• 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.
• Additional feature in CSMA/CD is that the stations can detect the collisions. The stations abort their transmission as soon as they detect a collision. In CSMA this feature is not present. The stations continued their transmission even though they find that the collision has occurred. This leads to the wastage of channel time.
• However this problem is handled in CSMA/CD. In CSMA/CD, the station that places its data onto the channel after sensing the channel continues to sense the channel even after the data transmission. If collision is detected, the station aborts its transmission and waits for predetermined amount of time & then sends its data again.
• As soon as a collision is detected, the transmitting station releases a jam signal.
• Jam signal will alert the other stations. The stations are not supposed to transmit immediately after the collision has occurred. Otherwise there is a possibility that the same frames would collide again.
• After some back-off delay time the stations will retry the transmission. If the collision occurs again then the back off delay time is increased progressively.
• Therefore the CSMA/CD method consists of alternating transmission period and collisions with idle periods when none of the stations is transmitting.
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.