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Slide Notes
RRC and RAB are without a doubt one of the most important concepts in all generations of mobile telephony from 3G onwards.
Their importance is as great as their documentation, present in practically all 3GPP specifications, as they participate in the control, establishment and maintenance of calls.
Whether they are voice (CS) or data (PS).
Trying to explain it in all its scope is not an easy task - but if we understand the concept, everything becomes easier.
So let’s see today an analogy that allows us to understand the importance and purpose of RRC and RAB in modern networks, including 4G and 5G.
As a spoiler, RRC (Radio Resource Control) is a layer 3 protocol used between the UE and the Base Station, on the air interface, and is used to control the radio resources of a device, such as allocating and releasing resources for data transmission and reception. It manages the connection between a device and the network, including establishing, maintaining and releasing the connection, as well as configuring and modifying radio parameters. The RRC also provides information to higher layers of the protocol stack, such as mobility and security information, and receives control information from higher layers to control radio resources. It is specified by 3GPP in TS 25.331 for UMTS, in TS 36.331 for LTE and in TS 38.331 for 5G New Radio.
RAB (Radio Access Bearer) is a logical channel that carries user data between the mobile device and the main network. RABs are established and released by the “nodes” (NodeB, eNodeB, gNodeB), and are responsible for end-to-end Quality of Service (QoS) and are used to differentiate traffic types and ensure that the proper service level is provided for each type of traffic.
But going back to our analogy, imagine in a real world two people separated by a cliff.
On the left side, one guy who wants to go shopping. On the right side, a saleswoman who has a warehouse (or a supermarket).
The person on the left side wants to buy something, and first passes the information on to the salesperson. For example, he ties a note to a rock and throws it to the right.
The salesperson, on the right, receives the order, sees what the client asked for, and checks her warehouse to see if she has what he needs.
Considering she has what he asked for, she builds a bridge (a path) between them.
With the bridge constructed, the buyer confirms the order.
And the seller sends (for example, a gift) in one or more shopping carts.
What we did then:
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We had a conversation, a communication in a common language for both, where resources were allocated (and released) for sending and receiving things. Equivalent to RRC (Radio Resource Control) in mobile networks.
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We created a bridge so that things (of different types) could be transported, with guarantee. Sort of like RAB (Radio Access Bearer) does on mobile networks.
Of course, our example is quite simplified! But that’s the “idea”.
All communication on mobile networks is based on conversations (protocols) that allow the definition of paths (channels) through which things (data) flow, within a certain guarantee of quality.
As we said at the beginning, RRC and RAB are extremely important, and it is worth having this concept well understood.
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