Adding voice to a WLAN

Q. How do I build a wireless LAN that supports converged voice/data services?

How do I build a wireless LAN that supports converged voice/data services?

— Tom, Dallas

Regardless of whether you are deploying voice over a WLAN (VoWLAN) today or you expect converged voice/data to be a future wireless networking requirement, there are several prerequisites that must be addressed in your WLAN planning:

• VoWLAN quality must be comparable with that of wired voice.

• You should have the capability to partition a portion of the bandwidth or tag the voice service to deliver the appropriate quality of service (QoS).

• Wireless coverage must be complete and continuous across the enterprise.

• System capacity should be sufficient to support data and voice.

• The network has to be secure.

The 802.11 community is working on a standard, 802.11e, which provides QoS features and multimedia support to the existing 802.11 standards. The 802.11e draft specifies traffic categories for priority-based traffic and, in brief, marks packets to indicate a specific premium service requirement. Unfortunately, 802.11e is behind schedule, and even a wizard cannot predict the final ratification date.

That isn’t stopping some WLAN system vendors, however, who are working closely with device and voice application providers to provide viable solutions for real-time applications support.  Most notably, we’ve heard of  systems that dramatically reduce network latency, with guarantees of less than 150 milliseconds of delay between client devices and the network, and less than 75 milliseconds of delay when roaming across subnets within the WLAN itself.  These performance levels let you create enterprise networks that can easily support the addition of voice applications.

With respect to network coverage, capacity, and security, all of these functions are dependent upon the WLAN system itself.  Your wireless infrastructure should be addressing these issues in a real-time fashion whether you are running voice or data traffic. But the importance of dynamically adjusting to changing network conditions is highlighted when a time-sensitive application, such as voice, is introduced. 

For example, a small coverage hole may not affect your users’ ability to receive e-mail, but it will cause a voice call to be dropped.  Similarly, network congestion may not have a noticeable effect when surfing the Web, but it will make a phone conversation unintelligible.  Therefore, when delivering converged voice/data, intelligence must be built into the WLAN system to optimize performance under heavy loads, balance traffic, detect and repair coverage holes, etc.  Otherwise, your help desk will spend a lot of time fielding e-mail from troubled VoWLAN users.