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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.