Overview
Signalling Radio Bearers (SRBs) are Radio Bearers used exclusively for the transmission of RRC and NAS messages. They are categorized based on logical channel type, purpose, priority, and security handling.
SRB Definitions (Clause-by-Clause)
|
SRB |
Purpose |
Logical Channel |
Key Characteristics |
|
SRB0 |
RRC messages |
CCCH |
Used for initial access messages. Exception: Not used for L2 U2N Remote UE or L2 Intermediate U2N Relay UE (multi-hop). |
|
SRB1 |
RRC messages (may include piggybacked NAS) + NAS messages before SRB2 setup |
DCCH |
Handles connection setup/establishment. Exception: Not used for L2 U2N Remote UE or L2 Intermediate U2N Relay UE (multi-hop). |
|
SRB2 |
NAS messages + RRC messages with logged measurement info |
DCCH |
Lower priority than SRB1; configured after AS security activation. Exception: Not used for L2 U2N Remote UE or L2 Intermediate U2N Relay UE (multi-hop). |
|
SRB3 |
Specific RRC messages for (NG)EN-DC or NR-DC |
DCCH |
Used only in Dual Connectivity scenarios. |
|
SRB4 |
RRC messages with application layer measurement reports |
DCCH |
Lower priority than SRB1; configured only after AS security activation. |
|
SRB5 |
RRC messages with application layer measurement reports |
DCCH |
Lower priority than SRB1 and SRB3; configured only by SN serving SCG in NR-DC, after AS security activation. |
|
SRB6 |
RRC messages with logged measurement info for network-side data collection |
DCCH |
Lower priority than SRB1; configured only after AS security activation. |
NAS Piggybacking Rules
· Downlink: Piggybacking of NAS messages is used only for dependent procedures (bearer establishment/modification/release) — i.e., joint success/failure.
· Uplink: Piggybacking of NAS messages is used only for transferring the initial NAS message during:
o Connection Setup
o Connection Resume
Note: NAS messages on SRB2 are contained in RRC messages but do not include any RRC protocol control information.
Security (Integrity & Ciphering)
Once AS security is activated:
· All RRC messages on SRB1, SRB2, SRB3, SRB4, SRB5, and SRB6 — including those containing NAS messages — are:
o Integrity protected by PDCP
o Ciphered by PDCP
· NAS messages additionally have independent integrity and ciphering applied by NAS layer (see TS 24.501).
Split SRB Support:
|
SRB |
Split SRB Support |
|
SRB0 |
❌ Not supported |
|
SRB1 |
✅ Supported (MR-DC & MP) |
|
SRB2 |
✅ Supported (MR-DC & MP) |
|
SRB3 |
❌ Not supported |
|
SRB4 |
❌ Not supported |
|
SRB5 |
❌ Not supported |
|
SRB6 |
❌ Not supported |
Applicable for: All MR-DC options and Multipath (MP) operations.
Channel Access Priority Class (CAPC) – Shared Spectrum
For operation with shared spectrum channel access in FR1:
· SRB0, SRB1, SRB3 → Assigned highest priority CAPC = 1
· SRB2 → CAPC is configurable
L2 U2N Relay Operations (Sidelink)
For NR sidelink L2 U2N relay (not involving MP) or L2 multi-hop U2N relay:
· SRB0, SRB1, SRB2 of:
o L2 U2N Remote UE
o L2 Intermediate U2N Relay UE
· Are not transmitted via Uu CCCH/DCCH logical channels.
· Instead, they are transmitted via:
o PC5 Relay RLC channels (over PC5 interface)
o Uu Relay RLC channels (over Uu interface)
Summary Table – SRB Characteristics:
|
SRB |
Logical Channel |
Priority |
Security |
Split SRB |
CAPC (FR1 shared) |
L2 U2N Relay |
|
SRB0 |
CCCH |
– |
❌ (pre-security) |
❌ |
1 (highest) |
❌ (relay path) |
|
SRB1 |
DCCH |
High |
✅ (post-security) |
✅ |
1 (highest) |
❌ (relay path) |
|
SRB2 |
DCCH |
Low (vs SRB1) |
✅ |
✅ |
Configurable |
❌ (relay path) |
|
SRB3 |
DCCH |
– |
✅ |
❌ |
1 (highest) |
N/A |
|
SRB4 |
DCCH |
Low (vs SRB1) |
✅ |
❌ |
– |
N/A |
|
SRB5 |
DCCH |
Low (vs SRB1 & SRB3) |
✅ |
❌ |
– |
N/A |
|
SRB6 |
DCCH |
Low (vs SRB1) |
✅ |
❌ |
– |
N/A |
Reference: TS 38.331