56 Planningchannels, which is detrimental to voice devices. As a general policy, forlarge amounts of data, use 802.11a for data and 802.11b for voice, butleave 802.11g disabled.Alternately, if there are few 802.11b/g data devices and the WLAN is to beused primarily for voice, consider enabling 802.11g support. The goal is tocarefully control the number of data devices that share radio resourceswith voice devices.For example, if a large number of laptops exist in a campus and if 802.11gmode is enabled, it is probable that a large proportion of those laptops use802.11g (2.4 GHz) for connectivity, which makes it much more difficult toprovide good quality voice for handsets. If 802.11g is disabled, it is probablethat a large proportion of those laptops use 802.11a (5 GHz) because itoffers much higher throughput compared with 802.11b, and voice qualitybenefits.Access Point interferenceWhen more than three APs are deployed, the APs themselves are asignificant source of interference. This is known as cochannel interference.Therefore, it is important to consider how channel reuse impacts networkcapacity.To maximize the distance between APs operating on the same channel, tilethe channels. To scale capacity, add more APs in the same geographicregion and at the same time, reduce the transmit power of each AP.However, the overall throughput increase does not increase proportionallywith the number of APs that are added because each individual AP losesthroughput, even though the number of APs per square foot is increasing.Note that the biggest loss of per-AP throughput occurs when going fromnonchannel-reuse to reusing channels. For more information about thissubject, see the whitepaper available from www.nortel.com.The goal is to achieve the required call density for the number of callsper square foot. Getting the most calls per AP is not a useful objective ofcapacity planning. The parameters that must be tuned to engineer a voicenetwork for capacity are:• channel reuse factor (that is, the number of channels in the channel plan)• transmit power of each AP• the radius of the cell (that is, based on the physical distance betweenAPs)Because of the complexity of this topic and the simulation data that isrequired, it is not possible to discuss tuning all three variables or even twovariables at a time. An example of a light to medium office environment(mostly cube space but some walls) is provided instead.Nortel Communication Server 1000WLAN IP Telephony Installation and CommissioningNN43001-504 01.02 StandardRelease 5.0 15 June 2007Copyright © 2004-2007, Nortel Networks.
