Voice over wireless LAN: Ready or not?
- 16 September, 2003 09:49
WLANs (Wireless LANs) are still in the experimental phase at most companies. Why? Because wireless security standards remain in flux. But a more obvious obstacle to wholehearted adoption is the lack of a compelling need for wireless in the average office, which already has a perfectly functional, wired LAN in place.
Well, here's a thought: How about replacing or supplementing your current phone system with a VoWLAN (Voice over WLAN) system? Just take an ordinary wireless network and add a VoWLAN server along with laptops, PDAs, or newfangled Wi-Fi phones to run the client. Instead of workers wasting time playing phone tag, they can field calls wherever they roam on campus -- or even on the road, if there's a Wi-Fi cloud nearby.
VoWLAN is a natural extension of VolP (Voice over Internet Protocol), a technology that has already taken root in enterprise telecommunications. (Today, more IP-based PBXs are sold than conventional models.) Yet VoWLAN presents its own unique QoS (quality of service) challenges relating to fluctuating wireless throughput and roaming among APs (access points), which is why most of today's local wireless voice systems are bundles of proprietary wireless network hardware and software.
Industries with highly mobile workers -- such as retail, manufacturing and healthcare -- can justify the premium for a proprietary network. Yet the proliferation of Wi-Fi and its increasing reliability opens the possibility of deploying VoWLANs across commodity WLAN setups at much lower costs.
Technical hurdles remain. But on the hardware side, at least, everyone seems to be getting in the game. Cisco Systems Inc. recently introduced its first VoWLAN handset and a slew of vendors including NEC, Qualcomm, Motorola and Dell Inc. promise hybrid phones next year that use both Wi-Fi and mobile phone networks.
A Very Local Exchange
"It's just a matter of time before (VoWLAN) catches on in the mainstream enterprise," says Ben Guderian, director of marketing for SpectraLink. SpectraLink recently introduced a new, lightweight, less rugged handset designed for mainstream enterprise users. Its lowest priced handset costs US$399, which compares to around $350 for many desk phones with standard wiring.
Proprietary wireless voice vendors such as SpectraLink and Symbol have been among the first to release VoWLAN solutions for commodity wireless infrastructure. But there's a new breed of VoWLAN provider as well: the softphone developers. These companies, such as TeleSym Inc., IP blue, and VLI have built software that can be loaded onto PDAs or laptops, enabling users to initiate and receive voice calls over WLANs.
Basically, VoWLAN systems work in two different ways. Offerings from SpectraLink, Symbol Technologies Inc. , and Cisco route calls from the phone to the WLAN AP to a VoIP gateway -- one that may already be in use to deliver VoIP over the wired network -- which translates calls between the IP network and the PBX. That setup allows all regular PBX functions that are available on workers' wired desk phones to be available on the VoWLAN phones. Calls that are made to phones outside the enterprise go through the PBX to the PSTN (Public Switched Telephone Network).
By contrast, softphone developers such as TeleSym provide systems that route calls outside the enterprise over the Internet. In this scenario, road warriors could use the softphone on their PDA or laptop to place calls from a hotel that offers a WLAN. The call could route entirely over the Internet. "Then the call is completely free," said Raju Gulabani, TeleSym's CEO.
The downside is QoS. Like any voice call that uses the Internet, users can't control the quality of the connection.
Unfortunately, even over the local network, VoWLAN has its shortcomings. The problem is that the 802.11 standards -- including those that cover QoS, security, and roaming -- simply weren't designed to support voice.
For example, 802.11 entirely lacks a QoS mechanism. This lack means that no special priority is given to voice, so when traffic spikes on the network, dropouts may occur. The IEEE is working on 802.11e, a QoS standard that should be finalized next year. In the meantime, Symbol and SpectraLink have implemented their own proprietary QoS mechanisms through their hardware.
But it's security, or a lack thereof, that's the biggest deal breaker for VoWLAN today. Enterprises view today's wireless security measures (such as the Wireless Encryption Protocol) as weak, forcing them to opt for wireless VPNs. But VoWLAN offerings that use phones, as opposed to those that use PDAs or laptops, often can't employ VPNs because the client software usually can't be loaded onto the phone. (Several vendors, including NEC and Cisco, plan to produce phones next year with VPN support built in.)
But using VPNs for voice has other problems. "A VPN tends to encapsulate voice packets, so it obscures them from the network and it can't discern high priority voice from low priority data," says Ron Seide, product line manager for Cisco's wireless networking business unit. In addition, VPN's cause latency that degrades the quality of voice, says Richard Watson, an engineer at Symbol and director of telephony product marketing.
Instead of using VPNs, users can protect against unwanted intruders by putting voice and data on different subnets or VLANs, allowing clients in the voice subnet to communicate only with the telephone gateway, Watson says. With this separation, the widely used encryption and authentication standards offer good enough security, say some vendors. That's because the threat of eavesdroppers is remote as the listener would have to be within range of the same AP as the phone user. "The joke here is watch out if you have someone walking behind you with an antenna," says Watson. "It's an overblown thing."
An added challenge, though, is that encryption and authentication must happen fast enough so that a call isn't dropped or degraded when a user moves from the coverage area of one AP to the next. The existing 802.11 standard can't support that handoff fast enough and neither can the security solutions developed by gateway vendors. Even 802.1x, the upgraded security standard in development, won't work quickly enough. "When standards bodies developed all this, they were more concerned with security on laptops," says Watson.
The handoff between APs should happen in under 50 milliseconds to cleanly support a voice call, says Doug Klein, CTO of Vernier Networks Inc., a security gateway vendor. Vernier's gateway does the transfer in a matter of a few hundred milliseconds. "It's not optimum," admits Klein.
As a result, vendors have developed their own fast authentication processes. These generally work by dispensing a certificate to the client proving that it is authenticated, so that when a user moves into range of another AP, the client offers that certificate as proof of authentication.
Cisco employs a similar scheme, where one AP is designated as a master AP within a subnet. Rather than require each AP to acquire an encryption key from a backend server as the user roams, the master AP gets a master key and from that, each AP in the subnet spawns a session key. But even Cisco's solution takes 150 milliseconds.
Maybe Next Year
The available VoWLAN security solutions aren't robust enough for many enterprises. "I don't think it's likely that we'll be deploying VoIP on Wi-Fi networks anytime soon," says Joshua Wright, senior network and security architect for Johnson and Wales University, which already has an extensive WLAN for data users. Current offerings don't enable the kind of reliability he wants in a voice network.
The efficiency of the security and QoS also affects the load that APs can handle. Most AP vendors say each AP can support 10 simultaneous calls, but when pressed, will usually admit to half that number in live deployments. Chris Kozup, META Group Inc.'s program director of technology research service, says his clients who have tested VoWLAN report on average four or possibly five calls without any data use per AP. "It becomes a problem because there are finite resources here in terms of channel and frequency," Kozup indicates. "It's not as easy as lighting up another AP."
New standards, including the faster 802.11g and the QoS specification, 802.11e, should increase the number of users an AP can support. Combined, those upgrades could allow 15 to 25 simultaneous calls, says Cisco's Seide.
Ultimately, although the hurdles of deploying a robust VoWLAN are many, they are also surmountable. The key to robust and reliable VoWLAN networks lies in the ability to support security and all capabilities fast enough to hand off between APs seamlessly. As more customers demand support for voice, vendors say they'll deliver it. "We'll get around to doing the work that needs to be done," said Vernier's Klein.
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