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UMTS (RRC, RLC, MAC, and PHY) - 22

RRC, RLC, MAC, and PHY [Under UMTS]
» Radio interface layer - 25.301 «
» RRC - 25.331 «
» RLC - 25.322 «
» MAC - 25.321 «

In earlier articles, we looked at number of aspects of access stratum like MAC and PHY interface (described in terms of TF/TFC), PHY structures of few initial channels (SCH, P-CCPCH, PRACH etc.), Logical to Transport to Physical channel mapping, RLC modes for channels, RRC states, RRC connection establishment etc. We know roughly: channel mapping is done based on RRC state, RLC take care of acknowledged/unacknowledged modes, MAC uses TF/TFC for multiplexing of logical channels, PHY works on concept CCTrCh, due to TF/TFC/CCTrCh UMTS can support various kinds of services.

In our flow of discussion, we have seen RRC signaling during RRC Signaling Connection establishment. Next step is how radio bearers are setup (you may want to take a quick look at article on signaling briefly).

But before we do that, we will list down some of the major functions (few of which we have already seen) of RRC, RLC, MAC, and PHY from UE point of view (refer diagram from article on radio layers).

RRC

- Managing RRC connection
- Mobility handling during RRC connected mode
- Cell selection and re-selection
- Interpreting broadcast system information
- Managing radio bearers
- Measurement reporting and control
- Ciphering control

- Outer loop power control (setting rough target for closed loop power control)
- Overall configuration and control of access stratum

RLC

- Support of Acknowledged/Unacknowleded/Transparent modes
- Segmentation and reassembly (PDU size can match to TF sizes) for larger size SDUs
- Concatenation of smaller RLC SDUs
- Padding (when required)
- Ciphering for non-transparent modes (Ack/Unack)
- Flow control
- Re-transmission if error detected for Ack mode

MAC

- Providing transport for logical channels or mapping of logical channels to transport channels
- Multiplexing/demultiplexing higher layer PDUs onto/from transport blocks given to/taken from PHY
- Selecting appropriate TF/TFC
- Deciding ASC for RACH (except for RRC Connection Request)
- Identification of UEs on common transport channels
- Ciphering for transparent RLC mode
- HARQ for HS-DSCH

PHY

- Scanning, Synchronisation
- Multiplexing of transport channels onto CCTrCh and reverse of it
- Rate matching
- Coding, Interleaving, Spreading, Modulation and reverse of it
- Mapping of CCTrCh onto physical channels
- Measurement reporting to RRC
- Closed loop power control

References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.

Copyright © Samir Amberkar 2010§

MAC PDU « UMTS Index » Radio bearer establishment