Carrier sense multiple access with collision detection
Carrier sense multiple access with collision detection (CSMA/CD) is the method used in wired networks to allow two transmitting stations to achieve error-free communication across the shared medium. It is based on the IEEE 802.3 standard that has become the basis for Ethernet. However, it is not a hardwired, dedicated connection between the stations and should not be construed as such. The connection is logical and temporary, and is set up exclusively for the purpose of transmitting data between any two attached stations. As the number of attached stations increases, contention for the use of the medium increases, eventually resulting in simultaneous transmissions by multiple stations. These are called collisions and result in corrupted or truncated data frames.
The first step in data transmission onto the network is when data is passed down the stack from the network layer (layer 3) to the logical link sublayer of the data link layer (Layer 2); the logical link control (LLC) requests the transmission and passes the data to the medium access control sublayer of the data link layer. The media access control (MAC) sublayer attaches, or encapsulates, the packet with a preamble and a field called the start frame delimiter (SFD), which indicates the beginning of what is now called a frame. The MAC sublayer also appends source and destination addresses, padding, and other data, such as a frame check sequence, to the frame to ensure proper frame length and error control, respectively.
The frame is sent to the physical layer, where it is converted to the proper signaling method for the particular attached medium being used. Prior to putting the data onto the medium, the physical layer monitors the medium for any traffic (carrier sense); hearing none, it begins the physical transmission process. The signaling component of the physical layer turns the frame data into bits that are put onto the medium as electrical signals. The initial stage of the transmission, called the collision window, is the period of time within which a station determines if the medium is free and whether another station has begun to transmit.
If no collision occurs, then the station successfully acquires the medium and completes its transaction. Subsequent collisions are avoided because all other stations, if operating properly, have determined the medium is in use through carrier sense. At the receiving station, the physical layer receives the signal (frame), synchronizes with the frame preamble, and passes the data to the data link layer. If the frame is addressed to that station, the encapsulation data is stripped off and sent up the stack in the reverse order of transmission. If the frame is intended for another station, it is ignored or discarded. Being a broadcast network, all stations hear all transmissions.
If two stations are transmitting at the same time during the collision window, a collision occurs between the two transmissions. The transmitting station detects the collision and turns on the collision detect signal, which is detected by the MAC sublayer; this initiates the collision handling response. The transmitting station ceases the transmission of data and transmits a Jam signal onto the network. This is a unique bitstream signaling to all stations on the medium that a collision has occurred. The Jam signal is transmitted long enough to allow stations party to the transmission to be aware of the collision.
On a normally functioning network, all stations back off before attempting additional transmissions. The transmitting station waits an arbitrary amount of time, called the back-off time, and attempts retransmission. If another collision occurs, then the back-off time is increased until eventually the transmission is successful or an upper limit on back-off time is reached and the transmission attempt is abandoned; this usually occurs if the medium has failed or the network is loaded beyond design capacity.
– 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 email@example.com. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Characteristics of IEEE 802.11n and 802.11ac
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.