Vendor-Specific Topology Extensions

The vendor-specific topology extensions are an enablement of additional network functionality by way of vendor-defined protocols, devices, and topologies. In this section you will learn how workgroup bridges, wireless repeaters, outdoor wireless bridges, and wire- less mesh networks through the use of wireless controllers can enhance the functionality and capability of your wireless deployment.

Workgroup Bridges
You will most likely have times when you have an isolated network that needs access to the rest of the network for access to the server farm and the Internet. You might not be able to run an Ethernet cable to the isolated network, or you might not own the property so you can’t drill holes in the walls, and so on. In this scenario, you would use a WGB topology such as the one shown in Figure 4-6.

Notice that the WGB is used to bridge a wired network to an AP that connects to a distri- bution system.

Cisco offers two types of workgroup bridges:

• Autonomous Workgroup Bridge (aWGB): The aWGB was originally just called a workgroup bridge, but Cisco later changed the name when it introduced the Univer- sal WGB. The aWGB is supported in IOS AP version 12.4(3G)JA and later. The aWGB connects only to upstream Cisco APs, and the AP sees multiple Ethernet clients.
  

• Universal Workgroup Bridge (uWGB): The uWGB is supported on IOS AP version 12.4(11)XJ and later. It allows bridging to upstream non-Cisco APs and appears as a single client.

Repeaters
Recall that in an Extended Service Set (ESS), multiple APs connect clients. This is all well and good until you have clients roaming about who get into areas where coverage is neces- sary but not possible. The solution of a WGB doesn’t work, because a WGB connects users who are wired. An example is a worker at a warehouse who carries a barcode scan- ner or even a wireless Cisco IP Phone. There are scenarios where you can’t run a cable into a location to install an AP. This is where you want to use a wireless repeater. A wireless repeater is simply an AP that doesn’t connect to a wired network for its connectivity to the distribution network. Instead, it overlaps with an AP that does physically connect to the distribution network. The overlap needs to be 50 percent for optimal performance. Figure 4-7 shows an example. A repeater is allowing a client to connect to the network when in fact the client would normally be out of the service area of the AP.

You can get APs that act as a repeater as well, which is how the Cisco solution works. The catch is that you need a Cisco AP as the upstream “root” device, and only one SSID is supported in repeater mode. Additionally, the overall throughput is cut in half for each re- peater hop.


Outdoor Wireless Bridges

When you have two or more LANs within a few miles of each other and you want to link them, you can use a wireless bridge. Because you are “bridging,” the technology works at Layer 2. This means that the LANs do not route traffic and do not have a routing table.

You can connect one LAN directly to another in a point-to-point configuration, as shown in Figure 4-8, or you can connect many LANs through a central hub, as shown in Figure 4-9.


Each end of a point-to-multipoint topology would have to communicate through the hub if it wanted to communicate with the others. Cisco offers the Cisco Aironet 1300 series wireless bridge and the Cisco Aironet 1400 series wireless bridge. When using a 1400 se- ries, you can bridge only networks, but if you use a 1300 series, you can allow clients to connect as well as bridge networks. The 1300 series operates in the 2.4-GHz range, and the 1400 series operates in the 5-GHz range.


Outdoor Mesh Networks

As you can see, bridges are a good way to connect remote sites. However, suppose that you are operating in a point-to-multipoint topology, and the central site experiences con- gestion. Who suffers? Just the central site? Just the remote site? No; the answer is every- one. When two remote sites communicate through a central site, the central site makes all the difference.

Assume that the central site goes down, as shown in Figure 4-10.


Now the remote sites can’t communicate with each other or the central site. This can be a major issue to contend with. The solution is to deploy a mesh network such as the one illustrated in Figure 4-11.

The mesh solution is appropriate when connectivity is important, because multiple paths can be used. The IEEE is currently working on a mesh standard (802.11s). However, the solution discussed here is a Cisco solution in which a wireless controller, also shown in Figure 4-11, is involved.

When you have a mesh network, some nodes (another term for APs in a mesh network) are connected to a wired network. Some nodes simply act as repeaters. A mesh node re- peats data to nearby nodes. More than one path is available, so a special algorithm is used to determine the best path. The alternative paths can be used when there is congestion or when a wireless mesh node goes down.