All the data transported over an Ethernet is carried in a packet that conforms to the standardized format. For all aims and functions, this packet format defines Ethernet. It has persisted since the early days and offers commonality across all the different Ethernet flavors.
The packet preamble is normally generated by the Ethernet hardware, which also adds the frame check sequence or checksum, a redundant series of bits that guarantees data integrity during transmission. Software is responsible for putting the destination and source addresses, as well as the data that is transported in the frame’s payload.
At the beginning of every frame, there is the preamble, which is a series of flipping ones and zeroes that can be used by the Ethernet receiver to acquire bit synchronization. Next comes the start of frame delimiter, a series of flipping ones and zeroes that ends with two consecutive ones and is used to get byte alignment.
The address information follows. The first part of this is the destination Ethernet address, which shows the address of the intended receiver. If this field is set to all ones, then the content is broadcast to all attached stations. The next part of address information is the source Ethernet address. This is the globally unique Ethernet address of the sending device (that is, the unique identity of the PC, workstation, router, or whatever other device initiates communication).
There are two interpretations for the next field-it can denote either message length or message type field. The reason for this duality is that the IEEE ethernet standard is slightly different from the original, proprietary spec from Xerox. The latter did not need a length field because all of the vendor protocols that used it (XNS, DECnet, IPX, and IP) had their own length fields. However, the IEEE committee needed a standard that did not depend on the good behavior of other communications protocols, so they substituted the two-byte type field with a two-byte length field.
The reason the two definitions of this field can coexist is that Xerox had not assigned any upper layer protocol-type values below the decimal value of 1,500. Since the maximum length of an Ethernet frame is 1,500 bytes, all possible length values can be set without any conflict or overlap. Hence, any Ethernet frame with a type/length field less than 1,500 is in IEEE format (with length set as a value between 64 and 1,500). Any frame in which the field’s value is greater than 1,500 must follow the Xerox format (with predefined type values such as 0×800 for IP packets or 0×600 for DECnet).
The actual information sent over the network follows next in the data field. This part of the frame is where an IP packet (or any other type of data) would be carried. The data field can be up to 1,500 bytes in length-and this sets the upper limit on the amount of data that can be transported inside any one frame. There is also a minimum frame size, so the data field is padded up to 46 bytes if needed.
There is good reason for defining maximum and minimum frame sizes. If the frame is too long, it can block other users from getting fair access-other users continuously detect a potential collision and thus back off from sending. If the frame length is too short, the last bit of it can leave the sender before the first bit has arrived at the recipient, thereby making it difficult to test when the network is free for use. The shortest Ethernet frame is 6 + 6 + 2 + 46 + 4 = 64 bytes and the longest frame is 6 + 6 + 2 + 1,500 + 4 = 1,518 bytes.
The last field is a frame check sequence. This is a 32-bit cyclic redundancy check that operates on the whole frame (except for the preamble and itself). It serves to let the receiver of the frame test know whether any errors have occurred in its transmission.
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