Radio Bearer in LTE

Bearer is just a virtual concept. It defines how the UE data/signaling are treated when it travels across the network. Network might treat some data in a special way and treat others normally.

Radio Bearer Types

• SRB (Signaling Radio Bearers), used to carry C-Plane (control plane) traffic, i.e. RRC signaling messages.
  • RRC messages can contain other messages, such as NAS messages or HRPD/1xRTT messages.
  • There are three SRBs, all MUST be established.

• DRB (Data Radio Bearers), used to carry U-Plane traffic (user data).
• Carry IP packets
• Default Bearer vs Dedicated Bearer

Signaling radio bearers: 

·         SRBs are used only for transmission of RRC or NAS messages

·         Three SRBs are defined:

·         SRB0 is for RRC messages using the CCCH logical channel use RLC TM in DL whereas in UMTS system it SRB 0 is sent on RLC UM entity over CCCH logical channel in DL.

·         SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel. SRB1 has higher priority.

·         SRB2 is for RRC messages which include logged measurement information as well as for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by E-UTRAN after security activation.

·         All RRC messages are integrity protected and ciphered by PDCP.

·         NAS independently applies integrity protection and ciphering to the NAS messages.

In LTE there is only three SRB is defined i.e. SRB0, SRB1 and SRB2 whereas in UMTS system RRC has 4 SRBs i.e. SRB0, SRB1, SRB2 and SRB3 (optional).

In 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

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

GBR: Radio Bearer with a guaranteed bit rate

VoIP: Radio bearer to carry the VoIP

Data Radio Bearers:

Default Bearer vs Dedicated Bearer

While trying to get a better understanding of VoLTE and IMS , I came across bearers stuff which I thought might be useful to share with you guys. Before we talk about what default and dedicated bearer is, let’s understand what bearer itself is :
Bearer is just a virtual concept. It defines how the UE data is treated when it travels across the network. Network might treat some data in a special way and treat others normally. Some flow of data might be provided guaranteed bit rate while other may face low transfer. In short, bearer is a set of network parameter that defines data specific treatment   e.g. Person A will always get at least 256 Kbps download speed on his LTE phone while for person B there is no guaranteed bit rate and might face extremely bad download speed at times

Default Bearer in LTE

• A default bearer is bearer able to carry all kinds of traffic (no filter) without QoS. It is typically created during the attach procedure
• When LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached. Default bearer is best effort service. Each default bearer comes with an IP address.UE can have additional default bearers as well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer.

Dedicated Bearer

• A dedicated bearer is a bearer to carry a specific data flow, identify by the TFT (Traffic Flow Template), with a given QoS.
• To put it simple, dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer on top of default bearer. It does not require separate IP address due to the fact that only additional default bearer needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR). For services like VoLTE we need to provide better user experience and this is where dedicated bearer would come handy. Dedicated bearer uses Traffic flow templates (TFT) to give special treatment to specific services.

Question: My question is, why the numbers of SRB’s and DRB’s are fixed as 3 and 8 respectively LTE? Why not more than this?