34 Planning• rate scaling and variable capacity• power adjustments and variable capacity• Quality of Service (QoS)High overhead of 802.11Unlike many other 802.n standards, 802.11 has a very high amount ofoverhead associated with transmitting a packet. To compare an 802.3network with an 802.11 network, the difference in overhead for transmittingline-rate minimum frame sizes compared to the line-rate maximum framesizes on an 802.3 network can be significant, yet not nearly as significant ason an 802.11 network.For 802.11, the difference in effective throughput varies dramatically withpacket size because of the amount of overhead involved in transmittinga frame. Therefore, the effective throughput of the medium is potentiallyhigher for data clients that use very large packet sizes than it is for voiceclients that use smaller packets. As an example, using very conservativeassumptions for average frame size, no rate scaling, and no contention orcollisions, transmission overhead consumes as much as 67% of the total802.11 medium capacity. By contrast, in an 802.3 network using the sameassumptions, the overhead is about 8%.Rate scaling and variable capacity802.11b supports four transmission rates or data rates. Usually, as ahandset gets farther from an Access Point (AP), both devices scale down tolower transmission rates to compensate for a weaker signal. As a result,a transmission at the 5.5 megabits per second (Mb/s) data rate takesapproximately twice as long as the same size packet transmitted at the 11Mb/s data rate. Longer transmission times mean less transmission time forother handsets. Therefore, rate scaling compromises the overall throughputof the medium.Rate scaling is necessary to extend the coverage of the AP beyond a verytight region around the AP, but the effects must be taken into account whendetermining medium capacity. For example, if the maximum call capacity foran 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 APreduces the total call capacity of that AP to roughly 10. This factor makesengineering the number of APs for the network difficult, because handsetsare roaming around and rate scaling up and down as necessary. Handsetsare moving, and as they do, the engineering target of call capacity becomesa moving target.Nortel Communication Server 1000WLAN IP Telephony Installation and CommissioningNN43001-504 01.02 StandardRelease 5.0 15 June 2007Copyright © 2004-2007, Nortel Networks.
