6.6 Medium Access Control and Radio Link Control Layer

03.643GPPGeneral Packet Radio Service (GPRS)Overall description of the GPRS radio interfaceStage 2TS

The Medium Access Control (MAC) and Radio Link Control (RLC) layer operates above the Physical Link layer in the reference architecture. MAC/RLC layer messages and signalling procedures are defined in GSM 04.60 [7] and GSM 04.08 [6].

6.6.1 Layer Services

The MAC function defines the procedures that enable multiple MSs to share a common transmission medium, which may consist of several physical channels. The MAC function provides arbitration between multiple MSs attempting to transmit simultaneously and provides collision avoidance, detection and recovery procedures. The operations of the MAC function may allow a single MS to use several physical channels in parallel.

The RLC function defines the procedures for a bitmap selective retransmission of unsuccessfully delivered RLC Data Blocks.

The RCL/MAC function provides two modes of operation:

– unacknowledged operation; and

– acknowledged operation

6.6.2 Layer Functions

The GPRS MAC function is responsible for:

– Providing efficient multiplexing of data and control signalling on both uplink and downlink, the control of which resides on the Network side. On the downlink, multiplexing is controlled by a scheduling mechanism. On the uplink, multiplexing is controlled by medium allocation to individual users (e.g., in response to service request).

– For mobile originated channel access, contention resolution between channel access attempts, including collision detection and recovery.

– For mobile terminated channel access, scheduling of access attempts, including queuing of packet accesses.

– Priority handling.

The GPRS RLC function is responsible for:

– Interface primitives allowing the transfer of Logical Link Control layer PDUs (LLC-PDU) between the LLC layer and the MAC function.

– Segmentation and re-assembly of LLC-PDUs into RLC Data Blocks.

– Backward Error Correction (BEC) procedures enabling the selective retransmission of uncorrectable code words.

  • Transmission of code words according to the channel conditions,i.e link adaptation.

NOTE: The Block Check Sequence for error detection is provided by the Physical Link Layer.

In EGPRS incremental redundancy (IR) mode, RLC function is also responsible for:

– Storing soft values of the erroneous RLC Data Blocks and combining them with the retransmitted RLC Data blocks.

6.6.3 Service Primitives

Table 6 lists the service primitives provided by the RLC/MAC layer to the upper layers:

Table 6: Service primitives provided by the RLC/MAC layer to the upper layers

Name

request

indication

response

confirm

comments

RLC/MAC-DATA

x

x

used for the transfer of upper layer PDUs. Acknowledged mode of operation in RLC is used. The upper layer shall be able to request high transmission quality via a primitive parameter.

RLC/MAC-UNITDATA

x

x

used for the transfer of upper layer PDUs. Unacknowledged mode of operation in RLC is used.

RLC/MAC-STATUS

x

used to indicate that an error has occurred on the radio interface. The cause for the failure is indicated.

6.6.4 Model of Operation

Each PDCH is a shared medium between multiple MSs and the Network except in dual transfer mode, in which a PDCH may be dedicated to one MS. Direct communication is possible only between an MS and the network.

The GPRS radio interface consists of asymmetric and independent uplink and downlink channels. The downlink carries transmissions from the network to multiple MSs and does not require contention arbitration. The uplink is shared among multiple MSs and requires contention control procedures.

The allocation of radio resources by the PLMN and the use of these resources by the MSs can be broken down into two parts:

– The PLMN allocates radio resources for the GPRS (uplink and downlink) in a symmetric manner.

– The allocated uplink and downlink radio resources for point-to-point, point-to-multipoint multicast or group call service types are used independently of each other. Dependent allocation of uplink and downlink shall be possible, in order to allow simple MSs to transfer data simultaneously in both directions. Allocation of several PDTCHs for one MS is possible.

The access to the GPRS uplink uses a Slotted-Aloha based reservation protocol.

The Network Protocol Data Units (N-PDU) are segmented into the Subnetwork Protocol Data Units (SN-PDU) by the Subnetwork Dependent Convergence (SNDC) protocol and SN-PDUs are encapsulated into one or several LLC frames. See GSM 03.60 [3] for information on SNDC and LLC. The details on SNDC can be found in GSM 04.65 [9] and the details on LLC can be found in GSM 04.64 8. LLC frames are segmented into RLC Data Blocks. At the RLC/MAC layer, a selective ARQ protocol (including block numbering) between the MS and the Network provides retransmission of erroneous RLC Data Blocks. When a complete LLC frame is successfully transferred across the RLC layer, it is forwarded to the LLC layer.

Figure 20: Transmission and reception data flow for GPRS and EGPRS

Transmission and reception data flows are same for GPRS and EGPRS, except for EGPRS MCS-9, MCS-8 and MCS-7, where four normal bursts carry two RLC blocks (one RLC block within two bursts for MCS-9 and MCS-8), as shown on Figures 21 and 22.

Figure 21: Transmission and reception data flow for EGPRS MCS-7

Figure 22: Transmission and reception data flow for EGPRS MCS-9 and MCS-8

6.6.4.1 Multiplexing MSs on the same PDCH

6.6.4.1.1 Uplink State Flag: Dynamic Allocation
6.6.4.1.1.1 Multiplexing of GPRS or EGPRS MSs

The Uplink State Flag (USF) is used on PDCH to allow multiplexing of Radio blocks from a number of MSs. USF is used in dynamic and extended dynamic medium access modes. USF is used only in downlink direction.

The USF comprises 3 bits at the beginning of each Radio Block that is sent on the downlink. It enables the coding of 8 different USF states which are used to multiplex the uplink traffic.

On PCCCH, one USF value is used to denote PRACH. The other USF values are used to reserve the uplink for different MSs. On PDCHs not carrying PCCCH, the eight USF values are used to reserve the uplink for different MSs. One USF value shall be used to prevent collision on uplink channel, when MS without USF is using uplink channel. The USF points either to the next uplink Radio Block or the sequence of 4 uplink Radio Blocks starting with the next uplink Radio Block.

6.6.4.1.1.2 Multiplexing of GPRS and EGPRS MSs

The GPRS and EGPRS MSs can be multiplexed dynamically on the same PDCH by utilising the USF. When uplink resources are allocated to a GPRS mobile, the network must use GMSK, i.e. CS-1 to CS-4 or MCS-1 to MCS-4, and the USF must point to the sequence of four uplink Radio Blocks starting with the next uplink Radio Block.

The dynamic allocation using USF granularity requires that a GPRS MS can read the USF in an EGPRS GMSK block. This is enabled by setting the stealing bits in the EGPRS GMSK blocks to indicate CS-4. The coding and interleaving of the USF is done as defined for CS-4. This leads to:

1. A standard GPRS MS will be able to detect the USF in EGPRS GMSK blocks. The risk that the rest of the block will be misinterpreted as valid information is assumed to be low.

  1. An EGPRS MS can not differentiate CS-4 blocks and EGPRS GMSK blocks by only looking at the stealing bits. This is however not needed for USF detection, since the USF is signalled in the same way. Further, assuming that the EGPRS MS knows if it is in EGPRS or standard GPRS mode, it will only have to try to decode the remainder of the GMSK blocks in one way in order to determine if they were aimed for it.

A mobile station in EGPRS TBF mode shall be able to detect the USF that assigns the uplink to that mobile station. The network may use either GMSK modulation or 8-PSK modulation, i.e. CS-1 to CS-4, MCS-1 to MCS-4 or MCS-5 to MCS-9 in those blocks.

For mobile station synchronization reasons, if GPRS MSs are multiplexed on the PDCH, at least one downlink radio block every 360ms shall be transmitted to each MS with a coding scheme and a modulation that can be decoded by that MS.

6.6.4.1.2 Fixed Allocation

Fixed allocation where the Uplink part of the PDCH is reserved only for one MS during a certain period of time can be used to multiplex GPRS and EGPRS MSs on the same PDCH on the Uplink.

For MS synchronization reasons, if standard GPRS MSs are multiplexed on the PDCH, at least one Radio Block every 360ms on the Downlink must use GMSK (i.e. standard GPRS or MCS-1 to MCS-4).

6.6.4.1.3 Exclusive Allocation

Exclusive allocation is used to reserve the uplink part of the PDCH for only one MS during the life of the Temporary Block Flow. In exclusive allocation, all the uplink blocks of the uplink part of the PDCH are available to the MS for transmission.

6.6.4.2 Temporary Block Flow

A Temporary Block Flow (TBF) is a physical connection used by the two RR entities to support the unidirectional transfer of LLC PDUs on packet data physical channels. The TBF is allocated radio resource on one or more PDCHs and comprise a number of RLC/MAC blocks carrying one or more LLC PDUs. A TBF is temporary and is maintained only for the duration of the data transfer.

6.6.4.3 Temporary Flow Identity

Each TBF is assigned a Temporary Flow Identity (TFI) by the network. The assigned TFI is unique among concurrent TBFs in each directions and is used instead of the MS identity in the RLC/MAC layer. The same TFI value may be used concurrently for TBFs in opposite directions. The TFI is assigned in a resource assignment message that precedes the transfer of LLC frames belonging to one TBF to/from the MS. The same TFI is included in every RLC header belonging to a particular TBF as well as in the control messages associated to the LLC frame transfer (e.g. acknowledgements) in order to address the peer RLC entities.

6.6.4.4 Medium Access modes

Four medium access modes are supported:

– Dynamic allocation,

– Extended Dynamic allocation,

– Fixed allocation, and

– Exclusive allocation.

The Dynamic allocation medium access mode or Fixed allocation medium access mode shall be supported by all networks that support GPRS. The support of Extended Dynamic allocation and Exclusive allocation medium access modes is optional.

The Dynamic allocation and Fixed allocation modes shall be supported in all mobile stations. Exclusive allocation shall be supported in all mobile stations supporting DTM.

6.6.4.5 Acknowledged mode for RLC/MAC operation

6.6.4.5.1 GPRS

The transfer of RLC Data Blocks in the acknowledged RLC/MAC mode is controlled by a selective ARQ mechanism coupled with the numbering of the RLC Data Blocks within one Temporary Block Flow. The sending side (the MS or the network) transmits blocks within a window and the receiving side sends Packet Uplink Ack/Nack or Packet Downlink Ack/Nack message when needed. Every such message acknowledges all correctly received RLC Data Blocks up to an indicated block sequence number (BSN), thus “moving" the beginning of the sending window on the sending side. Additionally, the bitmap that starts at the same RLC Data Block is used to selectively request erroneously received RLC Data Blocks for retransmission. The sending side then retransmits the erroneous RLC Data Blocks, eventually resulting in further sliding the sending window.

The Packet Ack/Nack message does not include any change in the current assignment (and thus does not have to be acknowledged when sent on downlink). A missing Packet Ack/Nack is not critical and a new one can be issued whenever. In Packet Downlink Ack/Nack message, the MS may optionally initiate an uplink TBF. In Packet Uplink Ack/Nack message , the network can assign uplink resources for mobile station using a fixed allocation.

When receiving uplink data from a MS the network shall, based on erroneous blocks received from MS, allocate additional resources for retransmission.

The acknowledgement procedure of the LLC layer is not combined with the acknowledgement procedure on the underlying RLC/MAC layer.

6.6.4.5.2 EGPRS

The transfer of RLC Data Blocks in the acknowledged RLC/MAC mode can be controlled by a selective type I ARQ mechanism, or by type II hybrid ARQ (incremental redundancy: IR) mechanism, coupled with the numbering of the RLC Data Blocks within one Temporary Block Flow. The sending side (the MS or the network) transmits blocks within a window and the receiving side sends Packet Uplink Ack/Nack or Packet Downlink Ack/Nack message when needed.

According to the link quality, an initial MCS is selected for an RLC block. For the retransmissions, the same or another MCS from the same family of MCSs can be selected. E.g. if MCS-7 is selected for the first transmission of an RLC block, any MCS of the family B can be used for the retransmissions. The selection of MCS is controlled by the network.

In the EGPRS type II Hybrid ARQ scheme, the information is first sent with one of the initial code rates (i.e., the rate 1/3 encoded data is punctured with the puncturing scheme (PS) 1 of the selected MCS). If the RLC Data Block is received in error, additional coded bits (i.e., the output of the rate 1/3 encoded data which is punctured with PS 2 of the prevailing MCS) are sent and decoded together with the already received codewords until decoding succeeds. If all the codewords (different punctured versions of the encoded data block) have been sent, the first codeword (which is punctured with PS 1) is sent. Alternatively, it is possible to use incremental redundancy modes called MCS-5-7 and MCS-6-9, in which the initial transmissions are sent with either MCS-5 or MCS-6 (respectively) and the retransmissions are sent with MCS-7 or MCS-9 (respectively). Header part is robustly coded so that the receiver is able to determine the block identities for all transmissions, even if the payload cannot be decoded.

In the EGPRS type I ARQ, the operation is similar to the one of the EGPRS type II hybrid ARQ, except that the decoding of an RLC Data Block is solely based on the prevailing transmission (i.e., erroneous blocks are not stored).

Type II hybrid ARQ is mandatory in EGPRS MS receivers and the associated performance requirements are specified in GSM 05.05 [14].

6.6.4.6 Unacknowledged mode for RLC/MAC operation

The transfer of RLC Data Blocks in the unacknowledged RLC/MAC mode is controlled by the numbering of the RLC Data Blocks within one Temporary Block Flow and does not include any retransmissions. The receiving side extracts user data from the received RLC Data Blocks and attempts to preserve the user information length by replacing missing RLC Data Blocks by dummy information bits.

The same mechanism and message format for sending temporary acknowledgement messages is used as for acknowledged mode in order to convey the necessary control signalling (e.g. monitoring of channel quality for downlink channel or timing advance correction for uplink transfers). The fields for denoting the erroneous RLC blocks may be used as an additional measure for channel quality (i.e. parameter for link adaptation). The sending side (the MS or the network) transmits a number of radio blocks and then polls the receiving side to send an acknowledgement message. The Packet Uplink Ack/Nack and Packet Downlink Ack/Nack message does not include any change in the current assignment. A missing acknowledgement message is not critical and a new one can be obtained whenever. In Packet Downlink Ack/Nack message, the MS may optionally initiate an uplink TBF. In Packet Uplink Ack/Nack message , the network can assign uplink resources for mobile station using a fixed allocation.

6.6.4.7 Mobile Originated Packet Transfer

6.6.4.7.1 Uplink Access
6.6.4.7.1.1 On the (P)RACH

This sub-clause applies to all mobile stations in packet idle mode. It also applies to class A mobiles stations in dedicated mode if they are not DTM capable.

Figure 23: Access and allocation for the one or two phase packet access, uplink packet transfer

An MS initiates a packet transfer by making a Packet Channel Request on PRACH or RACH. The network responds on PAGCH or AGCH respectively. It is possible to use one or two phase packet access method (see Figure 23).

In the one phase access, the Packet Channel Request is responded by the network with the Packet Uplink Assignment reserving the resources on PDCH(s) for uplink transfer of a number of Radio blocks. The reservation is done accordingly to the information about the requested resources that is comprised in the Packet Channel Request. On RACH, there is only two cause values available for denoting GPRS, which can be used to request limited resources or two phase access. On PRACH, the Packet Channel Request may contain more adequate information about the requested resources and, consequently, uplink resources on one or several PDCHs can be assigned by using the Packet Uplink Assignment message.

In the two phase access, the Packet Channel Request is responded with the Packet Uplink Assignment which reserves the uplink resources for transmitting the Packet Resource Request. A two phase access can be initiated by the network or a mobile station. The network can order the MS to send Packet Resource Request message by setting parameter in Packet Uplink Assignment message. Mobile station can require two phase access in Packet Channel Request message. In this case, the network may order MS to send Packet Resource Request or continue with a one phase access procedure.

The Packet Resource Request message carries the complete description of the requested resources for the uplink transfer. The MS can indicate the medium access method, it prefers to be used during the TBF. The network responds with the Packet Uplink Assignment reserving resources for the uplink transfer and defining the actual parameters for data transfer (e.g. medium access mode).

If there is no response to the Packet Channel Request within predefined time period, the MS makes a retry after a random backoff time.

On PRACH there is used a 2-step approach including a long-term and a short-term estimation of the persistence (see Figure 24). The optimal persistence of the mobile stations is calculated at the network side.

Figure 24: Basic principle of random access traffic control

The actual persistence values depend on:

– the priority i of the packet to be transmitted;

– the amount of traffic within higher priority classes;

– the amount of traffic within the own priority class.

Optionally, the existing backoff algorithm on RACH can be used on PRACH.

On RACH, the existing backoff algorithm shall be used.

Occasionally, more Packet Channel Requests can be received than can be served. To handle this, a Packet Queuing Notification is transmitted to the sender of the Packet Channel Request. The notification includes information that the Packet Channel Request message is correctly received and Packet Uplink Assignment may be transmitted later. If the Timing Advance information becomes inaccurate for an MS, the network can send Packet Polling Request to trigger the MS to send four random access bursts. This can be used to estimate the new Timing Advance before issuing the Packet Uplink Assignment.

6.6.4.7.1.2 On the main DCCH

This clause only applies to mobile stations in dedicated mode that support DTM. This procedure moves the mobile station from dedicated mode to dual transfer mode.

Figure 24b: Access and allocation for the packet request procedure, uplink packet transfer

A DTM mobile station initiates a packet transfer while in dedicated mode by sending a DTM Request message on the main DCCH.

The DTM Request message carries a description of the requested resources for the uplink transfer. The DTM Request message is responded by the network with one of the following DTM assignment messages:

  • DTM Assignment Command: when the network allocates a TBF and reallocation of the resource of the RR connection is needed.
  • Packet Assignment: when the network allocates a TBF and no reallocation of the resource of the RR connection is needed.

If there is reallocation of the resource of the RR connection (through a DTM Assignment Command message), the MS sends an Assignment Complete message on the new main DCCH after it is established.

6.6.4.7.2 Dynamic/Extended Dynamic allocation
6.6.4.7.2.1 Uplink Packet Transfer

The Packet Uplink Assignment message includes the list of PDCHs and the corresponding USF value per PDCH. A unique TFI is allocated and is thereafter included in each RLC Data and Control Block related to that Temporary Block Flow. The MS monitors the USFs on the allocated PDCHs and transmits Radio blocks on those which currently bear the USF value reserved for the usage of the MS.

If the resource assigned by the network in the case of extended dynamic allocation does not allow the multislot MS (see GSM 05.02 [11], annex B) to monitor the USF on all the assigned PDCHs, the following rules shall apply:

– Whenever the MS receives its USF on one downlink PDCH (e.g. on timeslot 0 while timeslots 0, 2 and 3 were assigned), it shall consider the corresponding uplink block and all subsequent ones from the list of assigned PDCHs as allocated (e.g. on 0, 2 and 3). Hence, if the network allocates a block to this MS on an assigned PDCH, it shall also allocate blocks to this MS on all subsequent PDCHs in the list. For each allocated block, the network shall set the USF to the value reserved for the usage of that MS. These rules apply on a block period basis.

– During block periods where it is transmitting, the MS shall monitor the USF on each PDCH in the list of assigned PDCHs, up to and including the first PDCH currently used for transmission. This rule applies on a block period basis. For example, if timeslots 0, 2 and 3 have been assigned and blocks are currently allocated on timeslots 2 and 3, then during this block period the MS monitors USF on timeslots 0 and 2. If the reserved value of USF is found on timeslot 0, then the next allocated blocks shall be on timeslots 0, 2 and 3. If the reserved value of USF is found on timeslot 2, then the next allocated blocks shall be on timeslots 2 and 3. And so on for the subsequent block periods. Because each Radio Block includes an identifier (TFI), all received Radio blocks are correctly associated with a particular LLC frame and a particular MS, thus making the protocol highly robust. By altering the state of USF, different PDCHs can be "opened" and "closed" dynamically for certain MSs thus providing a flexible reservation mechanism. Additionally, packets with higher priority and pending control messages can temporarily interrupt a data transmission from one MS.

The channel reservation algorithm can also be implemented on assignment basis. This allows individual MSs to transmit a predetermined amount of time without interruptions.

The MS may be allowed to use the uplink resources as long as there is queued data on the RLC/MAC layer to be sent from the MS. It can comprise a number of LLC frames. In that sense the radio resources are assigned on the initially “unlimited" time basis. Alternatively, the uplink assignment for each assignment may be limited to a number of radio blocks (e.g. in order to offer more fair access to the medium at higher loads).

The selective ARQ operation for the acknowledged RLC/MAC mode is described in Subclause 6.6.4.5. The unacknowledged RLC/MAC mode operation is described in Subclause 6.6.4.6.

Figure 25 shows an example of message sequence for the (multislot) uplink data transfer with one resource reallocation and possible RLC Data Block re-transmissions.

Figure 25: An example of dynamic allocation uplink data transfer

6.6.4.7.2.2 Release of the Resources

The release of the resources is normally initiated from the MS by counting down the last couple of blocks.

For the normal release of resources for RLC connection carrying a mobile originated packet transfer, the mechanism based on acknowledged final Packet Uplink Ack/Nack combined with timers is used.

After the MS has sent its last RLC Data Block (indicated by the countdown field), the acknowledgement is expected from the network side. By sending the last block, the MS may no longer use the same assignment unless a negative acknowledgement arrives. It also means that the network side may reallocate the same USF(s) to some other user as soon as all the RLC Data Blocks belonging to that Temporary Block Flow are correctly received; that regardless of the possible later errors in the acknowledgements.

The next step, in the case of all RLC Data Blocks being correctly received, is that the network sends Packet Uplink Ack/Nack which is to be immediately acknowledged by the MS in the reserved uplink block period. It must be possible for the network not to use the mechanism of acknowledgement for Packet Ack/Nack in which case the release of the resources procedure relies only on timers. The TFI can be reused for another assignment either upon the reception of the acknowledgement for Packet Ack/Nack or after expiry of the guard timer.

Further, the premature release or change of assignment for one MS may be initiated:

– by the network with an explicit message

– in dual transfer mode, by the release of the RR connection (i.e. handover, assignment and channel release procedures) or

– by the establishment of an RR connection.

In the case of release, the MS is ordered to interrupt the Temporary Block Flow. The MS shall then reorganise the uplink buffer and issue a new Packet Channel Request to continue the uplink transfer with the RLC Data Blocks containing untransferred (i.e. on the RLC/MAC layer unacknowledged) LLC frames.

A change in assignment may also be initiated by the network, in which case the Packet Uplink Assignment, Packet Timeslot Reconfigure, DTM Assignment Command or Packet Assignment message is issued.

6.6.4.7.3 Fixed Allocation

Fixed allocation uses the Packet Uplink Assignment message to communicate a detailed fixed uplink resource allocation to the MS. The fixed allocation consists of a start frame, slot assignment, and block assignment bitmap representing the assigned blocks per timeslot. The MS waits until the start frame indicated and then transmits radio blocks on those blocks indicated in the block assignment bitmap. The fixed allocation does not include the USF and the MS is free to transmit on the uplink without monitoring the downlink for the USF. Unused USF value is used to prevent other mobiles to transmit. If the current allocation is not sufficient, the MS may request additional resources in one of the assigned uplink blocks. A unique TFI is allocated and is thereafter included in each RLC data and control block related to that Temporary Block Flow. Because each Radio Block includes an identifier (TFI), all received Radio blocks are correctly associated with a particular LLC frame and a particular MS.

The number of blocks an MS requests in the initial and subsequent allocation request messages shall only account for the number of data and control blocks it intends to send. The MS shall not request additional blocks for the retransmission of erroneous blocks. The network can repeat the allocation of radio resources by setting the parameter in the Packet Uplink Assignment or the Packet Uplink Ack/Nack message.

The selective ARQ operation for the acknowledged RLC/MAC mode is described in Subclause 6.6.4.5. The unacknowledged RLC/MAC mode operation is described in Subclause 6.6.4.6.

Figure 25 shows an example of message sequence for the (multislot) uplink data transfer with one resource reallocation and possible RLC Data Block re-transmissions.

6.6.4.7.4 Exclusive Allocation

Exclusive allocation uses the Packet Uplink Assignment, Packet Timeslot Reconfigure, DTM Assignment Command or Packet Assignment messages to communicate a exclusive uplink resource allocation to the MS. The exclusive allocation consists of a start frame and a slot assignment. The MS waits until the start frame indicated and then starts transmitting the radio blocks. The exclusive allocation includes neither the USF nor the block assignment bitmap. The MS is free to transmit on the uplink without monitoring the downlink for the USF. Unused USF values may be used to prevent other mobiles from transmitting. A unique TFI is allocated and is thereafter included in each RLC data and downlink control block related to that Temporary Block Flow. Because each Radio Block includes an identifier (TFI or TLLI), all received Radio blocks are correctly associated with a particular LLC frame and a particular MS.

For a close ended TBF, the number of blocks an MS requests in the initial and subsequent allocation request messages shall only account for the number of data blocks it intends to send. The MS shall not request additional blocks for the retransmission of erroneous blocks.

The selective ARQ operation for the acknowledged RLC/MAC mode is described in Subclause 6.6.4.5. The unacknowledged RLC/MAC mode operation is described in Subclause 6.6.4.6.

Figure 25 shows an example of message sequence for the (multislot) uplink data transfer with one resource reallocation and possible RLC Data Block re-transmissions.

6.6.4.7.5 Contention Resolution

Contention resolution is an important part of RLC/MAC protocol operation, especially because one channel allocation can be used to transfer a number of LLC frames. Contention resolution applies for both dynamic and fixed allocation medium access modes. Contention resolution does not apply to the packet request procedures while in dedicated mode for an MS supporting DTM.

There are two basic access possibilities, one phase and two phase access as defined in Subclause 6.6.4.7.1.

The two phase access is inherently immune for possibility that two MSs can perceive the same channel allocation as their own. Namely the second access phase, the Packet Resource Request, uniquely identifies the MS by its TLLI. The same TLLI is included in the Packet Uplink Assignment/Packet Downlink Assignment and no mistake is possible.

The one phase access is somewhat insecure and an efficient contention resolution mechanism has to be introduced.

The first part of the solution is the identification of the MS. The identification of transmitting MS on the RLC/MAC level is necessary not only for contention resolution but also to be able to establish RLC protocol entity for that Temporary Block Flow on the network side. Additionally, the TLLI is necessary to be able to match simultaneous uplink and downlink packet transfers by taking into consideration multislot capability of that MS.

In order to uniquely identify the MS when sending on uplink, the RLC Header for all the RLC Data Blocks on uplink is extended to include the TLLI until the contention resolution is completed on the MS side.

The second part of the solution is the notification from the network side about who owns the allocation. That is solved by the inclusion of the TLLI in the Packet Uplink Ack/Nack/Packet Downlink Ack/Nack. This message shall be sent in an early stage, even before the receive window for RLC/MAC protocol operation is full. By doing so, the contention is resolved after the first occurrence of Packet Ack/Nack. The possibility of RLC Data Blocks being captured from “wrong" MS, thus destroying the LLC frame, shall be covered for by retransmissions on the LLC layer.

6.6.4.8 Mobile Terminated Packet Transfer

6.6.4.8.1 Packet Paging

The network initiates a packet transfer to an MS that is in the Standby state by sending one or more packet paging request messages on the downlink PPCH or PCH. The MS responds to one packet paging request message by initiating a mobile originated packet transfer, as described in section 6.6.4.7. This mobile originated packet transfer allows the MS to send a packet paging response to the network. The packet paging response is one or more RLC/MAC data blocks containing an arbitrary LLC frame. The message sequence described in Figure 26 below is conveyed either on PCCCH or on CCCH. After the packet paging response is sent by the MS and received by the network, the mobility management state of the MS is Ready.

The network can then assign some radio resources to the MS and perform the downlink data transfer as described in section 6.6.4.8.2.

Figure 26: Paging message sequence for Paging, downlink packet transfer

6.6.4.8.2 Downlink Packet Transfer

The transmission of a packet to an MS in the Ready state is initiated by the network using a packet downlink assignment message. In case there is an uplink packet transfer in progress, the packet downlink assignment message is transmitted on PACCH. Else, in case there is PCCCH allocated in the cell, the Packet Downlink Assignment message is transmitted on PCCCH. Else, the Immediate Assignment message is transmitted on CCCH. Else, for a DTM MS in dedicated mode, the assignment message is transmitted on the main DCCH. The packet downlink assignment message includes the list of PDCH(s) that will be used for downlink transfer. The Timing Advance and Power Control information is also included, if available. Otherwise, the MS may be requested to respond with a Packet Control Acknowledgement (see also Subclause 6.5.7 on timing advance procedures). The MS multislot capability needs to be considered.

The network sends the RLC/MAC blocks belonging to one Temporary Block Flow on downlink on the assigned downlink channels.

Multiplexing the RLC/MAC blocks destined for different MSs on the same PDCH downlink is enabled with an identifier, e.g. TFI, included in each RLC/MAC block. The interruption of data transmission to one MS is possible.

The acknowledged (i.e. selective ARQ operation) and unacknowledged RLC/MAC mode operation is described in Subclauses 6.6.4.5 and 6.6.4.6. The sending of the Packet Downlink Ack/Nack message is obtained by the occasional network initiated polling of the MS. The MS sends the Packet Downlink Ack/Nack message in a reserved radio block which is allocated together with polling. Unassigned USF value is used in the downlink radio block which corresponds to the reserved uplink radio blocks. Further, if the MS wants to send some additional signalling or uplink data, it may be indicated in the Packet Downlink Ack/Nack message.

Figure 27 shows an example of message sequence for (multislot) downlink data transfer with one resource reallocation and possible RLC Data Block re-transmissions.

Figure 27: An example of downlink data transfer

6.6.4.8.3 Release of the Resources

The release of the resources is initiated by the network by terminating the downlink transfer and polling the MS for a final Packet Downlink Ack/Nack message.

A mobile station in dual transfer mode shall abandon the packet resources when the RR connection is released.

It is possible for the network to change the current downlink assignment by using the Packet Downlink Assignment or Packet Timeslot Reconfigure message, which then has to be acknowledged by the MS in a reserved radio block on the uplink.

The handling of TFI is steered with the same timer that runs on both the MS and the network side after the last RLC Data Block is sent to the MS. When it expires, the current assignment becomes invalid for the MS and TFI can be reused by the network. Further, upon the reception of the final Packet Downlink Ack/Nack from the MS, another timer is started on network side. When it expires, the current assignment becomes invalid for the MS and TFI can be reused by the network.

6.6.4.8.4 Packet Paging Notification

The network initiates a packet transfer to a DTM MS that is in the Standby state and in dedicated mode by sending one or more packet paging notification messages on the downlink main DCCH. The MS responds to one packet paging notification message by initiating a mobile originated packet request, as described in section 6.6.4.7. This mobile originated packet transfer allows the MS to send a packet paging response to the network. The packet paging response is one or more RLC/MAC data blocks containing an arbitrary LLC frame. After the packet paging response is sent by the MS and received by the network, the mobility management state of the MS is Ready.

The network can then assign some radio resources to the MS and perform the downlink data transfer as described in section 6.6.4.8.2.

6.6.4.9 Simultaneous Uplink and Downlink Packet Transfer

During the ongoing uplink Temporary Block Flow, the MS continuously monitors one downlink PDCH for possible occurrences of Packet Downlink Assignment or Packet Timeslot Reconfigure messages on PACCH (see Figure 25). The MS is therefore reachable for downlink packet transfers that can then be conveyed simultaneously on the PDCH(s) that respect the MS multislot capability.

If the MS wants to send packets to the network during the ongoing downlink Temporary Block Flow, it can be indicated in the acknowledgement that is sent from the MS. By doing so, no explicit Packet Channel Requests have to be sent to the network. Further, the network already has the knowledge of which PDCH(s) that particular MS is currently using so that the uplink resources can be assigned on the PDCH(s) that respect the MS multislot capability. This method may introduce an extra delay when initiating the uplink packet transfer but only for the first LLC frame in a sequence.