High Overhead Of 802.11; Rate Scaling And Variable Capacity; Power Adjustments And Variable Capacity - Avaya Communication Server 1000 Installation And Commissioning Manual

Planning

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. To compare an 802.3 network with an 802.11 network, the difference
in overhead for transmitting line-rate minimum frame sizes compared to the 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 because
of the amount of overhead involved in transmitting a frame. Therefore, 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 packets. As an example, using very conservative assumptions
for average frame size, no rate scaling, and no contention or collisions, transmission overhead
consumes as much as 67% of the total 802.11 medium capacity. By contrast, in an 802.3
network using the same assumptions, the overhead is about 8%.

Rate scaling and variable capacity

802.11b supports four transmission rates or data rates. Usually, as a handset gets farther from
an Access Point (AP), both devices scale down to lower transmission rates to compensate
for a weaker signal. As a result, a transmission at the 5.5 megabits per second (Mb/s) data
rate takes approximately twice as long as the same size packet transmitted at the 11 Mb/s data
rate. Longer transmission times mean less transmission time for other handsets. 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 must 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 Mb/
s physical (PHY) layer, two handsets scaling down to 5.5 Mb/s 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 handsets are roaming around and rate scaling
up and down as necessary. Handsets are moving, and as they do, the engineering target of
call capacity becomes a moving target.

Power adjustments and variable capacity

A WLAN has dynamic mechanisms in place for adjusting channels, adjusting power, and filling
coverage holes, all in response to changes in the Radio Frequency (RF) environment. All of
these mechanisms present challenges to the engineering of voice networks.
Dynamic adjustments work well for guaranteeing minimum coverage and connectivity of
devices, particularly data devices. Voice requires more planned engineering.
44 Avaya WLAN IP Telephony Installation and Commissioning November 2010