In traditional Ethernet, why the minimum frame length and maximum frame length limit

Why is there a limit to the minimum frame length and maximum frame length in traditional Ethernet?

Ethernet (IEEE 802.3) frame format:

1, Preamble code: 7 byte 0x55, a string of 1, 0 interval, for signal synchronization

2, Frame starting delimiter: 1 bytes 0xd5 (10101011), indicating the beginning of a frame

3. DA (Destination mac): 6 bytes

4, SA (Source mac): 6 bytes

5, type/Length: 2 bytes, 0~1500 reserved as Length field value, 1536~65535 reserved as Type field value (0X0600~0XFFFF)

6. Data: 46~1500 bytes

7. Frame Check Sequence (FCS): 4 bytes, using CRC to calculate the checksum of the content from the destination Mac to the data field. A class of LANs with CSMA/CD as the MAC algorithm is called Ethernet. CSMA/CD Conflict Avoidance method: First listen to the post, edge-to-ear, random delay after the re-hair. Once a conflict occurs, each host must be detected. The minimum send gap and minimum frame length are also required to avoid collisions.

Consider the following scenario, the host sends a small frame, and the two conflicting hosts are far apart. B begins sending a frame when the frame transmitted by host a transmits to the first moment of B. Thus, when A's frame reaches B, b detects a conflict and sends a conflict signal. If A's frame has already been sent before the conflict signal in B is transmitted to a, then a will not detect the conflict and mistakenly think it has been sent successfully. Because the signal propagation is time-delay, it also takes a while to detect the conflict. This is also why there must be a minimum frame length limit.

by standard, 10Mbps Ethernet with repeaters, the maximum length of the connection is 2500 meters, up to 4 repeaters, so the minimum transmission time for a frame of 10Mbps Ethernet is 51.2 microseconds. The data that can be transmitted during this time is 512 bits, so it is also known as 512-bit time. This time is defined as an Ethernet timeslot, or a collision time slot. 512 bits = 64 bytes, which is the reason for the minimum 64 bytes of Ethernet frames.

512-bit is the time when the host captures the channel. If a host sends 64 bytes of a frame without conflict, there will be no further conflicts, which is called the host capturing the channel.

Because the channel is shared by all hosts, if the data frame is too long, the host will not be able to send the data for a long time, and some of the sending data may exceed the buffer size at the receiving end, causing buffer overflow. To avoid a single host occupying a channel for too long, the maximum frame length for an Ethernet frame is 1500.

When the time slot of the 100Mbps Ethernet is still 512 bits, the Ethernet specifies that the minimum transmit time for a frame must be 5.12μs. The time slot of the 1000Mbps Ethernet is increased to 512 bytes, or 4,096 bits, 4.096μs.

For 1000MB/S Gigabit Ethernet, the MAC layer has two options, either preserving the CSMA/CD or not using it. If the CSMA/CD protocol is retained, the collision detection problem must be faced, which will reduce the maximum effective transmission distance of the network to 25 meters. Of course, you can not shorten the distance of the network, but increase the degree of a frame, as we began to analyze 100mb/s Ethernet, so that a frame lasts long enough time. But because there is not so much data on the top, there is a need to populate the MAC layer with useless data to satisfy this requirement.

Finally, we derive the following proportional relationships:

Minimum frame length/transfer rate proportional to network maximum transmission distance/speed of light

The maximum transmission distance for a network is also known as the conflict domain, and the time taken to transmit a minimum frame (minimum frame length/transfer rate) is proportional to the time that 1 bits of information traverses the conflict domain.

The maximum frame length of the Ethernet is 1518,1522,1536. So how did these values come about?

The following is the Ethernet frame structure defined for the original 802.3:

Preamble (7-bytes)--leading bit

Start Frame Delimiter (1-byte)--delimiter

Dest. MAC Address (6-bytes)--Destination address

SOURCE MAC Address (6-bytes)--Origin address

Length/type (2-bytes)--length or frame type

MAC Client Data--the maximum of 1500, or MTU

(0-n bytes) Pad--fill field, p Max is 46

(0-p bytes) Frame Check Sequence (4-bytes)--Frames Check code

In fact, Ethernet V2 as the actual standard. So our most common is that the source address is the type, not the length.

In the 802.3AC proposed in 1998, support for VLANs was added. That is, in the case of a VLAN with the source address after adding more than 4 bytes, so there is 1522 this frame length.

Preamble (7-bytes)--leading bit

Start Frame Delimiter (1-byte)--delimiter

Dest. MAC Address (6-bytes)--Destination address

SOURCE MAC Address (6-bytes)--Origin address

Length/type = 802.1Q Tag Type (2-byte)--Indicates VLAN, this is 8100

Tag Control Information (2-bytes)--3-bits User priority field;1-bit Canonical Format Indicator (CFI); 12-bits VLAN identif IER (VID)

Length/type (2-bytes)--length or frame type

MAC Client Data--the maximum of 1500, or MTU

(0-n bytes) Pad--fill field, p Max is 46

(0-p bytes) Frame Check Sequence (4-bytes)--Frames Check code

The last thing to mention is the length of 1536. We know that 802.3 specifies that if the value of Length/type is greater than 0x600, it is a type, and this value is 1536.

Application in the 802.3z proposed in 1998. 802.3 presents a extension field at the very end. Its main role is in the short package (such as 64) transmission, due to the acceleration of the 1000M speed, resulting in a reduction in transmission time, the destruction of the original conflict detection mechanism, shorten the effective transmission moment away, so to increase some bytes, so that the length reached 512. Of course, this is only used in half a pair of hours.

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In fact, in 802.3 does not specify the size of the value to expand, so the maximum can be extended to 1536.

There is also a 802.1ad. The so-called "Q-in-q", is the multiple VLAN, mainly for network operators to manage, to realize the user's VLAN and carrier VLAN isolation. This will also increase the length of the existing packet, adding 4 bytes. Ethernet Maximum frame length min. frame Length-sacrifice-babyunion

There is also a jumbo frame. This thing is not the standard of 802.3, and some manufacturers proposed to improve the transmission efficiency of 1000M. Because they are non-standard, the size of the support varies. In general, it is now generally possible to support 9k. A frame bursting is also presented in the 802.3z. This thing can improve the transmission efficiency of short packets. Under 1000M Ethernet, limit ". The gap is filled with extension bits between the frame and the frame to indicate that the line is in a occupied state. Burst mode The only special is that the first frame is added with a "extension field".

New changes can be found in 802.3as-2006.

With burst mode, one end can be continuously contracted until the 65,536 bit times (8192 byte times) is reached burst

In traditional Ethernet, why the minimum frame length and maximum frame length limit