Interpacket gap
In computer networking, a minimal pause may be required between network packets or network frames. 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 known as the interpacket gap (IPG), interframe spacing, or interframe gap (IFG).[1] A brief recovery time between packets allows devices to prepare for reception of the next packet. The standard minimum interpacket gap is 96 bit times (the time it takes to transmit 96 bits of raw data on the medium), which is
- 9.6 µs for 10 Mbit/s Ethernet,
- 0.96 µs for 100 Mbit/s (Fast) Ethernet,
- 96 ns for gigabit Ethernet,
- 9.6 ns for 10 Gigabit Ethernet and
- 0.96/2.4 ns for 100/40 Gigabit Ethernet, respectively.[1]
Some adapter manufacturers design their adapters with a smaller interpacket gap for higher data transfer rates,[2] which can lead to a high rate of collisions. An example is Intel EtherExpress 100B.[3]
Smaller IPG are possible on repeaters towards the DTE (Data terminal equipment) and MII (Media Independent Interface) into the MAC (Media Access Control), that is when the packet is received by a node. This is to account for variable network delays, added preamble bits, and clock tolerances.
On reception, some interpacket gaps may be smaller due to variable network delays, clock tolerances, and the presence of repeaters.[1]
- For 40/100 Gigabit Ethernet received IPG can be reduced to a period of 8 bit times (1 byte).
- For 10 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 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 sequence consists of at least 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).[5]
References
- 1 2 3 "IEEE 802.3-2012 4.4.2 MAC parameters". Retrieved 2013-12-24.
- ↑ "Interframe Gap and Spacing". WildPackets. Retrieved 2015-03-20.
- ↑ "Intel EtherExpress 100B - High rate of collisions on 100-megabit networks". Microsoft co. Retrieved 2010-01-16.
- ↑ FC-PH REV 4.3, June 1, 1994, Clause 17.1 Frame Transmission
- ↑ FC-PH REV 4.3, June 1, 1994, Table 25 Primitive Signals