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RRC vs RAB | difference between RRC and RAB in UMTS,WCDMA,LTE

This page on RRC vs RAB describes difference between RRC and RAB terms used in UMTS and LTE. It also provides difference between RB,RAB and SRB and mentions UMTS, WCDMA and LTE related links.



RRC stands for Radio Resource Control. As we know whenever UE needs resources from UTRAN network, it requests for the same. UE needs resources when it wants to establish either CS call or PS call. In order to do this following three steps are performed as shown in the figure-1.

• RRC Connection Request: From UE to RNC
• RRC Connection Setup: RNC assigns traffic channel(i.e. transport channel) and creates SRB(Signaling Radio Bearer).
• RRC Connection Complete

Refer WCDMA CS Call and PS Call.

Difference between RAB vs RB vs SRB


RAB stands for Radio Access Bearer. AS entity provides NAS to take care of information transfer between UE and Core Network(CN). This is known as RAB.

Figure-2 depicts RAB across the UMTS system end to end from UE(i.e. mobile) to MSC/SGSN. Based on functionality and position in the system, RAB are of two main types. RB and lu bearer. RB(Radio Bearer) is between UE and RNC entities while lu bearer is between RNC and Core Network(CN).

RB is used for both user data as well as signalling. The RB used only for signaling purpose is known as SRB(Signaling Radio Bearer) used in control plane. There is another term 'TRB' used to refer Traffic Radio Bearer. TRB is used in User Plane.

Based on circuit switched call (CS call) or packet switched call (PS call), RAB type lu-interface carry either voice or data information. Following table compares RRC vs RAB.

Feature RRC
(Radio Resource Control)
(Radio Access Bearer)
Function used for signaling transfer used for information transfer
Similarity with GSM Same as SDCCH(Standalone Dedicated Control Channel) Same as TCH(Traffic Channel)

The terms RRC and RAB is similar to SDCCH and TCH channels used in GSM technology. So engineers who are familiar with SDCCH signalling channel and TCH traffic channel can easily interpret the functional difference between RRC and RAB used in UMTS/LTE networks.

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