A subtle consideration in Ethernet is the minimum packet size
P
min
P
min
.
The time required to transmit such packets equals
P
min
C
P
min
C
, where CC is the
Ethernet's capacity in bps. Ethernet now comes in two different
types, each with individual specifications, the most
distinguishing of which is capacity: 10 Mbps and
100 Mbps. If the minimum transmission time is such that the
beginning of the packet has not propagated the full length of
the Ethernet before the end-of-transmission, it is possible that
two computers will begin transmission at the same time and, by
the time their transmissions cease, the other's packet will not
have propagated to the other. In this case, computers in-between
the two will sense a collision, which renders both computer's
transmissions senseless to them, without the two transmitting
computers knowing a collision has occurred at all! For Ethernet
to succeed, we must have the minimum packet transmission time
exceed twice the voltage propagation time.
P
min
C>2Lc
or
P
min
>2LCc
P
min
C
2
L
c
or
P
min
2
L
C
c
(1)
Thus, for the 10 Mbps Ethernet having a 1 km maximum
length specification, the minimum packet size is about 70 bits.
The 100 Mbps Ethernet was designed more
recently than the 10 Mbps alternative. To maintain the
same minimum packet size as the earlier, slower version,
what should its length specification be? Why should the
minimum packet size remain the same?
The cable must be a factor of ten shorter: It cannot exceed
100 m. Different minimum packet sizes means different
packet formats, making connecting old and new systems
together more complex than need be.