Conducting a WLAN Site Survey and Implementation for the Cisco® Unified Wireless Network

Author: Keven Dech

Abstract

When preparing to conduct a site survey for a Cisco® wireless LAN (WLAN) installation, follow the Cisco network design methodology: Prepare, Plan, Design, Implement, Operate, and Optimize (PPDIOO). PPDIOO addresses network implementation in phases. This paper will address the first four phases of this methodology: 1) Prepare, 2) Plan, 3) Design, and 4) Implement.

1. Prepare

The Prepare phase addresses specifics such as what the organization can justify financially and what the wireless network design should support. What services are to be supported on the wireless network: data, voice, and/or location-based services? What clients are to be supported: laptops, handhelds PDA's, active RFID tags? You need to have the answer to all these questions prior to entering the Planning phase.

2. Plan

During the Planning phase a project plan is developed to manage tasks, responsible parties, resources, and milestones. If there is a current network in place, how will the existing network be impacted by adding controllers and points? Are there existing, available ports for access points and controllers to be connected to? Or will additional switches be added to the network to support the controllers and access points? Do current switches for access points support Power over Ethernet (PoE)? Will there be N+1, N+N redundancy, or N+N+1 redundancy?

If this is to be a new network then the site survey and design phase will dictate what equipment is needed to support the controller-based WLAN. If you purchased the Wireless Control System (WCS) from Cisco, upload buildings and floor plans that will support WLANs into WCS in the supported file format. Use the Planning Tool feature in WCS to determine the number of access points and placement for the services you plan to support: data, voice, and/or location-based services.

The WCS can generate access point density and coverage based on the services you have decided to support, as well as the amount of users that will access the WLAN. After you have determined the amount of access points needed this will dictate the amount of controllers needed to support the access points.

Once you have accomplished the above you are ready to move forward with the site survey which includes the following four components:

2.1 Layer 1 Sweep with Cisco Spectrum Expert

Cisco's Spectrum Expert can identify and locate any non-802.11 devices that may interfere with the proposed WLAN, be it in the 2.4 or 5 GHz spectrum or both. Sweep all areas where access points will be deployed with Cisco Spectrum Expert to identify any devices that may interfere with the proposed WLAN. These devices could be cordless phones, microwave ovens, or any other non-IEEE 802.11 devices that use the license-free Industrial Scientific Medical (ISM) band or Unlicensed National Information Infrastructure (UNII 1-3) bands. Once you have identified any devices that can cause Interference, they can be removed or replaced by devices that do not operate in the 2.4 or 5 GHz frequency bands. Cisco Spectrum Expert will also identify other 802.11 devices that may be heard from surrounding WLAN installations in the area.

2.2 Performing a WLAN Site Survey

Use AirMagnet® Survey PRO for the site survey. This survey program gives you all the data you need to ensure a proper site survey for data, voice or location-based services. Survey PRO version 6.0 and later can also perform Greenfield mode surveys for 802.11n technology.

The site survey is a process of identifying access point placement for coverage so that additional access points can be placed to achieve the appropriate signal-to-noise ratio (SNR) for WLAN services supported. The site survey process ensures the necessary signal levels and overlap with minimal interference to other access points.

Prior to performing a site survey you must know what devices are being supported on the WLAN to determine appropriate signal levels. These vary depending on the following:

• Data
  WLAN data-only networks are dependent only on SNR for data rate supported and proper cell overlap (Normally 10 to 15 percent. Twenty percent is recommended).
• Voice
  WLAN voice networks are critical to SNR, data rate supported, and proper overlap (20 percent). Quality of Service (QOS) also must be configured on controllers at installation for voice. Data rates below 18 Mbps should be disabled. It is recommended to use the 5 GHz band due to the amount of channels available (23 in all). This helps to avoid co-channel and adjacent channel interference. If you are surveying for data and voice only, ensure your cell edges are at a minimum of -67 dBm with 20 percent overlap. Access points on the same channel should have at least a 19 dBm SNR ratio of separation from each other. For more information on deploying WLAN voice refer to the Cisco Unified Wireless IP Phone 7921G Deployment Guide available at Cisco.com.

Related Courses

IUWNE - Implementing Cisco Unified Wireless Networking Essentials v1.0
CWLF - Cisco Wireless LAN Fundamentals v2.0
CUWSS - Conducting Cisco Unified Wireless Site Survey v1.0
CWLAT - Cisco Wireless LAN Advanced Topics v2.0