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Feature Article
January 2004

Building Voice Ready Wireless LANs


A certain excitement is starting to build around the potential intersection of three technologies: VoIP, wireless LANs, and a new generation of cellular handsets. Individually, each has established a proven market, but when converged, they represent an application that offers substantial new possibilities for enterprise networks. The result of this convergence is Voice over Wireless LAN (VoWLAN). Like many other promising technology trends that preceded it, VoWLAN is currently in the hype stage, with industry pundits offering varying predictions about when widespread market adoption will occur. Still, most agree that VoWLAN promises large enough cost savings and productivity gains to make it an exciting future direction for networking.

Today�s VoWLAN deployments provide cordless phone or walkie-talkie functionality in specific vertical markets such as health care and manufacturing. But the real future of VoWLAN lies in newly announced cellular phones that integrate Wi-Fi connectivity. With these phones, users can maintain their ability to access the cellular network when outside the corporate office, and then talk to each other over the WLAN and use their corporate PBX when in the office.

There are two key benefits of VoWLAN. First, it consolidates the cellular phone and landline phone into one device that is always at hand, allowing employees to be reached immediately without having to be tracked down through two or more landline and cellular phone numbers. Second, employees can reduce their cellular minutes while they�re in the office, since internal calls are carried over the WLAN and outside calls take advantage of the economic benefits of the PBX. This results in both productivity gains and cost benefits.

To realize the benefits of VoWLAN, however, network administrators must properly architect their entire network, paying special attention to the wireless LAN infrastructure. When implementing a WLAN of any scope, IT administrators have several issues to consider, including security, performance, and scalability. To properly prepare for VoWLAN, they also need to understand voice traffic requirements to ensure high-quality calls over their entire network. Whether they�re planning imminent VoWLAN deployments or just watching the technology for another year or two, IT administrators can ease deployment by ensuring that their WLAN infrastructure can provide blanket wireless coverage, seamless mobile connectivity, and guaranteed voice bandwidth.

Blanket Wireless Coverage
Like cellular users, VoWLAN users expect phone coverage throughout the corporate office. Network administrators must consider blanket wireless coverage as a requirement for their wireless LAN deployment. But providing blanket wireless coverage can be expensive and difficult if the infrastructure isn�t properly designed.

With most access points (APs), it is a complicated matter to provide blanket coverage due to the co-channel interference inherent in 802.11. Unfortunately, APs interfere with each other -- APs too close together cause interference, and APs too far apart cause coverage holes. This makes it difficult to simultaneously provide coverage while avoiding interference. To deal with co-channel interference, network administrators must do RF site surveys to help identify potential areas of interference, and then strategically place APs and manage RF channels. Administrators who have deployed basic islands of wireless coverage (in conference rooms, perhaps) will face a complex challenge in scaling their WLANs to provide blanket coverage.

To build a WLAN that scales from limited coverage to blanket coverage, network administrators should look for infrastructure products that eliminate co-channel interference.

Seamless Mobility
Although blanket wireless coverage is a necessary step, it�s not enough to ensure that the WLAN is capable of delivering high-quality voice calls. Since VoWLAN users are mobile, they expect high-quality calls even when they move throughout the corporate office. This requirement is difficult to meet with today�s APs, since they don�t meet the low-latency mobility requirements for high-quality voice connections. As clients leave the coverage area of one AP, they must scan for a new AP, associate with that AP, and in most cases re-authenticate with the network. This AP-to-AP handoff process usually takes between 200 and 500 microseconds. While these delays are hardly noticeable with data applications, they are quite noticeable for voice calls, since the human ear notices delays ranging from 50 to 100 microseconds. (Note: the AP-to-AP handoff delays become an even greater concern for users looking to implement the 802.11i security specification, requiring network administrators to make concessions between security and voice quality).

To build a wireless LAN capable of providing seamless mobility, network administrators should look for WLAN infrastructure products that eliminate AP-to-AP handoff issues. By eliminating handoffs, network administrators can provide a high-quality experience throughout the duration of the voice call, regardless of where the user goes. In addition, getting rid of AP-to-AP hand-offs provides seamless support for the upcoming 802.11i security protocol. Since users are not passed from AP to AP, they don�t have to deal with the additional delays associated with re-authenticating via 802.11i. As a result, eliminating handoffs allows the network administrator to implement both tight security and high-quality voice without making concessions between the two.

Wireless Quality of Service
Once blanket coverage and seamless mobility are established, the network administrator�s next focus is guaranteeing the quality of voice traffic between the VoWLAN client and the IP-PBX. The convergence of voice traffic onto the data network requires bandwidth guarantees and prioritization to ensure high-quality voice calls, and the network administrator must address two infrastructure areas to meet these requirements: the wireless network and the wireline network.
In the wireless LAN, there are two separate approaches to providing traffic management across the airwaves. The first method is straightforward: completely separate voice and data traffic by separating the traffic onto different channels. With this method, network administrators can guarantee quality of service because the voice traffic never contends with the data traffic.

However, it�s not always possible to use separate voice and data channels. For instance, users who want to use their laptops or PDAs for both voice and data require a mechanism to prioritize voice ahead of data on the same channel. This mechanism is currently being defined in the IEEE�s new 802.11e specification. Until 802.11e is ratified, however, network administrators should implement WLAN infrastructure products that enable blanket wireless coverage on each of several RF channels with separation of voice traffic, along with a clean upgrade path to 802.11e.

Wireline Quality of Service
The network administrator also needs to provide quality of service when the wireless traffic hits the wired network. Fortunately, there are well-established methods already in place for prioritizing and guaranteeing traffic on wired Ethernet networks. With the standards-based VLAN tagging and prioritization embedded in most Ethernet switches, network administrators can queue voice traffic ahead of data traffic throughout the wired LAN.

The early indications are positive for VoWLAN, and the promise of economic benefits and productivity gains are quite compelling for enterprise customers. Today, most network administrators are either considering WLANs or have deployed basic islands of coverage. As they begin to consider voice as an application for their existing enterprise networks, administrators should understand the requirements for voice along with the solutions that are available. Voice over WLAN can be challenging to implement, but there are solutions that deliver wireless data connectivity today while also providing the blanket coverage, scalability, and quality of service that will be required for wireless voice.

Brian Jenkins is vice president of product marketing at AirFlow Networks. AirFlow Networks makes secure wireless LANs that are designed to be as easy to deploy and manage as wired Ethernet LANs. For more information, visit the company Web site at www.airflownetworks.com.

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