Original 802.11 Topologies

Although the previous sections discussed network topologies that you might encounter, it was a very general discussion. You also need to understand the original topologies, defined by the 802.11 committees, including the following:

• Ad hoc mode
• Infrastructure mode

The following sections give more details on these topologies.

Overview of Ad Hoc Networks

When two computers want to communicate directly with one another, they do so in the form of an ad hoc network. Ad hoc networks don’t require a central device to allow them to communicate. Rather, one device sets a group name and radio parameters, and the other uses it to connect. This is called a Basic Service Set (BSS), which defines the area in which a device is reachable. Because the two machines don’t need a central device to speak to each other, it is called an Independent Basic Service Set (IBSS). This type of ad hoc network exists as soonas two devices see each other. Figure4-2 shows an ad hoc network.

Each computer has only one radio. Because there is only one radio, the throughput is lower and acts as a half-duplex device, because you can’t send and receive at the same time.

You don’t have much control in these networks, so you’re stuck when it comes to methods such as authentication. In addition, you need to address who starts the conversation and who decides on the order of communication, to name just a couple issues.

Network Infrastructure Mode

In wireless networks, an access point acts as a connection point for clients. An AP is actually a cross between a hub and a bridge. Here’s why:

• There is one radio, which cannot send and receive at the same time. This is where the AP is likened to a hub. It’s a half-duplex operation.
• APs have some intelligence that is similar to that of a bridge. That is how an AP can see a frame and decide to forward it based on MAC addresses.

What is different on an AP versus a bridge is that wireless frames are more complex. Standard Ethernet frames have a source MAC address and a destination MAC address. Wireless frames can have three or four MAC addresses. Two of them are the source and destination MAC addresses, and one is the AP’s MAC address that is tied to a workgroup.


The fourth that could be present is a NEXT_HOP address in the event that you are using a workgroup bridge (WGB).

An AP is actually just one type of wireless station. This terminology could cause some confusion between an AP and a client on a network, so to differentiate between them, a client is called a station (STA), and an AP is called an infrastructure device.

So what does a typical wireless topology look like? Of course, wireless clients are associated with an AP. In the wireless space, the coverage area of the AP is called a Basic Service Area (BSA), which is also sometimes known as a wireless cell. They mean the same thing. When only one AP exists, this coverage area is called a BSA, as shown in Figure 4-3. That AP then usually has an Ethernet connection to an 802.3 LAN, depending on the function of the AP.

Assuming that the AP has an Ethernet connection, it bridges the 802.11 wireless traffic from the wireless clients to the 802.3 wired network on the Ethernet side.

The wired network attached to the AP’s Ethernet port is a path to a wireless LAN controller (or controller for short). The client traffic is passed through the controller and then is forwarded to the wired network, called the distribution system. The distribution system is how a client accesses the Internet, file servers, printers, and anything else available on the wired network.
When more than one AP is connected to a common distribution system, as shown in Figure 4-4, the coverage area is called an Extended Service Area (ESA).


Why would you want more than one AP connected to the same LAN? There are a few reasons:

• To provide adequate coverage in a larger area.
• To allow clients to move from one AP to the other and still be on the same LAN.
• To provide more saturation of APs, resulting in more bandwidth per user.

This process of a client moving from one AP to another is called roaming. For roaming to work, the APs must overlap. You might wonder why they need to overlap, because interference in a wireless network is a common issue. The reason for the overlap is so that a client can see both APs and associate to the one with the stronger signal. As soon as the signal from the associated AP hits the threshold built into the client, the client looks for another AP with a better signal.


Service Set Identifiers

Think about how you connect to a wireless network. On your laptop, you might see a popup that says “Wireless networks are available” or something to that effect. When you look at the available networks, you see names. On older Cisco autonomous APs, the network was called “Tsunami.” On a store-bought Linksys, the network is actually called “linksys.” So the client sees a name that represents a network.

On the AP, the network is associated with a MAC address. This network or workgroup that your clients connect to is called a Service Set Identifier (SSID). So on an AP, the SSID is a combination of MAC address and network name. This MAC address can be that of the wireless radio or another MAC address generated on the AP. When an AP offers service for only one network, it is called a Basic Service Set Identifier (BSSID). APs offer the ability to use more than one SSID. This would let you offer a Guest Network and a Corporate Network and still use the same AP. When the AP has more than one network, it is called a Multiple Basic Service Set Identifier (MBSSID). You can think of it as a virtual AP. It offers service for multiple networks, but it’s the same hardware. Because it’s the same hardware and the same frequency range, users on one network share with users on another and can collide if they send at the same time.

Now let’s return to the roaming discussion. To get roaming to work, the BSA of each AP must overlap. The APs also need to be configured for the same SSID. This enables the client to see that the same network is offered by different MAC addresses, as illustrated in Figure 4-5.


When a client roams and moves from one AP to the other, the SSID remains the same, but the MAC address changes to the new AP with a better signal.

Another issue to consider when roaming is the possibility of interference between the two overlapping APs. Even though they offer the same SSID, they need to be on different channels, or frequency ranges, that do not overlap. This prevents co-channel interference, which should be avoided. The 2.4 spectrum allows only three nonoverlapping channels. You must consider this fact when placing APs.