CSMA/CD

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision.

CSMA/CD method, a station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again.

To better understand CSMA/CD, let us look at the first bits transmitted by the two stations involved in the collision. Although each station continues to send bits in the frame until it detects the collision, we show what happens as the first bits collide. In below figure stations A and C are involved in the collision.

At time t1, station A has executed its persistence procedure and starts sending the bits of its frame. At time t2, station C has not yet sensed the first bit sent by A. Station C executes its persistence procedure and starts sending the bits in its frame, which propagate both to the left and to the right. The collision occurs sometime after time t2. Station C detects a collision at time t3 when it receives the first bit of A’s frame. Station C immediately (or after a short time, but we assume immediately) aborts transmission. Station A detects collision at time t4 when it receives the first bit of C’s frame; it also immediately aborts transmission. Looking at the below figure, see that A transmits for the duration t4 − t1; C transmits for the duration t3 − t2.

Waiting for Backoff Time 

·         After the collision, the transmitting station waits for some random amount of time called as back off time.

·         After back off time, it tries transmitting the data packet again.

·         If again the collision occurs, then station again waits for some random back off time and then tries again.

·         The station keeps trying until the back off time reaches its limit.

·         After the limit is reached, station aborts the transmission.

·         Back off time is calculated using Backoff Algorithm.

Throughput

The throughput of CSMA/CD is greater than that of pure or slotted ALOHA. For the 1-persistent method, the maximum throughput is around 50 percent. For the non-persistent method, the maximum throughput can go up to 90 percent.

Traditional Ethernet

One of the LAN protocols that used CSMA/CD is the traditional Ethernet with the data rate of 10 Mbps. But it is good to know that the traditional Ethernet was a broadcast LAN that used the 1-persistence method to control access to the common media.