SIP femtocell architecture Part 1

What’s needed to roll out a SIP- based femtocell network, and what are the advantages? This series of articles explains what SIP and IMS are, how they might find their way into mobile and other networks, and how this relates to femtocells.

What is SIP anyway?

SIP is a standard for making fixed and mobile telephone calls which specifies a set of call setup and management procedures, all of which run over IP (Internet Protocol). The same set of messages and protocols can be used to handle calls in both mobile and fixed networks – all that is required is an IP connection. Unlike the existing mainstream call setup used today, which is based on Signalling System Number 7 (SS7), SIP can be used to establish any type of voice or data interactive session including video calls.

IMS (IP Multimedia Subsystem) is a complete network specification which incorporates SIP, and also specifies other aspects including everything from billing record formats through to policy control of managing traffic priorities between different classes of use. Although IMS was originally created by the 3GPP, the mobile phone standards body, it has also been adopted by fixed network and cable network standards bodies at the heart of their nextgen architecture (TISPAN, MSF, Packet Cable).

Where does SIP apply in Mobile Networks?

There are four potential areas for SIP to move into mobile networks:
a)    Providing additional services on 3G networks to suitably capable phones
b)    4G LTE networks for voice and other SIP based services
c)    SIP based femtocells and fixed/mobile convergence
d)    Providing voice services “over the top” from 3rd party VoIP providers using the data channel

Has any mobile operator used it yet?

No major mobile operator has yet used IMS for the majority of their call traffic. Many have run trials, and some have provided a commercial “push to talk” type of service (which operates similar to walkie-talkie radios) using it. Many are using it today for fixed line business and residential VoIP services, from Vodafone Portugal to China Mobile.

Some mobile network operators already use SIP-based VoIP inside their networks for long distance traffic, for example carrying long distance traffic between switching centres and media gateways over more efficient IP bearers. This is transparent to the end user, and uses only SIP rather than a full IMS solution.

I’ve also come across several wireline operators running office telephone applications for fixed line business customers using it, and some large operators are actively planning large scale migration to SIP in the coming years.

Providing additional 3G services using the Rich Communications Suite

One of the first ways in which SIP is being introduced into mobile networks is through a new set of SIP based features promoted by the GSMA (GSM Association) known as RCS (Rich Communication Suite ). This provides a collection of services based on existing SIP standards including Instant Messaging, Presence and Video Sharing. RCS version 2.0 has more features including legacy service interworking.
These services require a 3G or LTE mobile data connection and a suitable SIP client in the handset.

It’s fair to say that some commentators are sceptical of IMS/SIP in mobile networks, particularly around additional services on 3G. This article about RCS from Telco 2.0 is an example – read the comments at the bottom for additional insights.

RCS services were launched in Spain first, early in 2012, with features including enhanced video, ability to share/swap files and chat during sessions - much like Skype or Microsoft communicator. At the same time, Telefonica also launched their Tu Me service which can be downloaded and used by mobile subscribers in any country and offers similar features. [Update: Tu Me was withdrawn in August 2013]