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Fire Controlman Volume 06-Digital Communications | Next
Token Passing |
DISTRIBUTED STAR
Figure 6-4.—A distributed star (tree) network topology.
The distributed star, or tree, topology is shown in
figure 6-4. It provides many of the advantages of
both bus and star topologies. It connects workstations
to a central point called a hub. This hub can support
several workstations or hubs which, in turn, can
support other workstations.
Distributed star
topologies can be easily adapted to the physical
arrangement of the facility site. If the site has a high
concentration of workstations in a given area, the
system can be configured to more closely resemble a
star topology.
If the workstations are widely
dispersed, the system can use inexpensive hubs with
long runs of shared cable between hubs, similar to the
bus topology.
RING NETWORK
In a ring network, all the components or nodes are
connected to the main cable, and the cable forms a
ring, as shown in figure 6-5. This topology allows a
node to send a message to another node on the ring.
However, the message must be transmitted through
each node until it reaches its destination. Messages
proceed from node to node in one direction only.
Should anode fail on the network, data can no longer
be passed around the ring unless the failed node is
either physically or electronically bypassed. Using
bypass software, the network can withstand the failure
of a workstation by bypassing it and continuing to
maintain the integrity of the network. One of the
major issues in a ring topology is the need for making
sure all workstations have equal access to the
network.
One of the major disadvantages of ring topologies
is the extreme difficultly of adding new workstations
while the network is in operation. Normally, the
entire network has to be brought down while a new
node is added and cabling reattached. However, this
particular problem can be overcome by the installation
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
Figure 6-5.—A ring network topology.
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