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1-22
Chapter 1
Planning a Network Topology
The Ethernet MAC mechanism is called Carrier Sense Multiple Access with
Collision
Detection (CSMA/CD). With this mechanism, a computer that has data to
transmit lis-
tens to the network to see whether any other computer is transmitting. If
the network
is free, the computer proceeds to transmit its data. During the
transmission, the com-
puter continues to monitor the incoming signals from the network. If the
transmitting
computer detects a signal from another computer during its transmission
(indicating
that the two computers started sending data at nearly the same moment), a
collision is
said to have occurred. Both systems stop transmitting, pause for a randomly
deter-
mined period of time, and then begin the entire transmission process over
again. If one
(or both) of the systems is involved in another collision, it waits twice as
long before
attempting to transmit again. This introduces a measure of waste, as the
computers end
up transmitting the same data more than once. With CSMA/CD, a certain number
of
collisions are normal and expected. As the traffic level on the network
increases, so
does the number of collisions. For this reason, Ethernet becomes a less
effective pro-
tocol on a heavily trafficked network.
Token Ring, on the other hand, uses a MAC mechanism called token passing.
With this
method, a computer on the network generates a tiny packet called a token,
which cir-
culates endlessly around the network. Only the computer in possession of the
token is
permitted to transmit its data. When a system wants to transmit data, it
waits until the
token passes by, grabs the token, and then proceeds to transmit its data.
After complet-
ing the transmission, the computer then sends out the token again so that
another sys-
tem can use it. The protocol specifications determine how long the system
can keep
the token, how long it can circulate the token, and how it can generate a
new token if
one isn’t currently in circulation. The advantage of token passing is that
there are no
collisions whatsoever on a properly functioning network. Consequently, there
are also
no retransmissions, so no bandwidth is wasted. This also means that a
token-passing
network functions just as well under heavy traffic conditions as it does in
light traffic.
Off the Record
The limited speed of Token Ring networks and the higher hardware prices
have kept the protocol a distant second choice to Ethernet, but there are
enough Token Ring
advocates to keep the products on the market.
Mixing Media
When designing a network for a medium-to-large enterprise, it is not
necessary to
choose a single data-link layer protocol for the entire internetwork. A
router can
connect any type of network to any other, so it is possible to select a
different data-
link layer protocol for each LAN. However, for simplicity of construction
and ease of
maintenance, it is recommended that you use the same protocol wherever
practical.
You should use different protocols only when circumstances force you to do
so.
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