There are 2 types of Radio Bearers (RB) in LTE:
- To carry signaling. There are called the SRB (Signaling Radio Bearer)
- To carry user data. There are associated with an EPS Bearer
In LA1.X, the maximum number of RB per UE is 4
The following types of Radio Bearer are defined:
- SRB1: RRC signaling with high priority
- SRB2: RRC signaling and NAS signaling (lower priority)
- Best Effort: also defined as the default EPS Bearer
- GBR: Radio Bearer with a guaranteed bit rate
- VoIP: Radio bearer to carry the VoIP
In LA1.X the following combination are supported:
- SRB1
- SRB1+SRB2+Best Effort
- SRB1+SRB2+Best Effort + GBR
- SRB1+SRB2+Best Effort + VoIP
- SRB1+SRB2+Best Effort + Best Effort
At the RRC connection, the eNodeB scheduler creates a context for the UE containing the UEBearerList. this list is limited to 4 per user in LA1.X
Each bearer is identified by the LCID (Logical Channel ID)
Each bearer is associated with QoS parameters like :
- Max bit rate and guaranteed bit rate
- VoIP or not
- H-ARQ usage
Default Bearer vs Dedicated Bearer
- A default bearer is bearer able to carry all kinds of traffic (no filter) without QoS. It is typically created during the attach procedure
- A dedicated bearer is a bearer to carry a specific data flow, identify by the TFT (Traffic Flow Template), with a given QoS.
- Example: Voice, streaming
- It can be established:
- During the Attach procedure (depending on the user profile)
- After the Attach procedure, on demand
