asa response from the BTS, it starts to change the MS channel mode. If the BSCfails to receive the MODE MODIFY ACKNOWLEDGE message within thedefined time limit, or the message does not contain all the information expected,the BSC returns an ASSIGNMENT FAILURE message to the MSC with thecause equipment failure. The MSC either aborts with a CLEAR COMMANDmessage or retries.

If the BSC receives the MODE MODIFY NEGATIVE ACKNOWLEDGEmessage from the BTS, the BSC sends an ASSIGNMENT FAILURE message tothe MSC either with the cause equipment failure or requested transcoding or rateadaption unavailable. The cause value is mapped from the cause value receivedin the MODE MODIFY NEGATIVE ACKNOWLEDGE message. The MSCeither aborts with a CLEAR COMMAND message or retries.

CHANNEL MODE MODIFY message

After receiving the MODE MODIFYACKNOWLEDGE message from the BTS,the BSC sends the CHANNEL MODE MODIFY message to the MS and starts atimer. The CHANNEL MODE MODIFY message (e) is transparent to the BTSand is transferred as the DATA REQUEST message (d) over the Abis interface.

The CHANNEL MODE MODIFY message includes all the informationnecessary for the MS to change the channel mode from signalling to speech ordata. The message consists of the following:

. header

. channel description

. channel mode: speech full rate or speech half rate.

On receiving the CHANNEL MODE MODIFY message over the radio interface,the MS changes the channel mode and acknowledges this by sending theCHANNEL MODE MODIFYACKNOWLEDGE message (f) to the BSC. Thismessage is transparent to the BTS and is transferred over the Abis as a DATAINDICATION message (g).

The BSC checks the channel mode element of the CHANNEL MODE MODIFYACKNOWLEDGE message. The procedure has failed if the old channel mode isreturned instead of the one that was ordered. If the ordered channel mode isreturned, the BSC sends the ASSIGNMENT COMPLETE message (h) to informthe MSC of the successful completion of the mode modify procedure. See 3GPPTS 48.060 for more details.



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If the timer used to supervise the receival of the CHANNEL MODE MODIFYACKNOWLEDGE message expires, the BSC returns an ASSIGNMENTFAILURE message to the MSC with the cause radio interface message failure.The MSC either aborts or retries.



FACCH Call Set-up

3 User interface of FACCH Call Set-up

You can set the FACCH Call Set-up for each BSC via the man-machine interface(MMI).

Similarly, with the MMI you can set the New Establishment Causes support field(NECI) for new establishment cause values on or off for an individual cell. TheNECI field allows the MSs to use the new establishment cause values: when anMS requests for a channel, the request can include more accurate informationabout the establishment cause and channel type needed for the requested service.

The following calls and channel types are possible:

. emergency call (SDCCH or TCH/F)

. call re-establishment (SDCCH, TCH/F or TCH/H)

. answer to paging (SDCCH, TCH/F or TCH/H indicated by MS)

. mobile originating speech/data call (SDCCH, TCH/F or TCH/H)

. location updating (SDCCH)

. other procedures on SDCCH.

The following channel request values can be set in FACCH call set-up:

. emergency call

. answer to paging

. ordinary call

. re-establishment.

The channel request values in the NECI field also include an indication of thesuggested rate, such as the half rate support for the call. The BSC will allocate asuitable resource for a transaction according to the SDCCH or the TCH. For moredetails, see Half Rate in BSC.

For an overview, see Overview of FACCH Call Set-up.



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User interface of FACCH Call Set-up

Parameters

The parameters used in connection with FACCH Call Set-up are described below:

. FACCH_CALL_SET_UP : with this parameter you can activate ordeactivate the FACCH Call Set-up functionality. The parameter can beinput from the Parameter Handling MML or Nokia NetAct.

For more details, see PRFILE and FIFILE Parameter List.

. new establishment causes support (NECI) : with thisparameter you can set the value of the NECI field. This parameter can beoperated via the Base Transceiver Station Handling MML.

For more details, see Radio Network Configuration Management and BSSRadio Network Parameter Dictionary.

. enable emergency call on FACCH : with this parameter you canactivate or deactivate an emergency call to use FACCH Call Set-up. Thisparameter can be found from the radio network database and it can beoperated via the Base Station Controller Handling MML or Nokia NetAct.

. enable call re-establishment on FACCH : with this parameteryou can activate or deactivate call re-establishment to use FACCH CallSet-up. This parameter can be found from the radio network database and itcan be operated via the Base Station Controller Handling MML or NokiaNetAct.

. enable ordinary calls on FACCH : with this parameter you canactivate or deactivate a mobile originating call to use FACCH Call Set-up.This parameter can be found from the radio network database and it can beoperated via the Base Station Controller Handling MML or Nokia NetAct.

. enable answer to paging call on FACCH : with this parameteryou can activate or deactivate the mobile terminated call to use FACCHCall Set-up. This parameter can be found from the radio network databaseand it can be operated via the Base Station Controller Handling MML orNokia NetAct.

For more details on parameters enable emergency call on FACCH,enable call re-establishment on FACCH,enable ordinarycalls on FACCH and enable answer to paging call on FACCH , seeMSC documentation; Base Station Controller Handling in MSC and BSS RadioNetwork Parameter Dictionary.



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