Radio Resource Allocation

5.5 Radio Resource Allocation

This section describes the procedures enabling the assignment of radio resource to the mobile and in consequence the establishment of a TBF. These procedures correspond to the transition from packet idle mode to packet transfer mode. The procedures used for the report of measurements are also detailed.

As described in Chapter 3, two kinds of TBF can be established:

  • Uplink TBF assigning uplink radio resource to the mobile with a downlink signaling channel;

  • Downlink TBF assigning downlink resources on shared PDCHs with an uplink signaling channel.

Two simultaneous TBFs assigned to the same MS in opposite direction (uplink and downlink) are said to be concurrent.

5.5.1 Uplink TBF Establishment

The mobile triggers the establishment of an uplink TBF for three major reasons:

  • To perform an uplink data transfer;

  • To answer a paging;

  • To perform a GMM procedure (e.g., RA update procedure and GPRS attach procedure).

5.5.1.1 RACH/PRACH Phase

Access Request

The mobile triggers the establishment of an uplink TBF:

  • By sending a CHANNEL REQUEST message on the RACH when there is no PBCCH in the cell;

  • By sending a PACKET CHANNEL REQUEST message on the PRACH if PBCCH is present in the cell.

The mobile may also request the establishment of an uplink TBF during a downlink TBF on PACCH.

There are two codings for the PACKET CHANNEL REQUEST message. One format allows the transport of 8 bits information and the other one 11 bits information. The CHANNEL REQUEST message carries 8 bits of information.

The CHANNEL REQUEST message is used for both circuit-switched access and packet access. This message contains 8 bits of information, allowing a limited number of uplink combinations. As some of the combinations are reserved for GSM access, fewer combinations are available for packet access.

On PRACH, no circuit-switched access can occur. All the bit combinations of 8 or 11 bits of information within the access message are reserved for packet access only. Thus, access through the PRACH allows the mobile to send more precise information on its capability and requirements in order to establish as quickly as possible an uplink TBF.

Note 

On the RACH, the MS is unable to indicate its multislot capability in the CHANNEL REQUEST message. In order to use its maximum capability, it will be necessary to establish a TBF in two phases. The other possibility is to start the uplink transfer using only one time slot; once the mobile is identified, the BSS can then request the mobile capabilities to the SGSN and finally extend the allocation when they will be received. Even if the multislot capability of the mobile is given to the network during the GPRS attach procedure, the BSS does not have this information when it receives the access request. This information is only available at the SGSN side. The two-phase access procedure is described in the following sections.

As shown in Figure 5.5, the CHANNEL REQUEST and PACKET CHANNEL REQUEST messages are composed of two parts:

  • One establishment cause;

  • One random reference.

Click To expand Figure 5.5: Channel request and packet channel request format.

The random reference length is from 2 to 5 bits depending on the establishment cause.

The random reference reduces the probability that two MSs requiring the establishment of a TBF send exactly the same message in the same RACH or PRACH occurrence. This could happen, but the problem is solved by the contention resolution procedure described later.

When the MS requests the establishment of an uplink TBF, it randomly chooses the few bits that are sent within the random reference field of the access request message. When the network assigns the resources to the mobile it returns the request reference (random reference, establishment cause, and FN in which the AB was received) in order for the MS to correlate the assignment and the request. If two mobiles have sent an AB at the same time using different request references, the one that is not addressed by the returned request reference value will abort the access procedure.

The two basic procedures used for the establishment of an uplink TBF are:

  • The one-phase access procedure;

  • The two-phase access procedure.

Which type of procedure is used is indicated or deduced from the establishment cause of the access request message.

The different establishment causes that can be signaled by the mobile within the PACKET CHANNEL REQUEST message are:

  • One-phase access;

  • Two-phase access;

  • Short access (used when the mobile wants to transfer an amount of data that is less than 8 RLC/MAC blocks);

  • Page response (used in response to a paging);

  • Cell update (used to trigger a cell update procedure);

  • MM procedure;

  • Single block without TBF establishment (used to send a measurement report to the network).

For the short access, page response, cell update, and MM procedure access types, the TBF is established using one-phase access.

Within the CHANNEL REQUEST message, only two establishment causes are possible for packet access: the one-phase access and the single-block packet access. This last one is used to initiate the two-phase access procedure.

When the mobile requests a short access, one-phase access, or two-phase access using an 11-bits PACKET CHANNEL REQUEST message, it has to indicate the radio priority of the TBF. The highest radio priority is indicated when the TBF is used for signaling purposes.

If the mobile wants to establish a TBF in the RLC unacknowledged mode, it must request a two-phase access procedure. By default, the one-phase access requests a TBF in RLC acknowledged mode.

The two-phase access procedure is longer than the one-phase access, as it requires the exchange of four messages (see section below) rather than two as in the one-phase access.

The advantage of the two-phase access is that it allows the mobile to give more precision on its capabilities and requirements. As previously mentioned, during a one-phase access on the RACH, the mobile cannot provide its multislot capability. So in case of long uplink TBF, it is interesting for the mobile to request a two-phase access allowing a higher throughput thanks to the allocation of more time slots in uplink. On PRACH, the mobile can indicate its multislot class for a one-phase access within the PACKET CHANNEL REQUEST message.

Access Persistence Control on PRACH

It could happen that two MSs try to access the network at the same time and send their ABs on the same PRACH occurrence. In this situation, a collision is detected at the BTS side and if the two ABs are received with approximately the same power, neither of the two ABs is decoded. The access persistence control allows that the two mobiles avoid retransmitting ABs in the same PRACH occurrence in their next attempt. For that the occurrence in which the mobile will send the next AB is randomly determined. This mechanism is controlled dynamically by the network through the broadcast of access persistence control parameters on the PBCCH and PCCCH. The control of access persistence on PRACH is used to limit the collision probability on this channel.

The parameters involved in access persistence control are listed below.

  • MAX_RETRANS indicates the maximum number of PACKET CHANNEL REQUEST message retransmissions the mobile is allowed to do.

  • PERSISTENCE_LEVEL is a threshold whose usage is described below. The range of this parameter is {0, 1, 2, ..., 14, 16}.

  • S indicates the minimum number of PRACH occurrences between two consecutive PACKET CHANNEL REQUEST messages.

  • TX_INT defines the spreading interval (in terms of PRACH occurrences) of the random access.

Whenever the mobile attempts to send a PACKET CHANNEL REQUEST message, it has to draw a random value with uniform probability distribution in the set {0, 1, 2, ......., 15}. The MS is allowed to transmit a PACKET CHANNEL REQUEST message if the random value is greater than or equal to PERSISTENCE_LEVEL. The next PRACH occurrence in which the MS attempts to send a PACKET CHANNEL REQUEST message is determined by the parameters S and TX_INT. This is illustrated in Figure 5.6. After MAX_RETRANS + 1 attempts to send a PACKET CHANNEL REQUEST message, the mobile is not allowed to transmit any more access messages.

Click To expand
Figure 5.6: Access persistence control on PRACH.

Note that the PERSISTENCE_LEVEL and MAX_RETRANS parameters depend on the radio priority of the TBF. Four different radio priorities exist. Four values for each two parameters are broadcast by the network. The mobile chooses the one corresponding to the radio priority of the TBF it wants to establish.

Depending on the PRACH load and the number of collisions detected on the PRACH, the network can adjust the different access persistence control parameters, in order to regulate the load on PRACH and reduce the number of collisions. This ensures a constant throughput on the PRACH.

In order to reduce the load on the PRACH, the network can also forbid access to mobiles belonging to some access control classes. The access control class is a subscriber parameter. It is used to favor some subscribers in relation to others for accessing the network. The parameter ACC_CONTR_CLASS, which is broadcast on PBCCH, indicates which access control classes are allowed to access the network.

5.5.1.2 Uplink TBF Establishment on CCCH

When PBCCH is not present in the cell and a mobile in packet idle mode wants to establish an uplink TBF, it performs access on CCCH.

One-Phase Access Procedure on CCCH

Figure 5.7 describes the scenario for one-phase access uplink TBF establishment on CCCH. This procedure allows the allocation of only one time slot to the MS, even if its multislot class would have allowed more. This is due both to the impossibility of signaling the multislot class within the CHANNEL REQUEST message and to a limitation in the length of the IMMEDIATE ASSIGNMENT message.

Click To expand
Figure 5.7: One-phase access establishment scenario on CCCH.

The MS initiates this procedure by sending a CHANNEL REQUEST message on the RACH. The CHANNEL REQUEST message contains the establishment cause and the random value as described in the previous section. The establishment cause for this procedure is one-phase access.

Note that the network does not have to respect the one-phase access request of the mobile and may force a two-phase access procedure.

Upon reception of the CHANNEL REQUEST message, the BSS sends an IMMEDIATE ASSIGNMENT message on AGCH. This message contains the following information:

  • Request reference. This includes the contents of the CHANNEL REQUEST message and the FN in which it was received.

  • TA parameters. These include the TA index and the TA TN if continuous TA is implemented by the network and the initial TA. In this case the TA TN corresponds to the allocated PDCH.

  • Uplink TFI. This parameter identifies the uplink TBF.

  • Channel coding command. This parameter indicates to the mobile which coding scheme (CS-1, CS-2, CS-3, or CS-4) to use for uplink data transmission.

  • Packet channel description. This indicates the allocated TN, the training sequence code, and the frequency parameters.

  • TLLI block channel coding. This is used to indicate the coding scheme that must be used for data transmission during the contention resolution phase. The value could be CS-1 or the previous channel coding command. The contention resolution procedure is described later.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • Medium access parameters. These indicate the USF value in case of dynamic allocation and the fixed allocated bitmap in case of fixed allocation (refer to Section 3.3.2.3 for more details on the medium access methods).

Note that if the initial TA is not provided, the MS must await the reception of a correct TA value given during the continuous TA procedure before starting to transmit.

The assigned PDCH must be in the same frequency band as the BCCH, since the bands supported by the mobile are unknown at this time.

The network may request an acknowledgment from the mobile. The acknowledgment is requested by setting the polling bit in the IMMEDIATE ASSIGNMENT message. If the polling bit is set to 1, the mobile sends a PACKET CONTROL ACKNOWLEDGMENT message on the assigned PDCH, in the uplink block specified by the TBF starting time parameter. In this case the TBF starting time is used to indicate when the assigned PDCH becomes valid and when the uplink block for PACKET CONTROL ACKNOWLEDGMENT message is sent. The TBF starting time indicates the FN in which the uplink TBF starts.

The PACKET CONTROL ACKNOWLEDGMENT message is sent as either a normal RLC/MAC control block or as four consecutive identical ABs. The format of this message depends on the CONTROL_ACK_TYPE parameter value broadcast on BCCH or PBCCH.

Note that the network may request the sending of a PACKET CONTROL ACKNOWLEDGMENT message in order to be sure that the mobile has received the uplink assignment and to avoid the allocation of uplink resources that will not be used, particularly in case of dynamic allocation.

At the end of the assignment procedure, the mobile enters into contention resolution phase. This procedure is described later in this chapter.

Two-Phase Access Procedure on CCCH

The mobile must initiate a two-phase access procedure when it wants to establish a TBF in RLC unacknowledged mode. This procedure may be requested in other cases not specified in the GSM standard (e.g., if the mobile wants to establish an uplink TBF with multislot allocated on CCCH).

Note that when a two-phase access is requested by the mobile, the network must use this procedure to establish the uplink TBF.

Figure 5.8 describes the scenario for two-phase access uplink establishment on CCCH.

Click To expand
Figure 5.8: Two-phase access establishment scenario on CCCH.

The mobile initiates the two-phase access procedure by sending a CHANNEL REQUEST message on RACH requesting a single-block packet access. Upon receipt of the CHANNEL REQUEST message, the network sends an IMMEDIATE ASSIGNMENT message to the mobile on AGCH.

This message allocates one single uplink block to the mobile on a PDCH.

The IMMEDIATE ASSIGNMENT message contains the following parameters:

  • Request reference. This includes the contents of the CHANNEL REQUEST message and the FN in which it was received.

  • Packet channel description. This indicates the allocated TN, the training sequence code, and the frequency parameters.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • TBF starting time. This indicates the FN in which the mobile will start sending its uplink single block.

  • Initial TA. This is used to transmit in the single block allocation.

Up to this point, no TFI is allocated to the mobile. The network does not know the exact reason for the establishment cause (the mobile may have requested a single-block packet access for the mere purpose of sending a measurement report to the network; see Section 5.5.3).

In this uplink block occurrence, the mobile sends a PACKET RESOURCE REQUEST message in order to indicate the two-phase access request. It contains the following information:

  • TLLI. This parameter uniquely identifies the mobile.

  • Access type. This indicates the reason for requesting the access (in this example, two-phase access).

  • Channel request description. This indicates the peak throughput class for the PDP context of the LLC PDU, the radio priority, the RLC mode, the type of the first LLC PDU, and the number of RLC data octets of the requested TBF.

  • MS radio access capability. This indicates the mobile capabilities in terms of multislot class and RF power for the different frequency bands that are supported by the mobile.

With all this information, the network is able to allocate uplink resources in a more efficient way. The requested TBF properties are used by the network to select the multiplexing level matching. Depending on the QoS parameters, more or less bandwidth will be allocated to the mobile. The mobile that will share the same uplink PDCHs as other mobiles will have more or less uplink resources depending on these parameters. The radio access capability parameters are used to take maximum advantage of the mobile capabilities. The allocated PDCHs can be in any frequency band that is supported by the mobile.

Note that if the mobile indicates 0 for the number of RLC data octets that have to be transferred, the network interprets the TBF as an open-ended TBF. In this case, the number of uplink resources needed is undetermined. This information concerns only the fixed-allocation multiplexing scheme for which it is very important for the network to know the exact amount of uplink data in order to optimize the allocation of uplink resources. At this time in this procedure, the mobile does not know the uplink multiplexing scheme that is used by the network. Thus this information will always be provided.

Upon receipt of the PACKET RESOURCE REQUEST message, the BSS sends a PACKET UPLINK ASSIGNMENT message on PACCH assigning uplink resources to the mobile. This message contains the following parameters:

  • TLLI. This is used to address the block in downlink.

  • TA parameters. These include the TA index and the TA TN if continuous TA is implemented by the network.

  • Uplink TFI. This parameter identifies the uplink TBF.

  • Channel coding command. This parameter indicates to the mobile which coding scheme (CS-1, CS-2, CS-3, or CS-4) to use for uplink data transmission.

  • Packet channel description. This indicates the training sequence code and the frequency parameters.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • Medium access parameters. These include USF values on the different allocated time slots in case of dynamic allocation or extended dynamic allocation, and time slot allocation, downlink control timeslot (downlink PACCH), and the fixed-allocation bitmap parameters in case of fixed allocation.

The network provides a TBF starting time in case of fixed allocation. It indicates the FN in which the bitmap starts to be valid. In case of dynamic or extended dynamic allocation, the TBF starting time is optional. If it is provided, the mobile starts listening to the USF in the FN indicated.

The network can poll the mobile within the PACKET UPLINK ASSIGNMENT message to request the sending of a PACKET CONTROL ACKNOWLEDGMENT message. The polling is performed using the S/P and RRBP fields of the downlink control block MAC header.

5.5.1.3 Uplink TBF Establishment on PCCCH

When PBCCH is present in the cell and a mobile in packet idle mode wants to establish an uplink TBF, it performs access on PCCCH.

One-Phase Access Procedure on PCCCH

Figure 5.9 describes the scenario for one-phase access uplink establishment on PCCCH. The mobile triggers the establishment of an uplink TBF using the one-phase access procedure by sending a PACKET CHANNEL REQUEST message on PRACH indicating one of the following access types: one-phase access, short access, page response, cell update, or MM procedure.

Click To expand
Figure 5.9: One-phase access establishment scenario on PCCCH.

Note that despite the request of the mobile, the network can force a two-phase access procedure when a one-phase access is requested.

The PACKET CHANNEL REQUEST message contains either 8 or 11 bits of information depending on the ACCESS_BURST_TYPE parameter, which is broadcast on PBCCH. The establishment cause of the PACKET CHANNEL REQUEST message contains the multislot class of the mobile. Moreover, if the message is sent in the 11-bit format, it also contains the radio priority of the TBF. The network is then able to assign uplink resources matching the multislot capability of the mobile.

The radio priority parameter, if available, can be used by the network to prioritize the request in relation to others received from other mobiles. The network sends the PACKET UPLINK ASSIGNMENT message on the PCCCH on which the request has been received. This message contains the same parameters as described in Section 5.5.1.2 for one-phase access procedure on CCCH, except that the packet request reference is given instead of the request reference and the medium access parameters handle more than one time slot allocation.

The medium access parameters consists of:

  • USF values on the different allocated time slots in case of dynamic allocation or extended dynamic allocation;

  • Time slot allocation, downlink control time slot, and the fixed-allocation bitmap parameters in case of fixed allocation.

The allocated PDCH will be in the same frequency band as the PBCCH because the radio access technology types of the mobile are not known at this time.

When the mobile receives the PACKET UPLINK ASSIGNMENT message, it enters into packet transfer mode and the contention resolution phase starts. The network can poll the mobile within the PACKET UPLINK ASSIGNMENT message to request the sending of a PACKET CONTROL ACKNOWLEDGMENT message. The polling is performed by using the S/P and RRBP fields of the downlink control block MAC header.

Two-Phase Access Procedure on PCCCH

The mobile requests a two-phase access request when it wants to establish a TBF in RLC unacknowledged mode. This procedure may also be requested by the mobile to establish a TBF in RLC acknowledged mode. Figure 5.10 describes the scenario for two-phase access uplink TBF establishment on PCCCH when the mobile is in packet idle mode.

Click To expand
Figure 5.10: Two-phase access establishment scenario on PCCCH.

The MS requests the establishment of a TBF in two-phase access by sending a PACKET CHANNEL REQUEST message on PRACH. The establishment cause within the PACKET CHANNEL REQUEST message indicates two-phase access.

Note that the network must respect the request of the mobile.

If the AB is sent with the 11-bit format, the radio priority is also indicated within the establishment cause.

On receipt of the access request, the network allocates a single uplink block to the mobile by sending a PACKET UPLINK ASSIGNMENT message. This message contains the following information:

  • Packet request reference. This includes the contents of the PACKET CHANNEL REQUEST message and the FN in which it was received.

  • Packet channel description. This indicates the training sequence code and the frequency parameters.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • TBF starting time. This indicates the FN in which the mobile will send the PACKET RESOURCE REQUEST message.

  • Packet TA parameters. The initial TA is given to transmit in the single block allocation.

  • Time slot number. This indicates the time slot on which the block must be sent.

The end of the procedure is exactly the same as the one for two-phase establishment on CCCH (refer to Section 5.5.1.2).

Packet Access Queuing Notification Procedure

Whenever the network receives an access request from a mobile on PRACH and it cannot satisfy the request, a queuing notification may be sent to the mobile. This will indicate to the mobile that its request has been correctly received by the network and taken into account, but the network is temporarily unable to satisfy it. From a network point of view, this procedure reduces PRACH congestion. When the mobile receives the queuing notification, it will stop trying to access the cell. This procedure is only supported on PCCCH.

Figure 5.11 describes an example of uplink access establishment preceded by a temporary queuing phase.

Click To expand
Figure 5.11: Packet queuing notification procedure.

The mobile requests the establishment of a TBF by sending a PACKET CHANNEL REQUEST message on PRACH. Because of congestion or other reasons, the network is unable to satisfy the request. A PACKET QUEUING NOTIFICATION message is sent on the PCCCH on which the PACKET CHANNEL REQUEST message was received.

The PACKET QUEUING NOTIFICATION message contains the following information:

  • Packet request reference. This includes the contents of the PACKET CHANNEL REQUEST message and the FN in which it was received.

  • Temporary queuing identifier (TQI). This parameter identifies the request of the mobile.

When the mobile receives the queuing indication, it starts monitoring its downlink PCCCH. The queuing duration is handled by a timer at the MS and BSS side.

Whenever uplink resources again become available at network side, the BSS sends a PACKET UPLINK ASSIGNMENT message to the mobile assigning uplink resource corresponding to the mobile requests. The mobile is addressed within the PACKET UPLINK ASSIGNMENT message using the TQI.

5.5.1.4 Uplink TBF Establishment When Downlink TBF Is Already Established

The mobile has the possibility of establishing an uplink TBF when it is in packet transfer mode, thus during a downlink TBF. Figure 5.12 describes this procedure.

Click To expand
Figure 5.12: Procedure for uplink establishment when the MS is in Packet Transfer Mode.

The mobile requests the establishment of an uplink transfer during a downlink TBF by including a channel request description within the PACKET DOWNLINK ACK/NACK message. This message is used by the mobile to acknowledge the RLC data blocks received in downlink. Its sending on the PACCH is controlled by the polling mechanism on a network order.

The channel request description contains the same information as described in Section 5.5.1.2 for the two-phase access procedure on CCCH.

When the network detects the channel request description within a PACKET DOWNLINK ACK/NACK message, it allocates uplink resources (if available) to the mobile by sending a PACKET UPLINK ASSIGNMENT message, which contains the following information:

  • Downlink TFI or TLLI. This parameter is used to address the mobile in downlink.

  • TA information. In case of continuous TA, the network may allocate a new time slot and TAI for the procedure.

  • Channel coding command. This indicates the coding scheme for use in the uplink direction.

  • Medium access parameters. This was described in Section 5.5.1.2 for the two-phase access procedure on CCCH.

  • TBF starting time. This indicates the FN in which the uplink TBF starts.

  • Uplink TFI. This identifies the uplink TBF.

Note that:

  1. When the BSS allocates the uplink resources it must respect the multislot class of the mobile. As a downlink TBF already exists, the uplink resources assigned must be compatible with the downlink ones. If the network is not able to allocate uplink resources matching the downlink ones, it can reallocate at the same time the uplink and downlink resources by using the PACKET TIMESLOT RECONFIGURE message. This message allows reallocation of both uplink and downlink resources.

  2. When the MS receives the PACKET UPLINK ASSIGNMENT or PACKET TIMESLOT RECONFIGURE message, it will not enter in contention resolution phase since the mobile is already uniquely identified at the BSS side.

  3. The network could reject the request by sending a PACKET ACCESS REJECT message on the PACCH.

5.5.1.5 Uplink TBF Modification

The network can modify at any time the uplink TBF by sending either a new PACKET UPLINK ASSIGNMENT message as illustrated in Figure 5.13 or a PACKET TIMESLOT RECONFIGURE message on the PACCH. The PACKET TIMESLOT RECONFIGURE message is only used when there are two concurrent TBFs already established. Through this procedure, the network can modify the multislot allocation of the mobile.

Click To expand
Figure 5.13: Modification of the uplink TBF initiated by the network.

Such a procedure could be triggered when there is a change of service requested by the MS. In fact, during an uplink transfer, upper layers may request the transfer of an LLC PDU with a different radio priority or peak throughput class. The modification of the service demand is requested by the mobile by sending a PACKET RESOURCE REQUEST message on the PACCH including the channel request description.

This procedure can also be triggered by the network in the following situations:

  • In case of modification of the allocated PDCHs in the cell (e.g., increase of circuit-switched traffic and thus reduction of the number of PDCHs in the cell);

  • In case the link quality on the allocated PDCHs for the mobile is not sufficient.

The network can provide a new TFI assignment. The new allocation takes effect in the FN indicated by the TBF starting time.

5.5.1.6 Contention Resolution Procedure

It could happen that two mobiles trying to access the network send the same CHANNEL REQUEST message (respectively PACKET CHANNEL REQUEST message) within the same RACH occurrence (respectively PRACH occurrence). It may also happen that the two mobiles use the same establishment cause and the same packet reference request.

If the two messages are received at the BTS side with approximately the same level, the BTS will detect a collision on RACH or PRACH and then no access message will be decoded. The two mobiles will try to access the network later on, and the probability that the same occurrence is chosen is low. This does not cause any problem.

However, if one mobile is near the BTS and the other one is far away, due to the capture effect there is a high probability that the BTS decodes the message that was sent by the nearest mobile. In this case, the BTS will allocate uplink resources by sending a PACKET UPLINK ASSIGNMENT message. Since the two mobiles have transmitted the same message in the same RACH or PRACH occurrence, they will both identify their packet reference request and behave as if the assignment message was for them. In this case the two MSs will transmit in the same uplink PDTCH occurrence as described in Figure 5.14.

Click To expand
Figure 5.14: Contention at TBF establishment.

In order to avoid having one of the two MSs receive data that is not intended for it, the mobile, during the contention resolution phase, must not accept the establishment of a concurrent TBF. It must also not accept control messages such as PACKET CELL CHANGE ORDER message and PACKET POWER CONTROL/TIMING ADVANCE. For security reason, this shall be avoided.

Contention Resolution at One-Phase Access

In order to avoid this problem after uplink TBF establishment, the MS must insert its TLLI within each uplink RLC data blocks until the contention resolution period is finished (see Figure 5.15). The TLLI uniquely identifies an MS within one RA.

Click To expand
Figure 5.15: Contention resolution at one-phase access.

The uplink RLC data blocks, which contain the TLLI, are coded using the TLLI channel coding command, which is indicated in the assignment message. At the decoding of the first uplink RLC data block including a TLLI, the network sends a PACKET UPLINK ACK/NACK with the TLLI that was received in the uplink RLC data block. At this time, the contention resolution is completed at network side.

If two mobiles had performed an access on the same RACH or PRACH occurrence and sent the same message, the contention resolution would fail for the mobile that received the PACKET UPLINK ACK/NACK message including its assigned TFI but with a TLLI value other than that which the MS included in the RLC header of the uplink data blocks.

The contention resolution is completed at the mobile side when the mobile to which the TLLI belongs receives the PACKET UPLINK ACK/NACK message. It continues the uplink transfer without including the TLLI in the uplink RLC data blocks and using the channel coding command for data block encoding.

Contention Resolution at Two-Phase Access

In this case, the contention resolution is completed on the network side when it receives the PACKET RESOURCE REQUEST message including the TLLI. The contention resolution is completed at mobile side when it receives the second uplink assignment message with its TLLI included. The contention resolution fails at MS side when it receives the second assignment message with a TLLI value other than that which the MS has included in the PACKET RESOURCE REQUEST message.

5.5.2 Downlink TBF Establishment

The establishment of a downlink TBF is performed on the CCCH if there is no PBCCH in the cell; otherwise, it is performed on PCCCH. It can also be performed on PACCH during an uplink transfer.

The BSS initiates the establishment of a downlink TBF when it receives a downlink LLC PDU from the SGSN that must be transmitted to a mobile, and this MS is not already in downlink transfer. The transmission of an LLC PDU from the SGSN to the BSS is only allowed when the mobile is located at cell level within the SGSN. If this is not the case, the SGSN must start a paging procedure from which it will recover this information (see Chapter 3). At the end of the paging procedure, the mobile is in GMM READY state.

When the SGSN transmits a downlink LLC PDU that must be sent to the mobile, it indicates to the BSS the cell in which the mobile is located, the TLLI identifying the mobile, the MS radio access capability parameters (indicating the multislot class of the MS, the RF power capability, and the supported frequency bands) and the QoS profile. These parameters are used by the BSS to address the mobile and derive the downlink allocation for the transfer of the LLC PDU.

5.5.2.1 Downlink TBF Establishment on CCCH

Figure 5.16 describes an example of downlink TBF establishment on CCCH when the mobile is in packet idle mode.

Click To expand
Figure 5.16: Downlink TBF establishment on CCCH.

When the network receives a downlink LLC PDU to transmit to the mobile, it initiates the establishment of a downlink TBF by sending an IMMEDIATE ASSIGNMENT message to the MS on CCCH. The IMMEDIATE ASSIGNMENT message is sent on any block of the CCCH if the mobile is in non-DRX mode; otherwise, it is sent on one block corresponding to the paging group of the mobile.

The IMMEDIATE ASSIGNMENT message contains the following parameters:

  • TLLI. This indicates the identity of the mobile for which the message is intended.

  • Downlink TFI. This is the identifier of the downlink TBF.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • Packet channel description. This indicates the allocated TN, the TS code, and the frequency parameters.

  • RLC mode. This indicates whether the TBF is in RLC acknowledged mode or RLC unacknowledged mode.

  • TA parameters. These include the TA index and the TA TN if continuous TA is implemented by the network. The initial TA may not be present, as the network has no information on it except perhaps from a previous TBF.

Within the IMMEDIATE ASSIGNMENT message, the BSS is not able to allocate more than one time slot in downlink to the MS despite the fact that it knows the MS multislot class received from the SGSN. This is due to a limitation in the IMMEDIATE ASSIGNMENT message size.

Initial TA Problem

In most cases, no initial TA value can be provided to the mobile in the first assignment message. The mobile is not allowed to transmit before having received a correct TA value. If the continuous TA procedure is used, it may take up to two seconds before the MS receives a usable TA value and then is allowed to transfer in uplink. This could delay the TBF.

In order to accelerate the acquisition of the initial TA value, the network can request the sending of a PACKET CONTROL ACKNOWLEDGMENT message by setting the polling bit to 1 in the IMMEDIATE ASSIGNMENT message.

The mobile sends the PACKET CONTROL ACKNOWLEDGMENT message on the assigned PDCH, in the uplink block specified by the TBF starting time parameter. The TBF starting time indicates when the assigned PDCH becomes valid for the downlink transfer.

The PACKET CONTROL ACKNOWLEDGMENT message can be sent as either a normal RLC/MAC control block or as four consecutive identical ABs. The format of this message depends on the CONTROL_ACK_TYPE parameter value, broadcast on BCCH or PBCCH.

If the CONTROL_ACK_TYPE indicates AB type, the mobile sends the four consecutive ABs with a TA of zero. The BTS deduces the initial TA at the reception of these ABs. This estimation can be sent in a PACKET POWER CONTROL/TIMING ADVANCE message. The network will not use the combination polling plus CONTROL_ACK_TYPE set to AB type, as the mobile is not allowed to transmit without a valid TA value. Figure 5.17 describes this procedure.

Click To expand
Figure 5.17: Downlink TBF establishment with initial TA computation.

Note that another solution consists of sending a PACKET DOWNLINK ASSIGNMENT message rather than sending a PACKET POWER CONTROL/TIMING ADVANCE message. This message can be used to provide both the initial TA and a PDCH allocation that takes advantage of the MS multislot class.

5.5.2.2 Downlink TBF Establishment on PCCCH

Figure 5.18 describes an example of downlink TBF establishment on PCCCH when the mobile is in packet idle mode.

Click To expand
Figure 5.18: Example of downlink TBF establishment on PCCCH.

When the network receives a downlink LLC PDU to transmit to the mobile, it initiates the establishment of a downlink TBF by sending a PACKET DOWNLINK ASSIGNMENT message to the MS on PCCCH. The PACKET DOWNLINK ASSIGNMENT message is sent on any block of the PCCCH where paging may appear if the mobile is in non-DRX mode; otherwise, it is sent on one block corresponding to the paging group of the mobile.

The PACKET DOWNLINK ASSIGNMENT message contains the following parameters:

  • TLLI. This identifies the mobile for which the message is intended.

  • Downlink TFI. This is the identifier of the downlink TBF.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • MAC mode. This indicates the medium access method, which will be used if an uplink TBF is established during this downlink transfer.

  • RLC mode. This indicates whether the TBF is in RLC acknowledged mode or RLC unacknowledged mode.

  • Time slot allocation. This indicates the time slot assigned to the downlink allocation.

  • Packet TA parameters: These include the TA index and the TA TN if continuous TA is implemented by the network. The initial TA may not be present because the network has no information on it except perhaps from a previous TBF.

  • Frequency parameters. These give the frequency or the list of frequencies that are used on PDCHs.

  • TBF starting time. If present, this indicates the FN in which the downlink TBF starts.

The BSS polls the mobile within the PACKET DOWNLINK ASSIGNMENT message requesting the sending of a PACKET CONTROL ACKNOWLEDGMENT message in four ABs. The polling is performed by using the S/P and RRBP fields of the downlink control block MAC header.

The BTS computes the initial TA that is indicated to the mobile by sending a PACKET POWER CONTROL/TIMING ADVANCE message on PACCH.

5.5.2.3 Downlink TBF Establishment When Uplink TBF Is Already Established

Whenever the BSS receives a downlink LLC PDU to transmit to a mobile for which an uplink TBF is already established, the BSS initiates the downlink TBF on the PACCH of the uplink TBF.

The BSS can initiate the downlink TBF establishment by sending either a PACKET DOWNLINK ASSIGNMENT message or a PACKET TIMESLOT RECONFIGURE message. The first allocates downlink resources without changing the uplink allocation. The second allows the network to change the uplink allocation if necessary and allocate downlink resources.

Figure 5.19 describes an example of downlink TBF establishment when the mobile is in packet transfer mode.

Click To expand
Figure 5.19: Packet downlink establishment on the PACCH.

The BSS sends a PACKET TIMESLOT RECONFIGURE message on the downlink PACCH in order to establish a downlink TBF. This message can be used to reassign uplink resources to the mobile.

The PACKET TIMESLOT RECONFIGURE message contains the following information:

  • Uplink TFI. This is used to identify the uplink TBF; in this case, to address the mobile on the downlink PACCH.

  • Downlink assign TFI. This is the assigned TFI for the downlink TBF.

  • Channel coding command. This parameter indicates to the mobile which coding scheme (CS-1, CS-2, CS-3, or CS-4) is to be used for uplink data transmission.

  • Power control parameters. These indicate the downlink power control mode and the uplink power control parameters.

  • Downlink time slot allocation. This is the PDCH allocation for the downlink transfer.

  • Downlink RLC mode. This indicates either RLC acknowledged mode or RLC unacknowledged mode.

  • Packet TA parameters. Includes the TA index and the TA TN if continuous TA is implemented by the network, and optionally the initial TA.

  • TBF starting time. This indicates in which FN the resource reassignment takes place.

  • Frequency parameters. These parameters are optionally given if the network wants to change the frequency configuration of the PDCHs.

  • Medium access parameters. These describe the uplink access method, either fixed allocation or dynamic allocation or extended dynamic allocation.

5.5.2.4 Downlink TBF Modification

The BSS initiates resource reassignment by sending a PACKET DOWNLINK ASSIGNMENT message or a PACKET TIMESLOT RECONFIGURE message on PACCH. The PACKET TIMESLOT RECONFIGURE message is only sent when there is both an uplink and downlink TBF established (or the network wants to establish an uplink TBF during downlink transfer).

During this procedure, the network is not allowed to change the RLC mode or the TBF mode (medium access method). A change of RLC mode or TBF mode can only occur after a release of the TBF.

The resource reassignment takes effect in the FN indicated by the TBF starting time of the assignment message.

5.5.3 Measurement Report in Packet Idle Mode

In packet idle mode, as long as the mobile is in GMM READY state and the cell reselection mode is NC1 or NC2, the mobile regularly reports measurements to the network. The MEASUREMENT REPORT message fits in one radio block. As the importance of this message is not vital for the system (single messages can be lost), it has been decided to transfer this message without TBF establishment and without acknowledgment of the message (no RLC procedure is used). In order to transfer its measurements, the mobile simply requests one uplink radio block occurrence from the network.

5.5.3.1 Measurement Report Procedure on CCCH

When the MS is in packet idle mode and it must send a PACKET MEASUREMENT REPORT message, it requests via the CHANNEL REQUEST message a single block packet access. Upon receipt of the request, the network sends an IMMEDIATE ASSIGNMENT message, assigning one single uplink block to the mobile. The mobile sends a PACKET MEASUREMENT REPORT message in this occurrence. The information that is provided to the mobile in the assignment message is the same as that in the first assignment message during the two-phase access on CCCH. Figure 5.20 describes the measurement report procedure on CCCH.

Click To expand
Figure 5.20: Measurement report procedure on CCCH in packet idle mode.

5.5.3.2 Measurement Report Procedure on PCCCH

On PCCCH, the mobile initiates the measurement report procedure by sending a PACKET CHANNEL REQUEST message indicating "single block without TBF establishment" as the establishment cause.

The BSS allocates one uplink block occurrence by sending a PACKET UPLINK ASSIGNMENT message. The mobile transmits a PACKET MEASUREMENT REPORT message in the allocated uplink occurrence. Figure 5.21 describes this procedure.

Click To expand
Figure 5.21: Measurement report procedure on PCCCH in packet idle mode.


 
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