VoLTE Flows and CS Network

Before I’ll finish a new post about 5G I decided to take a break and create a presentation which would complement VoLTE Basic Flows.

Btw. I can’t find the option on slideshare which would allow to update the material .. does anyone know??

VoLTE Flows – Basics

I’ve just recently changed my job and that reminded me, what it means to start from the beginning again. To help those of you who have just started with VoLTE/IMS I’ve created a short presentation.

Let me know if it works (and maybe one day I’ll find some to make a recording too ūüôā ). Good luck!

Other presentations:

How to read tcpdump – Registration

In telco tracing has always been very important. It helps us to understand the flows, the content and – sure – to find bugs. SS7 used to be much more strict and we didn’t need to go in the detail traces that often. The flow and a few fields was usually enough. With SIP and SDP it is a different story. SIP is a very flexible protocol. That means powerful but also complex and sometimes quite confusing.

Let’s take a look on IMS Registration. I’m sure you remember the idea behind from a previous posts about Registration and 3rd party registration¬†and at least the basic flow:


IMS Registration

IMS Registration

The registration flow is very important Рin practice if something goes wrong, it is most probably registration. Once is a subscriber correctly registered,  the other flows (call, sms, fileshare) usually works fine.

There are many documents describing the message flow and important values. For VoLTE I’d recommend to go through VoLTE Service Description and Implementation Guide¬†or A Definitive Guide to Successful Deployments.¬†Would you have any doubts about a particular header please refer to¬†http://www.iana.org/assignments/sip-parameters/sip-parameters.xhtml. Now let’s go through the particular messages.

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Much Ado about Registration

Registration is something what was missing in the times of wires. Ok, not completely but it was done more or less once when an MSISDN was assigned to some line.

IMS Registration

IMS Registration

In contrast to the previous types of the networks the 4G network is ‘user-centric’. It means the user can use multiple devices and identities and we have to deal with it. The main purpose of the registration is to create a binding between user (her public identity) and IP address of the device, so we know where we can send the data to. That’s why there is a Contact header in the SIP REGISTER message. The Contact header contains an address which identifies the current location of the user (Point-of-Presence – e.g. IP/FQDN of a particular client). During the registration is the Contact Address¬†is linked to¬†a Public Identity (IMPU, AOR in SIP terminology). The IMPU is an equivalent to MSISDN in GSM and has to be present in the To header of the SIP REGISTER. One identity can by used by more terminals and as each terminal can have different capabilities this has to be also taken into account. This information is part of the Contact header.

Contact: <sip:119560022547@;transport=tcp>;

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RCS – Rich Communication Services

As this is a blog, I’m not going to recap all the stuff¬†you can find in wikipedia about RCS. Wikipedia is a great source of information but sometimes you can’t see the forest for the trees. The RCS is really a big theme so I guess it would be better¬†to write several posts than to try put all the relevant information in one. I’ll probably use some simplifications, the details and exceptions will be described later.


So what is the RCS?

Update: For the information about RCS Universal profile read the post¬†GSMA Advanced Messaging ‚Äď RCS Universal¬†Profile.

RCS is an attempt of mobile operators and their vendors to be more than just connectivity providers. With the¬†3rd generation of mobile networks many people started to believe that they’ll not need the operators soon¬†– as long as they can connect to the Internet they have VoIP, Instant messaging, Emails, Facebook, etc (Over-the-top ¬†applications OTT). Why should anyone pay for roaming or use SMS anymore? But for the operators Voice services and SMS were/are the main sources of income. VoLTE and RCS should give their customers the user experience they know from Skype, Whatsapp, Viber or Line and enable them to stay relevant.

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