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.
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.
C ON T E N T S Preface CHAPTER 1 Overview v 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 CHAPTER 2 Survey Tools 2-1 Survey Tools for Packet Jitter CHAPTER 3 2-11 Surveying for Cell Capacity and Channel Re-Use Survey Design for Cell Capacity CHAPTER 4 1-8 Conducting a W
Contents Site Survey Guide for Deploying Cisco 7920 IP Phones iv OL-6315-01
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.
Preface Site Survey Guide for Deploying Cisco 7920 IP Phones vi OL-6315-01
C H A P T E R 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.
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.
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 noise. Any 2.
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.
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.
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.
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.
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 collect survey data.
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
Chapter 1 Overview Recommendations for Successful VoIP Surveys Site Survey Guide for Deploying Cisco 7920 IP Phones 1-10 OL-6315-01
C H A P T E R 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.
Chapter 2 Figure 2-2 Survey Tools 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.
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.
Chapter 2 Survey Tools The Cisco CB21 ADU reports signal strength in dBm. Figure 2-4 shows the ADU Advanced Status window.
Chapter 2 Survey Tools Figure 2-5 AirMagnet Survey Tool Site Survey Guide for Deploying Cisco 7920 IP Phones OL-6315-01 2-5
Chapter 2 Survey Tools The AirMagnet Survey packet decode tool shows signal strength in dBm. Figure 2-6 shows the AirMagnet Survey Packet Decode window. Figure 2-6 AirMagnet Packet Decode Window The AiroPeek trace shows the signal dBm value and signal percentage with the PCM 350 client radio. It does give relative time between packets so that jitter can be investigated if you suspect inter-packet delays. Figure 2-7 shows the AiroPeek window.
Chapter 2 Survey Tools Figure 2-7 AiroPeek Window AiroPeek has options to show real-time graphs of current signal strength and current noise levels both in dBm and percentages, but it does not have a survey tool like ACU or AirMagnet. Figure 2-8 shows the AiroPeek real-time graphs.
Chapter 2 Figure 2-8 Survey Tools AiroPeek dBm and Signal Percentage Real-Time Graphs Sniffer Pro from Network Associates shows the signal strength in a percent value. There is no option to show RSSI or dBm values. Sniffer Pro is trace utility with no support for a site survey. Figure 2-9 shows the Sniffer Pro window with signal strength at 28 percent.
Chapter 2 Survey Tools The Cisco access point displays the signal strength of the packets from the client. The signal strength of the access point as reported by the client utility should be relatively close to the signal strength reported by the access point for the client. It is important that the signals between the access point and the 7920 be relatively the same. This design avoids a call in which one side of the call has poor connectivity.
Chapter 2 Survey Tools Figure 2-11 shows that the 7920 hears three access points and the RSSI values of those access points are 18, 31, and 48. Figure 2-11 Cisco 7920 IP Phone Survey Screen You can also use AirMagnet Surveyor to determine channel separation. Figure 2-12 shows the two access points on channel 1 with 1-mW transmit power. The dBm values from 0 to -70 dBm are shown in brown. Any signal below that is shown in dark grey.
Chapter 2 Survey Tools Survey Tools for Packet Jitter Survey Tools for Packet Jitter Typically, voice packets are sent every 20 ms. Voice calls are usually bursty, consisting of a group of tightly packed packets in a transmit direction followed by a tightly packed group in a receive direction. The users of WIPT are typically mobile users who have active calls while moving between WLAN coverage cells and hence roaming between coverage cells.
Chapter 2 Survey Tools Survey Tools for Packet Jitter Figure 2-14 AirMagnet Jitter Tool in an Enterprise Office with High 802.11 Traffic Figure 2-15 and Figure 2-16 show the LiveCapture window and the Station window results in a home office with low 802.11 traffic. Figure 2-15 AirMagnet in a Home Office with Low 802.
Chapter 2 Survey Tools Survey Tools for Packet Jitter Home office utilization is 2.18 percent and the throughput is 246 kbps for 11 Mbps-only calls, with a single active WIPT call, but utilization is 15.41 percent with throughput of 248 kbps for 1 Mbps-only calls. Figure 2-16 AirMagnet in a Home Office with Low 802.11 Traffic The 14.15 percent utilization result reflects a call at 1 Mbps.
Chapter 2 Survey Tools Survey Tools for Packet Jitter Site Survey Guide for Deploying Cisco 7920 IP Phones 2-14 OL-6315-01
C H A P T E R 3 Surveying for Cell Capacity and Channel Re-Use This chapter describes how to perform a site survey to design a wireless LAN for cell capacity and channel re-use. Survey Design for Cell Capacity Cisco recommends that you install enough access points to provide all users with quality calls. Maintaining high call quality might require small coverage cells that limit the number of calls per access point to 7 to meet the QBSS level on access points below 35.
Chapter 3 Surveying for Cell Capacity and Channel Re-Use Survey Design for Cell Capacity Note If the Cisco access point is configured to disable data rates 1, 2, and 5.5, clients are not required to transmit only at 11 Mbps. In fact, older client devices might not be programmed to recognize the access point configuration for data rates. Cisco recommends that customers update their 802.11 clients to run the newest firmware and drivers available.
C H A P T E R 4 Conducting a WIPT Survey This chapter describes specific steps for performing a site survey for WIPT. How To Survey for the WIPT Cisco recommends that the first step of a survey is to work with administrators at the local site to determine the requirements of the RF network. Determine what the coverage areas are or will become. Determine the number of RF network users by location in the site.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Figure 4-1 Ideal WIPT Environment A typical deployment showing a 15-20% overlap from each of the adjoining cells. Provides almost complete redundancy throughout the cell The radius of the cell should be: 67 dBm The separation of same channel cells should be: 19 dBm 7920 RSSI=20 Channel 1 67dB 86dB 135075 Channel 11 Channel 6 Channel Re-Use Figure 4-2 shows the typical channel re-use for 802.11b and 802.11g WLANs.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Manual Survey for the WIPT You can use passive or active mode when you use ACU to run a site survey. Figure 4-3 shows the ACU Site Survey windows. Figure 4-3 ACU Site Survey Windows To set up active mode, click the Setup button at the bottom of the screen. If the Setup button is unavailable, the client is using a version of firmware that does not support active mode, and you should upgrade the firmware.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Figure 4-4 ACU Site Survey Active Mode Click OK to return to the Site Survey screen. Click Start to start the site survey in active mode. This information updates constantly during the survey: • Percent Complete—the percentage of packets that have been sent. When you select continuous link test, this field shows the percentage of packets that have been sent. When it reaches 100 it starts over.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT The AirMagnet surveyor works very well for surveying a site with an installed WLAN or for surveying a site without a WLAN. The tool works with the existing transmit power of the access points, but allows you to redisplay coverage areas with simulated transmit power settings, another time-saving feature over manual surveys. Figure 4-5 shows the AirMagnet Surveyor main window. Figure 4-6 shows the Active Mode window.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Figure 4-6 AirMagnet Survey Active Mode To survey a group of access points using the Active Mode tool, configure the tool with an SSID that is configured on the access points. To begin the data collection in active mode, select a starting position near the perimeter of the area to be covered. Click this location on the floor map to indicate the starting point. A small stick figure appears on that spot.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Figure 4-7 AirMagnet Client Walkabout When you select display mode on the bottom tool bar, SiteViewer displays data collected during the survey. Two new panels appear in this mode: the Filter area in the center left panel, and the map zoom box in the lower left. You can display one or more survey data sets by clicking the appropriate check box in the data catalog area in the upper left of the window.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Figure 4-8 AirMagnet Showing Access Points and Merged Display Figure 4-9 shows SiteViewer’s four-panel display. Viewing more than one survey data type helps you compare measurement factors.
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Figure 4-9 AirMagnet Four-Panel Display Site Survey Guide for Deploying Cisco 7920 IP Phones OL-6315-01 4-11
Chapter 4 Conducting a WIPT Survey How To Survey for the WIPT Site Survey Guide for Deploying Cisco 7920 IP Phones 4-12 OL-6315-01
C H A P T E R 5 Positioning Access Points and Antennas for Site Surveys When you perform a site survey, it is important to position wireless equipment for optimal coverage. This chapter describes best practices for positioning access points and antennas. Access Point Positioning In industrial environments such as warehouses, a common installation point for access points is on girders or I-beams.
Chapter 5 Positioning Access Points and Antennas for Site Surveys Antennas and Antenna Positioning The access point in Figure 5-2 provides better coverage because the reflected signal produces fewer nulls and less multipath than the I-beam placement. However, because there is only one floor in this industrial building, there is no need for the antennas to be above the access point. The signal could be improved by inverting the access point so that the antennas point to the ground.
Chapter 5 Positioning Access Points and Antennas for Site Surveys Antennas and Antenna Positioning Figure 5-4 shows a patch antenna mounted on a wall. The antenna cables are routed above the suspended ceiling and are attached to a hidden access point. Figure 5-4 Patch Antenna Mounted on a Wall Figure 5-5 shows a dual-radio access point mounted on a wall. The removable antennas are pointed up, and the paddle antenna is folded against the body of the access point.
Chapter 5 Positioning Access Points and Antennas for Site Surveys Antennas and Antenna Positioning Figure 5-6 shows a patch antenna mounted on a suspended ceiling. The cables are hidden and are routed above the ceiling to the access point.
A P P E N D I X A Additional Sources of Information This appendix lists documents that provide additional information about site surveys: • Cisco Wireless IP Phone 7920 Deployment Recommendations http://www.cisco.com/en/US/products/hw/phones/ps379/products_white_paper0900aecd800f6d97. shtml • Solution Reference Network Designs: Best Practices for Building a Cisco AVVID Network Infrastructure http://www.cisco.com/warp/public/779/largeent/it/ese/srnd.
Appendix A Additional Sources of Information Site Survey Guide for Deploying Cisco 7920 IP Phones A-2 OL-6315-01
