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In today's society, where various "things" are connected to the Internet and information is exchanged, countless data such as audio, video, and e-mail are flying around in the Internetwork. Under such circumstances, there are various conventions to ensure that packets are not dropped between the sending device and the receiving device. This time, I will talk about packets and packet transmission intervals.
How is the frame-to-frame transmission interval determined?
Frame configuration of Ethernet
There are "DIX Ethernet (Ethernet II) Frame" and "IEEE802.3 Frame" in the Frame Format. Currently, the most commonly used frame is "DIX Ethernet (Ethernet II) Frame", and when we say Ethernet Frame, it usually means "DIX Ethernet (Ethernet II) Frame". The difference between each Frame is that "SFD" is added after Preamble and "EtherType" is "Size/EtherType", but most network devices can recognize either format. Therefore, there is no problem even if they are mixed.
Various information is stored in one Ethernet Frame according to regulations. The Ethernet Frame that flows over the LAN Cable has the shape shown in the figure below, and the actual data points from the destination MAC address to the end of the FCS. The value is a variable value from 64 bytes to 1518 bytes, and the Ethernet Frame size is expressed in "number of bytes".
In addition, there are Preamble, SFD, and IFG as states in which data is not sent.
- Preamble
In order to notify the receiver of the Ethernet Frame that "data is coming!" and give the timing for synchronization, the Ethernet Frame is
It is a 56Bit (7octet) Bit string of "101010...1010" added to the beginning.
- SFD (Start Frame Delimiter)
"01010111" is added to indicate the end of the preamble, which is the first field of the Ethernet Frame, and the beginning of the Ethernet Frame.
Bit string of 8Bit (1octet).
- IFG (Interframe Gap)
It is the interval from sending an Ethernet Frame to sending the next Ethernet Frame. The minimum value is determined and its value is 12Byte (96Bit). at each communication speed
The specific times are as follows.
10Mbps Ethernet 9.6 μsec
100Mbps Ethernet 0.96 μsec
1Gbps Ethernet 96ns
10Gbps Ethernet 0.96ns
Why IFG is necessary 1 ~ Frame Recovery Time ~
Models with power-saving features are available at low prices for recent network equipment.
As a power saving measure, a low power consumption mode (Sleep Mode) is realized by limiting power supply to unused LAN ports when no signal is being received. In order to return to the normal communication state from this state, the Ethernet Frame receiver side recovers the clock and uses IFG during the time to prepare for frame reception.
Also, when Ethernet frames are sent one after another, the receiving side needs "recovery time" to prepare to accept the next frame. IFG provides time for the receiver to prepare for frame acceptance.
Why IFG is necessary 2 - Ethernet communication is asynchronous communication -
Unlike SONET/SDH, Ethernet communication is an asynchronous communication standard that does not match the clock timing of the data sender and receiver. In the case of asynchronous communication, clock deviation (clock error) occurs because the data sender and receiver operate with their respective clocks. IEEE802.3u Fast Ethernet allows an error of up to ±100ppm with a 125MHz clock, so a maximum error of ±200ppm will occur between adjacent devices. Even with this clock deviation, IFG adjusts so that the receiving device can receive data correctly.
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