Service Set Identifier SSID

SSID is used to logically separate WLANs.

The SSID must match on client and access point.

Access point broadcasts one SSID in beacon.

Client can be configured without SSID.

Client association steps:

1. Client sends probe request.

2. A point sends probe response.

3. Client initiates association.

4. A point accepts association.

5. A point adds client MAC address to association table.

The SSID is the name of the wireless cell. It is used to logically separate WLANs. It must match exactly between the client and the access point.

The access point broadcasts the SSID in the beacons. Beacons are broadcasts that the access points send to announce the available services. Therefore, clients can be configured without an SSID (null-SSID), detect all access points, and learn the SSID from the beacons of the access point.

SSID broadcasts can be disabled on the access point, but this approach does not work if the client needs to see the SSID in the beacon.

Client Association Steps

The table shows how the client associates to the access point.

Step

Action

1.

Client sends probe request.

2.

Access point sends probe response or beacon.

3.

Client initiates association process.

4.

Access point accepts association of the client.

5.

Access point adds client MAC address to association table.

6-16 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Typical WLAN Topologies

This topic explains the WLAN topologies used for wireless client access.

WLAN Access Topology

This figure shows the WLAN topology for wireless client access.

The basic service area is the area of radio frequency (RF) coverage provided by an access point. This area is also referred to as a "microcell." To extend the basic service area, or to simply add wireless devices and extend the range of an existing wired system, you can add an access point. As the name "access point" indicates, this device is the point at which wireless clients can access the network.

The access point attaches to the Ethernet backbone and communicates with all the wireless devices in the cell area. The access point is the master for the cell and controls traffic flow to and from the network. The remote devices do not communicate directly with each other; they communicate with the access point.

If a single cell does not provide enough coverage, any number of cells can be added to extend the range. This range is known as an extended service area.

It is recommended that the extended service area cells have 10 to 15 percent overlap to allow remote users to roam without losing RF connections. For wireless voice networks, an overlap of 15 to 20 percent is recommended.

Bordering cells should be set to different nonoverlapping channels for best performance.

More recently, wireless deployments have moved from "microcell" to "pico cell." Pico cells further reduce access point coverage area by reducing power and increasing the total number of access points deployed.

© 2006 Cisco Systems, Inc. Wireless LANs 6-17

The resulting benefits are better coverage, less interference, higher data rates, and fault tolerance through convergence. When an adjacent access point goes down, the neighboring access points expand their coverage by increasing their RF power to cover the area that is lost by the access point that went down.

It is important that not only the access points can reduce their transmit power settings but also the clients can reduce their transmit power. Both access points and clients should use a comparable transmit power so that the client associates to the nearest access point.

6-18 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

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