Carrier Sense Multiple Access / Collision Detection (CSMA/CD) is a very basic and important protocol used in Ethernet and Local Area Networks (LAN) to detect the mingling of signals between multiple servers, clients and routers. This control scheme is built into every Ethernet router to detect and possibly avoid (using Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA) protocol) a collision of signals and frames between multiple Ethernet servers. It is simply a backup procedure of multiple Ethernet routers for effective data and frame transfer between router to client, with minimal data loss and time loss.
Methods of collision detection vary from media to media, but on the electrical wiring of Ethernet networks, the basic idea is to see if any other router is transmitting in the same wire at the same time. If the answer is yet, a jamming signal is sent to both the routers and after a random delay, they both try to send again. This process is repeated till the data is eventually sent.
The basic steps of the CSMA/CD procedure are as follows, in a simplified algorithm.
1. Start: There is data to be sent by the user. The user inputs and codifies this data into the stream.
2. Frame Assembly: MAC addresses and physical addresses are used to transform the data into a frame for proper transmission through the Ethernet layers.
3. Attempt to transmit: The router attempts to transmit the data into the data stream, and increments the iterative statement of the number of tries.
4. Check – is another router transmitting? This checks through the electrical voltage to see if another data packet is being sent. If yes, then it waits for a short while and goes to step 3.
5. If no, then the router transmits a small packet of the frame into the stream and waits.
6. Check – collision detected? The router checks if another router has started off its data packet at the same time. If yes then it runs the collision recovery algorithm that tries to revert to its original state. If that fails, then the data sending is failed.
7. If no, then transmit the next bit of the frame and go to step 6 till the entire frame is sent.
8. End the sending process.
Collisions are usually detected by a standard procedure known as collision Detect that provides a signal in the physical layer for the same process. This greatly reduces the loss of data through collisions in a large Ethernet ring.
