When preparing to conduct a site survey for a Cisco? wireless LAN (WLAN) installation, follow the Cisco network design methodology. Cisco's Prepare, Plan, Design, Implement, Operate, and Optimize (PPDIOO) addresses the network in a phased approach. When conducting a site survey and installation the Prepare, Plan, Design and Implement phases should be followed.
The prepare phase addresses specifics such as what the organization can justify financially. What the wireless network design should support. What services are to be supported on the wireless network, data, voice, 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.
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, what will the impact be by adding controllers and access points to the existing network? Is 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, load campus buildings and floor plans that will support WLANs into WCS. Use the planning tool feature in WCS to determine the amount of access points and placement for the services you plan to support, data, voice, 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. Once 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 perform a site survey which includes the following:
Cisco's Spectrum ExpertT can identify and locate any non 802.11 devices that may cause interference 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 ExpertT to identify any devices that may interfere with the proposed WLAN. These devices could be cordless phones, microwave ovens or any other devices that use the license free Industrial Scientific Medical Band (ISM band) or Unlicensed National Information Infrastructure bands (UNII 1-3) but, are not IEEE 802.11 devices. Once any of these devices are identified 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 ExpertT will also identify other 802.11 devices that may be heard from surrounding WLAN installations in the area.
Use Ekahau Site Survey software for the site survey. This survey program gives you all the data you need to insure a proper site survey for data, voice or location based services.
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 insures needed 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 and 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) must also 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.
No comments:
Post a Comment