Subject: [NEXTEL-1] Building Voice-Ready Wireless LANs

From: [email-masked]
Date: Tue, 02 Sep 2003 01:22:33 -0400

Building Voice-Ready Wireless LANs
BY HARRY BIMS
SEPTEMBER 1, 2003

Most network administrators who have deployed wireless local area networks have
had to face a plethora of issues ranging from interference to RF channel
management to security to performance. Unfortunately for them, the 802.11
specification simply wasn't designed to handle the requirements of
enterprise-class wireless networks. The enterprise issues stem from the need to
deploy multiple access points to provide coverage that spans the enterprise.
Deploying multiple APs means deploying APs on non-overlapping channels to avoid
interference, and this, in turn, affects the clients' ability to mobilize,
especially when dealing with real-time applications such as voice.

To better understand the real issues facing today's mobile client, a brief
tutorial of mobile connectivity is required. First, it is the client's
responsibility to associate with an AP, and then as the client mobilizes away from
its AP, the client's performance deteriorates. This not only affects the
mobilizing client but also affects all other clients associated with the same AP.
After the mobile client's performance drops below a minimum threshold, the client
decides to find a new AP by scanning the other RF channels. When the client finds
another AP, it reassociates with the AP and in most cases re-authenticates to get
its security profile.

The delays associated with the hand-off, reassociation and reauthentication can
take 500 microseconds or more, and these delays will become even worse when users
implement the 802.11i security specification. While these delays are an annoyance
with data applications, they can be fatal when implementing voice.  Because voice
applications typically require 100 microseconds or less to provide high-quality
connections, there is obviously a mismatch between today's WLAN infrastructure
options and voice over WLAN applications.

However, there is a way to implement WLANs that overcome these hand-off and
reassociation issues by simply eliminating them. The key requires rethinking how a
wireless LAN gets deployed. Instead of providing coverage with a lot of APs that
require non-overlapping channels, build the WLAN with a single AP that provides
scalable coverage through distributed antennas. With this architecture, an entire
enterprise WLAN can be deployed on a single channel without any co-channel
interference. This not only eliminates the interference issues, but the hand-off
and reassociation issues as well.
In this single-AP architecture, the client remains associated with the centralized
AP even during mobility. The centralized AP handles the client's mobility by
assigning it to the remote antenna with the best signal strength, thereby
eliminating the mobile client's need for AP-to-AP hand-offs and reassociation.

Not only does this provide the client with better overall performance (because the

client doesn't have to drop to a minimum threshold), but it also gives clients
seamless voice capabilities that are well within timing requirements. In addition,

this architecture provides the benefits of 802.11i security without negatively
impacting voice performance because the client doesn't have to reassociate or
re-authenticate.

The economic benefits of voice over WLAN are compelling to enterprise customers,
and network administrators need to consider voice when building an
enterprise-class WLAN. Fortunately for them, there is an architecture they can
deploy that provides a scalable WLAN that offers significant benefits for both
voice and data applications.
Harry Bims is founder and chief technical officer at AirFlow.

http://www.wirelessweek.com/index.asp?layout=article&articleid=CA319429&spacedesc=Departments&stt=000

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