Medium arbitration helps prevent wireless interference
Medium arbitration, or medium contention, is the collection of methods used to ensure that all users can access attached resources without interference from another user. The contention mechanisms ensure that all users are able to use the medium in a shared manner without having to worry about an instance where one user monopolizes the medium. This is a basic problem when using a shared medium. Multiple users have differing needs, which will utilize more or less of the network resources available. When too many users attempt to use the medium, the throughput slows because of the algorithms used to parse and assign access to the various users.
Without some form of medium arbitration resolution, data communication networks as we know them would not exist. Without a shared medium, each individual station would require a hardwired connection and immense switches to effect a physical, dedicated connection between two entities requesting a connection. This was how the early telephone systems operated. The caller, in effect, operated a mechanical switch (the Strowger switch) while dialing. These switches completed the physical connection to the receiver, if available. If the receiver was on another call (a collision), the system returned an error (busy) signal. The caller then called back later (backoff time). The switches occupied large pieces of real estate and consumed enormous amounts of energy.
Early systems used a token to grant exclusive permission to a client for access and control. In the most common type of system, called "token ring," the network was arranged into a logical ring. Each user received a window—or token—where he or she could retrieve or transmit information. The user could interface the network only during that time; all other access times would be blocked. This type of network worked well for a while, but required proprietary protocols and equipment to function properly. Overhead became an issue and the large networks were unwieldy, at best.
Another method is polling, in which a master device regularly addresses each attached device to allow an opportunity to transmit or receive data. This system was rendered ineffective because of the higher speeds demanded by users. Polling is still used in some instrumentation systems, particularly in limited, low-bandwidth applications.
Enter carrier sense, multiple-access with collision detection (CSMA/CD). This system is better known as IEEE 802.3 — Ethernet. CSMA/CD requires clients to monitor the network and transmit at will if the medium is idle. If a collision is detected—if two stations begin to transmit at the same instant—then all devices follow a procedure where everyone backs off and tries again. CSMA/CD is the primary wired medium arbitration method in use.
With the introduction of wireless technology, it became apparent that collisions could not be as readily detected as they could within a closed network. In an open ("unbounded") system like wireless, a method had to be devised to allow multiple users to access the medium and gain control for the duration of their transaction. In a wireless medium, an access point (AP) serves as something of an arbitrator; only one device can be connected to an AP at any one time. To accomplish effective and reliable communication, a technique called carrier sense multiple access with collision avoidance (CSMA/CA) was created. CSMA/CA utilizes several techniques to ensure the medium is available and not in use by another wireless device, which would cause corruption of both messages. Monitoring the medium, back-off algorithms and message acknowledgment play key roles in executing this medium arbitration method.
In both cases, medium access arbitration operations are performed in the physical and data link layers (layers 1 and 2) of the network model. The physical layer is unique to the type of medium being used. CSMA/CD and CSMA/CA both use carrier sensing and retransmission back-off periods based on the concept of slot times.
The following two segments will discuss CSMA/CD and CSMA/CA, respectively. A discussion of quality of service in wireless communication will follow them.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). He can be reached at firstname.lastname@example.org. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, email@example.com.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Characteristics of IEEE 802.11n and 802.11ac
Understanding modulation and coding schemes
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.