Operating Modes - Practices

There are two main client operating modes in the 802.11 family of standards: Infrastructure and Ad-Hoc. Two additional modes, Master and Monitor, are discussed in later chapters.

Infrastructure Mode requires the use of a wireless access point. At a minimum, this is a device with a radio that operates in Infrastructure Mode and has a connection to a wired network. This is also known as the Basic Service Set (BSS). There is also an Extended Service Set (ESS) for use with multiple access points.

A typical 802.11b access point consists of a radio, external antenna, and at least one Ethernet port. There are many variations on this theme, with models sporting 4-port Ethernet switches, connectors for other external antennas, and higher-power radios.

When operating in Infrastructure Mode, an access point is the master of any client radios that are associated with the access point. The client radios are also operating in Infrastructure Mode, in a different sub-mode. The access point is programmed with a Service Set Identifier (SSID); this is the network name for the access point. The access point broadcasts the SSID as an advertisement of the network name.

Clients operating in Infrastructure Mode identify an access point by these SSID broadcast frames. Once a client is associated with an access point, the access point manages all communication over the radio link. When multiple clients are associated with a single access point, the access point has a set of algorithms for controlling traffic to and from the access point radio.

Ad-Hoc Mode, or peer-to-peer mode, is designed specifically for client-to-client communication. To use Ad-Hoc Mode, you need at least two radio clients. In this example, let’s say we have two Linux notebooks with PCMCIA radio cards. Both cards are configured to work in Ad-Hoc Mode, and both clients must use the same SSID. Ad-Hoc clients do not advertise themselves with the same broadcast frames used by an access point.

While Ad-Hoc Mode is very useful for client-to-client communication, it introduces a difficult situation known as the Hidden Node problem. Ad-Hoc Mode does not provide an access point to control communications between other client machines, so any client using Ad-Hoc Mode may decide to transmit data on its own rather than being told when it is clear to transmit. Figure 1-7 illustrates the problem.

Figure 1-7. A Hidden Node problem with three clients in Ad-Hoc Mode

As shown, node A can hear node B, but it cannot hear node C. Node C can also hear node B, but it cannot hear node A. Because 802.11 is a shared-access physical medium, only one device can transmit at any given time. The Hidden Node problem is that node A and node C cannot hear each other, and neither node will detect a collision. Hidden Node issues reduce throughput in this example by at least 50%.  

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