Page 73 - M. Abrahim Thesis

Computer Network 2021

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:

• On a 10 Mbps Ethernet, 1 bit at the MAC sublayer requires 100 nanoseconds (ns)
to transmit.
• At 100 Mbps, that same bit requires 10 ns to transmit.

• At 1000 Mbps, it takes only 1 ns.

Table 3-2 summarizes the bit-time of different types of Ethernets.

Table 3-2 shown the bit-time of different types of Ethernet

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