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Site Survey Guide for Deploying Cisco 7920 IP Phones
OL-6315-01
Chapter 1 Overview
Recommendations for Successful VoIP Surveys
degradation. This quality requirement also dictates the need for a fast roaming solution between access
points where the network can reassociate a client to a new access point within 100 ms (that is, without
suffering at most one packet delay or loss). People often walk around while talking on the phone, so users
making voice calls tend to roam more than users of wireless computers. Users of wireless computers
roam, but because very few users walk and use their computers at the same time, computer users are not
using their computers when the roam occurs.
Because it might not be possible to have a VoIP network and WLAN network in place before the WIPT
site survey, this document also describes a WIPT site survey process using standard wireless client cards
and utilities.
Cisco recommends that all WIPT site surveys use diversity enabled access points and diversity antennas
because a diversity configuration provides better throughput performance. A simple throughput test
using an FTP data transfer of 50 MB in an office environment (with low multi-path signals) shows a
3-second improvement when using a diversity configuration. Diversity configurations are especially
important in environments such as shop floors and hospitals, which often have heavy multi-path signals.
Cisco also recommends that you use 256-byte packets when you perform receive and transmit tests
during the survey. Voice traffic consists of two-way transmissions, usually a burst of packets in one
direction followed by a burst of packets in the other direction. You should check performance, error
rates, and signal levels in both directions.
This document also describes how to use signal levels and signal-to-noise ratios (SNRs) in WIPT site
surveys. WLAN site surveys that use only signal level tests are designed to identify the wireless network
coverage area. Previous site surveys have shown that signal level tests alone are not sufficient for voice
traffic and many times are not sufficient for data traffic. WIPT site surveys must also consider voice call
capacity within a wireless cell. Currently, call loading is 7 active voice calls per channel using the G711
codec. This number is based on simultaneous requirements for data clients and quality voice calls with
current data rates and packet sizes. Cisco recommends that all WIPT site surveys and installations use
non-overlapping channels. For 802.11b/g operation the non-overlapping channels are a minimum of 5
radio channels apart. Cisco also recommends that you use a data rate of 11 Mbps to determine the cell
size for the Cisco 7920 WIPT handset. If other data rates are allowed, call capacity might be substantially
lower.
Getting started
Part of any site survey is measuring the noise level within a wireless cell. Noise levels vary from site to
site and also within different locations of a site. The noise level affects a radio’s ability to receive data
or voice packets. Figure 1-1 uses the Cisco Aironet Client Utility (ACU) and a Cisco PCM350 client
adapter to identify the noise level, signal strength, and SNR at a specific location with a wireless cell.
The ACU window shows that the signal strength is –48 dBm, the noise level is –92 dBm, and the SNR
is 44 dB.
