Ethernet is a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation (DEC), Intel Corporation and Xerox. The access mechanism used in an Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In CSMA/CD, before a station transmits data, it must check the medium where any other station is currently using the medium. If no other station is transmitting, the station can send its data. If two or more stations send data at the same time, it may result in a collision. Therefore, all stations should continuously check the medium to detect any collision. If a collision occurs, all stations ignore the data received. The sending stations wait for a period of time before resending the data. To reduce the possibility of a second collision, the sending stations individually generate a random number that determinates how long the station should wait before resending data.
As an access method, the token is passed from station to station in sequence until it encounters a station with data to send. The station to be sent data waits for the token. The station then captures the token and sends its data frame. This data frame proceeds around the ring and each station regenerates the frame. Each intermediate station examines the destination address, finds that the frame is addressed to another station, and relays it to its neighbouring station. The intended recipient recognises its own address, copies the message, checks for errors and changes four bits in the last byte of the frame to indicate that the address has been recognised and the frame copied. The full packet then continues around the ring until it returns to the station that sent it.
