Specifications

Ethernet Data Frames
4-4 Ethernet Data Frames
• Data Field: The data field follows the length field. It is 46 bytes
minimum to a maximum of 1500 bytes in length. This field contains the
actual data being sent across the network along with some control
information. If the data to be sent is less than the minimum 46-byte
packet size, a special bit pattern called PAD is used to fill in up to the
46-byte minimum. The minimum packet size set by the IEEE 802.3
specification is explained below.
• Cyclic Redundancy Check (CRC): The CRC follows the data field and
is 32 bits (4 bytes) in length. Also knows as the Frame Check Sequence
(FCS), this field is used to check the integrity of the frame. Before
placing a frame out on the wire, the sending station takes all the bytes
within the frame, performs a mathematical calculation, and places the
result at the end of the frame. When the frame arrives at the destination,
the receiving station performs the same mathematical calculation and
should receive the same result. If not, it assumes something has been
corrupted and discards the frame.
Data Frame Size
IEEE defines both a minimum and a maximum frame size. The minimum
frame size is 64 bytes (12 address bytes, 2 length bytes, 46 data bytes and 4
CRC bytes). The maximum frame size is 1,518 bytes (same as above with
1,500 byte data field).
The minimum frame size has been determined to give the best bridge
switch speed on heavily used networks by minimizing the amount of time
a station must defer to other transmissions. It also increases the amount of
overhead involved in completing a transmission. The minimum frame
size will move quickly because of its size, but two full size frames move as
much information as 66 minimum size frames containing only 46 bytes of
data each. Therefore, the two large frames require only 36 bytes of
overhead (Preamble, SFD, addresses, etc.) while the small frames require
1,188 bytes of overhead: nearly half the size of the original transmission.
This also doesn’t include the problems involved in trying to transmit 66
times in an operating network with collisions. The minimum frame size
also plays an important role in the detection of collisions and determining
the maximum network size.
In an Ethernet network, a station must still be transmitting its data to
detect that it was involved in a collision. We know that a minimum size
frame is 64 bytes in length which equates to 512 bits (8 bits per byte). We
also know that each bit time in an Ethernet network is defined as 0.1 µs.
Multiplying 512 bits by 0.1 µs yields 51.2 µs to transmit a 64-byte
minimum size frame.