If we want to connect two digital devices with a
network, we would have a kind of network known as "point-to-point:"
For the sake of simplicity, the network wiring is symbolized as a single
line between the two devices. In actuality, it may be a twisted pair of
wires, a coaxial cable, an optical fiber, or even a seven-conductor BogusBus.
Right now, we're merely focusing on the "shape" of the network, technically
known as its topology.
If we want to include more devices (sometimes called nodes) on
this network, we have several options of network configuration to choose
from:
Many network standards dictate the type of topology which is used, while
others are more versatile. Ethernet, for example, is commonly implemented in
a "bus" topology but can also be implemented in a "star" or "ring" topology
with the appropriate interconnecting equipment. Other networks, such as
RS-232C, are almost exclusively point-to-point; and token ring (as you might
have guessed) is implemented solely in a ring topology.
Different topologies have different pros and cons associated with them:
Point-to-point
Quite obviously the only choice for two nodes.
Bus
Very simple to install and maintain. Nodes can be easily added or removed
with minimal wiring changes. On the other hand, the one bus network must
handle all communication signals from all nodes. This is known
as broadcast networking, and is analogous to a group of people
talking to each other over a single telephone connection, where only one
person can talk at a time (limiting data exchange rates), and everyone can
hear everyone else when they talk (which can be a data security issue).
Also, a break in the bus wiring can lead to nodes being isolated in groups.
Star
With devices known as "gateways" at branching points in the network, data
flow can be restricted between nodes, allowing for private communication
between specific groups of nodes. This addresses some of the speed and
security issues of the simple bus topology. However, those branches could
easily be cut off from the rest of the "star" network if one of the gateways
were to fail. Can also be implemented with "switches" to connect individual
nodes to a larger network on demand. Such a switched network is
similar to the standard telephone system.
Ring
This topology provides the best reliability with the least amount of
wiring. Since each node has two connection points to the ring, a single
break in any part of the ring doesn't affect the integrity of the network.
The devices, however, must be designed with this topology in mind. Also, the
network must be interrupted to install or remove nodes. As with bus
topology, ring networks are broadcast by nature.
As you might suspect, two or more ring topologies may be combined to give
the "best of both worlds" in a particular application. Quite often,
industrial networks end up in this fashion over time, simply from engineers
and technicians joining multiple networks together for the benefit of
plant-wide information access. |
