Wireless Operations


The wireless network mode refers to the WLAN protocols: 802.11a, b, g, or n. Because 802.11g is backward compatible with 802.11b, access points support both standards. Remember that if all the clients connect to an access point with 802.11g, they all enjoy the better data rates provided. When 802.11b clients associate with the access point all the faster clients contending for the channel have to wait on 802.11b clients to clear the channel before transmitting. When a Linksys access point is configured to allow both 802.11b and 802.11g clients, it is operating in mixed mode.
For an access point to support 802.11a as well as 802.11b and g, it must have a second radio to operate in the different RF band.
A shared service set identifier (SSID) is a unique identifier that client devices use to distinguish between multiple wireless networks in the same vicinity. Several access points on a network can share an SSID. The figure shows an example of SSIDs distinguishing between WLANs, each which can be any alphanumeric, case-sensitive entry from 2 to 32 characters long.
The IEEE 802.11 standard establishes the channelization scheme for the use of the unlicensed ISM RF bands in WLANs. The 2.4 GHz band is broken down into 11 channels for North America and 13 channels for Europe. These channels have a center frequency separation of only 5 MHz and an overall channel bandwidth (or frequency occupation) of 22 MHz. The 22 MHz channel bandwidth combined with the 5 MHz separation between center frequencies means there is an overlap between successive channels. Best practices for WLANs that require multiple access points are set to use non-overlapping channels. If there are three adjacent access points, use channels 1, 6, and 11. If there are just two, select any two that are five channels apart, such as channels 5 and 10. Many access points can automatically select a channel based on adjacent channel use. Some products continuously monitor the radio space to adjust the channel settings dynamically in response to environmental changes.
802.11 Topologies
Wireless LANs can accommodate various network topologies. When describing these topologies, the fundamental building block of the IEEE 802.11 WLAN architecture is the basic service set (BSS). The standard defines a BSS as a group of stations that communicate with each other.
Click the Ad Hoc button in the figure.
Ad hoc Networks
Wireless networks can operate without access points; this is called an ad hoc topology. Client stations which are configured to operate in ad hoc mode configure the wireless parameters between themselves. The IEEE 802.11 standard refers to an ad hoc network as an independent BSS (IBSS).
Basic Service Sets
Access points provide an infrastructure that adds services and improves the range for clients. A single access point in infrastructure mode manages the wireless parameters and the topology is simply a BSS. The coverage area for both an IBSS and a BSS is the basic service area (BSA).
Extended Service Sets
When a single BSS provides insufficient RF coverage, one or more can be joined through a common distribution system into an extended service set (ESS). In an ESS, one BSS is differentiated from another by the BSS identifier (BSSID), which is the MAC address of the access point serving the BSS. The coverage area is the extended service area (ESA).
Common Distribution System
The common distribution system allows multiple access points in an ESS to appear to be a single BSS. An ESS generally includes a common SSID to allow a user to roam from access point to access point.
Cells represent the coverage area provided by a single channel. An ESS should have 10 to 15 percent overlap between cells in an extended service area. With a 15 percent overlap between cells, an SSID, and non-overlapping channels (one cell on channel 1 and the other on channel 6), roaming capability can be created.
Client and Access Point Association
A key part of the 802.11 process is discovering a WLAN and subsequently connecting to it. The primary components of this process are as follows:
Beacons - Frames used by the WLAN network to advertise its presence.
Probes - Frames used by WLAN clients to find their networks.
Authentication - A process which is an artifact from the original 802.11 standard, but still required by the standard.
Association - The process for establishing the data link between an access point and a WLAN client.
The primary purpose of the beacon is to allow WLAN clients to learn which networks and access points are available in a given area, thereby allowing them to choose which network and access point to use. Access points may broadcast beacons periodically.
Although beacons may regularly be broadcast by an access point, the frames for probin, authentication, and association are used only during the association (or reassociation) process.
The 802.11 Join Process (Association)
Before an 802.11 client can send data over a WLAN network, it goes through the following three-stage process:
Stage 1 - 802.11 probing
Clients search for a specific network by sending a probe request out on multiple channels. The probe request specifies the network name (SSID) and bit rates. A typical WLAN client is configured with a desired SSID, so probe requests from the WLAN client contain the SSID of the desired WLAN network.
If the WLAN client is simply trying to discover the available WLAN networks, it can send out a probe request with no SSID, and all access points that are configured to respond to this type of query respond. WLANs with the broadcast SSID feature disabled do not respond.
Stage 2 - 802.11 authentication
802.11 was originally developed with two authentication mechanisms. The first one, called open authentication, is fundamentally a NULL authentication where the client says "authenticate me," and the access point responds with "yes." This is the mechanism used in almost all 802.11 deployments.
A second authentication mechanism is based on a key that is shared between the client station and the access point called the Wired Equivalency Protection (WEP) key. The idea of the shared WEP key is that it gives a wireless link the equivalent privacy of a wired link, but the original implementation of this authentication method was flawed. Although shared key authentication needs to be included in client and access point implementations for overall standards compliance, it is not used or recommended.
Stage 3 - 802.11 association
This stage finalizes the security and bit rate options, and establishes the data link between the WLAN client and the access point. As part of this stage, the client learns the BSSID, which is the access point MAC address, and the access point maps a logical port known as the association identifier (AID) to the WLAN client. The AID is equivalent to a port on a switch. The association process allows the infrastructure switch to keep track of frames destined for the WLAN client so that they can be forwarded.
Once a WLAN client has associated with an access point, traffic is now able to travel back and forth between the two devices.

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