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Packet IN
November 2002


Voice Over Wireless LAN

BY RAVI RAVISHANKAR


Third generation (3G) wireless wide-area network (WWAN) technology promises to bring high-speed packet services to mobile subscribers and pave the way for true converged voice and data wireless services. However, the 3G-network rollout has been plagued with delays caused by huge infrastructure costs, lack of 3G-compatible mobile phones, and uncertainty about subscriber�s willingness to pay for the services.

SOME LIKE IT HOT
Wireless LAN (WLAN) technology, an evolution of local-area networking (LAN), offers an attractive alternative to 3G WWAN for delivering mobile high-speed packet applications. The technology has been around for over a decade and has gained some grassroots support in the home and enterprise markets. WLAN is beginning to gain traction with service providers who are eyeing the technology to cash in on increasingly mobile, data-hungry consumers. 

The new interest is being spurred by several factors. Improved standards are yielding faster data rates -- now in excess of 10 Mbps. The infrastructure cost is a tiny fraction that of 3G. And, while the technology�s transmission range is limited -- 500 feet indoors and 1,000 feet outdoors -- it�s sufficient for many business applications. Standard equipment prices are being driven down by volume production, and many PC and PDA manufacturers are already equipping their equipment with WLAN cards to enable wireless communication. Network installation is relatively simple and painless. No cables are required, and since it operates in the 2.4-GHz radio band, no licenses are necessary.

WLAN hot spots that deliver wireless Internet connections for people on the go are springing up in coffee shops, hotels, and airports. Some analysts predict that the number of users of public hot spot networks will rise from just over half a million this year to over 21 million by 2006. The initial focus for these deployments is high-speed data access, but riding voice applications over the same airwaves would create truly converged services for mobile subscribers and complement future 3G data offerings. 

CONNECTING THE DOTS
The success of WLAN technology hinges on several factors. One of the biggest challenges is to provide seamless, universal coverage. Current deployments are like a patchwork quilt pieced together with offerings from an assortment of carriers and providers. Of equal importance is making an account easy to use and manage. It is unlikely that subscribers will be willing to handle individual accounts at each and every hot spot that they frequent. 

By separating the access layer from the authentication, authorization, and administration (AAA) functions, new business models can evolve that will make the technology easier for subscribers to access and manage. Decoupling the access and AAA functions also creates more deployment options for operators and providers. 

The market already is seeing variety of deployment options. Providers like WayPort are building their own hot-spot networks. Aggregators such as Boingo and hereUare are working to create large national footprints by tying together the network islands created by wireless ISPs and large enterprises. Aggregators simplify things for consumers by allowing users to log on to multiple networks with a single account, user name, and password. 

Wireless operators are teaming with and investing in both hot-spot providers and aggregators to push the technology to data hungry consumers. Sprint announced its investment in Boingo, and T-mobile (formerly VoiceStream) has completed its acquisition of the WLAN service provider MobileStar and is offering broadband WLAN access in hundreds of hot spots.

WI-FI TELEPHONY
It is likely that many wireless operators will offer access to WiFi networks as part of their service packages. They can leverage their existing subscriber relationships and offer WLAN to compliment their GPRS/CDMA mobile data services as well as evolving 3G applications. Success will be contingent on the ability of subscribers to seamlessly access voice, data, and other multimedia services whether they are roaming in a 3G wide area network or WLAN hot spot. This requires two things:

Availability of mobile terminals, which are compatible with both WLAN and 3G access. A common service architecture for the WLAN and 3G

The 3G Partnership Project (3GPP) has defined the IP multimedia subsystem (IMS) for the 3G networks. By adapting and extending the same architecture to multimedia services over WLAN, the services can be access independent of the technology. 

The 3GPP IMS architecture designates two service areas -- the home network and the visited network. Extending that concept to the WLAN, a subscriber accessing the service from a WLAN hot spot would be considered to be roaming in a visited network. The WLAN network provides the network connectivity, and the home network provides the service control. The subscriber data and service logic reside in the home network. 

The essential component of the IMS architecture is the call session control function (CSCF), which is a Session Initiation Protocol (SIP) server. The CSCF SIP server provides the call control and routing logic required to complete multimedia sessions. All signaling information passes through the SIP server at the home operator network, which allows the home operator to control the service subscription, authentication, routing, and billing. The actual voice payload may or may not go through the home operator packet network. The IP transport network for the voice payload can be provided either by the wireless operator or a third-party managed IP network with which the WLAN operator has a direct service relationship.

In the IMS architecture, the media gateway control function (MGCF) provides the interworking between the SIP server and public switched telephone network (PSTN) for calls originating/terminating in the PSTN. This functionality is essentially identical to the mature packet-tandem solutions employed today by the wireline networks. 

The 3GPP is addressing the WLAN -- UMTS interoperability requirements in Release 6, which is expected in mid 2003. The initial requirement will cover the common billing between the two systems where as the future work will address the service interoperability, security, roaming, and other aspects.

CHALLENGES
A large number of IP addresses will be required to support increasing numbers of mobile handsets. This means that IPV6, which enables 128-bit addressing, must be supported by the end-user terminals, servers, and the WLAN access networks to avoid IP address exhaust. Limited spectrum availability, quality of service, and security are issues that also must be addressed. Work is underway at the standard bodies such as IETF in United States, the IEEE, and the ETSI BRAN project in Europe to deal with these issues. Upgraded versions of the WiFi standard 802.11 are in the works. 802.11e will add QoS support, and security is addressed in 802.11I.

CONCLUSION
While a wireless operator-centric view of voice over WLAN is presented here, other business model will likely evolve driven by non-wireless operators such as ISPs or other independent operators. The winning business model, to some extent, will determine the emerging WLAN architecture. Whatever the outcome, SIP will remain a key component of the future voice over WLAN service architecture.

Ravi Ravishankar is director, Advanced Technology Planning, at Tekelec. His focus is on defining signaling solutions and products for the next-generation packet telephony and 3G wireless networks. Tekelec is a leading developer of telecommunications signaling infrastructure, softswitches, testing and diagnostic solutions, and service applications. Please visit their Web site at www.tekelec.com.

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