01-04 immediate assignment

4 Immediate Assignment

About This Chapter

This topic describes immediate assignment including its internal BSC signaling procedure andabnormal cases.

4.1 Immediate Assignment ProcedureThe purpose of immediate assignment is to establish a Radio Resource (RR) connection betweenan MS and the network on the Um interface. The immediate assignment procedure is initiatedby the MS.

4.2 Internal BSC Signaling Procedure of Immediate AssignmentThis topic describes the internal BSC signaling procedure of immediate assignment.

4.3 Abnormal Immediate Assignment CasesThis topic describes the abnormal immediate assignment cases.

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4.1 Immediate Assignment ProcedureThe purpose of immediate assignment is to establish a Radio Resource (RR) connection betweenan MS and the network on the Um interface. The immediate assignment procedure is initiatedby the MS.

Figure 4-1 shows the immediate assignment procedure.

Figure 4-1 Immediate assignment procedure

Channel Request(1)

RACH

Channel Required(2)

Channel Activation(3)

MS BTS BSC

Channel Activation Acknowledge(4)

Immediate Assignment Command(5)

AGCH

Start T3101SABM(6)

main DCCH

UA(7)

main DCCHEstablish Indication(8)

Stop T3101

1. The MS sends the BTS a Channel Request message on the RACH.

The Channel Request message contains the Establish Cause and Random Referenceparameters. It has eight bits. Bits 3 to 6 indicate the access cause, and bit 5 to 2 carry theidentifier. Based on a Channel Request message, a maximum of 32 MSs are discriminated.The UA response messages on the Um interface are used to discriminate more MSs.

2. The BTS sends the BSC a Channel Required message.

The Channel Required message contains the Request Reference and Access Delayparameters. The Request Reference parameter contains the random access reference valueand the absolute frame number of the received access burst. The Access Delay is the delayof the access burst.

3. Upon receiving the Channel Required message, the BSC assigns a signaling channel andsends the BTS a Channel Activation message to activate a radio channel. The ChannelActivation message carries the activation type, channel mode, level, and TA.

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4. Upon receiving the Channel Activation message, if the channel type is correct, BTS2 turnson the power amplifier on the specified channel to receive information in the uplinkdirection and sends a Channel Activation Acknowledge message to the BSC.

5. The BSC sends through the BTS to the MS an Immediate Assignment Command message.

The Immediate Assignment Command message is sent on the AGCH. It carries theImmediate Assignment Extend message and the assignment information of two MSs. TheImmediate Assignment Command message has the following information elements:

l Dedicated mode or TBF

The Dedicated mode or TBF information element requests the MS to decode the rest ofthe message to an Immediate Assignment message or to allocate a Temporary BlockFlow.

l Page Mode

The Paging Mode information element controls the action of the MSs in a paging group.

l Channel Description

The Channel Description information element describes an assigned channel and thecorresponding SACCH. It carries the Channel Type, TDMA offset, timeslot, andabsolute RF channel number.

l Request Reference

The Request Reference information element contains the random access reference valueand the absolute frame number of the received access burst. The Access Delay is thedelay of the access burst.

l Time Advance

This Time Advance information element provides the initial timing advance value.

NOTE

If the BSC has no available channels, it sends the MS an Immediate Assignment Reject messageor an Immediate Assignment Extend Reject message.

l After receiving the Immediate Assignment Reject message, the MS starts timer T3122 uponreceiving one of the last three channel requests. Before timer T3122 expires, the MS is notpermitted to access the network nor initiate access attempts except emergency calls in thesame cell. It returns to idle mode and waits on the paging channel.

l If the Immediate Assignment Reject message is received for the first time, the MS starts timerT3126. Timer T3126 starts when the number of Channel Request messages sent by the MSreaches the maximum value. When timer T3126 expires, the immediate assignmentprocedure fails.

6. The MS sends an SABM frame on the main DCCH.

The SABM is a frame in the LAPDm protocol at the data link layer. It is the first frame toestablish a multiple frame acknowledged mode.

The messages on the Um interface come in two types, signaling messages and shortmessages. They are identified by the Service Access Point Identifier (SAPI). The serviceaccess point is the point at which the data link layer provides services to layer 3. SAPI 0supports the transfer of signaling, and SAPI 3 supports the transfer of short messages. Inthe GSM network, an SABM frame carries a signaling message, the layer 3 service requestmessage. The layer 3 service request message contains the identity of MS and indicateswhich service MS is requesting. The layer 3 service request messages are categorized intoCM service requests, location update requests, IMSI detach, and paging responses. Thesemessages contain the IMSI, detailed access cause, and classmarks.



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NOTE

The main DCCH is the primary signaling channel and is generally the SDCCH, FACCH, andSACCH.

7. The BTS responds with a UA frame on the main DCCH.The UA frame is an unnumbered acknowledge frame of the LAPDm protocol at the datalink layer. It confirms the SABM frame.When the Channel Request messages sent by two MSs have the same contents, the twoMSs may be assigned the same dedicated channel. Upon receiving the SABM frame, theBTS sends the MS a UA frame that contains the same information as the received SABMframe. If the IMSI contained in the SABM frame is different from that in the UA frame,the MS leaves this channel and starts another access attempt. If the IMSI contained in theSABM frame is the same as that in the UA frame, the MS stays on the assigned channel.

8. The BTS sends the BSC an Establish Indication message.This Establish Indication message indicates the establishment of a radio connection inmulti-frame mode and contains parameters Link Identifier and L3 Information. The LinkIdentifier parameter identifies the signaling channel type and the message priority of SAPIand SAPI 0. The L3 Information parameter is a complete L3 message.

4.2 Internal BSC Signaling Procedure of ImmediateAssignment

This topic describes the internal BSC signaling procedure of immediate assignment.

l Upon receiving the Channel Required message from the BTS, the BSC assigns a signalingchannel according to the specified channel type and channel algorithm.

l During random access, upon receiving the Establish Indication message from the BTS, theBSC sends the MSC a CM Service Request message that carries the target CGI based onthe MCC, MNC, LAC, and CI parameters in the Cell Attributes window.

4.3 Abnormal Immediate Assignment CasesThis topic describes the abnormal immediate assignment cases.

4.3.1 Failure in Receiving an Establish Indication Message After Channel ActivationThis topic describes the fault, probable causes, and handling suggestions.

4.3.2 BSC Sending an Immediate Assignment Reject MessageThis topic describes the fault, probable causes, and handling suggestions.

4.3.1 Failure in Receiving an Establish Indication Message AfterChannel Activation

This topic describes the fault, probable causes, and handling suggestions.

FaultThe BSC sends the MS an Immediate Assignment Command message but does not receive anEstablish Indication message from the BTS.

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Probable CausesThe probable causes are as follows:

1. The design of the MS does not comply with the protocols. The MS sends the ChannelRequest message for many times and the BSS assigns and activates multiple signalingchannels.

2. Even when the BSS is functional, the MS may send several Channel Request messagesduring one RR connection attempt. The BSS activates multiple signaling channels, but theMS uses only one of them. The BSC cannot receive the Establish Indication messages fromother channels and therefore releases the channel after timer T3101 expires. If the numberof slots for spread transmission (Tx-integer) is set properly, the reason may be that the BTSreceives the uplink signals correctly but the MS cannot receive the downlink signals.Meanwhile, as indicated in the signaling tracing on the Um interface on the MS side, theBSC does not receive a response message from the BTS after sending a Channel Requestmessage.

3. The Tx-integer and the CCCH are not set correctly. The settings of the Tx-integer and theCCCH affect the interval at which an MS resends the Channel Request messages.

Handling Suggestionsl Use another MS to determine whether the problem lies in the MS itself.

l Check whether the received level and received quality on the uplink and downlink arenormal. If the MS is near the BTS, but the received level and received quality is low, checkwhether the antenna system, MS antenna, and MS battery are functional.

l Check whether the Tx-integer and the CCCH are set correctly.

4.3.2 BSC Sending an Immediate Assignment Reject MessageThis topic describes the fault, probable causes, and handling suggestions.

FaultUpon receiving the Channel Required message, the BSC sends the MS an Immediate AssignmentReject message.

Probable CausesThe probable causes are as follows:

1. The BSC finds no suitable signaling channel, that is, a SDCCH or a TCH, to assign to theMS. This is because that the channels are busy or blocked.

2. Upon receiving a Channel Activation message, the BTS sends the BSC many ChannelActivation Negative Acknowledge messages.

Handling Suggestionsl Check whether the channels are busy or blocked and whether the TRXs are functional. Add

TRXs to the BTS, modify the access threshold, and enable the directed retry.

l Check whether the channel states on the BSC and the BTS are the same. If not, you caninfer that the transmission on the Abis interface is unstable. Check also the BTS boards.



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