Interframe spacing and backoff

Interframe spacing and backoff
6.2.4 This page explains how spacing is used in an Ethernet network for data transmission.


The minimum spacing between two non-colliding frames is also called the interframe spacing. This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame.

After a frame has been sent, all stations on a 10-Mbps Ethernet are required to wait a minimum of 96 bit-times (9.6 microseconds) before any station may legally transmit the next frame. On faster versions of Ethernet the spacing remains the same, 96 bit-times, but the time required for that interval grows correspondingly shorter. This interval is referred to as the spacing gap. The gap is intended to allow slow stations time to process the previous frame and prepare for the next frame.

A repeater is expected to regenerate the full 64 bits of timing information, which is the preamble and SFD, at the start of any frame. This is despite the potential loss of some of the beginning preamble bits because of slow synchronization. Because of this forced reintroduction of timing bits, some minor reduction of the interframe gap is not only possible but expected. Some Ethernet chipsets are sensitive to a shortening of the interframe spacing, and will begin failing to see frames as the gap is reduced. With the increase in processing power at the desktop, it would be very easy for a personal computer to saturate an Ethernet segment with traffic and to begin transmitting again before the interframe spacing delay time is satisfied.

After a collision occurs and all stations allow the cable to become idle (each waits the full interframe spacing), then the stations that collided must wait an additional and potentially progressively longer period of time before attempting to retransmit the collided frame. The waiting period is intentionally designed to be random so that two stations do not delay for the same amount of time before retransmitting, which would result in more collisions. This is accomplished in part by expanding the interval from which the random retransmission time is selected on each retransmission attempt. The waiting period is measured in increments of the parameter slot time.

If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer. Such an occurrence is fairly rare and would happen only under extremely heavy network loads, or when a physical problem exists on the network.

The next page will discuss collisions.