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Carrier sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in local area networking using early Ethernet technology. It uses a carrier sensing scheme in which a transmitting data station detects other signals while transmitting a frame, and stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.〔(【引用サイトリンク】title=Carrier Sense Multiple Access Collision Detect (CSMA/CD) Explained )〕 CSMA/CD is a modification of pure carrier sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted. ==Procedure== The following procedure is used to initiate a transmission. The procedure is complete when the frame is transmitted successfully or a collision is detected during transmission. # Is my frame ready for transmission? If yes, it goes on to the next point. # Is medium idle? If not, wait until it becomes ready〔On Ethernet, stations must additionally wait the 96 bit interframe gap period.〕 # Start transmitting and monitor for collision during transmission # Did a collision occur? If so, go to collision detected procedure. # Reset retransmission counters and end frame transmission. ;Collision detected procedure The following procedure is used to resolve a detected collision. The procedure is complete when retransmission is initiated or the retransmission is aborted due to numerous collisions. # Continue transmission (with a jam signal instead of frame header/data/CRC) until minimum packet time is reached to ensure that all receivers detect the collision # Increment retransmission counter # Was the maximum number of transmission attempts reached? If so, abort transmission. # Calculate and wait random backoff period based on number of collisions. # Re-enter main procedure at stage 1. This can be likened to what happens at a dinner party, where all the guests talk to each other through a common medium (the air). Before speaking, each guest politely waits for the current speaker to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time (in Ethernet, this time is measured in microseconds). The hope is that by each choosing a random period of time, both guests will not choose the same time to try to speak again, thus avoiding another collision. Methods for collision detection are media dependent, but on an electrical bus such as 10BASE5 or 10BASE2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Carrier sense multiple access with collision detection」の詳細全文を読む スポンサード リンク
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