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What is RRC and rab?
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INTRODUCTION
To work with modern wireless networks such as UMTS and LTE, it is
essential that the telecom professional has full understanding of
its basic concepts, such as those that control the call
establishment and maintenance, whether it is voice (CS) or data
(PS).
In this scenario, RAB and RRC are two of the most important
concepts because they are responsible for all the negotiation
involved in those calls.
In addition to RAB and RRC, we still have some other terms directly
involved in context, as RB, SRB, TRB, among others. These terms are
also important concepts, since without them RAB and RRC could not
exist.
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ANALOGY
To start, we can divide a call into two parts: the signaling (or
control) and data (or information). Already ahead of key concepts,
we can understand the RRC as responsible for the control, and the
RAB as responsible for the information part.
As mentioned, other auxiliary concepts are involved in calls, but
our goal today is to learn the most basic concepts - RRC and RAB,
allowing us to evolve in our learning later.
Oddly enough, even professionals who already work with UMTS-WCDMA
and LTE networks have trouble to fully understand the concepts of
RRC and RAB. And without this initial understanding, hardly they
can evolve with clarity and efficiency in their daily work
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Let's start imagining the following scenario: two people are
cut off by a cliff. On the left side, a person (1) want to
buy
some things that are for sale in a store (2) or deposit on the
right side.
In the right side, in addition to the deposit, we also have a
seller (3),
which will help the buyer to contact (negotiable) with the
deposit.
As additional or auxiliary objects (4), we have some iron bars with
different sizes, and some cars - some like train wagon, others like
remote control cars.
In short, we have the situation outlined in the image below.
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Let's continue with a possible solution: the buyer on the left
write his request in a note, tie on a small stone that he found on
the floor, and send (1) it to the seller on the other side. So, the
stone carry the information or initial request.
The seller receives the request, but she need to send it to the
deposit, in order for the shopping to be sent. She sends the
request on a remote control car (1), which run a previously
demarcated path to the deposit.
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So that we can proceed with our call, let's consider a positive
response. That is, what the buyer is willing, or the 'resources'
are available.
Seller realizes that to fulfil the request, and be able to send the
purchases, she will need to build a 'path' (1) between the two ends
of the cliff, so the wagons could carry over with the
orders/receipts and purchases. Then, the seller uses some of its
iron bars and creates a link between the two sides.
Once established all the way between those involved, requests can
be sent from both sides as well as the purchases or any other
information can be transferred by different paths and
wagons/cars!
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Although analogies are not perfect, it help us a lot to understand
the complex functioning of these networks, especially in relation
to new concepts such as RRC and RAB, but also a very often used,
the 'bearer' — so much that it's worth talking a little bit about
it.
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What is Bearer?
If we search the word 'bearer' in the dictionary, we'll find
something like transporter, or carrier. In a simple way: one who
carries or conveys something from some point to another point. In a
restaurant, we can compare the 'bearer' to a waiter.
But from the telecommunications point of view, 'bearer' is best
understood as a 'pipe' that connects two or more points in a
communication system, through which the data flows.
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Technically speaking, it is a channel that carries Voice or Data, a
logical connection between different points (nodes) that ensures
that the packets that are traveling have the same QoS attributes.
Explaining better: for each 'bearer' we have several associated
parameters, such as the maximum delay and packet loss limit – and
these attributes that make sure each packet going in the same
channel have the same QoS attributes.
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Now that we know what is bearer, let's go back to the analogy
presented earlier, but now bringing it to the real, more technical
side.
All that we'll talk can be summarized in a single figure, having
all the concepts seen today, and that will be detailed from now
on.
Note: If you manage to understand the concepts that will be
explained in the figure below, you will be with a great base for
both WCDMA and LTE networks. This is because, in order to
facilitate we use WCDMA nomenclatures, but the principle is pretty
much the same in LTE. Just do the equivalent replaces, like NodeB
for eNB
General Flowchart - RRC, RAB and Others
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On that fictitious scenario, the seller is the UTRAN, responsible
for creating and maintaining the communication between the UE
(buyer) and CN (deposit) so
that the QoS requirements of each are met.
UTRAN: UMTS Terrestrial Radio Access Network
NodeB
RNC
SGSN: for packet-switched services
The cliff is the Uu Interface between the UE and the UTRAN, and the
road through the remote control car goes until the deposit is the
Iu Interface, between the UTRAN and CN.
Sending requests and receipts is part of signaling, or the RRC. The
shipment of purchases is the data part, or the RAB. In our
scenario, the RRC are the Rails, and RAB is the full service of
sending data between the UE and the CN.
RRC: Radio Resource Control
RAB: Radio Access Bearer
Note: the RRC is in Layer 3 - control plane, while the RAB occurs
between the UE and CN, in the user plane.
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The railcars are the RBs, and convey the information in the radio
path. These wagons define what type of thing will be transported,
and in what quantity. Similarly, the RBs define what type of data
will in the RRC, which can be Data or Signaling. When the QoS
attributes change, then the Rbs associated with that RRC connection
need to be reconfigured.
The remote control cars are the Iu bearer, and carry information on
Iu Interface (between the UTRAN and the CN), either CS or PS.
RB: Radio Bearer
Note: RAB is the combination of RB and Iu bearer.
As examples of RAB for some services and different rates we
have
The Conversational RAB and the Interactive RAB can be used
together, and in this case we have a case of MultiRAB.
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The RB is a layer 2 connection between the UE and the RNC, and can
be used for Signalling and control User Data. When it is used for
Signalling or Control Messages is called SRB. And when it is used
for user data is called TRB.
SRB: Signalling Radio Bearer (Control Plane)
TRB: Traffic Radio Bearer (User Plane)
Note: in an optimized network, we can find much of the traffic
being handled by HSPA bearers, even MultiRAB. This option frees
resources from CE (Channel Elements), relieving the load on R99
(that can only use these resources). However, it should be done
with caution, because if improperly configured it can degrade the
Performance Indicators with Blockage (Congestion) and
Failures.
As you've probably noticed, we're talking about several new
technical terms, but these terms are what you'll find for example
when reading UMTS or LTE call flowcharts. But if you can understand
at least in part the concepts presented today, everything will be
much easier.
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Let us then take a look again on our figure, and continue our
analogy.
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A RRC connection exists when an UE performs the call establishment
procedure, and get resources from the UTRAN. When a RRC connection
is established, the UE will also get some SRBs. (If for some reason
the initial request is not accepted, the UE can make a new request
after some time).
Since the SRB was established between the UE and the CN, the RNC
checks a series of information such as the UE identity, what is the
reason for the request and whether the UE is able to handle the
requested service.
The RNC that maps the requested RABs into RBs, to transfer between
the UE and the UTRAN. In addition it is also check the attributes
of the RABs: if they can be met by the available resources, and
even whether to activate or reset radio channels (reconfiguration
of lower layers services ) based on the number of Signaling
Connections and RABs to be transferred.
This way, it creates the impression that there is a physical path
between the UE and the CN. Remembering again that no matter how
many signaling and RABs connections there are between the ue and
the CN - there is only a single RRC connection used by the RNC to
control and transfer between the UE and the UTRAN.
Now that we have seen a lot about RRC and RAB, let's learn only a
few more concepts today – after all, we already have enough
information presented. Let's talk about the AS and NAS.
AS – Access Stratum is a group of specific protocols of access
network
NAS – NON Access Stratum: so, are the other protocols, or those
that are not access network
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At this point of view, the AS provides the RAB to the NAS, or
information transfer service.
The UE and CN need to communicate (events/messages) with each other
to perform several procedures with many purposes. And the
'language' of this conversation between them is called
protocols.
The protocols are then responsible for allowing this conversation
between the UE and CN, and cause the CN do not worry about the
method of access (be it GSM/GPRS, UTRAN, LTE). In our case the RNC
acts as a protocol - between the UTRAN and CN.
According to what we learned today, the RAB is carried:
Between the UE and the UTRAN: within the RRC connection. The RRC
Protocol is responsible for negotiating the (logical) channels of
Uu and IuB interfaces, and for the establishment of signaling
dedicated channels as SRBs and RBs among these interfaces.
Between the RNC and the CN: after being negotiated and mapped, in
the RANAP protocol connection, through Iu interface (CS/PS).
RANAP: Radio Access Network Application Part
As we have seen above, the RNC maps requested RABs into RBs using
current radio network resources information, and controls the
services of lower layers. To optimize the use of these resources,
as well as the network band and physical resource sharing between
different entities, the UTRAN can also perform the function of CN
messages distribution.
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RRC Connection and RAB example
To complete for today, let's see (always in simplified form) a
simple RRC connection and RAB.
Whenever the UE needs the UTRAN resources, he asks. So that these
resources are allocated, it establishes a RRC connection with some
SRBs.
In this case, a RAB connection is created to enable the transfer of
user data. We remind you that the RAB consists of RB + Iu bearer.
The RAB is created by CN, with a specific QoS request.
For a single UE, there may be multiple RAB for NAS service (CS or
PS).
But let's just stick to the initial procedure, that is, how is
performed the 'RRC Setup' procedure, from the UE's request.
The following figure shows this more straightforward.
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ΑΒΓΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨαβγδεζηθικλνξορςΣΤΥΦΧΨΩ Do not add objects or
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The RRC has always 3 steps:
The UE requests a new connection in the Uplink (‘RRC CONNECTION
REQUEST’);
With sufficient resources available, the 'RRC Downlink CONNECTION
SETUP' message is sent, including the reason, along with the SRB
configuration; (Note: otherwise, if the RRC connection cannot be
established, the message sent is 'RRC CONNECTION SETUP
REJECT').
If all goes well, the UE sends the message in the Uplink: ‘RRC
CONNECTION SETUP COMPLETE’.
And after this, the ‘MEASUREMENT CONTROL’ message are being sent in
the Downlink, for the communication continuity.
After the RRC connection is established, the UTRAN makes the checks
between the CN and the UE, for example the authentication and
security operations.
And so, the CN informs the RAB to UTRAN in accordance with
requirements of the service requested by the UE. As we have seen,
RAB occurs after the RRC, and without a RRC connection no RAB may
be established.
Slide title 44 pt Text and bullet level 1 minimum 24 pt Bullets
level 2-5 minimum 20 pt Characters for Embedded font:
!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~¡¢£¤¥¦§¨©ª«¬®¯°±²³´¶·¸¹º»¼½ÀÁÂÃÄÅÆÇÈËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿŒœŠšŸƒˆ˜–—‘’‚“”„†‡•…‰‹›⁄€™−≤≥
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Conclusion
We have seen today a simplified explanation that covers a number of
concepts involved in the communication of the most modern existing
mobile networks, primarily related to RRC and RAB.
With this conceptual base, we will continue to evolve in the next
tutorials with examples that make the assimilation of these complex
concepts in a task far less exhaustive than normal.
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