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6.1 | |
Ethernet Fundamentals | |
| 6.1.7 | |
Ethernet frame fields |
Some of the fields permitted or required
in an 802.3 Ethernet Frame are:
The Preamble is an alternating pattern
of ones and zeroes used for timing synchronization in the asynchronous
10 Mbps and slower implementations of Ethernet. Faster versions of
Ethernet are synchronous, and this timing information is redundant but
retained for compatibility.
A Start Frame Delimiter consists of a one-octet field that marks the end of the timing information, and contains the bit sequence 10101011. The Destination Address field contains the MAC destination address. The destination address can be unicast, multicast (group), or broadcast (all nodes). The Source Address field contains the MAC source address. The source address is generally the unicast address of the transmitting Ethernet node. There are, however, an increasing number of virtual protocols in use that use and sometimes share a specific source MAC address to identify the virtual entity. The Length/Type field supports two different uses. If the value is less than 1536 decimal, 0x600 (hexadecimal), then the value indicates length. The length interpretation is used where the LLC Layer provides the protocol identification. The type value specifies the upper-layer protocol to receive the data after Ethernet processing is completed. The length indicates the number of bytes of data that follows this field. If the value is equal to or greater than 1536 decimal (0600 hexadecimal), the value indicates that the type and contents of the Data field are decoded per the protocol indicated. The Data and Pad field may be of any length that does not cause the frame to exceed the maximum frame size. The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size. The content of this field is unspecified. An unspecified pad is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length. Ethernet requires that the frame be not less than 46 octets or more than 1518 octets. A FCS contains a four byte CRC value that is created by the sending device and is recalculated by the receiving device to check for damaged frames. Since the corruption of a single bit anywhere from the beginning of the Destination Address through the end of the FCS field will cause the checksum to be different, the coverage of the FCS includes itself. It is not possible to distinguish between corruption of the FCS itself and corruption of any preceding field used in the calculation.
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