Technical data

www.westermo.com Theoretical and general applications 35

Topology

The expression topology refers to how a network is structured; the physical or logical

placement of the nodes. There are five basic topologies: point to point, ring, star, bus

and combined network. The choice of topology is important, as it is a long term infra-

structure that shall manage and transport important data without downtime.

In addition, it must be possible to adapt and expand the network as and when condi-

tions change.

Serial point to point

Point to point data communications, i.e. between two communicating devices is

one of the most common applications. Both in basic applications, such as com-

puter to printer, and more complex applications, where you choose to allow each user

to communicate on their own line for reasons of security. The standard RS-232/V.24

interface is not recommended for transmission distances greater than 15 metres

(50 ft). For this reason a modem is used to extend the line and suppress disturbance

for communication up to 18 km (11 mi).

Star network

A network with many connected point to point users is known as a star network.

Each device communicates with the central unit in the centre on its own line. The

star network offers the advantage of high reliability. When a line goes down the

others are not affected. One disadvantage is that more cable is required, resulting

in a higher cost, and that all communications must go through the central unit.

Ring network

A ring network connects all units in series with each other in a closed ring. This

means that all communications must pass “through” all devices on the ring in order to

be forwarded to the receiver. An “empty slot” is sent round the network to avoid col-

lisions. The transmitting node checks whether it is empty, attaches its address and

adds its data information. The next node in the ring checks whether the contents

of the slot are intended for it, if not is passed on. When the receiver receives its

slot, he empties its contents and inserts a receipt and sends this out on the net-

work again. The transmitting device checks that the message has been received

and acknowledged, it then sends the empty slot forward for new traffic. Token Ring

is an example of a ring network from a signal standpoint, which is physically con-

nected as a distributed star network. Ring networks offer high performance, but

can be more complicated to build and adapt compared to a bus network.

Node

Node Node

Node

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