Concept of SRB

8/10/2019 Concept of SRB

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Prepared by

Riyaz A. Khan

[email protected]


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SRBs are associated with the control plane protocol

stack.

The control plane protocol stack includes RRC, RLC,

MAC, Frame Protocol and Physical layers.


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The concept of Signaling Radio Bearer (SRB) is used to define the logical signaling

connection between the RLC layer in the UE and the peer RLC layer in the RNC.


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An SRB is a control plane version of a Radio Bearer (RB). Every radio bearer has an

identity and radio bearer identities 1 to 4 are reserved for SRB. It is possible to configure

more than a single SRB to simultaneously link a UE to an RNC. Each SRB has its ownlogical channel between the RLC and MAC layers. The MAC layer is used to multiplex the

multiple SRB onto a single transport channel which is then processed by the physical

layer. This multiplexing operation requires the MAC layer to include a header to specify

which SRB is using the transport channel at any point in time.


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3GPP TS 25.331 specifies that all messages sent on the Common Control Channel (CCCH)

logical channel make use of SRB0. SRB0 is always encapsulated by the RACH transport

channel in the uplink direction and the FACH transport channel in the downlink direction.


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UE in RRC Idle mode are limited to using only SRB0. This SRB is used when establishing an

RRC Connection and making the transition from RRC Idle mode to RRC Connected mode.

SRB0 uses Transparent Mode (TM) RLC in the uplink direction and Unacknowledged Mode(UM) RLC in the downlink direction. Both of these RLC modes rely upon re-transmissions

being provided by layer 3 rather than layer 2, i.e. by the RRC layer rather than the RLC

layer.

This is in contrast to Acknowledged Mode (AM) RLC which allows re-transmissions from

the RLC layer.

The use of TM and UM RLC means that entire RRC messages

have to be re-transmitted rather than only the individual transport blocks which have been

received in error. Re-transmissions may be required if the air-interface conditions are

relatively poor.


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Concept of Signaling Radio Bearer


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3GPP TS 25.331 specifies that SRB1, 2, 3 and 4 make use of Dedicated Control Channel (DCCH)

logical

channels.

If all four of these SRB are used then four DCCH logical channels are multiplexed onto a single

transport channel within the MAC layer. These DCCH SRB are configured during RRC

Connection

establishment within the RRC Connection Setup message. They can also be configured when

making

RRC Connected mode state changes, e.g. within a Radio Bearer Reconfiguration message when

moving

from CELL_DCH to CELL_FACH.


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When a UE is in RRC Connected mode SRB1, 2 and 3 are configured as a minimum whileSRB4 is optional.


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SRB1 is used for all messages sent on the

DCCH which use

Unacknowledged Mode (UM) RLC. SRB2

is used for all messages sent on the

DCCH which use Acknowledged

Mode (AM) RLC and which do not contain

NAS messages. SRB3 and SRB4 are used

for all

messages sent on the DCCH which

contain NAS messages. SRB3 and 4 use

AM RLC in the same way asSRB2. SRB3 is used rather than SRB 4

either when the NAS indicates that a

message has high priority, or

when SRB4 has not been configured. SRB4

is used when it has been configured and

the NAS indicatesthat a message has low priority. The

priority levels are used within the MAC

layer when multiplexing the

set of SRB onto a single transport channel,

i.e. the priority determines which

messages are sent first.


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Summary of signaling radio bearers


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RRC messages associated with each SRB

Always SRB0: Cell Update, RRC Connection Request, RRC Connection Setup,

RRC Connection Reject, URA Update

Always SRB1: None


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Always SRB2: Active Set Update Complete, Active Set Update Failure, Assistance Data

Delivery,Cell Change Order From UTRAN, Cell Change Order from UTRAN Failure, Counter

Check, Counter Check Response, Handover from UTRAN Command, Handover from

UTRAN Failure, Handover to UTRAN Complete, Measurement Control, Measurement

Control Failure, Paging Type 2, Physical Channel Reconfiguration Complete, Physical

Channel Reconfiguration Failure, Radio Bearer Reconfiguration Complete, Radio Bearer

Reconfiguration Failure, Radio Bearer Release Complete, Radio Bearer Release Failure,Radio Bearer Setup Complete, Radio Bearer Setup Failure, RRC Connection Setup

Complete, RRC Status, Security Mode Command, Security Mode Complete, Security Mode

Failure, Signaling Connection Release, Signaling Connection Release Indication,

Transport Channel Reconfiguration Complete, Transport Channel Reconfiguration failure,

Transport Format Combination Control Failure, UE Capability Information, UTRAN

Mobility Information Confirm, UTRAN Mobility Information Failure


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Always SRB3:Initial Direct Transfer

Always SRB4: None

SRB0 or SRB1: Cell Update Confirm, RRC Connection Release, URA Update Confirm

SRB1 or SRB2: Active Set Update, Measurement Report, Physical Channel Reconfiguration,

Radio Bearer Reconfiguration, Radio Bearer Release, Radio Bearer Setup, RRC Connection

Release

Complete, Transport Channel Reconfiguration, Transport Format Combination Control,

UE Capability Enquiry, UE Capability Information Confirm, UTRAN Mobility Information

SRB3 or SRB4: Downlink Direct Transfer, Uplink Direct Transfer


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The SRB associated with each RRC

message can be deduced from the

logical channel type,

RLC mode and whether or not the RRC

message includes a NAS message. The

majority of RRC

messages make use of SRB2, i.e. the

DCCH logical channel with AM RLC andwithout including a

NAS message. There are no messages

which always use SRB1 or SRB4. The RRC

Initial Direct

Transfer message is always sent with high

priority whereas the priority of the Uplinkand Downlink

Direct Transfer messages depends upon

the type of encapsulated NAS message.


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SRB 1 to 4:

The RB represents the logical connection between the UE and the

RNC. The concept of a user plane

RB is the same as the concept of a control plane SRB. RB identities 1 to4 are reserved for SRB and so

user plane RB have identities which are greater than 4.


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Documents

Concept of SRB