MAC PDU [Under UMTS]
» UTRAN
description - 25.401 «
» Radio
interface layer - 25.301 «
» MAC -
25.321 «
As noted in RRC establishment procedure, UE receive a (temporary) identity, called RNTI, which is used in distinguishing UE on common transport channels in further communications. RNTI is part of MAC header. In addition to RNTI, MAC header has information related to mapping of multiple logical channels onto a transport channel. Let us quickly look at MAC header from this perspective.
Below diagram shows MAC PDU structure (for non-HS-DSCH):
| TCTF | UE-Id type | UE-Id | C/T | MAC SDU |
TCTF (Target Channel Type Field) identifies type of logical channel on FACH and RACH transport channel. For example, on FACH, 000 indicate BCCH, 01100 indicate DCCH or DTCH over FACH.
When multiple (dedicated) logical channels are carried on same transport channel, C/T field is used to denote number of logical channel. Size of C/T field is fixed to 4 bits (so 15 logical channels).
UE-Id identifies UE on common transport channel. Two types are possible: U-RNTI and C-RNTI.
U-RNTI (UTRAN Radio Network Temporary Identity) is given to UE having RRC connection. It identifies UE in UTRAN. It is of 32 bits and composed of SRNC identity and S-RNTI. S-RNTI (Serving RNC RNTI) is specific to serving RNC and it also used by DRNC (Drifting RNC) to identify UE to SRNC.
U-RNTI is used only by network i.e. in downlink direction only. U-RNTI may be used in DCCH mapped over common transport channel (like FACH).
C-RNTI (Cell RNTI) is valid/unique in a cell and is used on DTCH and DCCH in uplink, and may be used on DCCH in downlink (when mapped onto common transport channels like FACH). C-RNTI is given to UE by controlling RNC upon accessing (new) cell.
MAC header and MAC header fields may or may not be present depending on channel mapping, type transport channel and multiplexing of logical channels. For example, BCCH mapped to BCH transport, MAC header is not needed whereas BCCH mapped to FACH transport, MAC header is needed (with TCTF). Refer TS 25.301 Sec 5.3.5 and TS 25.321 Sec 9.2 for more details.
References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.
| Copyright © Samir Amberkar 2010 | § |
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