It uses carrier -sensing to defer transmissions until no other stations are transmitting. This is used in combination with collision detection in which a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame.
When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame. The following procedure is used to initiate a transmission.
The procedure is complete when the frame is transmitted successfully or a collision is detected during transmission. The following procedure is used to resolve a detected collision. The procedure is complete when retransmission is initiated or the retransmission is aborted due to numerous collisions.
Methods for collision detection are media dependent. On a shared, electrical bus such as 10BASE5 or 10BASE2collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus.
The collision recovery procedure 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.
The jam signal or jamming signal is a signal that carries a bit binary pattern sent by a data station to inform the other transmitting stations of the collision and that they must not transmit. The maximum jam-time is calculated as follows: The maximum allowed diameter of an Ethernet installation is limited to bits.
This makes a round-trip-time of bits. As the slot time in Ethernet is bits, the difference between slot time and round-trip-time is 48 bits 6 byteswhich 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 bit combinations.
Note: The size of this jam signal is clearly above 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 bit MAC CRC, this causes the other receivers to discard the frame due to a CRC error.
A late collision is a type of collision that happens further into the packet than is allowed for by the protocol standard in question. In megabit shared-medium Ethernet, if a collision error occurs after the first bits of data are transmitted by the transmitting station, [11] a late collision is said to have occurred.
Importantly, late collisions are not re-sent by the NICunlike collisions occurring before the first 64 octets; it is left for the upper vilken nätverkstyp använder sig av csma cd of the protocol stack to determine that there was loss of data. A local collision is a collision that occurs at the NICas opposed to on the wire.
A NIC cannot detect local collisions without attempting to send information. On UTP cable, a local collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair.