Specifications

Ethernet Data Frames 4-7
Ethernet Data Frames
The IEEE 802.3 frame format is almost identical to the Ethernet II format.
The only difference is that a length field is used in place of the type field.
This field indicates the length of the data portion of the 802.3 frame, with
the maximum length being 1,518 decimal.
A network device can decipher the difference between an 802.3 “raw”
frame and an Ethernet II frame by looking at this portion of the packet (the
length or type field). As it turns out, the assigned values for the Ethernet II
type field are always greater than 1,500 decimal. Since the maximum
frame size for Ethernet is 1,518 bytes, with 18 bytes of overhead, the length
field always contains a value less than that.
Ethernet 802.2 Frame Type
Without a protocol type field, it is impossible to determine what protocol
to use for interpreting the encapsulated data in an 802.3 “raw” frame. If
more than one upper-layer protocol exists on the network, the packet may
be incorrectly routed. Therefore, sometime after the 802.3 standard was
released, IEEE came out with the 802.2 standard. Table 4-3 describes each
of the Ethernet 802.2 frame fields.
Table 4-2. Ethernet “Raw” Frame Type
Field Name Field Size Field Definition
Preamble 7 bytes Signals beginning of the frame.
Start Frame
Delimiter
1 byte Signals start of data.
Destination
Address
6 bytes
Contains the address of the destination of
the frame.
Source Address 6 bytes
Contains the address of the frame’s
origin.
Length Field 2 bytes Specifies the length of the data field.
Data 46–1500 bytes Contains the data to be transferred.
Frame Check
Sequence
4 bytes Determines the integrity of the frame.