Interpacket gap

In computer networking, a minimal pause may be required between network packets or network frames. This time between packets is known as the interpacket gap (IPG), interframe spacing, or interframe gap (IFG). Depending on the physical layer protocol or encoding used, the pause may be necessary to allow for receiver clock recovery, permitting the receiver to prepare for another packet (e.g. powering up from a low-power state) or another purpose.

Ethernet

Ethernet devices must allow a minimum idle period between transmission of Ethernet packets.[1] A brief recovery time between packets allows devices to prepare for reception of the next packet. While some physical layer variants literally transmit nothing during the idle period, most modern ones continue to transmit an idle pattern signal. The standard minimum interpacket gap for transmission is 96 bit times (the time it takes to transmit 96 bits of data on the medium), which is

Some manufacturers design adapters with a smaller interpacket gap for slightly higher data transfer rates. That can lead to data loss when mixed with standard adaptors.[2][3]

On reception, some interpacket gaps may be smaller due to variable network delays, clock tolerances, and the presence of repeaters (10 and 100 Mbit/s only).[1]

  • For 40, 50, 100, 200, and 400 Gigabit Ethernet received IPG can be reduced to a period of 8 bit times (1 byte).
  • For 2.5, 5, 10, and 25 Gigabit Ethernet received IPG can be reduced to a period of 40 bit times (5 bytes).
  • For Gigabit Ethernet received IPG can be reduced to a period of 64 bit times (8 bytes).
  • For Fast Ethernet received IPG reduction is not specified. Standard is 96 bit times (12 bytes).
  • For Classic Ethernet received IPG can be reduced to a period of 47 bit times.

Fibre Channel

For Fibre Channel, there is a sequence of primitives between successive frames, sometimes called interframe gap as well. The minimum sequence consists of six primitives, IDLE|IDLE|R_RDY|R_RDY|IDLE|IDLE.[4] Each primitive consists of four channel words of 10 bits each for 8b/10b encoded variants (1–8 Gbit/s), equivalent to four data bytes.[5]

References
  1. "IEEE 802.3 4.4.2 MAC parameters". Retrieved 2013-12-24.
  2. "Interframe Gap and Spacing". WildPackets. Retrieved 2015-03-20.
  3. "Intel EtherExpress 100B - High rate of collisions on 100-megabit networks". Microsoft co. Retrieved 2010-01-16.
  4. FC-PH REV 4.3, June 1, 1994, Clause 17.1 Frame Transmission
  5. FC-PH REV 4.3, June 1, 1994, Table 25 Primitive Signals
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