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CARRIER SENSE/MULTIPLE ACCESS WITH COLLISION DETECTION (CSMA/CD)


This media access technology with the extremely long acronym is probably the most common. When a protocol that uses CSMA/CD has data to transmit,


it first senses if a signal is already on the wire (a carrier), indicating that someone is transmitting currently. That’s the “Carrier Sense” part. If no one else is transmitting, it attempts a transmission and then listens to hear if someone else tried to transmit at the same time. If someone else transmits at the exact same time, a collision occurs. Both senders “back off” and don’t transmit until some random period of time has passed. Then they both retry. That’s the “Collision Detection” part. The final part (“Multiple Access”) just means that more than one station can be on the network at the same time. CSMA/CD is the access method used in Ethernet and wireless Ethernet networks.


TOKEN PASSING


This media access method uses a special packet called a token. The first computer that is turned on creates the token. It then passes on the token to the next computer. The token passes around the network until a computer that has data to send takes the token off the network, modifies it, and puts it back on the network along with the data it has to send. Each station between the sender and the receiver along the network reads the destination address in the token. If the destination address doesn’t match its own, the station simply sends the package on its way. When the destination station recognizes its address in the destination address of the token, the NIC copies the data into the station’s memory and modifies the token, indicating it has received the data. The token continues around the network until the original sender receives the token again. If the original sender has more data to send, the process repeats itself. If not, the sender modifies the token to indicate that the token is “free” for anyone else to use. With this method, there are no collisions (as in CSMA/CD networks) because everyone has to have “permission” to transmit (via the token).

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