of additional connectors when the network is initially
set up. These connectors enable you to add or remove
nodes while the network remains intact and in
operation.
ACCESS METHODS
Another decision the designer makes is that of
which access method to use. Access methods are the
arrangements used to make sure each workstation has
fair and equal access to the network. The access
method used is governed primarily by the topology
and the protocol of the network. The principal access
methods are contention and token passing.
Contention
The contention method features Carrier Sense
Multiple Access (CSMA) and Carrier Sense Multiple
Access with Collision Detection (CSMA/CD). The
CSMA/CD method is shown in figure 8-6. Access for
both is on a first-come, first-served basis. The CSMA
access scheme is very similar to that of a citizens
band (CB) radio. Stations with data to send listen to
the channel and wait until it is clear to transmit. With
CSMA/CD, when two or more workstations transmit
simultaneously, their messages will collide. As soon
as a workstation detects a collision, it ceases
transmission, monitors the network until it hears no
other traffic, and then retransmits. Most contention
networks assign a unique retry algorithm to vary the
wait-and-retry period. This algorithm reduces the
likelihood that after a collision, two workstations will
transmit retries simultaneously.
Token Passing
Token passing is an orderly access method and is
shown in figure 8-7. Each workstation passes on the
opportunity to transmit to its closest neighbor until a
station is found with a message to send. This
permission to transmit is called a token. When a
workstation with data to send is handed a token, part
of the token is changed, indicating it is carrying a
message, and then data is transmitted with the token.
The token is then passed around the network, and
every station checks whether the message is intended
for it. The receiving station copies the message from
Figure 8-6.—A bus network using the CSMA/CD access
method.
the token, but then passes the unchanged token along
the network. When the transmitting station receives
the same token, it knows the message has been passed
around the network. The transmitting station erases
the message and puts the empty token back into
circulation on the network. The amount of
information that may be transmitted during possession
of the token is limited so that all workstations can
share the cable equally.
PROTOCOLS
Network protocols are an important component
because they define how networks establish
8-10
