The Ethernet architecture consists
of a single coaxial cable terminated at either end by a resistor
having a value equal to the cable's characteristic
impedance. Computers attach to the Ethernet through an
interface known as a transceiver because it
sends as well as receives bit streams represented as analog
voltages.
Ethernet uses as its communication medium a single length of
coaxial cable (). This
cable serves as the "ether", through which all digital
data travel. Electrically, computers interface to the coaxial
cable () through a device
known as a transceiver. This device is
capable of monitoring the voltage appearing between the core
conductor and the shield as well as applying a voltage to
it. Conceptually it consists of two op-amps, one applying a
voltage corresponding to a bit stream (transmitting data) and
another serving as an amplifier of Ethernet voltage signals
(receiving data). The signal set for Ethernet resembles that
shown in BPSK
Signal Sets, with one signal the negative of the
other. Computers are attached in parallel, resulting in the
circuit model for Ethernet shown in .
From the viewpoint of a transceiver's
sending op-amp, what is the load it sees and what is the
transfer function between this output voltage and some other
transceiver's receiving circuit? Why should the output
resistor
Rout be large?
The transmitting op-amp sees a load or
∥RoutZ0RoutN, where N is the
number of transceivers other than this one attached to the
coaxial cable. The transfer function to some other
transceiver's receiver circuit is
Rout divided by this load.
'transceiver'
The top circuit expresses a
simplified circuit model for a transceiver. The output
resistance
Rout must be much larger than
Z0
so that the sum of the various transmitter voltages add to
create the Ethernet conductor-to-shield voltage that serves
as the received signal
rt
for all transceivers. In this case, the equivalent circuit
shown in the bottom circuit applies.
No one computer has more authority than any other to control
when and how messages are sent. Without scheduling authority,
you might well wonder how one computer sends to another without
the (large) interference that the other computers would produce
if they transmitted at the same time. The innovation of Ethernet
is that computers schedule themselves by a random-access
method. This method relies on the fact that all
packets transmitted over the coaxial cable can be
received by all transceivers, regardless of
which computer might actually be the intended recipient. In
communications terminology, Ethernet directly supports
broadcast. Each computer goes through the following steps to
send a packet.
The computer senses the voltage across the cable to
determine if some other computer is transmitting.
If another computer is transmitting, wait until the
transmissions finish and go back to the first step. If the
cable has no transmissions, begin transmitting the packet.
If the receiver portion of the transceiver determines
that no other computer is also sending a packet, continue
transmitting the packet until completion.
On the other hand, if the receiver senses interference
from another computer's transmissions, immediately
cease transmission, waiting a random amount of time to
attempt the transmission again (go to step 1) until
only one computer transmits and the others defer. The
condition wherein two (or more) computers'
transmissions interfere with others is known as a
collision.
The reason two computers waiting to transmit may not sense the
other's transmission immediately arises because of the
finite propagation speed of voltage signals through the coaxial
cable. The longest time any computer must wait to determine if
its transmissions do not encounter interference is
2Lc, where L is the coaxial
cable's length. The maximum-length-specification for Ethernet
is 1 km. Assuming a propagation speed of 2/3 the speed of
light, this time interval is 10 μs. As analyzed in
this
problem, the number of these time intervals required, on
the average, to resolve the collision is less than two!
Why does the factor of two enter into
this equation? (Consider the worst-case situation of two
transmitting computers located at the Ethernet's ends.)
The worst-case situation occurs when one
computer begins to transmit just before the other's
packet arrives. Transmitters must sense a collision before
packet transmission ends. The time taken for one
computer's packet to travel the Ethernet's length
and for the other computer's
transmission to arrive equals the round-trip, not one-way,
propagation time.
Thus, despite not having separate communication
paths among the computers to coordinate their transmissions, the
Ethernet random access protocol allows computers to communicate
without only a slight degradation in efficiency, as measured by
the time taken to resolve collisions relative to the time the
Ethernet is used to transmit information.