Radio Bearer in LTE

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