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