Posts Tagged rb

What is RAB?

RAB (Radio Access Bearer)


created to provide user plane data transfer.

RAB comprises

1. RB and

2. connection from the SRNC to the SGSN (Iu bearer).

The RAB is created on reequest from the SGSN with a specific QoS request.

For a given UE, there can be multiple RABs per NAS service (speech or PS data service).


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What is RB?

RB (Radio Bearer)

is a layer 2 connection between the UE and the RNC.

It is used for both control signaling and user data.

RB used for signaling is called an SRB.

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RRC connection vs RB

RRC connection establishment is initiated by UE all the time.

RB established is initiated by CN all the time.

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RB and SRB

Layer 2 provides upper layers with information transmission between the UE and UTRAN by means of Radio Bearers (RBs) and Signaling Radio Bearers (SRBs).

RBs are for transmission of user data.

SRBs are for transmission of control information that can be originated either in the RRC or in upper layers.

RB specifies the configuration and the parameters of layer 2 and physical layer depending on the characteristics of the service (voice, interactive service, etc.).

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Radio interface protocol structure

Radio interface protocol structure:


Note that RB vs. SRB

At the UTRAN side, RLC and above protocols are located at RNC.

MAC protocols: some functionalities are at Node B, others are at RNC.

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RAB vs. RB vs. SRB

The AS provides the NAS with a service of information transfer between UE and the CN in what is named a Radio Access Bearer (RAB).

RAB consists of two:

1. Radio bearer (RB) between the UE and RNC

2. Iu Bearer between RNC and CN

RB is only for user data either in CS or PS mode.

RB is for both user data and signaling.

Especially for signaling purpose RB, we call it SRB: In the control plane, the signalling information is transfered through the Signaling Radio Bearer (SRB).

However, RAB consists of Iu bearer and user data RB (not SRB).


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I'm confused! RRC connection, RB, SRB, signaling connection ?!

Now I got the idea!!!

RRC connection:

To transfer RRC messages (messages generated at RRC layer itself and NAS messages) between UE and UTRAN.

RRC messages are transferred by the lower layers through Signaling Radio Bearers (SRBs).

Thus, establishment of RRC connections mean establishments of different SRBs.

Only a single connection.


Signal connection:

Establishment is needed in order to transfer NAS signal messages. That is, messages are exchanged between UE and CN.

However, note that this NAS signal messages stillĀ use SRBs between UE and UTRAN!!



The upper layer signalling messages (exchanged by the UE and the CN) may request the initiation of a user service (beloning to certain service class and with different requirements) .Then, RB will be established. RB is a user data.


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