When the MAC and RLC layer functions are positioned remote to the BTS, the information between the Channel Codec Unit (CCU) and the remote Packet Control Unit (PCU) is transferred in frames with a fixed length of 320 bits (20 ms). In the present document these frames are denoted "PCU Frames"
and are an extension to the "TRAU frames"
defined in TS 48.060
. Within these frames both GPRS data and the RLC/MAC associated control signals are transferred.
The Abis interface should be the same if the PCU is positioned at the BSC site (option B in Figure 92
) or at the SGSN site (option C in Figure 92
). In option B, the PCU could be implemented as an adjunct unit to the BSC. In option C, the BSC should be considered as transparent for 16 kbit/s channels. In configurations B and C the PCU is referred to as being a remote PCU.
The remote PCU is considered a part of the BSC, and using BSC internal signals may provide the signalling between the BSC and the PCU. The in-band signalling between the CCU and the PCU functions, using PCU frames is required when the Abis interface is applied (options B and C in Figure 92
The PCU is responsible for the following MAC and RLC layer functions as defined in TS 43.064
LLC layer PDU segmentation into RLC blocks for downlink transmission;
LLC layer PDU reassembly from RLC blocks for uplink transmissions;
PDCH scheduling functions for the uplink and downlink data transfers;
PDCH uplink ARQ functions, including RLC block Ack / Nack;
PDCH downlink ARQ function, including buffering and retransmission of RLC blocks;
channel access control functions, e.g. access requests and grants; and
radio channel management functions, e.g. power control, congestion control, broadcast control information, etc.
The functions inside the Channel Codec Unit (CCU) are:
the channel coding functions, including FEC and interleaving;
radio channel measurement functions, including received quality level, received signal level and information related to timing advance measurements; and
for EGPRS, in case of incremental redundancy mode of operation, enhanced channel coding functions.
The BSS is responsible for allocation and de-allocation of radio resources. A PCU frame shall be transferred between the PCU and the CCU every 20 ms.
During the PS handover procedure the PS Handover Request Context containing packet flow specific information needs to be transferred between SGSNs. The detailed description of the procedures used during PS handover from GERAN A/Gb mode to GERAN A/Gb mode or GERAN A/Gb mode to Iu mode and vice-versa are described in TS 29.060