Interframe gap

From Wikipedia, the free encyclopedia

Ethernet devices must allow a minimum idle period between transmission of Ethernet frames known as the interframe gap (IFG) or interpacket gap (IPG). It provides a brief recovery time between frames to allow devices to prepare for reception of the next frame. The minimum interframe 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, 960 ns for 100 Mbit/s (fast) Ethernet, and 96 ns for 1 Gbit/s (gigabit) Ethernet.

The interframe gap is also known as interframe spacing. Several adapter manufacturers design their adapters with a smaller interframe gap for higher data transfer rates, which can lead to a high rate of collisions. An example is Intel EtherExpress 100B.

IFG Shrinking is allowed on repeaters towards the DTE and MII (Media Independent Interface) into the MAC (Media Access Control). This is to account for variable network delays, added preamble bits, and clock tolerances.

For 10 Gigabit Ethernet it can be reduced to a period of 40 bit times = 5 bytes
For Gigabit Ethernet it can be reduced to a period of 64 bit times = 8 bytes
For Fast Ethernet shrinking can't be done.
For Ethernet it can be reduced to a period of 47 bit times

IFG is a notable delay when packet-flooding. For example, on a Linux machine, one can perform a packet flood by entering the command ping and then either specifying the -f switch for flood, or by specifying an interval of zero: -i 0. The resulting ping statistics then include ipg/ewma which report the Interpacket Gap and the "Exponential Weighted Moving Average":

raptor:~ # ping wikipedia.org -i 0 -c 5
PING wikipedia.org (66.230.200.100) 56(84) bytes of data.
64 bytes from rr.pmtpa.wikimedia.org (66.230.200.100): icmp_seq=1 ttl=52 time=50.1 ms
64 bytes from rr.pmtpa.wikimedia.org (66.230.200.100): icmp_seq=2 ttl=52 time=47.9 ms
64 bytes from rr.pmtpa.wikimedia.org (66.230.200.100): icmp_seq=3 ttl=51 time=49.5 ms
64 bytes from rr.pmtpa.wikimedia.org (66.230.200.100): icmp_seq=4 ttl=52 time=49.9 ms
64 bytes from rr.pmtpa.wikimedia.org (66.230.200.100): icmp_seq=5 ttl=52 time=47.7 ms

--- wikipedia.org ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 46ms
rtt min/avg/max/mdev = 47.735/49.067/50.130/1.026 ms, pipe 5, ipg/ewma 11.517/49.574 ms
raptor:~ #