Windows 7 / Networking

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Wireless AP Placement

An important and time-consuming task in deploying a wireless LAN is determining where to place the wireless APs in your organization. Wireless APs must be placed to provide seamless coverage across the floor, building, or campus. With seamless coverage, wireless users can roam from one location to another without experiencing an interruption in network connectivity, except for a change in IPv4 and IPv6 addresses when crossing a subnet boundary. Determining where to place your wireless APs is not as simple as installing them and turning them on. Wireless LAN technologies are based on propagation of a radio signal, which can be obstructed, reflected, shielded, and interfered with.

When planning the deployment of wireless APs in an organization, you should take the following design elements into consideration (as described in the following sections):

• Wireless AP requirements
• Channel separation
• Signal propagation modifiers
• Sources of interference
• Number of wireless APs

Note: For additional specifications and guidelines for placing wireless APs, see the manufacturer's documentation for the wireless APs and the antennas used with them.

Wireless AP Requirements

You must identify the requirements for your wireless APs, which might include the following features:

• WPA
• WPA2
• 802.1X and RADIUS
• 802.11a, b, g, and n

Depending on your budget and bandwidth requirements, you might need wireless APs that support 802.11b, 802.11a, 802.11g, or a combination of technologies.

• Building or fire code compliance: The plenum area (the space between the suspended ceiling and the ceiling) is regulated by building and fire codes. Therefore, for plenum placement of APs and associated wiring, you must purchase wireless APs that are fire-rated and in compliance with building and fire codes. If you place your wireless APs in the plenum area, you must determine the best method for powering the wireless APs. Consult with the wireless AP manufacturer to determine how to meet the power requirements for the wireless APs. Some wireless APs can receive electrical power through the Ethernet cable that connects them to the wired network.
• Preconfiguration and remote configuration: Preconfiguring the wireless APs before installing them on location can speed up the deployment process and can save labor costs because less-skilled workers can perform the physical installation. You can preconfigure wireless APs by using the console port (serial port), Telnet, or a Web server that is integrated with the wireless AP. Regardless of whether you decide to preconfigure the wireless APs, make sure that you can access them remotely, configure the wireless APs remotely through a vendor-supplied configuration tool, or upgrade the wireless APs by using scripts.
• Antenna types: Verify that the wireless AP supports different types of antennas. For example, in a building with multiple floors, a loop antenna-which propagates the signal equally in all directions except vertically-might work best.
  Note: For information about which type of antenna will work best for your wireless WLAN deployment, see the documentation for your wireless APs.
• IPsec support: Although not a requirement, if possible, choose wireless APs that use Internet Protocol security (IPsec) and Encapsulating Security Payload (ESP) with encryption to provide data confidentiality for RADIUS traffic sent between wireless APs and RADIUS servers. Use Triple Data Encryption Standard (3DES) encryption and, if possible, certificates for Internet Key Exchange (IKE) main mode authentication.

Channel Separation

Direct communication between an 802.11b or 802.11g wireless network adapter and a wireless AP occurs over a common channel, which corresponds to a frequency range in the S-Band ISM. You configure the wireless AP for a specific channel, and the wireless network adapter automatically configures itself to the channel of the wireless AP with the strongest signal.

To reduce interference between 802.11b wireless APs, ensure that wireless APs with overlapping coverage volumes use unique frequency channels. The 802.11b or 802.11g standards reserve 14 channels for use with wireless APs.

Signal Propagation Modifiers

The wireless AP is a radio transmitter and receiver that has a limited range. The volume around the wireless AP for which you can send and receive wireless data for any of the supported bit rates is known as the coverage volume. (Many wireless references use the term coverage area; however, wireless signals propagate in three dimensions.) The shape of the coverage volume depends on the type of antenna used by the wireless AP and the presence of signal propagation modifiers and other interference sources.

With an idealized omnidirectional antenna, the coverage volume is a series of concentric spherical shells of signal strengths corresponding to the different supported bit rates.

Signal propagation modifiers change the shape of the ideal coverage volume through radio frequency (RF) attenuation (the reduction of signal strength), shielding, and reflection, which can affect how you deploy your wireless APs. Metal objects within a building or used in the construction of a building can affect the wireless signal. Examples of such objects include:

• Support beams
• Elevator shafts
• Steel reinforcement in concrete
• Heating and air-conditioning ventilation ducts
• Wire mesh that reinforces plaster or stucco in walls
• Walls that contain metal, cinder blocks, and concrete
• Cabinets, metal desks, or other types of large metal equipment

Sources of Interference

Any device that operates on the same frequencies as your wireless devices (in the S-Band ISM, which operates in the frequency range of 2.4 GHz to 2.5 GHz, or the C-Band ISM, which operates in the frequency range of 5.725 GHz to 5.875 GHz) might interfere with the wireless signals. Sources of interference also change the shape of a wireless AP's ideal coverage volume.

Devices that operate in the S-Band ISM include the following:

• Bluetooth-enabled devices
• Microwave ovens
• 2.4 GHz cordless phones
• Wireless video cameras
• Medical equipment
• Elevator motors

Devices that operate in the C-Band ISM include the following:

• 5 GHz cordless phones
• Wireless video cameras
• Medical equipment

Number of Wireless APs

To determine how many wireless APs to deploy, follow these guidelines:

• Include enough wireless APs to ensure that wireless users have sufficient signal strength from anywhere in the coverage volume.
  Typical wireless APs use antennas that produce a vertically flattened sphere of signal that propagates across the floor of a building. Wireless APs typically have indoor coverage within a 200-foot radius. Include enough wireless APs to ensure signal overlap between the wireless APs.
• Determine the maximum number of simultaneous wireless users per coverage volume.
• Estimate the data throughput that the average wireless user requires. If needed, add more wireless APs, which will:
  1. Improve wireless client network bandwidth capacity.
  2. Increase the number of wireless users supported within a coverage area.
     Based on the total data throughput of all users, determine the number of users that can connect to a wireless AP. Obtain a clear picture of throughput before deploying the network or making changes. Some wireless vendors provide an 802.11 simulation tool, which you can use to model traffic in a network and view throughput levels under various conditions.
  3. Ensure redundancy in case a wireless AP fails.

When designing wireless AP placement for performance, use the following best practices:

1. Do not overload your wireless APs with too many connected wireless clients. Although most wireless APs can support hundreds of wireless connections, the practical limit is 20-25 connected clients. An average of 2-4 users per wireless AP is a good average to maximize the performance while still effectively utilizing the wireless LAN.
2. For higher density situations, lower the signal strength of the wireless APs to reduce the coverage area, thereby allowing more wireless APs to fit in a specific space and more wireless bandwidth to be distributed to more wireless clients.