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e.g. The sequence of numbers below will convert to

65

A W E E T E S T

31.5 PARALLEL

•This I/O port uses multiple data lines (one for each bit) and extra lines for handshaking.

•These cables can permit very high data transfer rates.

•Because of the extra conductors the cables are large, most costly, length limited and much less flexible

31.6 NETWORKS

•A Network is a way of connecting different computers to allow data and message passing, and resource sharing.

•Early communication techniques included telegraphs (with morris code), telephones and teletypes.

•Advantages

-allows shared computers and peripherals

-provides communication and cooperation tools for people and computers

-increases reliability by having alternate computers available

-decreases isolation of many computers

•Disadvantages

-more sophisticated hardware and knowledge required

-additional costs

•Applications,

-allows remote access to databases and libraries

-electronic mail

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-distributed computational facilities

-remote control of applications

31.6.1 Introduction

• The relationship between various network types is shown below

Example LAN Topologies

Bus or Ring

Star

...

- Shared Media

- Distributed Control

- No media sharing

- Central Control

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•The Star Network topology uses many connections to a central server

-Central control

- Dedicated communication channels

- Single point of failure

- Often compatible with telephone wiring (voice/data integration)

-Typical minicomputer and mainframe environments

•When all computers are connected to the same network cable, there are a number of structures which may be used. The bus topology below is simpler, and the tree topology is more flexible

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and versatile. This approach uses a single cable which is easy to add to

...

Bus Topology

...

Tree Topology

R

R

R

R Repeater

• The ring topology below shows a closed loop of cable which can be simple to implement, but

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limited in capabilities

- Distributed control

-Entire communication capacity

capacity for any station

- Vulnerable to loss of shared communication facilities

-Primary vendors: IBM & Proteon

•An enhanced ring topology is pictured below

Characteristics

H O S T

N o . 3

- Control token passed from H O S T

N o . 2

node to node

- Only node with token can transmit

-	Token replaced on ring at the end					S	start of
	of message						message
		H O S T
-	Permits variable size messages	N o . 4
						H O S T
-	Only one message on the ring
					E	N o . 1

at a time

end of message

direction of message flow

• Significant issues in networks are,