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 🙂 ). Good luck!

Other presentations:

Comment: A Piece of Advice for Every (Telco) Company

As a trainer and blogger I have a chance to talk to many engineers from various (telco) companies. And I can hear a lot of similar complains. “We’re just reinventing a wheel, over and over again.”, “I feel like an investigator – to find the right information/documentation is so painful.”, “R&D doesn’t want to share anything.”,  “They are not answering to my questions, they’re just sending a lot of crap to keep me busy.”, etc. The common denominator is that companies don’t encourage engineers to work aloud and share their experience and knowledge. Why? So much effort and talent is wasted.. And by encouraging I mean also that they don’t provide the right tools (no, it’s not sharepoint) and managers don’t lead by their examples.

This can’t be fixed by any process or action points. It is about company/team culture. If managers treat engineers as “resources”, act as if they are more important, or if they say “we’re not at work to make friends” (and some are even so dump to share it on social networks), it is hard to imagine that others would willingly share their know-how. Yes, there can be some lessons-learn filled after project, some reports created – but honestly, what engineers read these documents??

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IMS Centralized Services – Overview

Have you heard about the IMS Centralized Services (ICS)? The basic idea is fairly simple. We want to apply services for IMS subscribers, regardless what access network they use. We know that in IMS we can do it for all IP-based access domains. But if the subscriber is accessing through the legacy CS network (e.g. because of a low LTE coverage in her area), we are still triggering the services in the CS Core network … right, unless we have the ICS in place. So ICS enables the IMS services even when one is using CS access for the media bearer.

IMS Centralized Services

The ICS is specified in GSMA IR.64 and 3GPP TS 23.292, 23.237 and 23.216. The scope of the specification includes:

  • Session establishment when using CS access for media transmission for an IMS service
  • Support of Service Continuity
  • Support of Single Radio Voice Call Continuity
  • Access Domain Selection (ADS)
  • IMS control of services where the media is transported via the CS network (e.g. managing of mid-call services)
  •  Service data management

The solution is applicable for UEs with or even without ICS functionality. As the first step all the sessions have to be anchored in the IMS. That is a job for Service Centralization and Continuity Application Server (SCC AS). The SCC AS is on the signalling path for both the originating and terminating services. Using the initial Filter Criteria (IFC), the SCC AS is triggered as the first AS for originating sessions and as the last for terminating sessions.

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NEWS: Number Portability & ENUM

Even in 2017 the telephone number remains the most universal identifier for real-time communication. And as the word is moving to be All-IP, we have to be able to translate the number into something more meaningful for routing in IP networks. The GSMA organization selected for this purpose the Electronic Number Mapping System (ENUM) and in 2007 released the first version of PRD IR.67.

ENUM based Routing

Moreover Mobile Operators in over 50 countries have to support Mobile Number Portability (MNP). Although for MNP is a great feature for end subscribers, it makes the signalling more complex and costly for the Operators. The MNP is not just a problem for signalling (routing) but also for billing and management of interconnect agreements. Last but not least it can be a significant issue for content and application providers who may not be aware of the change of the operator for a particular user.

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SIP Illustrated 5: SIP Session Routing

In the last post we have seen a basic SIP (VoLTE) session. This time we should analyze in more detail, what headers are used by network elements for their routing decisions and how they discover what port and IP to use. In practice that’s what people are not always sure about. They know the flow, order of signalling messages, but when something goes wrong, they are just guessing what could be the reason.

SIP Message Routing

Let’s recap what we have learnt so far. We use loose routing. So if a SIP message contains a Route header, we will use the top most one for the routing. If there is no Route header the routing is done based on the Request-URI, which contains the address of the final recipient. Don’t forget, network elements are able to add and modify the headers. The way how we handle the messages and modify their content within the IMS is described in 3GPP standards.

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SIP Illustrated 4: SIP Session

To describe a single SIP Session in IMS is not that easy as it maybe sounds and in the beginning it requires a lot of simplification. The purpose of the signaling over SIP is to establish a multimedia session over RTP (or MSRP). That means that SIP helps to locate the recipient and to negotiate the parameters of the RTP session. To do that we need one more protocol, called Session Description Protocol (SDP) which SIP carries in its body. The procedures for IMS describing this mechanism can be found in 3GPP 24.229.

SIP 3 Way Handshake

To set up a session the SIP protocol mandates the SIP INVITE request. It has to be answered by some final response – ideally with 200 OK. To confirm, that the client received the 200 OK message, it sends a special request SIP ACK. The SIP ACK is the only SIP request which doesn’t trigger any response on the server side. The procedure is also known as SIP 3-way handshake.

In this post we will go through a basic VoLTE flow from the SIP and SDP point of view.

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SIP Illustrated 3: Routing and IMS Registration

In the last post I promised that this time we will take a closer look on SIP headers which are related to routing. SIP routing is very flexible and most of the SIP textbooks are explaining it in a very general way. Here we focus only on routing in IMS context. We should also point out that there are two methods how to route SIP messages – so called strict routing and loose routing. As the strict rooting is obsolete since 2002 and 3GPP mandates only the loose routing for IMS, we will talk just about the loose routing.

The first and the most important header is the Request-Line, which contains the Request-URI. It allows us to route a SIP request directly to a Server. The response then follows the Via header. SIP Client and each proxy which wants to intercept the response adds itself into Via headers of the SIP request. During the processing of the response then each proxy removes its own Via record from the message. The Via header is also used to detect possible loops in signalling.

SIP Request - Response
SIP Request – Response

 

In practice the UAs can’t see directly one each other and we have to use the IMS network to provide the routing. The first scenario we should go through is the IMS Registration. A VoLTE UE initiates a SIP REGISTER to the P-CSCF, using the P-CSCF IP that was made available during the LTE Attach. The Request-URI is set to the SIP-URI of the domain name of the home network.

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SIP Illustrated 2: SIP Message

The general structure of a SIP message is based on Internet Message Format (RFC5322, RFC2822). With some minor differences in character sets the SIP message syntax is identical to HTTP 1.1 (but surely the SIP is not an extension of HTTP).

SIP Message
SIP Message

The SIP messages start with the Start-line, which is followed by a number of header fields in the format name:value. An empty line separates the header fields from the optional message body. As the SIP is a textual protocol, with just a little practice with Wireshark or a similar tool, it is not that difficult to analyze the massages. But to truly understand the headers in the context of IMS requires some experience. In this post we will highlight the most important message parameters.
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SIP Illustrated 1: Basics

In November 2000, the Session Initiation Protocol (SIP) was accepted by 3GPP as a signaling protocol of the IP Multimedia Subsystem (IMS) network for IP-based streaming multimedia services. Later it was extended for video conferencing, streaming multimedia distribution, instant messaging, presence information, file transfer, etc.

SIP in VoLTE
SIP in VoLTE

The SIP protocol is easy to understand as it is text-based and practically derived from the Hypertext Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol (SMTP). SIP is very flexible and well designed for Internet telephony, on the other hand it has also some disadvantages and limitations. If you’re new to IMS signalling or you just need some brief introduction into SIP, I hope you’ll find this post useful.

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GSMA Advanced Messaging – RCS Universal Profile

Rich Communication Services (RCS) has been around for a couple of years without any major success (actually the number of RCS deployments has been declining). There were many reasons. From the technical point of view it was caused by very ambitious design at the times where there was a little experience with IP-based technologies in operators’ networks, along with the lack of good enough RCS clients. That has changed and in the November 2016 we got a new standard called RCS Universal Profile (RCS UP) created by GSMA.

It contains a set of Advanced Calling and Messaging features and agreed enablers – such as application to person messaging (chatbots) and conversational commerce (the seamless integration of transactions and messaging).

Update: http://www.bbc.com/news/technology-43836504

Update: GSMA in June, 2017 released the version 2 of RCS Universal Profile. This document also introduces the key enablers for Messaging as a Platform (MaaP). MaaP includes support for Application-to-Person messaging, Rich Cards, privacy control and spam protection to open up an A2P RCS business.

 

To date 46 Mobile Network Operators and 12 manufacturers, covering a subscriber base of 4.7 billion people globally, have committed to supporting a single, standard implementation of the Universal Profile. That means that the profile will be implemented by most smartphones makers (similarly as in the case of VoLTE/VoWifi). They are Alcatel, ASUS, General Mobile, HTC, Intex Technologies, Lava International Ltd., LG Electronics, Lenovo/Motorola, Samsung Electronics and ZTE,. Moreover the standard is supported by mobile OS providers Google and Microsoft. Therefore the devices will be equipped (as from Q2 2017) with a built-in Advanced Messaging app, so consumers will be able to text, chat and share media without the need to download any special application. (However a usage of app is also an option supported by RCS Universal Profile.)

Update: We have got the first smartphone with RCS UP support – more here. 

It is worthwhile to mention that Google is playing an important role because it bought Jibe – one of a few useful RCS clients, and has developed a universal Android client based on the GSMA RCS UP. Google along with Sprint in US and Rogers in Canada announced that they’re rolling out with their RCS Messaging based on RCS UP at the end of the last year. Besides, as GSMA says, Google is offering a carrier hosted service for Operators to launch and manage Advanced Messaging services to their customers without deploying the RCS or even IMS infrastructure.

Update: We have got some more RCS  UP deployments in Europe and Asia this year – more here. 

Update: Australia – more here

Update: Sprint and Rogers now support RCS Interconnect! – more here. 

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