Ethernet Operation

Ethernet is a LAN technology that functions at the data link layer. Ethernet uses the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) mechanism to send information in a shared environment. Ethernet was initially developed with the idea that many devices would be connected to the same physical piece of wiring. The acronym CSMA/CD describes the actual process of how Ethernet operates on a shared medium.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980[1] to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET.
Carrier Sense and Multiple Access
In a traditional, or hub-based, Ethernet environment, only one NIC can successfully send a frame at a time. All NICs, however, can simultaneously listen to information on the wire. Before an Ethernet NIC puts a frame on the wire, it will first sense the wire to ensure that no other frame is currently on the wire. If the cable uses copper, the NIC can detect this by examining the voltage levels on the wire. If the cable is fiber, the NIC can detect this by examining the light frequencies on the wire. The NIC must go through this sensing process, since the Ethernet medium supports multiple access—another NIC might already have a frame on the wire. If the NIC doesn’t sense a frame on the wire, it will transmit its own frame; otherwise, if a frame is found on the wire, the NIC will wait for the completion of the transmission of the frame and then transmit its own frame.
Collision Detection
If two or more devices simultaneously sense the wire and see no frame, and each places its frame on the wire, a collision will occur. In this situation, the voltage levels on a copper wire or the light frequencies on a piece of fiber get messed up. For example, if two NICs attempt to put the same voltage on an electrical piece of wire, the voltage level will be different from that of only one device. Basically, the two original frames become unintelligible (or indecipherable). The NICs, when they place a frame on the wire, examine the status of the wire to ensure that a collision does not occur: this is the collision detection mechanism of CSMA/CD.
If the NICs see a collision for their transmitted frames, they have to resend the frames. In this instance, each NIC that was transmitting a frame when a collision occurred creates a special signal, called a jam signal, on the wire. It then waits a small random time period, and senses the wire again. If no frame is currently on the wire, the NIC will then retransmit its original frame. The time period that the NIC waits is measured in microseconds, a delay that can’t be detected by a human. Likewise, the time period the NICs wait is random to help ensure a collision won’t occur again when these NICs retransmit their frames.

Collision detected procedure

  1. Continue transmission until minimum packet time is reached (jam signal) 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 collision
  5. Re-enter main procedure at stage 1.

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