The RLC/MAC block is the basic transport unit on the air interface that is used between the mobile and the network. It is used to carry data and RLC/MAC signaling.
In the previous chapter, the structure of the 52-multiframe was presented, and the concept of radio blocks was introduced. A radio block is defined as an information block transmitted over four consecutive bursts on four TDMA frames on a given PDCH.
One RLC data block is mapped onto one radio block, which is always transmitted on a packet data subchannel (PDTCH). One RLC/MAC control block is transmitted into one radio block on a signaling subchannel (PACCH, PCCCH, PBCCH).
The RLC/MAC control block is used to transmit RLC/MAC control messages, whereas the RLC data block contains data. A MAC header is added at the beginning of each type of radio block. A block check sequence (BCS) for error control detection is added at the end of the radio block.
The block formats are presented in the following section, with a brief definition of the different fields. The use of these fields for the GPRS layers will be studied in detail later in the chapter.
The RLC/MAC block used for the transfer of control messages consists of a MAC header and an RLC/MAC control block, as shown in Table 5.1.
RLC/MAC control block
RLC/MAC blocks used for control are encoded using the coding scheme CS-1. The size of the RLC/MAC control block is 22 bytes; the size of the MAC header is 1 byte.
Table 5.2 shows the format of the RLC/MAC control block with its MAC header for the downlink direction. The RLC/MAC control block consists of a control message contents field and an optional control header.
The MAC header contains the following elements:
PT This indicates whether the block is a control block or a data block.
USE This is used as an uplink multiplexing means when dynamic or extended dynamic allocations are used. It authorizes or refuses a transmission in the next block or set of four blocks following that received by a mobile. A number of mobiles can thus share a given uplink PDCH, but a single mobile transmits on one block at a given time. When resources are allocated, a given USF is reserved for a mobile on a given PDCH. The USF field can be set to the value FREE in a block on a PDCH that would support PCCCHs. FREE is used to indicate the presence of a PRACH, thereby allowing all mobiles in packet idle mode to send access requests on the PRACH.
Relative reserved block period (RRBP). This indicates the number of frames that the mobile must wait before transmitting an RLC/MAC control block.
Supplementary/polling (S/P). This indicates whether the RRBP field is valid.
The RLC/MAC header contains the following elements:
Reduced block sequence number (RBSN), which gives the sequence number of the RLC/MAC control block;
Radio transaction identifier (RTI), which is used to identify an RLC/MAC control message that has been segmented into two RLC/MAC control blocks;
Final segment (FS), which indicates whether the RLC/MAC control block contains the FS of the segmented RLC/MAC control message;
Address control (AC), which indicates the presence of an optional byte containing the PR, TFI, and D fields;
Temporary flow identifier (TFI), which identifies a downlink or uplink TBF;
Direction (D), which indicates the direction of the TBF identified by the TFI field;
PR, which indicates the power reduction that has been used by the BTS to transmit the current block.
As its name indicates, the control message contents field contains an RLC/MAC control message.
Table 5.3 shows the format of the RLC/MAC control block for the uplink with its MAC header. The RLC/MAC control block consists of a control message contents field.
The MAC header contains:
PT, which indicates the type of data within the block;
R (retry), which indicates whether the mobile transmitted the access request message one time or more than one time during its most recent channel access.
The RLC/MAC block that is used for data transfer consists of a MAC header and an RLC data block. The RLC data block consists of an RLC header, an RLC data unit, and spare bits, as described in Table 5.4.
The size of the RLC block depends on the type of channel coding (CS-1, CS-2, CS-3, CS-4). A block can contain 184, 271, 315, or 431 bits, including the MAC header, and the number of spare bits is 0, 7, 3, 7 for CS1, CS2, CS3, and CS4 channel coding, respectively. The spare bits are set to 0 by the sending entity and ignored by the receiving entity.
The size of the RLC data block for each of the channel coding schemes is shown in Table 5.5.
RLC Data Block Size Without Spare Bits (Bytes)
Number of Spare Bits
Table 5.6 shows the format of the RLC data block with its MAC header for the downlink data transfer.
The MAC header, which is common to the RLC data block and RLC/MAC control block for the downlink, was described in Section 126.96.36.199.
The RLC header contains the following fields:
The PR indicates the power reduction used by the BTS to transmit the current block.
The temporary flow identifier (TFI) identifies the ownership of the block. When resources are allocated, the TFI is used to identify the TBF.
The final block indicator (FBI) indicates whether or not the RLC data block received is the last of the TBF.
The block sequence number (BSN) is the sequence number of the RLC block in the TBF.
Extension (E) indicates the presence of an optional byte.
More (M) indicates the presence of another LLC frame in the data unit part.
The length indicator (LI) makes it possible to delimit LLC frames within an RLC data block by giving the length of the data in the RLC data block belonging to an LLC frame. If this field is set several times, it indicates the length of the other LLC frames.
The RLC data field can contain bytes for one or more LLC frames.
Table 5.7 shows the format of the RLC block for the uplink data transfer.
The MAC header does not contain exactly the same fields for the uplink as for the downlink. It contains the following fields:
PT indicates whether the block transmitted is of the RLC data block type or the RLC/MAC control block type.
Countdown value (CV) gives the number of RLC blocks associated with a TBF remaining to be transmitted.
Stall indicator (SI) indicates an acknowledgment request from the mobile when the RLC protocol is stalled.
R indicates whether the mobile transmitted the access request message one time or more than one time during its most recent channel access.
The other fields are identical to the downlink RLC data block, with the exception of the following:
Temporary logical link identity (TLLI), which identifies a GPRS user;
TLLI indicator (TI), which indicates the presence of the TLLI field;
PFI indicator (PI), which indicates the presence of the optional byte that contains the PFI;
PFI, which identifies the packet flow context.
The E bit after the TFI field will be used in the future to indicate the presence of an additional byte due to an evolution of the protocol.
If the number of bytes corresponding to one or more LLC frames in the RLC data unit field cannot fill the RLC frame completely, it is filled with spare bits.