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Site Survey Guide for Deploying
Cisco 7920 IP Phones
April 2005
Corporate Headquarters
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA
http://www.cisco.com
Tel: 408 526-4000
800 553-NETS (6387)
Fax: 408 526-4100
Text Part Number: OL-6315-01
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THE SPECIFICATIONS AND INFORMATION REGARDING THE PRODUCTS IN THIS MANUAL ARE SUBJECT TO CHANGE WITHOUT NOTICE. ALL STATEMENTS, INFORMATION, AND RECOMMENDATIONS IN THIS MANUAL ARE BELIEVED TO BE ACCURATE BUT ARE PRESENTED WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED. USERS MUST TAKE FULL RESPONSIBILITY FOR THEIR APPLICATION OF ANY PRODUCTS. THE SOFTWARE LICENSE AND LIMITED WARRANTY FOR THE ACCOMPANYING PRODUCT ARE SET FORTH IN THE INFORMATION PACKET THAT SHIPPED WITH THE PRODUCT AND ARE
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CONTENTS Preface v CHAPTER 1 Overview 1-1 Recommendations for Successful VoIP Surveys 1-1 Getting started 1-2 Minimum Requirements for WIPT Cells 1-4 The Ideal WIPT Environment 1-5 Data Rate and Signal Strength Considerations 1-6 Multi-floor survey 1-7 Comparison of a Manual Survey and an Automated Survey 1-8 CHAPTER 2 Survey Tools 2-1 Survey Tools for Packet Jitter 2-11 CHAPTER 3 Surveying for Cell Capacity and Channel Re-Use 3-1 Survey Design for Cell Capacity 3-1 CHAPTER 4 Conducting a WIPT
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Contents Site Survey Guide for Deploying Cisco 7920 IP Phones iv OL-6315-01
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Preface This document provides instructions and guidelines for conducting a site survey for wireless LANs using Voice over IP (VoIP) on Cisco 7920 IP Phones. With the introduction of voice to a predominantly wireless data network the methodology of site surveys will have to be altered. Voice over WLAN is a new application to wireless technology. Many of the survey techniques used for WLAN must be updated. This document is intended to identify many of the recommended techniques and tools
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Preface Site Survey Guide for Deploying Cisco 7920 IP Phones vi OL-6315-01
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CH A P TER 1 Overview This document begins by identifying the specific radio environmental values that are needed for a successful voice deployment. Note Cisco offers a Cisco Aironet Wireless Site Survey class for technical individuals who will be performing site surveys for wireless LAN solutions. This white paper does not replace the information in the site survey class. Cisco recommends that wireless LAN survey technicians take the Aironet Wireless Site Survey class as a requisite. Cis
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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 be
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Chapter 1 Overview Recommendations for Successful VoIP Surveys Figure 1-1 ACU Site Survey Window Noise is defined as a 2.4-GHz signal that is not in an 802.11 DSSS format but is in the frequency range of the access point’s configured channel. The noise can originate from an 802.11 2.4-GHz frequency-hopping radio, a 2.4-GHz wireless phone, a 2.4-GHz HAM radio, a Microwave oven, or a Bluetooth radio. Signals from a distant out-of-network 802.11b or 802.11g radio may also be seen as nois
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Chapter 1 Overview Recommendations for Successful VoIP Surveys Figure 1-2 Signal to Noise Ratio Minimum Requirements for WIPT Cells Table 1-1 lists the minimum values for voice and data cells. For WIPT cells, Cisco recommends that the cell edge be –67 dBm and the SNR be 25 dB using a data rate of 11 Mbps. These values are much more restrictive than those needed for data because of the sensitivity to delays and retries for voice clients. Table 1-1 Minimum Signal Strength and Signal to N
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Chapter 1 Overview Recommendations for Successful VoIP Surveys difference in transmitter power and receiver sensitivity that matters because the gain of an antenna is reciprocal. To verify the cell coverage edge of –67 dBm while the client is actively sending and receiving voice size packets, check the access point’s Station Information and Status page for a dBm value of –67 dBm or higher. Figure 1-3 Transmit Power Imbalance 5mW 100mW The Ideal WIPT Environment Figure 1-4 shows the ide
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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
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Chapter 1 Overview Recommendations for Successful VoIP Surveys The characteristics of VoIP require a meticulous WIPT site survey. To avoid a call with jitter, the delay variation between packets should not exceed 30 ms. The one-way delay should not exceed 150 ms and packet loss should not exceed one percent. Surveying for a one percent packet loss is important because VoIP uses the Real-Time Transport Protocol (RTP), which does not retransmit for lost packets. Voice packets are small
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Chapter 1 Overview Recommendations for Successful VoIP Surveys Comparison of a Manual Survey and an Automated Survey Figure 1-6 shows an actual hospital survey done using AirMagnet Surveyor with an 802.11b Cisco PCM350 client card. Four panels on the AirMagnet window show the survey results. The top-left panel shows that the survey data was collected in passive mode. The middle left panel shows the access points seen by the survey client while doing the passive mode walkabout to colle
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Chapter 1 Overview Recommendations for Successful VoIP Surveys Figure 1-7 Hospital Floor Survey by a Survey Crew Site Survey Guide for Deploying Cisco 7920 IP Phones OL-6315-01 1-9
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Chapter 1 Overview Recommendations for Successful VoIP Surveys Site Survey Guide for Deploying Cisco 7920 IP Phones 1-10 OL-6315-01
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CH A P TER 2 Survey Tools AirMagnet offers many tools for analyzing 802.11 performance at a site. Figure 2-1 shows the AirMagnet Survey tool. Cisco recommends AirMagnet for data and WIPT surveys. Figure 2-1 AirMagnet Survey Tool An accurate survey requires packets sent in both directions. The size of the packets needs to equal the size of the packets used by the primary applications used at the survey site. Figure 2-2 shows a packet capture of the AirMagnet Survey tool survey packets. This
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Chapter 2 Survey Tools Figure 2-2 AirMagnet Packet Decode Tool The 7920 survey tool reports the signal strength of the signal from the access point as an RSSI value. Most client card utilities report signal strength as dBm or percent of signal strength. In this chapter, each of the tools tested will be reviewed showing how they measure signal strength. Table 2-1 provides a cross reference of RSSI, dBm, and percent of signal strength as reported by different cards and utilities. The nu
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Chapter 2 Survey Tools Table 2-1 RSSI, dBm, and Signal Strength Cross-Referenced for 7920 Phones (continued) 1200 Series Access Point @ 11 Mbps @ 20 mW with 2.2 dBm Antennas SniffPro access 350 ACU AirMagnet AiroPeek ADU CB21 Feet Signal % point dBm 7920 RSSI dBm 350 (PDA) CB21 dBm dBm 80 47 –74 34 –69 –67 –72 –69 90 59 –84 31 –68 –70 –71 –71 100 28 –80 31 –72 –78 –69 –75 110 28 –78 34 –78 –70 –77 –79 120 33 –84 32 –77 –71 –77 –78 The data in Table 2-1 was created from live tests. Each t
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Chapter 2 Survey Tools The Cisco CB21 ADU reports signal strength in dBm. Figure 2-4 shows the ADU Advanced Status window. Figure 2-4 ADU Advanced Status Window Site Survey Guide for Deploying Cisco 7920 IP Phones 2-4 OL-6315-01
