Wireles Networking is a practical guide to planning and building low-cost telecommunications infrastructure. See the editorial for more information....

IEEE 802.11 Wireless Networks

Before packets can be forwarded and routed to the Internet, layers one (the physical) and two (the data link) need to be connected. Without link local connectivity, network nodes cannot talk to each other and route packets.

To provide physical connectivity, wireless network devices must operate in the same part of the radio spectrum. As we saw in chapter two, this means that 802.11a radios will talk to 802.11a radios at around 5GHz, and 802.11b/g radios will talk to other 802.11b/g radios at around 2.4GHz. But an 802.11a device cannot interoperate with an 802.11b/g device, since they use completely different parts of the electromagnetic spectrum.

More specifically, wireless cards must agree on a common channel. If one 802.11b radio card is set to channel 2 while another is set to channel 11, then the radios cannot communicate with each other.

When two wireless cards are configured to use the same protocol on the same radio channel, then they are ready to negotiate data link layer connectivity. Each 802.11a/b/g device can operate in one of four possible modes:

  1. Master mode (also called AP or infrastructure mode) is used to create a service that looks like a traditional access point. The wireless card creates a network with a specified name (called the SSID) and channel, and offers network services on it. While in master mode, wireless cards manage all communications related to the network (authenticating wireless clients, handling channel contention, repeating packets, etc.) Wireless cards in master mode can only communicate with cards that are associated with it in managed mode.
  2. Managed mode is sometimes also referred to as client mode. Wireless cards in managed mode will join a network created by a master, and will automatically change their channel to match it. They then present any necessary credentials to the master, and if those credentials are accepted, they are said to be associated with the master. Managed mode cards do not communicate with each other directly, and will only communicate with an associated master.
  3. Ad-hoc mode creates a multipoint-to-multipoint network where there is no single master node or AP. In ad-hoc mode, each wireless card communicates directly with its neighbors. Nodes must be in range of each other to communicate, and must agree on a network name and channel.
  4. Monitor mode is used by some tools (such as Kismet, chapter six) to passively listen to all radio traffic on a given channel. When in monitor mode, wireless cards transmit no data. This is useful for analyzing problems on a wireless link or observing spectrum usage in the local area. Monitor mode is not used for normal communications.

When implementing a point-to-point or point-to-multipoint link, one radio will typically operate in master mode, while the other(s) operate in managed mode. In a multipoint-to-multipoint mesh, the radios all operate in ad-hoc mode so that they can communicate with each other directly.

Figure 3.5: APs, Clients, and Ad-Hoc nodes.

It is important to keep these modes in mind when designing your network layout. Remember that managed mode clients cannot communicate with each other directly, so it is likely that you will want to run a high repeater site in master or ad-hoc mode. As we will see later in this chapter, ad-hoc is more flexible but has a number of performance issues as compared to using the master / managed modes.

Now that your wireless cards are providing physical and data link connectivity, they are ready to start passing around packets on layer 3: the internet-working layer.

Last Update: 2007-01-24