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Unit 10: Networks.
Starter: Take a look at some components of a typical network system:
1. a backbone 2. a bridge 3. a file server 4. a gateway 5. a LAN
6. a modem
7. a router
Technical Terms.
analogue signal: a type of signal that can take any value between a maximum and a minimum.
application layer: the only part of a network communications process that a user sees. It prepares a
message for sending over a network by converting the message from human-readable
form into bits and attaching a header identifying the sending and receiving computers.
ASCII: acronym for American Standard Cod for Information Interchange. A standard character
encoding scheme.
asynchronous: not synchronized, i.e. occurring at irregular intervals.
backbone: the main transmission path handling the major data traffic connecting different LANs
together.
backup: 1- the process of storing a copy of data on a storage device to keep it safe; 2- the term used for
the copied data.
(data) block: a collection of data stored together and treated as a single unit.
bracketing: to set the boundaries of a message or part of a message by marking its beginning and its end
with special control bits.
bridge: a hardware and software combination used to connect the same type of networks or to partition a
large network into two smaller ones.
checksum: a calculated value that is stored with data to detect any errors that may occur when the data is
copied or transmitted.
compress: to reduce to a much smaller size.
control bit: a data bit used to mark parts of a transmitted signal so that the transmission can be controlled,
e.g. a ‘start’ bit marks the beginning of a character and a ‘stop’ bit marks its end.
data-link layer: the part of a network communications system that supervises the transmission. It
confirms the checksum then addresses and duplicates the packets. This layer keeps a
copy of each packet until it receives confirmation from the next point along the route that
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the packet has arrived undamaged.
decrypt: to recover the original text from an encrypted message.
dumb terminal: a network device with a keyboard and display screen that is used for sending and
receiving data but does not do any processing.
encrypt: to transform data into coded form to make it secure.
full-duplex: able to transfer data in both directions simultaneously, i.e. data can be transmitted and
received at the same time.
gateway: an interface that enables dissimilar networks to communicate such as two LANs based on
different topologies or network operating systems.
half-duplex: able to transfer data in both directions but only in one direction at a time, i.e. data can be
transmitted or received but not at the same time.
header: the first section of a message that contains information about the content and transmission of the
message including the sending and destination addresses.
hub: an electronic device at the center of a star network topology.
isolation adapter: an electronic device that isolates a network system from high voltages and allows it to
use the mains electricity cables for connecting computers together and transferring
data.
line driver: an electronic circuit that provides high currents or voltages to other electronic circuits.
log: 1)- to record the time that an event happened; 2)- a record of when an event happened.
NetPC: an industry specification for a low-cost basic Windows PC with an Intel processor designed for
use on a multi-user network system. It is managed centrally and has no floppy disk drive, CDROM drive or hardware expansion slots, i.e. it is a type of thin client.
Netscape Communicator: a widely-used web browser package.
network computer: a computer designed using the industry specification from Oracle and Sun
Microsystems for a low-cost basic personal computer that can have an Intel
processor or another type of processor and can use a Java-based operating system. It
is designed for use on a multi-user network system and is managed centrally. It has
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no floppy disk drive, CD-ROM drive or hardware expansion slots, i.e. it is a type of
thin client.
network layer: the part of a network communications system that forms the data into packets and selects
a route for the message.
network operating system: an operating system that is used to administer and control a network
allowing computers to share hardware and software while providing file
security and backup facilities.
node: a network terminal or point where a computer is connected to a network.
packet: a fixed size unit of data prepared for transmission across a network. Messages are normally
divided into packets before transmission.
Palm Pilot: a popular handheld personal organizer produced by 3Com.
physical layer: the part of a network communications system that encodes the packets into the medium
that will carry them and sends the packets along that medium.
presentation layer: the part of a network communications system that ensures the message is transmitted
in a language that the receiving computer can interpret.
route: 1)- the path that is used to transfer data in a network; 2)- to move data from node to node on a
network.
router: an electronic device that links different networks or parts of a network. It determines the path
that a signal should take to reach its destination.
segment: a subdivision of data created by a network communications transport layer for which a
checksum is generated.
session layer: the part of a network communications system that opens communications and has the job
of keeping straight the communications among all nodes on the network. It sets
boundaries for the beginning and end of a message and establishes whether the messages
will be sent half-duplex or full duplex.
site: a common name for a website.
start bit: a data bit that marks the beginning of a data block in a network transmission.
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stop bit: a data bit that marks the end of a data block in a network transmission.
string: a series of data characters which can be a mixture of letters or numbers.
synchronize: to cause different processes to occur at the same time.
terminal: a network device used to input and output data (usually a basic computer).
thin client: a low-cost centrally-managed basic computer with a keyboard and display screen processor
and memory but no CD-ROM drive, floppy disk drive or expansion slots, e.g. a NetPC or a
network computer (NC).
traffic: the volume of signals or data that passes through a network system.
transport layer: the part of a network communications system that protects the data being sent. It
subdivides the data into segments and creates checksum tests. It can also make backup
copies of data.
tuned: set to operate on the same frequency.
TV (set): abbreviation for television. A television set is the actual television device used for displaying
the video signals.
twisted-pair (cabling): a common type of network cable that uses two wires twisted together to reduce
interference from external signals.
verify: to check for accuracy.
Diagram 1: ‘Components of a typical LAN’.
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Task 1: With the help of ‘Diagram 1’ and some words from ‘Starter’ match the following
definitions.
( ) is a hardware and software combination used to connect the same type of networks. It can also
partition a large network into two smaller ones and connect two LANs that are nearby each other.
(
) is a special computer that directs communicating messages when several networks are connected
together. The high-speed ones can serve as part of the Internet backbone.
(
) is an interface that enables dissimilar networks to communicate, such as two LANs based on
different topologies or network operating systems.
(
) is the main transmission path, handling the major data traffic, connecting different LANs together.
(
) is a network contained within a small area, for example a company department.
(
) is a device for converting digital signals to analogue signals and vice versa to enable a computer to
transmit and receive data using an ordinary telephone line.
Reading: ‘A simple home network setup’.
The technology needed to set up a home network is here today. It is just a matter of connecting a number
of PCs equipped with Ethernet adapters to a hub using twisted-pair cabling which uses sockets rather like
phone sockets. Special isolation adapters can be fitted to allow existing mains lines to be used instead of
twisted-pair cabling. Most future home networks, however, are likely to be wireless network systems,
using tuned transmitter and receiver devices. The simplest networks allow basic file-sharing and multiplayer gaming as well as sharing of peripherals such as printers. Most advanced home networks are
likely to have a client/server structure, with low-cost terminals, or ‘thin’ clients, connected to a central
server which maintains the system’s storage capacity and, depending on whether the terminals are dumb
or processor-equipped network computers, its processing power. To make the most of such a network, it
must become part of an integrated home entertainment and control system. To the user, the desktop
becomes just one of many features accessible throughout the house. Tired of working in the study? Pop
down to the living room and reload it into the terminal there. Before you start work, call up the hi-fi
control program and have the music of your choice pumped through the living room speakers. Computer
and entertainment networks can be separate to allow control of the latter from the terminals. Future home
networks are more likely to have the entire system based on a single loop.
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Diagram 2: ‘Simple home network’.
Key to the Diagram 2.
1. Line receiver delivers home entertainment audio to speakers within the room.
2. TV set relaying digital TV broadcasts relayed from the receiver by the home entertainment system.
3. Network modem allowing clients to access the Internet simultaneously. Ideally this would be
replaced by an ISDN adapter or DSL modem fitted inside the server.
4. Thin client comprising a display, keyboard, mouse, floppy and CD-ROM drive. If the client is
NetPC-based, it will have its own processor and memory. A dumb terminal will simply act as an
Interface to the real computer, the server.
5. Network printer connected to any client.
6. Line driver connected to the home entertainment system: the cable TV player, DVD player , etc.
7. Home server. It contains roughly 5Gb of storage per terminal and one or more processors, depending
on whether it is connected to network computers or to cheaper dumb terminals.
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8. Entertainment system delivery network. This also hooks up to the server to control the system and
receive digital audio and video from it.
9. Entertainment network control pad. While the system can be controlled by a PC, there would be one
of these per connected room to ensure that the client does not need to be activated to use the system.
10. Data line linking clients to server.
Task 2: Re-read the text ‘A simple home network setup’ and study the Diagram 2. Match
the diagram key to the components of the network.
Task 3: Match the columns in order to complete the definitions.
1. A gateway is an interface...
2. A bridge is a hardware and software combination...
3. A backbone is a network transmission path...
4. A router is a special computer...
5. A network is a number of computers and peripherals...
6. A LAN is a network...
7. A server is a powerful computer...
8. A client is a network computer...
9. A thin client is a simple computer...
10. A hub is an electronic device...
(
(
(
(
(
(
)
)
)
)
)
)
(
(
(
(
)
)
)
)
...connecting computers over a small distance such as within a company.
...directing messages when several networks are linked.
...used for accessing a service on a server.
...connecting all the data cabling in a network.
...used to connect the same type of networks.
...comprising a processor and memory, display, keyboard, mouse and hard drives
only.
...linked together.
...enabling dissimilar networks to communicate.
...handling major data traffic.
...storing data shared by all the clients in the network.
Task 4: You will find some advantages and some disadvantages of a network. Separate
them.
1. Hardware and software can be shared.
2. Access to the system can be controlled.
3. Networks are more complex to set up.
4. Networks are more expensive to set up.
5. Maintenance is easier.
6. Networks are more vulnerable to viruses.
7. Users can communicate easily with each other.
8. It is easier to check for viruses.
9. The whole network depends on the central server.
10. It is easier to make backups.
11. Networks require more expertise to maintain.
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Ways to minimize disadvantages of a network.
1. Employ well trained computing staff.
2. Use standard systems.
3. Try to negotiate bulk discounts.
4. Use thin clients instead of full computers.
5. Install an anti-virus program on the server.
6. Schedule frequent virus checks on the server.
7. Buy a good quality server.
8. Buy as powerful a server as you can afford.
9. Purchase a server with hot-swappable components.
10. Install a RAID system on the server.
11. Have a good training scheme for computing personnel.
Linking some of the advantages and disadvantages of a network.
1. Although networks allow data to be shared, they permit viruses to spread quickly.
2. Users can share software on the server; however server failure means that no one can work.
3. Networks are more vulnerable to viruses; however it is easier to check for viruses.
4. Although maintenance is easier, networks require more expertise to maintain.
5. Networks are more complex to set up; however maintenance is easier.
6. Although access to the system can be controlled, networks are more vulnerable to viruses.
7. Hardware and software can be shared; however the whole network depends on the central server.
Technical Reading 1: ‘Asynchronous transmission’.
This method, used with most microcomputers, is also called start-stop transmission. In asynchronous
transmission, data is sent one byte (or character) at a time. Each string of bits making up the byte is
bracketed, or marked off, with special control bits. That is, a ‘start’ bit represents the beginning of a
character, and a ‘stop’ bit represents the end. As a means of checking that the whole character has been
transmitted, an error check bit is generated immediately after each character.
Transmitting only one byte at a time makes this a relatively slow method. As a result, asynchronous
transmission is not used when great amounts of data must be sent rapidly. Its advantage is that the data
can be transmitted whenever it is convenient for the sender.
Diagram 3: ‘Asynchronous transmission’.
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a
b
c
d
e
f
start bit
error check bit
stop bit
start bit
error check bit
stop bit
Technical Reading 2: ‘Synchronous transmission’.
Synchronous transmission sends data in blocks of characters. Start and stop bit patterns, called synch
bytes, are transmitted at the beginning and end of the blocks. These start and end bit patterns synchronize
internal clocks in the sending and receiving devices so that they are in time with each other. Error check
bytes are included immediately after each block of characters to ensure that the whole sequence of
characters has been correctly transmitted.
This method is rarely used with microcomputers because it is more complicated and expensive than
asynchronous transmission. It also requires careful timing between sending and receiving equipment. It
is appropriate for computer systems that need to transmit great quantities of data quickly.
Diagram 4: ‘Synchronous transmission’.
a
b
c
d
e
start synch bytes
character
character
error check bytes
stop synch bytes