CSMA/CD (Carrier Sense Multiple Access with Collision Detection):
The IEEE 802.3 defines the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as the standard access method for traditional Ethernet. Stations in a traditional Ethernet can be connected using a physical bus or star topology, but the logical topology is always a bus. It means that the medium is shared and only one station can use it at a single time. But the problem is the assurance that only one station is using the medium at a time is solve d as follows,
- Every station has an equal right on the medium.
- Every station senses the medium before sending a frame and if the medium is idle only then the station sends the frame.
- It may happen, that two stations senses the medium at a time and find it idle and send frame. This time a collision will occur. That’s why the protocol forces the stations to sense the medium even after the sending has begun and the stations are able to sense the collision. After sensing the collision the stations send a jam signal which destroys the frames on the line and the stations wait for some random amount of time and re-send their frames avoiding the collision.
Three factors are related to CSMA/CD;
- The Minimum Frame Length: The time the station needs to wait to be sure that there is no data on the medium is the minimum frame length.
- The Data Transmission Rate: The time required to send out the minimum frame length.
The Collision Domain: The maximum network distance.
(Minimum frame length / Transmission rate) ∝ (Collision domain / Propagation speed)