7.5 10-Mbps Configuration Guidelines: Model 2

The interframe gap is a 96 bit time delay provided between frame transmissions to allow the network interfaces and other components some recovery time between frames. As a series of frames travel through a LAN the variable timing delays in network components, combined with the effects of signal reconstruction circuits in the repeaters, can result in a shrinkage of the interframe gap. Too small a gap between frames can overrun the frame reception rate of network interfaces, leading to lost frames. Therefore, it's important to ensure that a minimum interframe gap is maintained.

The network model for checking the interframe gap shrinkage looks a lot like the round trip path delay model, except that it includes a "transmitting end" segment.

FIGURE 7.5 Network model for interframe gap shrinkage

When doing the calculations for interframe gap shrinkage, only the transmitting end and the middle segments are of interest, since signals on these segments must travel through a repeater to reach the receiving end computer. The final segment connected to the receiving DTE does not contribute any gap shrinkage and is not included in the interframe gap calculations.

The following table provides the values used for calculating interframe gap shrinkage.

In a network where the receive and transmit end segments are not the same media type, you use the end segment with the largest number of shrinkage bit times as the "transmitting end" for the purposes of this calculation. This will provide the worst-case value for interframe gap shrinkage. If the total is less than or equal to 49 bit times, the path passes the shrinkage test.

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