Voice over Wireless LAN Solution Guide v1.0 December 2005
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1. Executive summary Voice over Wireless LAN (VoWLAN) represents the coming together of two important and rapidly
growing technologies — WLAN and Internet Protocol (IP) Telephony. By seamlessly integrating
the IP Telephony system with WLAN infrastructure, VoWLAN provides users with high-quality
mobile voice and data communications throughout the workplace.
This document has two main purposes in defining the aspects of a VoWLAN product solution.
Network designers need to know the engineering limits of various permutations of a network
design. However, this introduces a lot of complexity into the document, making it less useful to a
general audience. The primary purpose of this guide is to provide simple design
recommendations that resolve most issues and common scenarios, while not limiting the
applicability of the document. Therefore, the second purpose of this guide is to describe possible
deviations from the basic design that may be necessary in specific customer environments.
These scenarios present the most challenges and typically do not have a one-size-fits-all answer.
And so, in lieu of specific recommendations, these deviations are presented in a more open-
ended manner that enables you to engineer customized solutions as necessary.
1.1 Challenges
Integrating voice applications on any data network poses some issues and challenges. WLANs
create a number of problems for voice above and beyond those inherent to most data networks.
This guide does not provide an exhaustive treatment of issues, but rather describes those that
are particular to supporting voice on 802.11 networks compared with typical data networks.
1.1.1 High overhead of 802.11
Unlike many other 802.n standards, 802.11 has a very high amount of overhead associated with
transmitting a packet. As a point of comparison, the difference in overhead for transmitting line
rate minimum frame sizes versus line rate maximum frame sizes on an 802.3 network can be
significant, yet not nearly as significant as on an 802.11 network. For 802.11, the difference in
effective throughput varies dramatically with packet size due to the amount of overhead involved
in transmitting a frame. This means that the effective throughput of the medium is potentially
higher for data clients that use very large packet sizes than it is for voice clients that use smaller
payloads. As an example, using very conservative assumptions in terms of average frame size,
no rate scaling, and no contention or collisions, transmission overhead consumes as much as 67
percent of the total 802.11 medium capacity. Taking the same assumptions on an 802.3 network,
the overhead is by contrast about 8 percent.
1.1.2 Rate scaling and variable capacity
802.11b supports four transmission rates or data rates. Usually, as a client gets farther from an
Access Point (AP), both devices scale down to lower transmission rates in order to compensate
for a weaker signal. As a result, a transmission at the 5.5 megabits per second (Mbps) data rate
will take approximately twice as long as the same size packet transmitted at the 11 Mbps data
rate. That means less transmission time for other devices. Therefore, rate scaling compromises
the overall throughput of the medium. Rate scaling is necessary to extend the coverage of the AP
beyond a very tight region around the AP, but the effects should be taken into account when
determining medium capacity. For example, if the maximum call capacity for an AP is 12 when all
handsets are using the 11 Mbps physical (PHY) layer, then two handsets scaling down to 5.5
Mbps as they move away from the AP reduces the total call capacity of that AP to roughly 10.
This factor makes engineering the number of APs for the network difficult, because devices will
be roaming around and rate scaling up and down as necessary. Devices are moving, and the
engineering target of call capacity likewise becomes a moving target.