Chapter 1 Overview
Recommendations for Successful VoIP Surveys
Figure 1-4 also illustrates the recommended overlap of 15 to 20 percent for WIPT cells (larger than the 10 to 15 percent for data cells). The reason for the increase in overlap is to provide enough capacity for quality calls, efficient roams, and better load balancing. With a 15% to 20% overlap the call capacity would be double that of a cell without overlap. However, it is important to avoid excess overlap because it can cause frequent roams by the WIPT clients which may result in lower quality calls.
Data Rate and Signal Strength Considerations
You should consider data rate and signal strength before beginning any WIPT site survey. Signal strength or transmitting power of the access point radio combines the configured transmit power of the access point and the antenna attached to it. If an additional antenna cable is placed between the access point and the existing antenna cable, there is a loss in transmitted power. Generally, the longer the cable between the access point and the antenna, the lower the transmitted signal strength will be. The combination of radio transmit power, antenna cable loss, and antenna gain is known as Effective Isotropic Radiated Power (EIRP). For example, if the 100mW transmit power of the radio equals 20 dBm, the loss of a
This example highlights the guidelines for surveying for the Cisco 7920 WIPT handset. Cisco recommends an access point data rate configuration of 11 Mbps for WIPT cells. The faster data rate means that packets take less time to be received and results in higher call quality. The
The data rates configured on the access point depend on the other types of client devices used at the customer site. The customer might have an installed base of legacy 802.11 clients that require support of a
Note Older clients might require long headers. The Cisco 7920 works correctly with long headers, as should all client devices.
802.11b might use a long
Note An 802.11 transmission channel is simplex only. Only one device can transmit at a time, exactly like shared Ethernet.
Site Survey Guide for Deploying Cisco 7920 IP Phones
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