34Planning
•rate scaling and variable capacity
•power adjustments and variable capacity
•Quality of Service (QoS)
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
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
802.11medium 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.
Nortel Communication Server 1000
WLAN IP Telephony Installation and Commissioning
Release 5.0 15 June 2007
