LTE - Signalling Radio Bearers

LTE - Signalling Radio Bearers

Lets talk about the Radio Bearers in LTE. The Radio bearers exactly comes in the RRC layer with a Logical Channel Mapping. The Radio bearers are classified as Signalling Radio Bearers - SRB's & Data Radio Bearers - DRB's

Signalling Radio Bearer's:

There are 3 SRB's - SRB0, 1 & 2

SRB0:
SRB0 is mapped to CCCH in Uplink & Downlink. The RRC messages RRC Connection request & RRC Connection Setup will be transmitted in SRB0

SRB0:
Direction: UL/DL
RLC - Mode: TM (Transparent Mode)
Logical Channel: CCCH
Logical Channel Group: 0

Messages Transmitted in SRB0:

1. RRC Connection request
2. RRC Connection setup
3. RRC Connection Reestablishment
4. RRC Connection Reestablishment Reject

SRB1:
SRB1 is mapped to DCCH in uplink & downlink. RRC messages & RRC with NAS piggybacked messages prior to the establishment of SRB2 will be transmitted in the SRB1

SRB1:
Direction: UL/DL
RLC - Mode: AM (Acknowledged Mode)
Logical Channel Group: 0

Messages Transmitted in SRB1:

1. RRC Connection setup Complete,
2. RRC Connection Reconfiguration,
3. RRC Connection reconfiguration Complete
4. RRC Connection Re establishment Complete.
5. Security Mode Command &
6. Security Mode Complete,
7. UE Capability Enquiry,
8. UE Capability Information,
9. Measurement reports. etc...

SRB2:

SRB2 is mapped to the DCCH in Uplink & Downlink, but after the integrity & Ciphering protected. All the NAS piggybacked messages for the UE, will be transfered in SRB2 if its present hereafter.

For the SRB2 & DRB's to establish, the AS has to be always Integrity Protected. SRB2 establishment will be taken place with the help of RRC connection reconfiguration coming from E-UTRAN to UE

SRB2:
Direction: UL/DL
RLC - Mode: AM (Acknowledged Mode)
Logical Channel: DCCH
Logical Channel Group: 0

Messages Transmitted in SRB2:

1. UL Information Transfer
2. DL Information Transfer

Note: SRB1 will always have the high priority in transferring the messages over SRB2. LCID is only for DRB's. SRB's will not have the Logical Channel Identity IE.