Signaling Radio Bearer basic Part-2

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In this section we will learn about SRB and its basics as well as interview questions. Please read our previous SRB article basic Part-1.


SRBs (Signaling Radio Bearers) are nothing but the Radio Bearers that are used only for the transmission of NAS and RRC messages. In more convenient way SRBs can be defined as follow:

In downlink piggybacking of NAS messages is used only for bearer establishment or modification or release. Whereas, in uplink piggybacking of NAS message is used only for transferring the initial NAS message during connection resume and connection setup.FFS Piggybacking of NAS messages in other procedures than bearer establishment or modification or release.

NOTE: The NAS messages transferred through SRB2 are also contained in RRC messages, which however do not include any RRC protocol control information.

As soon as the security is activated, all RRC messages on SRB1, SRB2 and SRB3, including those NAS messages, are integrity protected as well as ciphered by PDCP. Integrity protection and ciphering to NAS messages are applied independently by NAS itself.

The integrity protection and ciphering algorithm is common for SRB1 and SRB2. When not configured with any kind of DC, the integrity protection and ciphering algorithm is common for all SRBs that is SRB1, SRB2 and DRBs. The security configuration for all DRBs related to same PDU is same.

Does integrity Protection and ciphering are applied to SRB0?

The integrity protection and ciphering are not applied to SRB0.

What is the motivation for SRB3?

·        Below are the motivations for SRB3:
·        Lower signaling delay over NR interface
·        Less processing at MCG.
·        Can only be used for message that does not need coordination between MCG and SCG.

 
Do SCG SRB has higher priority than DRBs?

SCG SRB has higher scheduling priority than all DRBs.


What is the Range of LCID in LTE?

Logical channel ID range is 1 to 10.

What is a Control channel?

It is carrying control plane information (SRB) between UE and eNodeB.

LCID (logical channel ID) & SRB ID mapping:
LCID
SRB ID
1
1 (SRB 1)
2
2 (SRB 2)

What is a traffic channel?

It is carrying user plane information (DRB) between UE and eNodeB.


What is maximum range of DRB in LTE?

Maximum DRB as per spec 36.331 is 11 for LTE.

What is EPS-Bearer ID and its range?

NAS identifies the bearer base on the EPS-Bearer ID, which has a range from 0 to 15. But in that 0 to 4 are reserved, so 5 to 15 are used. 

LCID (logical channel ID) & DRB ID & EPS-Bearer ID mapping:
LCID
EPS-Bearer ID
DRB ID
3
5
3
4
6
4
5
7
5
6
8
6
7
9
7
8
10
8
9
11
9
10
12
10

Note: Most of the operator use same LCID and DRB-ID for better understand. It is not necessary to start DRB ID from 3, as its maximum range is 11 so we can give any DRB ID number between 1 to 11. 

What is maximum possible DRB ID, UE can establish in the LTE case?

Since LCID is having range from 3 to 10, hence as per above table maximum 8 DRB’s can be established per UE at a time in LTE.

In EN-DC, SRB3 can be used for measurement configuration and reporting, to configure and reconfigure MAC, RLC, physical layer and RLF timers and constants of the SCG configuration, and to reconfigure PDCP for DRBs associated with the S-KgNB or SRB3, given that the (re-)configuration does not require any MeNB involvement.




In the next section we will learn more about SRB. So keep reading……. And Stay tune……………


Pinal Dobariya………..





Reference:
3GPP TS 38.331
3GPP TS 37.340

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