Srg2-wireless resource bearer in LTE

In LTE
As a special wireless bearer (RB), Radio bearers-signaling is only used to transmit RRC and NAS messages. In Protocol 36.331, The srbs transmission channel is defined:

-- Srb0 is used to transmit RRC messages and transmitted on the Logical Channel ccch.

-- Srb1 is used to transmit RRC messages (may contain piggybacked
NAS messages), which has a higher priority than srb2 before the establishment of srb2. Transmission over the Logical Channel dcch.

-- Srb2 is used to transmit NAS messages, which has a lower priority than srb1 and is always configured after the security mode is activated. Transmission over the Logical Channel dcch.

 

 

Downstream piggybacked NAS messages are only used in the attachment process (for example, connection success/failure): The creation, modification, and release of the bearer. Upstream piggybacked
NAS messages initialize NAS messages during connection establishment (that is, initiate connection establishment, msg3)

Note: transmitting NAS messages through srb2 is also contained in RRC messages, but these NAS messages do not include any RRC Protocol control information, but are only included in RRC when RRC messages are transmitted, RRC is a carrier.

 

 

Once the security mode is activated, all RRC messages (including some NAS or 3GPP messages) of srb1 and srb2 are protected and encrypted through PDCP, NAS only protects and encrypts the integrity of NAS messages. In other words, layer-2 encryption and Protection of LTE: NAS only performs control signaling encryption, while PDCP performs both control plane and data plane protection and encryption at the same time,

 

Note the following link when using srb2: it is built on a dedicated bearer and uses the dcch logical channel.

 

Note:

Rrcconnectionreqest is transmitted on srb0, which always exists and is used to transmit the RRC Signaling mapped to ccch.

After the UE receives the rrcconnectionsetup signaling from NodeB, The srb1 between UE and NodeB is established.

Enodeb sends the rrcconnectionreconfiguration message to the UE to establish the srb2 and DRB