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IEEE 802.3 Interframe Spacing

Written by Administrator. Posted in Ethernet Protocol, CSMA/CD, Collisions

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The IEEE 802.3 specification states that before a station can attempt to transmit on the wire, it must first wait until it has heard 9.6 microseconds of silence. Many popular myths have arisen surrounding the reasons for the 9.6 microsecond interframe gap. The purpose of this section is to clarify the true reason for the 9.6 microsecond interframe gap.

The Truth

The sole reason for the 9.6 microsecond interframe gap is to allow the station that last transmitted to cycle its circuitry from transmit mode to receive mode. Without the interframe gap, it is possible that a station would miss a frame that was destined for it because it had not yet cycled back into receive mode.

There is, however, an interesting sidebar to this discussion and that is that most Ethernet cards in today's market are capable of switching from transmit to receive in much less time than 9.6 microseconds. This is an example of what can happen when 1970's specifications are applied to 1990's technology. In fact, some adapter manufacturers are designing their cards with a smaller interframe spacing, thereby achieving higher data transfer rates than their competitors.

The problem arises when cards with a smaller interframe spacing are mixed on a network with cards that meet the specifications. In this case, there is a potential for lost data.

The moral of the story is that a network administrator needs to know what is going on in his or her network and be aware that not all vendors will stick to the specs. Contact your vendors and find out what they're doing differently -- it'll pay off!

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