Wireless VoIP Demystified – Part 2: SIP

In the first part of this mini-series on Mobile VoIP, I’ve taken a look at UMA as one of the many flavours of Mobile Voice over IP (VoIP). This part is dedicated to the Session Initiation Protocol (SIP) architecture and how it can be used in mobile devices.

SIP Architecture

SIP is a true end to end VoIP system and has been initially conceived without mobile networks in mind. Today, it is already widely adopted in the ‘classic’ Internet as one of the main protocols for VoIP. The basic architecture of SIP is simple: A SIP server in the network forms the central element of a VoIP network. VoIP clients are either software clients on PCs or notebooks or standalone devices like phones with an Ethernet port. When connected to an IP network their first task is to register their IP address with the SIP server. To call another device, a SIP client then sends a request to the SIP server. The SIP server then retrieves the IP address of the destination device from its database and contacts the destination device. If the destination device is willing to accept the call, the server informs the originator of the IP address of the destination device and a direct IP connection is established between the two parties. For VoIP, the connection is used to carry a voice data stream. Note, that the SIP server is only required for signalling as the voice stream is directly exchanged between the two parties. This is a big difference to circuit switched voice calls which are always routed through a switching center of a fixed or mobile operator. To call non VoIP destinations, gateways are used to convert the IP voice data stream into a 64 kbit/s circuit switched voice data stream.

SIP is more than just Voice Telephony

As the name Session Initiation Protocol (SIP) already implies, it is not limited to setting up voice sessions. More sophisticated clients also use the protocol for video telephony or the exchange of other multimedia data such as Instant Messaging, pictures and file transfers in general. These functionalities, however, are only available if both clients support them.

Comparison to UMA

While UMA (see part one of this series) only replaces the GSM air interface with Wireless LAN and IP, SIP is a true end to end voice over IP system offering a wide range of applications from voice, to video, and to rich multimedia applications like instant messaging and file exchange. Also unlike in UMA, the user is not bound to the mobile operator but can select from an abundant number of SIP server operators on the Internet.

SIP in the Wireless World

In the wireless world, SIP has not been very popular so far due to a number of wireless network limitations. GPRS and other first generation wireless IP packet networks are too slow and the latency of the connection was too high. In addition, speech algorithms used by current SIP implementations use inefficient codecs which require a substantial amount of bandwidth. 3G networks such as UMTS offer higher bandwidths compared to earlier networks and are thus able to carry SIP voice calls over the air interface. A SIP call, however, uses around five times more bandwidth then a traditional circuit switched mobile voice call for which very bandwidth efficient codecs are used in the radio network. This fact together with the openness of SIP for the user to choose the operator of the SIP server himself explains the reluctance of wireless operators to support the application of SIP services in their 3G networks.

The Future of SIP in the Wireless World

In the near future, SIP clients will mostly be adopted on GSM/UMTS/Wifi smart phones such as the Nokia N80, where they can be used to make phone calls over a Wireless LAN access point connected to DSL or a company network. In effect, a SIP client in the mobile phone can replace the fixed line phone at home and I am actually waiting for the day when I can use a single phone at home and when underway.

When leaving the office or home, a SIP client can still be used for voice calls but many operators (carriers) try to restrict SIP for the reasons discussed above. Very near term evolutions of 3G networks to technologies like HSDPA (High Speed Data Packet Access) might change these policies in the mid term.

SIP also forms the core of the IMS (IP Multimedia Subsystem), a standard designed by the mobile industry for mobile operators to offer their own VoIP and multimedia applications. IMS will be the focus of part 3 of this mini-series, so stay tuned…

2 thoughts on “Wireless VoIP Demystified – Part 2: SIP”

  1. Excellent article, dude, Had no idea something like UMA existed, though now it makes perfect sense.

    I don’t share your disdain for UMA though (same ole shit repackaged is, I think, the gist of it) I think there is a business case and I’ll address it shortly (hopefully)

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