Site Surveys for Voice
As discussed previously in this chapter, WLANs used for voice require a little different survey approach. Table 11-3 indicated that the recommended signal strength, S/N ratio, and packet loss were much more conservative than those values used for a data-only WLAN. There is also more concern for same-channel cells that can overlap, or be close to overlapping. Figure 11-20 shows yet another parameter that should evaluated when installing a WLAN that will use wireless voice devices. That parameter is the signal level from the closed same-channel AP.
Figure 11-20. Same-Channel Isolation
 In Figure 11-20, a client that is placed at the edge (or anywhere in) Cell B should receive a signal from Cell A with a signal strength no greater than 87 dBm. This provides enough same-channel isolation to maintain a quality voice connection.Because of the critical nature of interference from adjacent same-channel cells, as well as the more stringent requirements for signal strength and S/N, it is highly recommended that assisted or automated site surveys are not used for WLANs that will include voice. If there is the remote chance that voice will be added to the WLAN at a later date, the survey should be performed with voice parameters. This normally requires a bit more work and possibly more APs, but this trade-off is necessary for a properly working voice system. |
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