Interframe Gap

The Truth About Interframe Spacing

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 document is to clarify the true reason for the 9.6 microsecond interframe gap.

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 today’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 on 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!

• Frame Formats
  The four ways that frames may be structured
• Media Access
  Taking turns accessing the cable using the rules of Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
• Collisions
  The results of simultaneous transmissions on the media: Fragments, Runts, CRC errors
• Propagaton Delay
  The relationship between maximum cable length and minimum frame size is based on the propagation delay of the signal
• Frame Corruption
  Troubleshooting coaxial Ethernet networks by examining the types of corruption patterns that result from specific events
• Interframe Gap
  The 9.6 microsecond interframe gap and an understanding of its purpose
• Signal Encoding
  Manchester Encoding for the electrical Ethernet signal