Post on 02-Jun-2018
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Prepared by
Riyaz A. Khan
riyazahmad79@gmail.com
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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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