WirelessMoves

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…

Nokia has managed quite successfully to bring UMA (Unlicensed Mobile Access), it’s preferred Wireless Voice over IP variant to the attention of the big press during the recent 3GSM congress in Barcelona. Most journalists, however, haven’t really understood what UMA is about as it is just one of at least four very different flavors of Wireless VoIP, each with its advantages, disadvantages, usage scenarios, and proponents that push the solution. To fill this gap and to show the benefits and drawbacks of the different flavors of Wireless VoIP for the user, I’ve decided to write a couple of blog entries in the days to come to compare the following technologies:

• UMA (Unlicensed Mobile Access)
• SIP (Session Initiation Protocol) implementations on mobile phones
• IMS (IP Multimedia Subsystem) clients
• Non standard Wireless VoIP systems such as Skype

First on the list is UMA, a 3GPP standard like GSM and UMTS, loved and feared alike by mobile operators (or carriers as you say in the U.S.). The principle of UMA is simple: It replaces the GSM radio technology on the lower protocol layers of the mobile phone with Wireless LAN. A call is then tunneled via a Wifi Access Point connected to a DSL/cable modem via the Internet and a gateway to the Mobile Switching Center (MSC) of a mobile network operator. The gateway between the Internet and the network of the mobile operator is called a UMA Network Controller and one of the companies developing such a network node is Kineto Wireless.

For me, UMA is a semi-VoIP service, as a call is only transported over IP on the link between the mobile phone and the UMA Network Controller. After the gateway, a traditional Mobile Switching Center (MSC) and a circuit switched connection is used to connect the call to the destination.

By always traversing the core network and an MSC of a mobile operator, UMA binds a mobile subscriber to his mobile network operator. This is the part mobile operators like most about UMA. What mobile operators don’t like about UMA in many cases is the fact that the DSL or cable connection is usually in the hands of other companies. In many cases users pay their DSL fees to an incumbent fixed line operator or cable company. Thus, in most cases UMA only makes sense if a mobile operator offers the service together with the fixed line operator that controls the DSL or cable access.

As described above, UMA replaces one radio technology with another and otherwise leaves the rest of the system as it is. This makes it difficult to price incoming calls differently for a caller while the called party is at home and using his (cheaper) Wifi/DSL/cable connection compared to calls the called party receives while roaming in the cellular network. This is due to the fact that mobile operators in Europe use special national destination codes in order to be able to charge a caller a different tariff for calls to a mobile phone user. In the U.S. charging incoming calls to a UMA user differently might be less of an issue as mobile networks use the same national destination codes as fixed line operators. There is no additional charge for the caller as the mobile phone user gets charged for incoming calls. As the mobile network is aware that the user is currently in his (cheaper) home Wifi cell, incoming calls can be charged accordingly.

Outgoing calls made via the Wifi access point and a DSL or cable connection are also under the control of the mobile operator. It is unlikely that mobile operators will offer outgoing calls for free as is usually the case for connections between two VoIP subscribers as the call will always be routed through a mobile switching center and a circuit switched connection instead of being transported via IP end to end. Consequently I think it’s going to be difficult for an operator to price the service competitively.

Last point on the downside for the user: As UMA is not an end to end VoIP technology there is no presence information and built in instant messaging capabilities as in other systems.

On the positive side, UMA offers a seamless experience for the user. From an application point of view UMA it is transparent to the user on the mobile as the same graphical user interface is used for both cellular and Wifi calls. The standard even offers seamless roaming between the two access technologies for ongoing calls, i.e. a call is handed over from Wifi to the cellular network when a user leaves the coverage area of a Wifi access point.

UMA also tunnels GPRS services into the core network of the mobile operator. Data speeds are much higher though, again producing a seamless or even better experience for the user while in a UMA Wifi cell, e.g. for web browsing on the phone, operator portal access, music downloads, etc.

Two other important positive sides of the technology are the use of the same phone number regardless of whether the phone is connected via the cellular network or Wifi and the fact that indoor coverage can be improved by deploying Wifi access points instead of more expensive cellular micro base stations.

As has been shown, UMA offers a lot to users. If operators figure out a way to offer the service at a competitive price users will surely like it.

In the next blog entry in this series I will give an overview of mobile SIP and how this end-to-end VoIP technology compares to UMA.