RF regulations.
Content 6.2 Describing Wireless LAN Topologies 6.2.1 WLAN Topologies WLANs replace the Layer 1 transmission medium of a traditional wired network (usually Category 5 cable) with radio transmission over the air. Cisco Aironet wireless products fit into three main categories: Wired LANs require that users locate in one place and stay there. WLANs are an extension to the wired LAN network. A WLAN can be an overlay to, or substitute for, a traditional wired LAN network. With Cisco Aironet WLANs, mobile users can do the following: 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. Figure shows how the client associates to the access point.
Content 6.2 Describing Wireless LAN Topologies 6.2.2 Typical WLAN Topologies Figure shows the WLAN topology for wireless client access. The basic service area (BSA) is the area of radio frequency coverage provided by an access point. This area is also referred to as a “microcell.” To extend the BSA, 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 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 (ESA). It is recommended that the ESA 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 non-overlapping channels for best performance. More recently, wireless deployments have moved from microcell to pico cell. Pico cells further reduce the access point coverage area by reducing power and increasing the total number of access points deployed. 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 was lost. Clients can also reduce their transmit power. Access points and clients both should use a comparable transmit power so that the client associates to the nearest access point. In an environment where you need extended coverage but access to the wired LAN is not practical or available, you can use a wireless repeater, which is simply an access point that is not connected to the wired LAN. This topology requires a 50 percent overlap of the access point on the wired LAN and the wireless repeater. The receive and retransmit time involved decreases the throughput by approximately half. The SSID of the root access point must be configured on the repeater access point. The repeater access point uses the same channel as the root access point. Note: Not all implementations support this feature. The Cisco Aironet Workgroup Bridge (WGB) connects to the Ethernet port of a device that does not have a WLAN NIC. The Cisco WGB provides a single MAC address connection into an access point and onto the LAN backbone. It cannot be used in a peer-to-peer mode connection and must communicate with an autonomous Cisco Aironet Access Point or Cisco Aironet Bridge in access point mode. The Cisco Aironet WGB does not operate with access points of other vendors. Another WGB configuration allows multiple wired machines to be attached to the same radio device. This configuration is ideal for connecting remote workgroups to a wired LAN. To use a WGB with multiple MAC addresses, you must connect the WGB to a hub or switch with an Ethernet patch cable. If the WGB is connected directly to an Ethernet client node, you must use an Ethernet crossover cable. Note: Not all WLAN implementations support this topology. The BSA can consist of a number of PCs, each with a wireless network card communicating directly without the use of an access point. Operating systems such as Windows have made this peer-to-peer network easy to set up. This setup can be used for a small office (or home office) to allow a laptop to be connected to the main PC or for several people to simply share files. The coverage is limited. Everyone must be able to hear everyone else. A problem is that peer-to-peer networks are difficult to secure. Note: Many clients default to ad hoc mode, which has a negative impact on infrastructure WLANs with regard to both bandwidth use and network security. The following summarizes the different WLAN topologies:
Content 6.2 Describing Wireless LAN Topologies 6.2.3 Roaming through Wireless Cells A typical WLAN can include PCs, laptop computers, pen-based computers, printers, and any other device that is normally found on a typical wired network. The WLAN consists of microcells, and the user can move anywhere that the RF coverage