Getting Ready for VoWLAN

For years Voice over Wireless LANS (VoWLAN) has been a niche product but with the increasing deployment of 802.11 wireless LANs in enterprises, VoWLAN is increasingly becoming a feasible alternative

December 6, 2003

6 Min Read
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Does this sound familiar? The CEO is never at his desk. The CEO's administrative assistant spends most of her time at the copier, filing room, attending meetings, and tracking down her boss. They're not the only ones who are hard to reach: You can't even find a shipping clerk in the warehouse where cell coverage doesn't reach.

This is an accessibility problem and, in many enterprises, the solution could be voice-over-WLAN (VoWLAN). For years it has been a niche product but with the increasing deployment of 802.11 wireless LANs in enterprises, VoWLAN is increasingly becoming a feasible alternative to wireline and cell coverage.

One nagging question with VoWLAN is whether current technology is up to the task. Proponents believe it is but skeptics point to a lack of standards insuring quality of service and other problems. But that hasn't stopped a host of vendors such as Cisco Systems and SpectraLink from investing heavily in this area.

In fact, SpectraLink has been in this niche for many years -- its 10-year-old 900 Mhz product is well established in some vertical markets like hospitals. Cisco just announced its new 7920 handset for 802.11b networks that includes virtually the same feature set you'd find on a desktop cordless phone.

With major players like these, plus add-on units from traditional phone system vendors like Nortel Networks, NEC and Alcatel enabling VoWLAN, is VoWLAN something your enterprise should be considering?Planning for Pitfalls

Not surprisingly, a key to a successful VoWLAN deployment is adequate wireless coverage so that users can connect from, at the very least, the most obvious places. As you've probably found already, positioning access points (APs) is part science and part trial and error. Keep in mind, though, that the areas needing coverage for laptops are much more obvious than those needed for phones.

Coverage for laptops is needed in gathering spots such as conference rooms while VoWLAN coverage needs to be more widespread. Laptop users rarely try to connect in stairwells, elevators, or bathrooms. But you can be sure as soon as people can walk away from their desks with their VoWLAN phones, they will be in exactly those places.

As a result, site surveys become more crucial than usual if you are planning to deploy VoWLAN. That's because contention and latency are bigger issues for voice than for data. Data traffic is fairly immune to short latency but voice traffic isn't. Also, while a single access point may support several laptop connections, there are thresholds for voice traffic that define the realistic number of simultaneous conversations.

Also, access point placement for VoWLAN has more restrictions than is the case for standard computers. VoWLAN doesn't take much bandwidth -- usually it is significantly below 64kb/s, which is a fraction of 802.11b's 11mb/s. However, each voice session needs an uncontended connection so latency doesn't become a factor. For that reason, most enterprise-class access point manufacturers specify no more than seven concurrent voice sessions per AP.Another reason that site surveys are crucial is that planning for crowded areas can be particularly tricky, especially areas likely to have intermittent overcrowding such as a lobby that is suddenly full after a meeting but is otherwise empty.

Some vendors such as Meru Networks offer proprietary solutions to some of the problems inherent in VoIP. For instance, Joel Vincent of Meru Networks says their new product line helps to reduce the number of APs required in any particular location by managing them intelligently.

Meru's management console combines information from each AP. This changes the standard hub, or contention-based methodology to a switch type control that assures the full-time connectivity required for voice calls.

The added efficiency of a managed environment increases the number of concurrent sessions from the normally recommended maximum to as many as 30. Since access is controlled at the management console rather than simply by proximity to an AP, designs such as Meru's perform functions similar to cellular technology and can transfer calls between APs to perform load balancing and to better support roaming.

Another potential limitation is quality of service (QoS), which negligible for most forms of data but critically important for voice and other forms of multimedia such as video. Video feeds may be a bit more impacted than voice is also relatively more forgiving.

A boost to overall WLAN QoS should come along with IEEE's ratification of the 802.11e specification, which is expected sometime in 2004. The current draft supports interoperability for home and business environments. It also adds QoS for voice and multimedia applications. In the meantime, proprietary solutions such as Meru's are available.Components

While some vendors such as Cisco can deliver a fully integrated phone system including the PBX portion all the way to wireless handsets, a significant part of the market is comprised of add-on components that add wireless capability to wired phone systems. Some of the basic components are: Phone system (PBX); IP gateway; wireless gateway/access points; and handsets.

Depending on the particular phone system, some of these components already exist in your enterprise. But the trick is connecting the wireless LAN and VoLAN elements to your traditional phone system. The following are some considerations.

IP Gateway. The IP Gateway is essentially a translator that converts digital ports on the PBX to IP data and inserts the calls into the LAN traffic. This is a many-to-one connection with each digital phone connection on the PBX translating to a single LAN connection as logical devices. Connections to wired IP phone sets are then made just as any other computing resource on the LAN. Most IP desk phones have built-in 2-port switches so a single LAN connection to the desktop can be used for both the phone and computer.

Wireless gateway / access points. Access points for VoWLAN are used in the same way any with data. Some systems require a management unit as well. The number and placement of the access points is likely the most important issue and makes a difference in the service your users get from their wireless handsets.Wireless handsets. Vendors such as SpectraLink have long used wireless phones based on the 900Mhz band. But now, with 802.11 become widely deployed, you have more flexibility. In fact, SpectraLink is, itself, switching to 802.11. Within the next year or so, expect major handset vendors to release phones that have both cellular and Wi-Fi capabilities.

With these considerations in mind, you can start thinking seriously about VoWLAN and its many advantages in terms of allowing access to valuable employees.

When you're ready to add VoWLAN to your phone system, make the best use of your vendor's testing and site survey capabilities, and get them to be around to check accessibility after the installation. Also don't forget those hidden locations below ground, in the parking garage, and in any out-buildings that need to be covered.

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