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According to In-Stat/MDR, the voice over WLAN (VoWLAN) market is growing more slowly than expected. Only 30,000
VoWLAN handsets shipped last year, primarily 802.11b-enabled phones sold to verticals like healthcare and retail. But In-Stat still expects this market to take off, surging from $16.5M in 2002 to $507M in 2007.
So why all the buzz?Many analysts expect VoWLAN to be the "killer app" that drives enterprise wireless deployment. A growing number of vendors appear to agree:
- Symbol NetVision and Spectralink NetLink Voice-over-IP phones digitize voice for transport over an 802.11 WLAN; PBX gateways relay these calls to the PSTN.
- TeleSym SymPhone and Vedic Technology VedicPhone are software programs that convert 802.11-enabled Pocket PCs into VoWLAN phones.
- Vocera Communications sells a small "wearable communications badge" for hands-free VoWLAN communication, with calls relayed through a Vocera Sever.
- Cisco's Wireless IP Phone 7920 dovetails with Cisco's Aironet 802.11 access points and Cisco's CallManager, supporting both wired and wireless VoIP.
Getting from here to thereSure, VoWLAN sounds attractive, but there are still obstacles that must be overcome:
- Security - Early 802.11 products were plagued by security concerns related key cracking and Wired Equivalent Privacy (WEP). Many vendors now offer upgrades to Wi-Fi Protected Access (WPA), a much stronger solution for confidentiality and integrity of wireless traffic. 802.1X port access control enables stronger authentication, but work remains to make (re)authentication faster and more tightly coupled with handset identifiers (SIM cards). Accelerating encryption in hardware will also be important to minimize latency.
- Roaming - No one should expect VoWLAN handsets to remain stationary; that means fast, seamless roaming between access points. Today this may be possible if your WLAN consists of single-vendor APs. There is no standard (yet) for fast association handoff between APs, but 802.11f will define an Inter-AP Protocol (IAPP) and recommendations that can facilitate multi-vendor roaming. According to CommsDesign, handoff times are now as long as 400ms; they need to be an order of magnitude smaller for VoWLAN.
- Quality of Service - Inability to control or guarantee QoS is the 800 pound gorilla for VoWLAN. The IEEE 802.11e specification is trying to wrestle this problem into submission, but no single clear-cut solution is nearing consensus. Voice networks must continually compensate for jitter and latency to deliver a continuous bit rate service. This is a challenge even in wired IP networks, but is much harder in wireless IP networks. Link characteristics are constantly changing due to multi-path, fading, and other wireless phenomena, making it hard to predict future WLAN behavior. Add competition for resources on a shared medium, and you have a really knotty problem.
Moving beyond the enterpriseThe products discussed thus far deliver VoWLAN within a single enterprise: to employees in an office building, nurses and doctors in a hospital, staff in a shopping mall. Ultimately, many would like to see VoWLAN extend beyond these walls, letting callers roam seamlessly between private WLANs and public carrier networks. For example, Motorola, Proxim and Avaya recently announced a partnership to develop dual-mode Wi-Fi/3G mobile phones and the network infrastructure needed to support them.
Fulfilling this vision is much harder than delivering VoWLAN within a single enterprise. Wireless carriers will have to buy into this scheme, and what's their motivation to offload paid calls from GPRS and CDMA2000 to Wi-Fi? Once again, the answer is cost. If a carrier owns both the Wi-Fi hotspot and the 3G network, and gets paid to carry the call either way, then offloading increases the carrier's profit margin by reducing resources required from more expensive 3G networks. Have you noticed T-Mobile, Verizon, and Sprint getting into the Wi-Fi hotspot business recently? Perhaps now you have a better idea about why.
ConclusionVoWLAN has plenty of promise. If your business incurs a high cost for internal communication - and especially if your employees use wireless cell phones to contact each other throughout the day - then it might pay you to start looking at VoWLAN. The rest of us should probably wait until challenges associated with delivering high-quality, secure, seamless VoWLAN are resolved.
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