To reduce the impact of collisions on the network performance, Ethernet uses an algorithm called CSMA with Collision Detection (CSMA / CD): CSMA/CD is a protocol in which the station senses the carrier or channel before transmitting frame just as in persistent and non-persistent CSMA. If the channel is busy, the station waits. it listens at the same time on communication media to ensure that there is no collision with a packet sent by another station. In a collision, the issuer immediately cancel the sending of the package. This allows to limit the duration of collisions: we do not waste time to send a packet complete if it detects a collision. After a collision, the transmitter waits again silence and again, he continued his hold for a random number; but this time the random number is nearly double the previous one: it is this called back-off (that is to say, the "decline") exponential. In fact, the window collision is simply doubled (unless it has already reached a maximum). From a packet is transmitted successfully, the window will return to its original size.
Again, this is what we do naturally in a meeting room if many people speak exactly the same time, they are realizing account immediately (as they listen at the same time they speak), and they interrupt without completing their sentence. After a while, one of them speaks again. If a new collision occurs, the two are interrupted again and tend to wait a little longer before speaking again.
The entire scheme of CSMA/CD is depicted in the fig.
Frame format of CSMA/CD
The frame format specified by IEEE 802.3 standard contains following fields.
1. Preamble: It is seven bytes (56 bits) that provides bit synchronization. It consists of alternating Os and 1s. The purpose is to provide alert and timing pulse.
2. Start Frame Delimiter (SFD): It is one byte field with unique pattern: 10 10 1011. It marks the beginning of frame.
3. Destination Address (DA): It is six byte field that contains physical address of packet's destination.
4. Source Address (SA): It is also a six byte field and contains the physical address of source or last device to forward the packet (most recent router to receiver).
5. Length: This two byte field specifies the length or number of bytes in data field.
6. Data: It can be of 46 to 1500 bytes, depending upon the type of frame and the length of the information field.
7. Frame Check Sequence (FCS): This for byte field contains CRC for error detection.
Fig. Shows a flow chart for the CSMA/CD protocol.
• The station that has a ready frame sets the back off parameter to zero.
• Then it senses the line using one of the persistent strategies.
• If then sends the frame. If there is no collision for a period corresponding to one complete frame, then the transmission is successful.
• Otherwise the station sends the jam signal to inform the other stations about the collision.
• The station then increments the back off time and waits for a random back off time and sends the frame again.
• If the back off has reached its limit then the station aborts the transmission.
• CSMA/CD is used for the traditional Ethernet.
• CSMA/CD is an important protocol. IEEE 802.3 (Ethernet) is an example of CSMNCD. It is an international standard.
• The MAC sublayer protocol does not guarantee reliable delivery. Even in absence of collision the receiver may not have copied the frame correctly.