Minimum Frame Length and Maximum Transmission Distance
Due to the limitation of the CSMA/CD algorithm, an Ethernet frame must be longer than or equal to a specified length. On the Ethernet, the minimum frame length is 64 bytes, which is determined jointly by the maximum transmission distance and the collision detection mechanism.
The use of minimum frame length can prevent the following situation: station A finishes sending the last bit, but the first bit does not arrive at station B, which is far from station A. Station B considers that the line is idle and begins to send data, leading to a collision.
The upper layer protocol must ensure that the Data field of a packet contains at least 46 bytes, so that the total length of the Data field, the 14-byte Ethernet frame header, and the 4-byte check code at the frame tail can reach the minimum frame length, as shown in Figure 1-2. If the Data field is less than 46 bytes, the upper layer must pad the field to 46 bytes.