11a Radio Link Control (RLC) procedures for FLO on UDCH, ADCH
3GPP44.160General Packet Radio Service (GPRS)Mobile Station (MS) - Base Station System (BSS) interfaceRadio Link Control / Medium Access Control (RLC/MAC) protocol Iu modeRelease 16TS
11a.1 General
This sub-clause describes the RLC procedures in UDCH TBF mode applicable in MAC-Dedicated state and MAC-DTM state.
In UDCH TBF mode, the following definitions apply in NT-RLC only:
RLC acknowledged mode
– Sequence Number Space (SNS): 1024.
– Window Size (WS): 512.
RLC unacknowledged mode
– Sequence Number Space (SNS): 16.
– Window Size (WS): 8.
11a.2 Procedures and parameters for peer-to-peer operation
11a.2.1 Send state variable V(S)
See 3GPP TS 44.060 sub-clause 9.1.1.
11a.2.2 Control send state variable V(CS)
See 3GPP TS 44.060 sub-clause 9.1.1a.
11a.2.3 Acknowledge state variable V(A)
See 3GPP TS 44.060 sub-clause 9.1.2.
11a.2.4 Acknowledge state array V(B)
See 3GPP TS 44.060 sub-clause 9.1.3.1.
11a.2.5 Block sequence number BSN
Each RLC data block contains a block sequence number (BSN) field that is 10 bits in length in RLC-AM and 4 bits in length in RLC-UM. At the time that an in‑sequence RLC data block is designated for transmission, the value of BSN is set equal to the value of the send state variable V(S).
11a.2.6 Reduced block sequence number RBSN
See 3GPP TS 44.060 sub-clause 9.1.4a.
11a.2.7 Receive state variable V(R)
See 3GPP TS 44.060 sub-clause 9.1.5.
11a.2.8 Receive window state variable V(Q)
See 3GPP TS 44.060 sub-clause 9.1.6.
11a.2.9 Receive state array V(N)
See 3GPP TS 44.060 sub-clause 9.1.7.1.
11a.2.10 Starting sequence number (SSN) and received block bitmap (RBB)
11a.2.10.1 General
The Packet Ack/Nack message (PACKET DBPSCH UPLINK/DOWNLINK ACK/NACK TYPE 2 message) contains a starting sequence number (SSN) and a reported bitmap. The Packet Ack/Nack message is sent by the RLC receiver and is received by the RLC transmitter. The SSN and reported bitmap are determined as defined in this sub-clause 11a.2.10 and transmitted in RLC acknowledged mode.
The BSN values specified in the reported bitmap are interpreted by substracting the bit position in the bitmap to the starting sequence number (SSN), modulo SNS (where the bit in first position in the bitmap has the bit position ‘1’).
A valid BSN value in the reported bitmap is one that is in the range [ V(A) £ BSN < V(S) ] modulo SNS. These inequalities shall be interpreted in the following way: BSN is valid if, and only if, [BSN – V(A) ] modulo SNS < [ V(S) – V(A) ] modulo SNS.
11a.2.10.2 Polling
Upon reception of polling in uplink (respectively downlink) TBFs, the network (respectively mobile station) may select any composition (first partial bitmap or next partial bitmap) of the Packet Ack/Nack message to send to the mobile station (respectively network).
11a.2.10.3 Determination of SSN and generation of the reported bitmap
If the receiving side is the network, the network may select any SSN within the receive window. If the receiving side is the mobile station, SSN shall be determined as follows: let PBSN represent a partial bitmap sequence number variable stored at the receiver which helps to determine the SSN for the next partial bitmap to be transmitted. SSN and PBSN shall be determined as follows.
First, a Full Received Bitmap (FRB) is built from the receive state array V(N) by extracting the part between V(Q) and V(R) similar to the GPRS case: it is assigned the elements whose indices in the receive state array V(N) at the receiver range from [V(Q)+ 1] to [V(R) -1] (modulo SNS). For each bit in the bitmap, the bit is assigned the value ‘1’ if the corresponding element in V(N) indexed relative to SSN has the value RECEIVED. The bit is assigned the value ‘0’ if the element in V(N) has the value INVALID.
Figure 11a.2.10.3.1: Representation of bitmaps and variables
From the FRB, a reported bitmap shall be generated. The FRB shall be recalculated before each reported bitmap is generated. For uplink TBFs, the network may transmit any reported bitmap size to the mobile station. For downlink TBFs, the mobile station shall follow the rules given in this sub-clause 11a.2.10.3. The reported bitmap is either uncompressed or compressed.
If the compressed reported bitmap covers more RLC data blocks than the uncompressed reported bitmap, the receiver shall send the compressed reported bitmap, otherwise the receiver shall send the uncompressed reported bitmap. If FRB can be reported uncompressed, the receiver shall send the uncompressed reported bitmap without attempting compression (see below). The compression algorithm is defined in 3GPP TS 44.060 sub-clause 9.1.10.
Let N be the maximum possible size in bits of the uncompressed reported bitmap in the acknowledgement message when no compressed bitmap is included.
If [V(R)-V(Q)] mod SNS £ N (i.e. FRB can be reported uncompressed), then SSN=V(R) and the reported bitmap covers N RLC data blocks (with a BSN smaller than SSN). PBSN shall then be set to V(Q). The reported bitmap shall be uncompressed. The bits corresponding to RLC data blocks that may fall outside the FRB shall be set to ‘1’, except for V(Q) which shall be set to ‘0’.
If [V(R) – V(Q)] mod SNS > N (i.e. FRB cannot be reported uncompressed), the mobile station shall determine whether a first partial bitmap or a next partial bitmap is transmitted, as follows:
– If [PBSN-V(Q)] mod SNS < WS and if [V(R) – (PBSN + PBSN_Threshold)] mod SNS < WS, then a next partial bitmap is sent.
– Otherwise a first partial bitmap is sent.
NOTE: The PBSN_Threshold value intends at identifying a limit for the difference between PBSN and V(R) beyond which a next partial bitmap is generated. Its value is MS implementation dependent.
If a first partial bitmap is determined, then SSN shall be set to the last sequence number + 1 (mod SNS) for which the Ack/Nack status can be indicated in the reported bitmap. The reported bitmap covers the RLC data blocks having a BSN within the interval [V(Q), SSN[. PBSN shall then be set to the last sequence number for which the Ack/Nack status can be indicated in the reported bitmap i.e. SSN-1.
NOTE: The first partial bitmap may cover the entire FRB when compression is used
If a next partial bitmap is determined, then
– If [V(R) – (PBSN + 1)] mod SNS > N then SSN shall be set to the last sequence number + 1 (mod SNS) for which the Ack/Nack status can be indicated in the reported bitmap and the reported bitmap covers the RLC data blocks having a BSN within the interval [PBSN+1, SSN-1]. PBSN shall then be set to the last sequence number for which the Ack/Nack status can be indicated in the reported bitmap i.e. SSN-1.
– If [V(R) – (PBSN+1)] mod SNS N, then SSN = V(R) and the reported bitmap covers as many RLC data blocks having a BSN smaller than the SSN as can fit in the reported bitmap. PBSN shall then be set to V(Q).
NOTE: The next partial bitmap may cover the entire FRB when compression is used
The beginning of window (BOW) bit shall be set to ‘0’ when the reported bitmap does not cover V(Q), and to ‘1’ otherwise. The end of window (EOW) bit shall be set to ‘0’ when the reported bitmap does not cover [V(R)-1], and to ‘1’ otherwise.
If V(Q) equals V(R), then SSN shall be set to the value SSN = V(R), BOW bit shall be set to the value ‘1’, EOW shall be set to the value ‘1’ and the reported bitmap size shall equal 0 bits.
11a.2.10.4 Interpretation of the bitmap
If a compressed bitmap is received, the bitmap shall first be decompressed according to the one-dimensional run length coding algorithm specified in 3GPP TS 44.060 sub-clause 9.1.10. The uncompressed bitmap shall be treated as follows:
The first bit in the uncompressed bitmap corresponds to the block SSN-1 (mod SNS). The second bit in the uncompressed bitmap corresponds to the block SSN-2 (mod SNS), and so on.
If the BOW bit in the Packet Ack/Nack message has the value ‘1’, then the bitmap acknowledges all blocks between V(A) and the block corresponding to V(Q)-1 (the block corresponding to V(Q) corresponds to the first ‘0’ starting from the end of the bitmap), and the corresponding elements in V(B) shall be set to the value ACKED.
If the EOW bit in the Packet Ack/Nack message has the value ‘1’, then bitmap value ‘0’ shall be assumed for all RLC blocks with a BSN value higher than the last entry in the bitmap but less than V(S) (i.e. [ V(R) – 1 < BSN < V(S)] modulo SNS).
For each bit in the uncompressed bitmap whose corresponding BSN value is within the transmit window, if the bit contains the value ‘1’, the corresponding element in V(B) indexed relative to SSN shall be set to the value ACKED. If the bit contains the value ‘0’, the element in V(B) shall be set to the value NACKED. A bit within the uncompressed bitmap whose corresponding BSN is not within the transmit window, shall be ignored. If the bit contains the value ‘0’, the last burst of the corresponding RLC data block has been physically transmitted in the TDMA frame number FNcomm, and the first burst carrying a segment of the Packet Ack/Nack message has been physically transmitted in the TDMA frame number FNresp where FNresp < FNcomm + Trmin +1 (i.e. the RLC data block was recently (re)transmitted and thus can not be validly negatively acknowledged in this particular acknowledgement), the element in V(B) shall not be modified. See sub-clause 9.2.1.3 for the definition of Trmin.
11a.2.11 Window Size
The window size (WS) shall be 512 in RLC acknowledged mode.
The window size (WS) shall be 8 in RLC unacknowledged mode.
11a.2.11a RLC buffer
See 3GPP TS 44.060 sub-clause 9.1.9.3.
11a.2.12 Segmentation of upper layer PDUs into RLC data units
See 3GPP TS 44.060 sub-clause 9.1.11.
Once an RLC data block has been transmitted over the physical link, should it be necessary to re-transmit the RLC data block, it shall be re-transmitted using the same payload and BSN as it had in the previous transmission.
If so ordered by RRC (CRLC-CONFIG-Req primitive), the RLC transmitter may discard:
– in RLC acknowledged mode, RLC SDU(s) not yet segmented into RLC PDUs. The RLC transmitter shall notify the higher layer of all discarded RLC SDUs, if indicated (RLC-AM-DATA-DiscardReq primitive).
– in RLC unacknowledged mode, RLC SDU(s).
11a.2.13 Re-assembly of upper layer PDUs from RLC data units
See 3GPP TS 44.060 sub-clause 9.1.12.
11a.2.14 Segmentation of RLC/MAC control messages into RLC/MAC control blocks
See 3GPP TS 44.060 sub-clause 9.1.12a.
11a.2.15 Re-assembly of RLC/MAC control messages from RLC/MAC control blocks
See 3GPP TS 44.160 sub-clause 10.2.15:
The duration of timer T3200 shall be so that, if the last burst of the RLC/MAC control block that triggered the start of timer T3200 has been physically transmitted in the TDMA frame number FNcomm, then the first burst carrying a segment of the RLC/MAC control block that triggered the stop of timer T3200 shall at the latest be physically transmitted in the TDMA frame number FNresp where:
FNresp = FNcomm + Trmax +1, with Trmax defined as shown in the table below
Table 11.2.15.1: Maximum difference between two segments of the same RLC/MAC control message
|
Logical channel |
Trmax |
|
SDCCH |
Four TRMIN_SDCCH |
|
SACCH |
332 |
|
SACCH |
Four TRMIN_SACCH |
|
FACCH/Full rate |
Four TRMIN |
|
FACCH/Half rate |
Four TRMIN |
|
ADCH/Full rate |
Four TRMIN_DCH |
|
ADCH/Half rate |
Four TRMIN_DCH |
11a.3 Operation during RLC/MAC control message transfer
RLC/MAC control blocks shall be used to transport RLC/MAC control messages. Segments of only one RLC/MAC control message shall be transported per RLC/MAC control block.
RLC/MAC control blocks shall be sent at a higher priority than RLC data blocks.
RLC/MAC control blocks shall be sent on ADCH.
The receiving side shall determine the length of the RLC/MAC control message contents by interpreting the RLC/MAC control block contents.
No general acknowledgement shall be made as part of the transfer of RLC/MAC control blocks or RLC/MAC control messages. The receiver shall not acknowledge an RLC/MAC control block except when it is polled by the transmitter as indicated by the polling (P) bit in the MAC header of this RLC/MAC control block. The receiver shall not acknowledge an RLC/MAC control message except when the RLC/MAC procedures explicitly specify an acknowledgement. Upon reception of a polling request, the receiver shall respond following the rules defined in sub-clause 9.2.3 and the requirements defined in sub-clauses 9.2.1.2 and 9.2.1.3.
A RLC/MAC control block header, may contain a Radio Transaction Identifier (RTI) field that is 2 bits in length and performs in effect a modulo 4 count of the downlink RLC/MAC control messages sent on ADCH. The RTI field shall be used to group the RLC/MAC control blocks that make up an RLC/MAC control message. The RTI field allows the transmitting and receiving entities to distinguish between up to 4 RLC/MAC control messages in a single transmit direction therefore allowing up to 4 parallel transactions per ADCH.
The network shall not use the same RTI value at the same time on the same DBPSCH for two separate RLC/MAC control messages. The network shall transmit both segments of a segmented control message on the same DBPSCH.
11a.4 Operation during RLC data block transfer
11a.4.1 General
The RLC ARQ functions are applicable in NT-RLC mode only and support two modes of operation: RLC acknowledged mode and RLC unacknowledged mode. RLC acknowledged mode operation uses retransmission of RLC data blocks to achieve high reliability. RLC unacknowledged mode operation does not utilize retransmission of RLC data blocks. No ARQ function shall apply in T-RLC mode.
A TBF may operate in either RLC acknowledged mode, RLC unacknowledged mode or RLC transparent mode.
For a URB, the RLC mode of the corresponding TBF is set to either RLC acknowledged mode, RLC unacknowledged mode or RLC transparent mode at set-up of this particular URB by means of primitive exchange between RRC and RLC (CRLC-CONFIG) (see 3GPP TS 44.118).
11a.4.2 Acknowledged mode operation
11a.4.2.1 General
The transfer of RLC data blocks in RLC acknowledged mode uses retransmissions of RLC data blocks. The transmitting side numbers the RLC data blocks via the block sequence number (BSN). The BSN is used for retransmission and for reassembly. The receiving side sends acknowledgement in order to request retransmission of RLC data blocks. The operation in RLC acknowledged mode shall be as described in sub-clause 11.2.
The transfer of RLC data blocks in RLC acknowledged mode is controlled by a selective type I ARQ mechanism coupled with the numbering of the RLC data blocks.
11a.4.2.2 Uplink
The selection of the transport format set is under control of the network and ordered by RRC during radio bearer set-up and reconfiguration procedures (see 3GPP TS 44.118).
The network shall send PACKET DBPSCH UPLINK ACK/NACK TYPE 2 messages on ADCH when needed. The mobile station may poll the network for sending a PACKET DBPSCH UPLINK ACK/NACK TYPE 2 message by setting the polling (P) bit in an uplink RLC data block. Upon reception by the network of a polling request, the network shall send a PACKET DBPSCH UPLINK ACK/NACK TYPE 2 message for the corresponding RLC entity to the mobile station in the next possible downlink occurence following the rules described in sub-clause 9.2.3 and the requirements defined in sub-clauses 9.2.1.2 and 9.2.1.3. Upon reception by the mobile station of a PACKET DBPSCH UPLINK ACK/NACK TYPE 2 message for this RLC entity, the mobile station shall reset counter N3106. If the mobile station does not receive any PACKET DBPSCH UPLINK ACK/NACK TYPE 2 message for this RLC entity before the response time specified in sub-clause 9.2.1.2, the mobile station shall increment counter N3106. If counter N3106=N3106max, the mobile station shall indicate a link failure to the RRC layer which in turn shall stop the corresponding RLC entity (see sub-clause 14.3 and 3GPP TS 44.118).
The mobile station shall indicate a transmit window stall condition when V(S)=V(A) + WS. Upon detecting a transmit window stall condition the mobile station shall set the Stall Indicator (SI) bit in all subsequent uplink RLC data block until the stall condition ceases to exist.
Upon detecting the stall condition the mobile station shall also start timer T3182. Timer T3182 shall be stopped upon reception of a PACKET DBPSCH UPLINK ACK/NACK TYPE 2 message that makes V(S)<V(A)+WS. If timer T3182 expires, the mobile station shall notify a link failure to the RRC layer which in turn shall stop the corresponding RLC entity (see sub-clause 14.3 and 3GPP TS 44.118).
11a.4.2.3 Downlink
The mobile station receives RLC/MAC blocks for data transfer on UDCH.
The selection of the transport format set is controlled by the network and ordered by RRC during radio bearer set-up and reconfiguration procedures (see 3GPP TS 44.118). The RLC/MAC block format used shall be the one corresponding to the transport channel (UDCH, CDCH, ADCH) on which it is sent (see sub-clause 12.8). The network may poll the mobile station for sending a PACKET DBPSCH DOWNLINK ACK/NACK TYPE 2 message by setting the polling (P) bit in a downlink RLC data block. Upon reception by the mobile station of a polling request, the mobile station shall send a PACKET DBPSCH DOWNLINK ACK/NACK TYPE 2 message to the network for the corresponding RLC entity in the next possible uplink occurence on ADCH following the rules described in sub-clause 9.2.3 and the requirements defined in sub-clauses 9.2.1.2 and 9.2.1.3. Upon reception by the network of a PACKET DBPSCH DOWNLINK ACK/NACK TYPE 2 message for this RLC entity, the network shall reset counter N3107. If the network does not receive any PACKET DBPSCH DOWNLINK ACK/NACK TYPE 2 message for this RLC entity before the response time specified in sub-clause 9.2.1.2, the network shall increment counter N3107. If counter N3107=N3107max, the network shall indicate a link failure to the RRC layer which shall in turn stop the corresponding RLC entity (see sub-clause 14.3 and 3GPP TS 44.118).
11a.4.3 Unacknowledged mode operation
11a.4.3.1 General
The transfer of RLC data blocks in RLC unacknowledged mode does not include any retransmissions. The block sequence number (BSN) in the RLC data block header is used to number the RLC data blocks for reassembly. The operation in RLC unacknowledged mode shall be as described in sub-clause 11.2.
11a.4.3.2 Uplink
The network shall send acknowledgements when needed.
11a.4.3.3.2 Downlink
The mobile station shall be able to receive RLC/MAC blocks in RLC unacknowledged mode on UDCH. The mobile station shall, in the RLC/MAC header, identify the TFI and decode the RLC data blocks belonging to the corresponding radio bearer.
11a.4.4 Transparent mode operation
When operating in transparent mode, the RLC protocol has no functionality. The incoming RLC SDUs are transferred to the MAC layer without being altered. No upper layer protocol information is removed. No RLC protocol information is added.