4 Dedicated channel management procedures

08.583GPPBase Station Controler - Base Transceiver Station (BCS-BTS) Interface Layer 3 SpecificationRelease 1999TS

4.1 Channel activation

This procedure is used to activate a channel at the BTS for an MS which later will be commanded to this channel by an IMMediate ASSIGN, an ASSIGN CoMmanD, an ADDitional ASSIGNment; a NOTIFICATION, a CHANNEL RELEASE (with a Channel description) a HANDOver CoMmanD or a CONFiguration CHange CoMmanD message.

In the handover case, the procedure is used between the target BSC and the target BTS to activate a channel for a subsequent handover from the old BTS.

4.1.1 Signalling Procedure

BSC determines what channel shall be used and starts up that channel at BTS by sending a CHANnel ACTIVation message to the relevant TRX. This message contains the reason for the activation (immediate assignment, assignment, asynchronous/synchronous handover, additional assignment, activation of a secondary channel in a multislot configuration), the identification of the channel to be used (channel no) and a complete description of the channel (full/half rate, speech/data, coding/rate adaption, hopping sequence, encryption key etc.).

If the Encryption Information field is present, the activation is done with ciphering active. If the Encryption Information element is not present, activation is done without ciphering.

After activating the channel as requested, TRX responds with the CHANnel ACTIVation ACKnowledge message. This message contains the current frame number at BTS. The frame number is used by BSC to determine the Starting Time parameter to be included in the following assignment message to MS. (A suitable number has to be added to current frame number to take all possible signalling delays into account).

If the TRX for some reason cannot activate the resource as requested by the CHANnel Activation message, the TRX shall return a CHANnel ACTIVation Negative ACKnowledge message with the most appropriate cause value.

Possible cause values may be:

– O&M intervention (e.g. channel blocked);

– resource not available (e.g. speech coder, encryption device);

– equipment failure;

– channel already activated;

– etc.

In the handover case, the procedure is initiated by the target BSC when this receives the HANDOver REQuest message from MSC (or autonomously by BSC for BSC internal handover). The BSC sends a CHANnel ACTIVation message to the relevant TRX. The message contains the Handover Reference value which can be used by the BTS to check the Handover Access from MS. After activation of the channel TRX responds with a CHANnel ACTIVation ACKnowledge message containing the current frame number at BTS.

The BSC can then determine the Starting Time parameter to be included in the HANDOver REQuest ACKnowledge message to MSC (and the HANDOver CoMmanD message to MS).

4.1.2 Activation for Intra-Cell Channel Change

This activation precedes the Immediate Assignment, Assignment or Additional assignment procedures. The Timing Advance element must be included in the CHANNEL ACTIVATION message.

BTS activates the channel and starts transmission and reception on the main channel in the indicated mode. Ciphering is started if so indicated in the encryption information.

The reception and transmission on SACCH is also started immediately.

If the BS and/or MS power elements and/or the Physical Context element are present, the reception and transmission processes and the L1-header of SACCH are initialized accordingly.

4.1.3 Activation for Asynchronous Handover

BTS starts transmission immediately on the main channel in the indicated mode and with encryption if so indicated. If the MS Power element is present the BTS may start transmission also on the SACCH.

When receiving a correct access burst with the correct handover reference, BTS starts the normal reception process on the main channel in the indicated mode and starts receiving (and sending if not started earlier) on SACCH. Deciphering is started if so indicated. The handover detection procedure towards BSC is also started.

4.1.4 Activation for Synchronous Handover

BTS starts transmission immediately on the main channel in the indicated mode and with encryption if so indicated. If the MS Power and Timing Advance element are present, BTS shall start transmission also on SACCH with the timing advance and MS power control parameters indicated. If only the MS power element is present the BTS may start transmission also on the SACCH.

When receiving a correct access burst with the correct handover reference, BTS starts the normal reception process on the main channel in the indicated mode, with deciphering applied if so indicated, and starts receiving (and sending if not started earlier) on SACCH. The handover detection procedure towards BSC is also started. Alternatively, the reception of a correctly decoded frame from the MS on the main channel, in the indicated mode and deciphering applied if so indicated, allows the start of sending on SACCH (if not already started) and starts the handover detection procedure towards the BSC.

NOTE: The activation for synchronous handover can be used for pseudo synchronized handover.

4.1.5 Activation for Secondary Channels in Multislot Configuration

BTS activates the channel and starts transmission and reception on the traffic and SACCH channels in the indicated mode. Ciphering is applied if so indicated in the encryption information.

If the BS and/or MS power elements and/or the Physical Context element are present, the reception if applicable and transmission processes and the L1-header of SACCH are initialized accordingly.

4.1.6 Channel reactivation

This procedure is used by BSC to request a reactivation of an active channel. During the reactivation, information flows, e.g., user information such as speech or data, that are common for the two phases of operation, are not interrupted.

BSC initiates the procedure by sending a CHANNEL ACTivation message to BTS where the activation type indicates "reactivation", the BTS shall reactivate the channel with the new parameters. After having successfully reactivate the channel with the parameters supplied the BTS responds with a CHANNEL ACTivation ACKnowledge message to BSC.

If the TRX for some reason cannot reactivate the channel as requested in the CHANNEL ACTIVATION message, the TRX shall return a CHANNEL ACTivation Negative ACKnowledge message with the most appropriate cause value.

4.2 Channel MODE MODIFY

This procedure is used by BSC to request a change of the channel mode and/or a change between uni‑directional and bi-directional channel types of an active channel. In addition, for secondary channels in a multislot configuration, the procedure can be used by BSC to request a change in the encryption information of an active channel.

BSC initiates the procedure by sending a MODE MODIFY message to BTS. The message contains the new mode to be used. After having changed to the new mode, BTS responds with a MODE MODIFY ACKnowledge message to BSC.

If the TRX for some reason cannot modify the channel as requested in the MODE MODIFY message, the TRX shall return a MODE MODIFY Negative ACKnowledge message with the most appropriate cause value.

4.3 Handover detection

This procedure is used between the target BTS and BSC when a handed over MS accesses the new BTS.

The procedure is initiated by BTS upon detection of an MS on a channel activated for handover as described in subclause 4.1.3 for the asynchronous handover and in subclause 4.1.4 for synchronous handover.

In case of an asynchronous handover, BTS builds the PHYsical INFOrmation message as specified in 3GPP TS 04.08, sends the message to MS in unacknowledged mode on the main signalling link and starts timer T3105. A HANDOver DETection message is sent to BSC. This message contains the measured delay of the access burst. If the timer expires before the reception of a correctly decoded frame from MS, BTS repeats the PHYSical INFOrmation message to MS as specified in 3GPP TS 04.08. If the PHYsical INFOrmation message has been repeated Ny1 times without a correctly decoded frame being received from MS, the BTS shall send a CONNECTION FAILURE message to BSC with the cause value "handover access failure".

In case of a synchronous handover, BTS only sends a HANDOver DETection message to BSC (no PHYsical INFOrmation message sent to MS). If the handover detection is based on the detection of an handover access burst with the correct handover reference, see subclause 4.1.4, the measured delay of the access burst is included in the HANDOver DETection message.

4.4 Start of encryption

This procedure is used to start encryption according to the procedure defined in 3GPP TS 04.08.

The procedure is initiated by BSC upon reception of the CIPHER MODE COMMAND message from MSC (see 3GPP TS 08.08).

BSC sends the ENCRyption CoMmanD message to the relevant TRX and channel. In case of a Multislot configuration the message is sent only to the TCH used as a main channel (defined in 3GPP TS 05.02). The message contains all information required to select and load the user data and encryption device with the appropriate key and also the complete Ciphering Mode Command message to be sent to MS.

After receipt of this message, TRX sends the CIPHering MODe CoMmanD message to MS in unciphered form and starts deciphering as described in 3GPP TS 04.08 and 3GPP TS 03.20. The start of deciphering and the sending of the Ciphering Mode Command message to MS must be done simultaneously.

When receiving the CIPHering MODe CoMmanD, MS starts both deciphering and enciphering and sends the CIPHering MODe COMplete message.

TRX starts enciphering upon reception of any correct layer 2 frame which is received after start of deciphering.

If the TRX for some reason can not perform the enciphering as requested in the ENCRYPTION COMMAND, the TRX shall return an ERROR REPORT message, e.g., with the cause "Encryption algorithm not implemented".

4.5 Measurement reporting

These procedures are used to report to BSC all parameters and measurement results required by BSC for handover requirement determination. One procedure is also used to report to the BSC extended measurements made by Mobile Stations.

MS measures downlink received signal level and quality from the serving cell and received signal level from surrounding cells as defined in 3GPP TS 05.05 and 3GPP TS 05.08. The measurement results are reported in Measurement Report messages sent in every SACCH block (every 480 ms) or, in case SACCH is used also for other signalling, in at least every second SACCH block (every 960 ms).

In addition, the MS which implements ECSD options shall use fast inband procedure for downlink quality reporting if the use of such procedure has been ordered by the BSC.

The TRX measures the received signal level and the quality on the uplink of the current channel. The averaging period is one SACCH block period (same as the basic period for MS).

These measurements made by MS and TRX form the basic raw data for the handover algorithms in BSC/MSC. The support of forwarding this raw data over the A-bis interface is mandatory for both BTS and BSC. The procedure to be used for this basic measurement reporting is defined in subclause 4.5.1.

In addition, the BTS and BSC may optionally support some pre-processing in BTS of these basic measurements. The additional and optional procedures required to support this pre-processing are defined in subclause 4.5.2.

Extended measurements made by MS shall be forwarded to the BSC, using the same procedure as for ‘normal’ measurements. This case is described in subclause 4.5.3.

4.5.1 Basic measurement reporting

This procedure is used by BTS to report the results of the basic radio measurements made by MS and TRX according to 3GPP TS 05.08 and 3GPP TS 05.05. The support of this procedure is mandatory in all BTS:s and all BSC:s. It is the default procedure to use unless otherwise indicated (see subclause 4.5.2.1).

TRX reports all these measurements in MEASurement RESult messages to BSC. The sending of the MEASurement RESult messages is synchronized with the reception of SACCH blocks from MS.

If an uplink SACCH block does not contain a MEASurement REPort or an EXTended MEASurement REPort (see subclause 4.5.3) from MS (e.g. when it sends a short message), only the uplink measurement results are included with an indication that the MS measurements are missing.

4.5.2 Measurement pre-processing

These additional and optional procedures are included to support some pre-processing in BTS of radio measurement data. When used, they may replace the basic procedure defined in subclause 4.5.1. However, it shall be possible to change back to the basic procedure.

Pre-processing in BTS must not affect the procedures on the A interface (e.g. the Handover Candidate Enquiry procedure).

4.5.2.1 Pre-processing configuration

This procedure is used by BSC to modify the pre-processing parameters according to reported communication conditions (e.g. degradation of the communication).

In order to change the parameters, BSC sends a PREPROCESS CONFIGURE message to BTS.

A parameter setting in the PREPROCESS CONFIGURE message indicates if the basic procedure defined in subclause 4.5.1 or pre-processing is to be used.

4.5.2.2 Pre-processed measurement reporting

This procedure is used by BTS to report the results of measurement pre‑ processing.

To report the results, BTS sends a PREPROCESSED MEASUREMENT RESULT message to BSC.

The conditions to send the message are set in the PREPROCESS CONFIGURE message.

4.5.3 Extended measurement reporting

This procedure is used by BTS to report the results of the extended measurements made by MS according to 3GPP TS 05.08.

If an uplink SACCH block contains an EXTended MEASurement REPort from MS, it shall be forwarded to the BSC using the same procedure as when reporting regular measurement reports from an MS. Though, no measurement pre-processing is possible.

4.6 Deactivate SACCH

This procedure is used by BSC to deactivate the SACCH at BTS according to the Channel Release procedure defined in 3GPP TS 04.08.

When sending the Channel Release message to MS, BSC also sends the DEACTIVATE SACCH message to BTS to deactivate the SACCH (see 3GPP TS 04.08, Channel Release procedure).

4.7 Radio channel release

This procedure is used by BSC to release a radio channel which is no longer needed.

When an activated radio channel is no longer needed, BSC sends an RF CHANnel RELease message to the relevant TRX and channel. After having released the addressed resources, the BTS sends a RF CHANnel RELease ACKnowledge to BSC.

4.8 MS power control

This procedure is used by BSC to set the MS power level or the parameters required by TRX for the control of MS power.

The initial parameters are set by BSC in the CHANNEL ACTIVATION message (see Channel Activation procedure). If these parameters are to be changed for an active channel, BSC sends a MS POWER CONTROL message to TRX.

The support of the power control performed by BTS is optional.

If power control is supported by BTS and it is to be used, this is indicated by optional parameters in the MS POWER CONTROL message (or the CHANNEL ACTIVATION message). Based on the measurements performed on the uplink, TRX then attempts to keep the power control parameters within the limits set by the MS POWER CONTROL message (or by the CHANNEL ACTIVATION message) by changing the MS Power Level field of the L1 header sent to MS in each SACCH block. MS confirms the power in the uplink L1 header.

When the BTS supports MS power control the BSC can modify the MS power parameters during the connection (e.g. because of a classmark change) by sending a MS POWER CONTROL message containing the new parameters.

The MS POWER CONTROL and the CHANNEL ACTIVATION message must also contain a maximum power permitted for the MS.

In addition, the BTS which implements ECSD option shall use fast inband procedure for fast power control if the use of such procedure has been ordered by the BSC.

4.9 Transmission power control

This is an optional procedure which is used between BSC and BTS to set the TRX transmission power level or the parameters required by TRX for the control of TRX transmission power.

The initial parameters are set by BSC in the CHANNEL ACTIVATION message (see Channel Activation procedure). If these parameters are to be changed for an active channel, BSC sends a BS POWER CONTROL message to TRX.

The support of the power control and the fast power control performed by BTS are optional.

If power control is supported by BTS and it is to be used, this is indicated by optional parameters in the BS POWER CONTROL message (or the CHANNEL ACTIVATION message). Based on the Measurement Report messages sent by MS, the TRX will then attempt to keep the power control parameters within the limits set in the BS POWER CONTROL message (or by the CHANNEL ACTIVATION message) by changing the transmitted power on that channel.

Based on the Fast Measurement Report messages sent by MS, the TRX will then attempt to keep the power control parameters within the limits set in the BS POWER CONTROL message (or by the CHANNEL ACTIVATION message) by changing the transmitted power on that channel.

If fast power control mechanism is supported by BTS and is to be used, this is indicated by optional paramters in the BS POWER CONTROL message (or by the CHANNEL ACTIVATION message).

The maximum power of the TRX is determined from network planning criteria. However, BSC may indicate a lower maximum power in the BS POWER CONTROL message (or the CHANNEL ACTIVATION message).

4.10 Connection failure

This procedure is used by BTS to indicate to BSC that an active connection has been broken (e.g. due to a radio link failure as defined in 3GPP TS 05.08).

When BTS detects that a connection has been broken, BTS sends a CONNection FAILure INDication message to BSC with the most proper cause value. Further actions are defined in 3GPP TS 04.08.

Some possible cause values are:

– radio link failure (as defined in 3GPP TS 05.08);

– hardware failure (e.g. transcoder failure);

– etc.

4.11 Physical context request

This is an optional procedure which allows the BSC to obtain information on the "physical context" of a radio channel just prior to a channel change. This information may be forwarded to the new TRX (possibly in another collocated cell).

The procedure is initiated by BSC sending a PHYsical CONTEXT REQuest message to TRX. TRX responds with a PHYsical CONTEXT CONFirm message which contains information on the "physical context" of the channel.

4.12 SACCH information modify

This procedure is used by the BSC to modify the SACCH filling information (System Information) sent on an individual SACCH channel. For this purpose, the BSC sends a SACCH INFO MODIFY message to the BTS. The SACCH filling information as given in the SACCH INFO MODIFY message shall be used on the indicated channel until the channel is released or the information is changed by another SACCH INFO MODIFY message.

4.13 Talker detection

The procedure is used by the BTS during the period the channel is activated for a voice group call. Upon detection of an MS on a channel activated for group call as specified in subclause 4.1, the BTS builds the VGCS UPLINK GRANT message as specified in 3GPP TS 04.08, sends the message to the MS in unacknowledged mode on the main signalling link and starts timer T3115. A TALKER DETection message is sent to the BSC. This message contains the measured delay of the access burst. If the timer expires before the reception of a correctly decoded frame from the MS, the BTS repeats the VGCS UPLINK GRANT message to the MS as specified in 3GPP TS 04.08. If the VGCS UPLINK GRANT message has been repeated Ny2 times without a correctly decoded frame being received from the MS, the BTS shall send a CONNECTION FAILURE INDICATION message to the BSC with cause value "talker access failure".

4.14 Listener detection

The procedure is used by the BTS during the period the channel is activated for a voice group call. Upon detection of an uplink access having the value reserved for replying to an uplink access request as specified in subclause 4.1. The BTS builds the LISTENER DETection message and sends the message to the BSC. This message contains the measured delay of the access burst.

4.15 Remote Codec Configuration

This procedure is used by the BTS to report to the BSC the codec information received from the remote BSS prior to a MultiRate Codec TFO establishment or within TFO. This information can be used by the BSC to solve potential codec type and codec mode mismatches.

4.16 Round Trip Delay Report

This procedure is used by the BTS to report to the BSC the BTS to Transcoder or BTS to Remote BTS round trip transmission delay. This information can be used by the BSC to set the correct timing for a pre-handover warning and to optimise the AMR Configuration and/or Adaptation algorithm.

4.17 Pre-handover Warning

This procedure is used by the BSC to notify the serving BTS that a handover is going to be performed and allow the serving BTS to take the necessary steps in preparation of the handover (adaptation freezing for MultiRate Codec, TFO discontinuation,….). The Pre-handover Warning may disable (withdraw the “general authorisation” of ) the RATSCCH mechanism between BTS and MS for a while or permanently to ensure consistent configurations in BSC, BTSs and MS during and after handover. The BSC shall provide to the BTS the codec configuration parameters after Handover.

In case the announced handover has failed, the BSC sends the Pre-handover Notification again to the serving BTS, this time with the codec configuration parameters as before the handover attempt in order to re-establish the BTS operation (adaptation enable for MultiRate Codec, TFO enable, RATSCCH authorisation, …).

4.18 MultiRate Codec Configuration Change

This procedure is used by the BSC to authorise the BTS to change the MultiRate Codec Configuration to solve a codec mismatch prior to TFO establishment or within TFO. The BTS shall use the RATSCCH inband signalling with the MS as defined by Rec 3GPP TS 05.09.

Once the MultiRate Codec Configuration has been changed, the BTS shall send a MultiRate CODEC MOD ACK to the BSC to acknowledge the execution of the codec change. The MultiRate CODEC MOD ACK contains the final MultiRate Codec configuration.

If for any reason, the BTS is not able to complete the MultiRate Codec Configuration change, it should send a MultiRate CODEC MOD Negative ACK with the most appropriate cause.

Possible causes are:

– tbd

4.19 MultiRate Codec Configuration Change Performed

This procedure is used by the BTS to report to the BSC that a MultiRate Codec Modification was performed using the RATSCCH in-band signalling capability. The BTS shall report the MultiRate codec configuration after the completion of the procedure.

4.20 TFO Report

This procedure is used by the BTS to report to the BSC that TFO (based on AMR) is established, or is not in operation anymore.

4.21 TFO Modification Request

This procedure is used by the BSC to change the TFO configuration while operating with AMR: enabling or disabling TFO.

This procedure shall be used only during an established call. At call setup, Channel Activation procedure shall be used. During an in-call modification from data to speech, Mode Modify procedure shall be used.