Technology Overview
Technology: Ethernet Frames
Basic Ethernet Frame
•
Preamble and SFD
(Start Frame Delimiter). Preceding each frame is a
preamble of 7 bytes and a 1-byte SFD. The preamble is a pattern of
alternating 1s and 0s (10101010) for all 7 bytes. The SFD has a pattern of
10101011. The preamble allows devices to detect and synchronize to
incoming Ethernet frames; the SFP marks the end of the preamble. For the
purposes of calculating frame lengths, the 8 bytes of Preamble and SFP are
not included.
•
Ethernet frame: C
onsists of a MAC (Media Access Control ) header, followed
by the frame payload, and ends with a FCS (Frame Check Sequence).
•
MAC header
:
14-bytes- consists of a 6-byte Destination Address, a 6-byte
Source Address, and a 2-byte Ethertype field (see
).
•
Ethertype field: U
sed as a frame length indicator or as protocol indicator,
depending on which Ethernet
is being used. IEEE 802.2 uses the
field to indicate the frame length (in hex). The DIX or Ethernet II standard
uses the field to indicate the type of data being transmitted. In most IP-based
applications, the Ethernet II standard is used and the field is set to an
Ethertype of 0x0800 to indicate an IP version 4 payload. Ethertype values:
http://standards.ieee.org/regauth/ethertype/eth.txt
•
Payload Size
: The minimum payload size is 46 bytes.
Frames with fewer payload bytes are considered undersized.
The minimum Ethernet frame size is 64 bytes.
The maximum frame size is 1518 bytes.
Frame sizes above 1518, called jumbo frames, are allowed by some systems,
and are an effective means of increasing the efficiency of the network.
The presence of VLAN tags changes the effective minimum and maximum
frame sizes .
•
FCS
: A 4-byte CRC performed over the entire Ethernet frame. Sometimes the
FCS is called the CRC field.
To avoid confusion, it is sometimes written as the FCS/CRC field. When an
Ethernet device receives a frame, it performs a CRC calculation and
compares it to the frame’s FCS field. If they match, the frame is processed.
If they do not match, the frame is discarded. Due to the limits of the error-
checking capabilities of a 4-byte CRC, the largest practical size for an
Ethernet frame is roughly 12,000 bytes.
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Summary of Contents for XTT 5000
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