Page 73 - Handout Computer Network.

• The actual data payload

• A checksum (FCS) used to ensure that the message was not corrupted along the
way

Stations receiving the frame recalculate the FCS to determine whether the incoming
message is valid, and hand good messages to the next higher layer in the protocol stack.

For 10-Mbps Ethernet and slower versions, which are asynchronous, each receiving
station uses the eight octets of timing information to synchronize its receive circuit to the
incoming data but then discard it. The 100 Mbps higher-speed implementations of Ethernet
are synchronous, so the timing information is not actually required at all. However, for
compatibility reasons, the preamble and SFD are present. All information following the SFD at
the end of the timing information is passed to the next higher layer.
A new checksum is calculated and compared with the checksum found at the end of
the received frame. If the frame is intact, it then must be interpreted according to the rules
for whichever protocol is indicated by the Length/Type field or the LLC-layer protocol indicated
by the first few octets of the data. A notable number of changes to the basic structure of
Ethernet were included in the 1998 and 2000 versions of the standard.
One significant change was that two-octet addresses explicitly were excluded,
although they were included in all previous versions. When used for Length, the maximum
value for the
802.3 Length/Type field clearly was specified as 1536 (600 hex), where it previously was
assumed to match the maximum MTU data size of 1500 (5DC hex).

For all speeds of Ethernet transmission at or below 1000 Mbps, the standard describes
how a transmission can be no smaller than slot time.

Slot time for 10 and 100 Mbps Ethernet is 512 bit-times (64 octets). Slot time for 1000
Mbps Ethernet is 4096 bit-times (512 octets, including the extension). Slot time is not defined
for 10 Gbps Ethernet because it does not permit half-duplex operation. Slot time is just longer
than the longest possible round-trip delay time when maximum cable lengths are used on the
largest legal network architecture and all hardware propagation delay times are at the legal
maximum; the 32-bit jam signal is used when collisions are detected.
In other words, slot time is just longer than the time it theoretically can take to go from
one extreme end of the largest legal Ethernet collision domain to the other extreme end,
collide with another transmission at the last possible instant, and then have the collision
fragments return to the sending station and be detected.
For the system to work, the first station must learn about the collision before it finishes
sending the smallest legal frame size. To allow 1000 Mbps Ethernet to operate in half duplex,
the Extension field was added when sending small frames, purely to keep the transmitter busy
long enough for a collision fragment to make it back.
This field is present only on 1000 Mbps half duplex links, and it allows minimum-size
64-octet frames to be stretched long enough to meet slot-time requirements. Extension bits
are discarded by the receiving station.
To examine the issue briefly, consider the following:

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