The territory for large-scale enterprise wireless LANs is mostly uncharted,
yet many are now looking at how to build a standards-based, enterprise-grade
WLAN for both voice and high-user-density data. But getting to the sort
of user density that large enterprises will expect for pervasive WLAN
deployments, and running voice across the same enterprise wireless network,
is not easy over traditional systems designed for basic data connectivity.
Nonetheless, such a network is possible.
A typical access point can support between 10 and 20 connections, a number
easily exceeded in high-density areas. For example, an access point that
serves a radius of 50 feet covers an area of 7,800 square feet, and if
there are more than 20 active contenders it may exceed the recommended
ratings for the system to achieve higher density.
By its very nature, voice is responsible for two problems in this scenario.
First, pervasive voice deployment will dramatically increase client density
because every enterprise user will carry an always-on device. Second,
voice is a demanding application with respect to missed or delayed packets.
Though not harmful to data, such events can destroy a voice call.
Specifically, to bring voice and data together in the enterprise, data
traffic needs predictable throughput, low loss and no disruption of service
while roaming. For its part, voice traffic needs low latency, low jitter,
toll-quality voice and imperceptible handoffs.
It turns out that the requirements for both data and voice can be satisfied
adequately without requiring any specialized hardware, firmware or software
upgrades at the clients over the IEEE 802.11-standard media-access control
(MAC) function. Instead, the requirements can be met through innovation
in both algorithms and architecture in the WLAN infrastructure.
Such innovation makes it possible to support more than thirty 64-kbit/second
voice calls/cell; a sustained end-to-end throughput of over 6 Mbits/s
aggregate per cell for up to 100 users per cell; a mix of high-density
voice and data traffic in the same network; and 2-millisecond handoffs
for clients. This can be accomplished on an 802.11b network, with 100
percent standard IEEE 802.11 Distributed Coordination Function (DCF) clients
and no specialized client software. For 802.11a/g clients, the benefits
are commensurately higher.
The importance of sticking with plain, standards-compliant DCF for a
WLAN design stems from the need to simplify the support required when
the network is deployed. There are many vendors, each selling different
types of cards with different firmware versions that fix some problems
and introduce others-and all of that behaving differently, depending on
the flavor of Windows each laptop or client uses. There is no unified
mechanism for enforcing firmware enhancement or compatibility. The safe
bet is to assume that clients are only using the basic IEEE 802.11 DCF
Peak transmission rates have risen dramatically, from 2 Mbits/s to 11
and 54 Mbits/s, and soon they will top out at over 100 Mbits/s. While
almost every client supports rates of more than 11 Mbits/s, the effective
performance in a cell with 20 clients nonetheless drops below 200 kbits/s
It turns out that all of that extra bandwidth goes to contention-the
fighting that each station does to get time on the air. The way to solve
this problem is by coordinating the protocols across access points (APs).
Infrastructure that does this can now combine both downlink and uplink
scheduling, in order to achieve statistically predictable channel access.
Unlike quality-of-service (QoS) tagging, this is needed for dense data
networks whether or not voice is present-let alone voice networks of any
In 802.11, the clients decide when and where to associate and hand off,
and they do so independent of one another. This causes a Ping-Pong effect,
where clients spend much of their time flopping from AP to AP rather than
transmitting or receiving data. Voice transmissions cannot afford the
losses involved with too many handoffs or of handoffs taking more than
a few milliseconds, which may reduce the quality to unacceptable levels,
as measured by the Mean Opinion Score.
If a network uses IPSec or other security protocols, the handoff can
also destroy the security context as the network interface goes down and
up. The renegotiation time could be in the seconds. The key here is for
the infrastructure, rather than the client, to take on the role of deciding
handoffs, as in a cellular network, and to perform these handoffs as quickly
as possible. The best way to do this is to create virtual access
points that span multiple radios but appear as one AP to the client.
In the end, it boils down to air traffic control, or the provisioning
of the air for downlink and uplink packets. In environments where the
air is well used, such as a converged enterprise network, air traffic
control is critical for any form of predictable or uniform end-to-end
In many vendors' minds, QoS equates just to priority scheduling. While
this thinking applies very well to switched networks, it does not work
for wireless networks, which use a shared, hublike medium. Wireless, of
course, uses multiple transmitters that make independent decisions. To
keep the independent transmitters from fighting, an access point must
take into account uplink and downlink scheduling. Maintaining control
of the channel is the key.
Just as with cellular networks, the actual control of who transmits and
who doesn't is what gives the network robustness and scalability.