Integrating your strategies for wireless LANs, wired LANs and IP telephony makes sense. But if you choose to deploy voice over WLAN technology, you will face extra costs and complexities

What You Need to Know
An organisation that combines separate strategies for IP telephony and wireless and wired LANs into a single comprehensive plan will reduce its costs. It will also establish the basis for a more flexible architecture. Network managers should take this into account before buying new equipment, but recognize that, until 2008, supporting IP voice calls over wireless LANs may only be cost-effective for organizations in specific industries and for some small offices. They must carefully weigh the benefits of providing this voice support against the extra cost it entails. 
 
Analysis
Voice and data traffic, and wired and wireless LANs are converging.

Network managers in most organisations that don't already have wireless LANs (WLANs) are planning to install them in office buildings. This type of network is becoming a mainstream technology.

As WLANs develop from second-generation access point architectures into their third generation, the "intelligence" in them is moving toward the edge, or even the core, of organisations' wired LANs. Mid-sized and large WLAN installations also have implications for wired LANs because they impose additional requirements on management systems, security and switches.

At the same time as organisations plan to deploy WLANs, it's often the case that IT managers in the same organisations are considering upgrading wired LANs to handle both voice and data traffic during the next two or three years. Significantly, most organisations' wired infrastructure will need upgrading to support IP telephony. This entails adding redundancy features, emergency power, Power over Ethernet (PoE) and quality-of-service (QOS) protection for voice traffic. Security may also have to be revised.

Currently, most purchases of PBX lines are for non-IP fixed handsets connected through a gateway to an IP-enabled PBX. PBX lines serving IP client devices account for less than a tenth of the lines in use. But this situation is changing fast. By 2009, over half the world's PBX user terminations will support IP.

Given the complexity of this situation, it makes sense for network managers to combine their strategies for WLANs, wired LANs and IP telephony into a single comprehensive plan for a cost-effective and flexible infrastructure. At the very least, they need an integrated plan for equipment purchasing. Those without one will have to rebuild their networks several times to add WLAN and IP telephony capabilities.

Voice telephony over wireless LANs comes at a price
Many organisations with plans for IP telephony and WLANs may think that their next move is to support voice calls over these networks. They hope to reap the rewards of a converged infrastructure and added mobility. But there's a price attached to getting voice-over-WLAN capability: infrastructure costs can rise significantly, as a much more complex network is needed to support voice calls throughout an office.

Organisations must carefully evaluate whether the potential benefits of IP telephony via WLAN outweigh the extra cost. The following six sections outline the main issues.

Technological standards
The current lack of a technological standard for voice-over-WLAN equipment is one factor that limits uptake to certain industry applications. The relevant standard, called 802.11e, is still being developed by the Institute of Electrical and Electronics Engineers (IEEE) and pays particular attention to QOS for voice calls. It has two components -- Wi-Fi Scheduled Media (WSM) and Wireless Media Extensions (WME) -- the latter especially important for business use.

Equipment based on this standard is expected to reach the market in the second half of 2005, from a wide range of vendors. The Wi-Fi Alliance will certify equipment that meets its requirements for interoperability, but vendors are likely to introduce proprietary innovations to improve performance and add features.

Quality of service
QOS is critical for voice calls, but the limited range and capacity of WLANs imposes difficulties not found in wired LANs. An important issue concerns the handoff between access points in office WLANs, which is sometimes delayed. These delays can severely disrupt voice calls.

Vendors already offer proprietary QOS support within their WLAN products, but this situation severely limits interoperability between different makes of access point and handset. Also, as noted above, though 802.11e will improve interoperability, proprietary features are likely to remain.

Handsets
Until 802.11e equipment arrives, customers wanting to transmit voice calls over WLANs have little choice but to buy expensive proprietary handsets. These cost from $400 (£219) to $600 or more in the first half of 2004, and, though prices are falling, it's doubtful whether they will drop below $200 for several years. However, there is one possible alternative to overpriced proprietary handsets: PDA softphones. These are less expensive, but buyers need to check that battery life is sufficient.

Power over Ethernet
Gartner recommends that PoE technology be used to supply electricity to WLAN access points in all but the smallest installations.

There is a technological standard for PoE: the IEEE's 802.3af. However, it allows for two configurations and two methods of power injection. This may create compatibility problems for organisations using equipment from more than one vendor.

PoE is also needed for IP telephony over wired LANs. Organisations can significantly reduce the number of PoE ports needed at LAN edge switches by using wireless handsets rather than fixed ones.

Site planning
Building a WLAN that supports high-quality transmission of voice traffic is over twice as expensive as creating one for basic data services. In contrast to WLAN coverage for data applications, which can be limited to specific parts of an office complex, coverage for voice calls must be wider. It has to include every area of the workplace where calls can be taken. And, to support both data and voice traffic, a denser configuration of access points is required. This means a bigger, costlier network.

Where practicable, organisations should consider dedicating a WLAN channel to voice services. They should also buy equipment that lets users "roam" between access points without dropping calls.

Choice of technology
Most WLANs based on IEEE 802.11b -- today's most common standard -- support only eight to 12 simultaneous voice calls. Also, 802.11b, like 802.11g, supports only three radio channels. As a result, much rests on efficient re-use of those channels. Clearly, equipment based on these standards isn't ideal for use in large, busy offices.

From 2005, 802.11a equipment will be the best choice for these locations, which need a dense network of access points. The larger number of channels supported by 802.11a will make site planning much easier and let network managers assign sets of channels to specific services, such as voice calls.

Predicted use of voice-over-WLAN systems
Wireless PBX systems aren't new to corporate users. Systems that comply with the Digital European Cordless Telephone (DECT) specification, for example, have been available for over 10 years, but have never accounted for more than 12 percent of the PBX lines in service. This is because the benefits of wireless PBX systems have outweighed the additional costs only in certain industry applications and some small offices.

We don't expect transmission of voice traffic over WLANs to be popular in most large offices until two conditions are fulfilled:

VoIP client devices will account for about a third of large-office voice connections in 2008, by which time vendors will have lowered the price of WLAN systems and reduced the complexity required to support voice traffic. In view of this, we don't expect mainstream deployment of voice-over-WLAN systems in large offices before 2009. Before then, however, this technology will be deployed more and more by organisations in sectors such as warehousing and healthcare, where the business benefits clearly outweigh the costs. These systems will also crop up in more small offices, where frequent changes of location make a "portable" network highly attractive

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