RRC vs RAB: Understanding Differences in UMTS and LTE

This article explains the difference between RRC (Radio Resource Control) and RAB (Radio Access Bearer) terms used in UMTS and LTE networks. It also clarifies the relationships between RB, RAB, and SRB.

Radio Resource Control (RRC)

RRC stands for Radio Resource Control. When a User Equipment (UE) needs resources from the UTRAN (UMTS Terrestrial Radio Access Network) network, it requests them through RRC procedures. This usually happens when the UE wants to establish either a Circuit Switched (CS) call (voice) or a Packet Switched (PS) call (data).

The RRC connection establishment involves these steps:

• RRC Connection Request: The UE sends a connection request to the Radio Network Controller (RNC).
• RRC Connection Setup: The RNC assigns a traffic channel (a type of transport channel) and creates a SRB (Signaling Radio Bearer).
• RRC Connection Complete: The UE confirms the connection.

Radio Access Bearer (RAB)

RAB stands for Radio Access Bearer. The Access Stratum (AS) entity utilizes the Non-Access Stratum (NAS) to manage the information transfer between the UE and the Core Network (CN). This entire process is encompassed by the RAB.

The diagram illustrates the RAB’s end-to-end path across the UMTS system, from the UE (mobile device) to the MSC (Mobile Switching Center) or SGSN (Serving GPRS Support Node).

RABs are categorized into two main types based on their function and position within the network:

• Radio Bearer (RB): This is the connection between the UE and the RNC.
• Iu Bearer: This is the connection between the RNC and the Core Network (CN).

The Radio Bearer (RB) handles both user data and signaling information. When the RB is used solely for signaling purposes (control plane), it is called a Signaling Radio Bearer (SRB).

The term ‘TRB’ (Traffic Radio Bearer) is sometimes used to specifically refer to Radio Bearers carrying user data (User Plane).

Depending on whether it’s a circuit-switched (CS) call or a packet-switched (PS) call, the Iu-interface carries either voice or data.

RRC vs RAB: Key Differences

The following table summarizes the key differences between RRC and RAB:

Analogy to GSM

The concepts of RRC and RAB are analogous to the SDCCH (Standalone Dedicated Control Channel) and TCH (Traffic Channel) used in GSM technology. Understanding the functional difference between SDCCH and TCH in GSM helps in grasping the difference between RRC and RAB in UMTS/LTE networks.