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UMTS (Radio bearer establishment) - 23

Radio bearer establishment [Under UMTS]
» RRC - 25.331 «

RRC connection (running over signaling radio bearer) provide signaling connection for NAS. User data transmission happen over radio bearers. As indicated in signaling briefly article, network trigger radio bearer establishment.

Below diagram shows signaling exchange for radio bearer establishment.

UE NB

Network does Iu bearer setup in UTRAN and trigger radio bearer setup over radio interface.

DCCH
RADIO BEARER SETUP
Main intent of this message is to inform UE: how access layers are to be configured. Configuration is decided by network taking into consideration the QoS requirements for the particular service. The message contain TF/TFC/TFCS/CCTrCh configurations, type of transport channel (common - RACH/FACH or dedicated DCH), scrambling code (DPCH), frequency etc. RRC (thru control primitives) configures access stratum layers (PDCP, RLC, MAC, and PHY) accordingly. DCCH could be FACH or DCH depending on RRC state.

DCCH
RADIO BEARER SETUP COMPLETE
After configuration, UE acknowledges it to network with this RRC message.

Radio bearers or transmission plane is ready to be used. RRC possibly will indicate this to NAS.


The radio bearers can later be released or reconfigured. This allows network to manage available resources effectively (may based on user load). As in case of establishment, release/reconfiguration is done for Iu bearers at UTRAN side before being initiated for radio interface.

Below diagram shows signaling exchange for radio bearer release.

UE NB

Network decide to release radio bearers (e.g. voice call is over).

DCCH
RADIO BEARER RELEASE
RRC configures access layers for release. It may inform NAS about release.

DCCH
RADIO BEARER RELEASE COMPLETE
RRC acknowledges it to network.


Reconfiguration can be done by RRC messages, RADIO BEARER RECONFIGURATION, TRANSPORT CHANNEL RECONFIGURATION, and PHYSICAL CHANNEL RECONFIGURATION. This may impact one or more access layers (including PDCP).

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

Copyright © Samir Amberkar 2010§

RRC, RLC, MAC, and PHY « UMTS Index » Bit about MM/GMM mobility