Carrier sense multiple access with collision detection

Simplified Algorithm of CSMA/CD

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.^[1]

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.

Main procedure

1. Is my frame ready for transmission? If yes, it goes on to the next point.
2. Is medium idle? If not, wait until it becomes ready^{[note 1]}
3. Start transmitting.
4. Did a collision occur? If so, go to collision detected procedure.
5. Reset retransmission counters and end frame transmission.

Collision detected procedure

1. 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.
2. Increment retransmission counter.
3. Was the maximum number of transmission attempts reached? If so, abort transmission.
4. Calculate and wait random backoff period based on number of collisions.
5. 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 10BASE-5 or 10BASE-2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus.

Jam signal

The jam signal is a signal that carries a 32-bit binary pattern sent by a data station to inform the other stations that they must not transmit.

The maximum jam-time is calculated as follows: The maximum allowed diameter of an Ethernet installation is limited to 232 bits. This makes a round-trip-time of 464 bits. As the slot time in Ethernet is 512 bits, the difference between slot time and round-trip-time is 48 bits (6 bytes), which is the maximum "jam-time".

This in turn means: A station noting a collision has occurred is sending a 4 to 6 byte long pattern composed of 16 1-0 bit combinations. Note: The size of this jam signal is clearly beyond the minimum allowed frame-size of 64 bytes.

The purpose of this is to ensure that any other node which may currently be receiving a frame will receive the jam signal in place of the correct 32-bit MAC CRC, this causes the other receivers to discard the frame due to a CRC error.

Applications

CSMA/CD was used in now obsolete shared media Ethernet variants (10BASE5, 10BASE2) and in the early versions of twisted-pair Ethernet which used repeater hubs. Modern Ethernet networks, built with switches and full-duplex connections, no longer need to utilize CSMA/CD because each Ethernet segment, or collision domain, is now isolated. CSMA/CD is still supported for backwards compatibility and for half-duplex connections. IEEE Std 802.3, which defines all Ethernet variants, for historical reasons still bears the title "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications".

Variations of the concept are used in radio frequency systems that rely on frequency sharing, including Automatic Packet Reporting System.

See also

• Carrier sense multiple access with collision avoidance (CSMA/CA)
• Late collision
• Ethernet

Notes

References

•  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
• IEEE 802.3