Telecommunications and computer networks

TELECOMMUNICATIONS AND COMPUTER NETWORKS

By M. Magomelo

Computer Science Dept

Communication is the transfer of information from one participant to another during a conversation. The information is transmitted via a medium.

Telecommunications is the sending of information in any form (e.g. voice, data, text and images) from one place to another using electronic or light –emitting media and

Data communication is a more specific term that describes the transmitting and receiving of data over communication links between one or more computer systems.

THE OBJECTIVES OF COMMUNICATION NETWORKS ARE:

To offer more timely interchange of information and data among users.

To reduce the effort and cost required to collect and transmit business data and information.

To support better performance of tasks and improved management control over an organization especially with several remote locations.

TRANSMISSION MODES

Various transmission modes are available for transmitting signals from a sending device to a receiving devise.

Network devices use three transmission modes (methods) to exchange data, or “talk” to each other, as follows: 

Simplex permits transmission in one direction only e.g. broadcasting of radio and TV.

Half – duplex permits transmission in either direction but in only one direction at a time e.g. conversation with walkie – talkies. 

Full – duplex permits transmission in both directions simultaneously e.g. a phone conversation the listener can interrupt the speaker at any time.

ANALOGUE TRANSMISSION• Is the transmission of a signal that takes a

continuous state e.g. speech.• Analogue signals are continuous sine waves,

which send a continuous 5-volt signal on a channel, but the signal will vary continuously from +5 to –5 volts.

• The number of cycles per second is the frequency of the signal and is expressed in hertz (Hz).

DIGITAL TRANSMISSION• In digital data only a limited number of discrete

values or levels for the variable are possible e.g. 2 levels 0 and 1 for binary data. The 0 and 1 can be used to represent the “Off” and “On” state of electrical pulses.

• A cycle consists of 2 pulses. The number of pulses per second is a unit called a baud rate.

Asynchronous Transmission• Is the transmission of characters one at a time at

irregular time intervals.• It uses start and stop bits to mark the beginning and

the end of characters.• Characters are transmitted independently with their own

start and stop bits.

Synchronous transmission• Is the transmission of data in blocks at fixed rates. A

block is a prepared set of characters that are transmitted together as a unit.

• It is more efficient when transmitting large quantities of data.

• Synchronization of sender and receiver is achieved by means of special synchronization characters, which align clocks at each terminal so that they function simultaneously.

TELECOMMUNICATION NETWORK MODEL

This is an arrangement of telecommunication equipment, media, including the software, where a sender transmits a message to a receiver over a channel consisting of some type of medium.

End user terminal• These include such devices as the VDU, terminal, and end-user workstations

used for inputting and outputting data. Telecommunications/network processors• These support data transmission and reception between terminals and

computers.• Devices such as modems, multiplexors, and front-end processors perform a

variety of control and support functions e.g. a modem converts digital data to analogue and back.

 Telecommunication channel and media• Are used to carry data and information from one point/place to another e.g.

copper wires, coaxial cable, fibber optic cables, micro–wave systems and telecommunication satellites.

 Computers• Carry out the information processing assignments e.g. a mainframe computer

can act as a network server or a host computer in a large network assisted by a mini–computer serving as a front-end processor.

 Telecommunication software• Consist of programs that control telecommunication activities and manage the

functions of telecommunication network. Examples are network operating system (Novell Network, Unix, Windows NT etc)

COMPUTER NETWORKS

A network is any collection of independent computers that communicate with one another over a shared network medium.

The purpose of networking:• To share resources like expensive peripherals

such as printers, optical disks and scanners.• For communication with between offices e.g.

electronic mail (e-mail).• To use/share multi-user software.• To access other computer systems such as

mainframes.

Computer Network types can be classified by:• Size or distance covered • Structure or topology

CLASSIFICATION BY SIZE

Networks can be divided into Local Area Networks (LANs), Metropolitan Area Networks (MANs) and Wide Area Networks (WANs).

The connection of two or more networks is called an internetwork.

The worldwide Internet is a well-known example of an internetwork.

LANS (LOCAL AREA NETWORKS) LANs are networks usually confined to a

geographic area, such as a single building or a college campus. LANs can be small, linking as few as three computers, but often link hundreds of computers used by thousands of people.

LANs normally: do not exceed tens of kilometres tend to use one type of transmission media entirely contained within the same building or the same

flow or the same floor of a multi-storey building is organisation owned. Provides high internal data rates (about 10Mbps to

100Mbps) compared to WANs LAN technologies include

Ethernet Token ring

LAN HARDWARE Network Workstation – are usually microcomputers or

terminals which are used to input and output data in the network:

File servers - these are computer systems connected to the network that controls access to and manages one or more hard disks to allow workstations to share disk space, programs and data.

It also controls all the operations/activities of the network. Print servers – these are computers that control access to and

manages the printer resources attached to the network Communication servers – these are computers that provide

and manages communications devices (modems and multipexors) in the network. They are usually microcomputers dedicated to handling communication devices.

Network Interface Card (NIC)

Each component in the network is attached to the network through the use of an NIC. These cards/adapters provide the necessary translation of signals to and from devices in the network.

METROPOLITAN AREA NETWORKS (MAN)

City-wide networks. Covers an entire city i.e. 5 – 50 kms Uses different hardware and transmission

media. Mainly used for interconnecting private LANs. Supports data rates of 100 to 1000 Mbps. Normally owned by someone else: an

independent or government service provider.

WANS (WIDE AREA NETWORKS) Wide area networking combines multiple LANs that are

geographically separate. This is accomplished by connecting the different LANs

using services such as dedicated leased phone lines, dial-up phone lines, satellite links, and data packet carrier services.

Wide area networking can be as complex as hundreds of branch offices globally linked using special routing protocols and filters to minimize the expense of sending data sent over vast distances.

WAN links are connected by switches. The switches relay information through the WAN.

Routers determine the most appropriate path through the internetwork (communication subnet) for the required data streams.

It supports data rates of 28.8 Kbps to 2 Gbps.

WHAT IS AN INTERNETWORK? An internetwork is a

collection of individual networks, connected by intermediate networking devices, that functions as a single large network.

Figure 1-1 illustrates some different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork.Figure 1-1: Different Network

Technologies Can Be Connected to Create an Internetwork

NETWORK CONNECTION DEVICES (FOR INTERNETWORKING)

Repeaters. Its primary purpose is to enable the network to expand beyond

the distance limitations of the transmission medium. A repeater regenerates a weak signal from one port onto the

other ports to which it is connected. It does not filter or interpret anything; instead it merely

regenerates a signal, passing all network traffic in all directions.

A repeater merely passes along bits of data, even if a data frame is corrupt

Hubs Hubs are used to connect together two or more network

segments of any media type. In larger designs, signal quality begins to deteriorate as segments exceed their maximum length.

Hubs provide the signal amplification required to allow a segment to be extended a greater distance.

A hub takes any incoming signal and repeats it out to all ports.

NETWORK CONNECTION DEVICES (CONT)

Bridges A bridge is used to connect similar networks. Bridges perform the same functions as repeaters but are a bit

clever. When a data packet/frame arrives, the bridge checks the

frame’s destination address (MAC) and forwards the frame to the segment that contains the destination address, if it cannot find the destination address in its routing table it forwards the frame to all segments except the source segment.

Routers A router is hardware that helps Local Area Networks (LANs)

and Wide Area Networks (WANs) achieve interoperability and connectivity, and can link LANs that have different network topologies (such as Ethernet and Token Ring).

Routers match packet headers to a LAN segment and choose the best path for the packet, optimizing network performance.

NETWORK CONNECTION DEVICES (CONT)

Gateways These are used to interconnect two or more dissimilar

or incompatible networks i.e. networks with different architectures (different hardware and software).

Gateways normally change the representation of data before passing it on while repeaters, bridges and routers do not change the data in any way. A gateway can change ASCII coding system to EDCDIC and vice versa.

A gateway translates between different transport protocols or data formats (for example, IPX and IP) and is generally added to a network primarily for its translation ability.

NETWORK CONNECTION DEVICES (CONT)Firewalls A firewall is a combination of hardware and software that

provides a security barrier between networks. Firewalls are generally set up to protect a particular network

from attack (or unauthorized access) by outside invaders. All traffic to or from the protected network must go through

the firewall.

User A might have the authority to connect to network 2 whilst User B might not. When someone tries to connect network 2, the firewall checks the authority of the user and if the user does not have any authority, the firewall prevents the connection.

CLASSIFICATION BY PHYSICAL NETWORK TOPOLOGY

A physical network topology defines the wiring or layout for a network. This specifies how the elements in the network are connected to each other.

Topology defines the physical configuration of computers.

A node is an active device connected to the network, such as a computer or a printer.

A node can also be a piece of networking equipment such as a hub, switch or a router.

STAR NETWORKS A star network is a network in which the

nodes are connected to a central component. This central component may be a switching device like a PABX (Private Automatic Branch Exchange), a computer with a switching or polling capability or just a wiring centre that is a common termination point for the nodes called a hub.

A PABX is a telephone switching system configured for communication in a private telephone network and with access to the public telephone system. It provides automatic switching.

A hub is a component that serves as a common termination point for multiple nodes and that can relay (store and forward) signals along the appropriate paths.

STAR TOPOLOGY (CONT)

Advantages of star networks Easy to add/remove nodes Because each node has its own link to the central

node, the star is more reliable. Easy to troubleshoot and isolate problems.

Disadvantages All traffic between 2 nodes passes through central

node. If the central node breaks down the whole network is down.

Number of ports of the central component limits the number of connected nodes

BUS TOPOLOGY Uses a trunk or backbone to which all of the computers on the

network connect. Systems connect to this backbone using T connectors or taps.

In a bus network the messages are broadcasted and travel in both directions and do not go through the individual nodes, but every node can hear each message as it goes past.

When this signal reaches an end of the bus, a terminator absorbs it, to keep it form travelling back again along the bus line, possibly interfering with other signals /messages already on the line.

BUS TOPOLOGY (CONT)

Advantages When a node breaks down the network does

not breakdown Little cable is used relative to other topologies Does not use any specialized network

equipment.Disadvantages Diagnosis /troubleshooting can be difficult. Network disruption when computers are

added or removed A break in the cable will prevent all systems

from accessing the network.

TREE TOPOLOGY / HIERARCHICAL

A tree topology is a hybrid physical topology that combines features of star and bus topologies. The nodes are connected in a tree line form.

If the nub goes down, then the nodes connected to it are no longer networked but other parts of the network keep on functioning.

RING TOPOLOGIES A ring network has nodes

connected in a circular way. Each node has 2 neighboring

nodes. Data/messages flow only in one direction.

A message is forwarded in one direction until it reaches the destination with intermediate nodes acting as relay (store and forward) units.

The destination node copies the message and passes the message again to the ring.

This message them continue to circulate around the ring to the source.

RING TOPOLOGY (CONT)

Advantages Collisions of packets are rare Little cabling needed compared to star Each node acts as a relay unit Cable faults are easily located, making

troubleshooting easier

Disadvantages Adding or removing nodes disrupts the

network. If one of the nodes breaks down, the whole

network will break down.

MESH TOPOLOGY This uses direct (point-to-point)communications

lines to connect some or all of the computers in the network to each other.

A fully connected mesh network with n(n-1)/2 devices has links.

Each network devices must have (n - 1) I/O ports.

MESH TOPOLOGY (CONT)

Advantages: No traffic problems Robust - provides redundant paths between devices Privacy & security Simple fault identification and isolation.Disadvantages: Wiring bulk Hard to manage and inflexible Expensive to setup.

WAN COMPONENTS/DEVICES

Perform a variety of support functions between the terminals and computers in a telecommunication network.

Modem Is a device that converts digital signals

from a computer transmission terminal at one end of the telecommunication link into analogue frequencies, which can be transmitted over ordinary telephone lines.

Modems at the other end of the communication link convert the transmitted data back into digital form and the whole process is called modulation and demodulation respectively.

WAN COMPONENTS/DEVICES(CONT)

An analogue signal is a signal that takes a continuous state e.g. speech generated signal.

A digital signal is any signal that takes a discrete or non-continuous state e.g. all signals from computer are digital in nature.

Note: Modem is an acronym for MOdulator/DEModulator

WAN COMPONENTS/DEVICES(CONT)

Modulation A carrier wave of high frequency is used to carry digital data. Some characteristics of the carrier wave are changed in order to carry the signals. Modulation is therefore a technique where you change some characteristic of a carrier signal for the purpose of transmitting information. Concisely we say modulation is conversion of digital to analogue signals, and demodulation, analogue to digital signals. 

WAN COMPONENTS /DEVICES(CONT)

Modulation Types Frequency Modulation

(FM)  In FM the frequency of the

carrier wave is adjusted so that it represents the binary 1and 0. A high frequency for a 1 and a low frequency for a zero.

Amplitude Modulation (AM) In AM the amplitude of the

carrier wave is modified so that it represent the binary 1 and 0 by using 2 different amplitudes. High amplitude for a 1 and low amplitude for a 0 (zero).

 Phase Modulation (PM) In PM the signals is adjusted

by a fixed amount so that binary 0 or 1 can correspond to different phase shifts.

WAN COMPONENTS/DEVICES(CONT

Multiplexor is a device that allows a single communication

channel to carry simultaneous data transmissions from many terminals.

Typically a multiplier merges the transmission of several terminals at one end of a communication channel while a similar unit separates the individual transmissions at the receiving end (demultiplexor).

TYPES OF MULTIPLEXORS

a) Frequency division Multiplexor This divides a high speed channels into multiple slow speed

channels The bandwidth of the high speed channel is divided into some

non-overlapping frequency bands.b) The division multiplexor (TDM) divides the time each terminal can use a high-speed channel

into very short time slots or time frames. Each user gets periodically the entire bandwidth of a

communication channels for that time slot. The multiplexing is done bit, byte for byte or sometimes block for block.

c) Statistical time division multiplexor (STDM) Instead of giving all terminals equal time slots, this multiplexor

dynamically allocates time slots only to active terminals. Time slots are allocated based on the traffic demand on

individual channels. There are no longer empty slots as in conventional TDM, then

information has to be transmitted with some identification to which/what channels the time slots belong.

WAN COMPONENTS/DEVICES(CONT)3) Concentrators With TDM it is likely that a high-speed line is underutilized. A

concentrator overcomes this deficiency by gathering the bits from each slow terminal or a group of slow terminals and holding them in its buffer store until they are sufficient to justify forward transmission.

Thus it accepts bits at a slow rate and then transmits them in a high – speed burst occupying a period of time division multiplexing.

4) Front End Processors (FEP) In communications, a computer that is located between

communications lines and a main (host) computer and used to relieve the host of tasks related to communications.

A front-end processor is dedicated entirely to handling transmitted information, including error detection and control; receipt, transmission, and possibly encoding of messages; and management of the lines running to and from other devices.

 The FEP relieves the host computer so that it can concentrate on its information processing functions.

WAN COMPONENTS/DEVICES(CONT)

5) Backend processors a computer system connected to the

mainframe or host computer which links the host to the database.

Its main purpose is to retrieve data from the database.

NETWORK OPERATING SYSTEM

Programs /software that control the computer systems and devices on a network and allow them to communicate with each other.

They also allow you to; Install each network device Install application software on the network To diagnose network problems Manage file, print and communication servers

WAN SWITCHING TECHNOLOGIES

WAN services are provided using the following primary switching technologies Circuit switching Message switching Packet switching

WAN SWITCHING TECHNOLOGIES(CONT)1. Circuit switching (CS) Provide a dedicated communication path between

two stations and offers bandwidth that cannot be infringed upon by other users.

An end-to-end path is established before communication can occur.

The path is a connected sequence of links between nodes.

Circuit switching has the following major phases:I. circuit establishment – a station to station

circuit established.II. Data transfer – data is now sent over the

dedicated channel.III. Circuit disconnect – connection termination.

WAN SWITCHING TECHNOLOGIES (CONT)

Advantages Circuit switching (CS) no congestion. dedicated transmission channel with guaranteed data

rate.  Disadvantages Circuit switching (CS)

channel reservation for duration of connection even if no data are being transferred is an inefficient media use process.

long delays in call setup. designed for voice traffic (analog).

WAN SWITCHING TECHNOLOGIES(CONT)2. Message Switching (MS) No dedicated path is established between the two stations for an

entire conversation. Messages are sent in their entirety with source and destination

addresses. There is often no real limit on the message / block size. A message switching node accepts complete messages from

originators, stores the messages, examines the address in the headers of the messages and then forwards / routes the each message to the next switching node or destination when circuits become available.

Mostly used for the E-mail system.Advantages more devices can share network bandwidth reduced traffic congestion one message can be sent to many destinations through broadcast

addresses Disadvantages often costly – must have large storage devices to hold potentially

long messages not compatible with most real time applications

WAN SWITCHING TECHNOLOGIES(CONT)

3. Packet switching (PS) PS differs from MS in that long messages are

broken down into smaller and manageable units called packets, which are then sent to the destination independently using the fastest routes.

Packets may travel in different routes to the destination.

A Packet Assembler / Dissembler (PAD) is used to break data into packets at sender and reassemble them at receiver.

No call set-up is required. Fast and suitable for interactive applications.

TELECOMMUNICATION MEDIA/ LINKS/ CHANNEL These are the means by which data and other

forms of communication are transmitted between the sending and receiving devices.

It is any medium through which data is transmitted from source to destination.

 Factors to consider when choosing the transmission media are; i. Transmission rate to be implemented on the line.ii. Line capacity or bandwidth. Bandwidth is the range of

usable frequencies that a medium can accommodate.iii. Transmission distances involved – this determine

attenuation of a signal along the cable. Attenuation is the loss of signal power as the signal moves along the communication medium.

iv. Cost of the medium and ease of installation v. Resistance to environmental conditions like EMI

(Electrical Magnet Interference).

TELECOMMUNICATION MEDIA/ LINKS/ CHANNEL (CONT) Channel Transmission Impairments All transmission media suffer the following major

problems, attenuation, noise, and distortion Attenuation – is the loss of power as a signal

propagates through a medium. Noise – noise is unwanted signals from sources other

than the signal. It is sometimes referred to as circuit interference.

 Distortion – means that the signals are deformed a more or less different signal as it propagates through the medium.

TRANSMISSION MEDIA

There are 2 basic categories of transmission media: guided and unguided.

1. Guided transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the cabling system. Guided media is also known as bound media. "Cabling" is meant in a generic sense, and is not meant to be interpreted as copper wire cabling only.

2. Unguided transmission media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and are therefore often called unbound media.

GUIDED TRANSMISSION MEDIA

There 4 basic types of guided media: Open Wire Twisted Pair Coaxial Cable Optical Fiber

1. OPEN WIRE

Open wire is traditionally used to describe the electrical wire strung along power poles.

There is a single wire strung between poles.

No shielding or protection from noise interference is used.

We are going to extend the traditional definition of open wire to include any data signal path without shielding or protection from noise interference.

This can include multi conductor cables or single wires.

This medium is susceptible to a large degree of noise and interference and consequently is not acceptable for data transmission except for short distances under 20 ft.

2. TWISTED PAIR WIRE These are ordinary telephone wires consisting of 2 insulated

copper wires twisted together in a helical from to reduce cross talk or electro-magnetic interference from similar pairs close by.

Widely used in established communication networks through out the world for both voice and data transmission.

Twisted pair is extensively used in home and office telephone systems and many LANS.

Characteristics It has big attenuation, which limits the possible distances, for larger distances amplifiers are needed. Low bandwidth Very much affected by noise Very sensitive to electromagnetic interference.

TWISTED PAIR WIRE(CONT)

Advantages Cheap to install and repair Easy to terminate

Disadvantages Very sensitive to noise Only effective for short distances Data can be easily tapped off (data security

is low)

TWISTED PAIR WIRE(CONT) STP

STP or shielded twisted pair is used with the traditional Token Ring cabling or ICS - IBM Cabling System. It requires a custom connector. IBM STP (shielded twisted pair) has a characteristic impedance of 150 ohms.

UTP

Has a thin layer of cover More extensive EMI than STP

cables Cross talk between UTP pairs limits

max cables length Very cheap (Costs the Least) Care must be taken to avoid

electrical noisy devices e.g. electrical motors around.

Flexible to handle A typical impedance for UTP is 100

ohm for Ethernet 10BaseT cable. UTP or unshielded twisted pair

cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc..)

3. COAXIAL CABLES

It consists of a stiff copper wire as the core surrounded by an insulating material.

A cyclical conductor often as a branded mesh encases the insulator.

The other conductor is covered in a protective plastic covering.

This installation minimizes interference and distortion of signals the cable carries.

Group of coaxial cables can be banded together in a big cable for easy of installation.

Coaxial cable is used extensively in television, radio, network and data communications.

They can also be used to connect or interconnect computer and peripheral devices.

COAXIAL CABLES(CONT)Characteristics of coaxial cable Higher bandwidth and data rates (high – speed transmission) Less attenuation More expensive than twisted pair Can be easily tapped Low error data rates Advantages Good noise immunity Easy to install Reasonably high bandwidth Low data error rates Disadvantages Data security is low (they can be easily tapped)

4. FIBER OPTIC CABLES

Data is transmitted in the form of light pulses.

It uses cables consisting of one or more hair – thin filaments of glass fiber wrapped in a protective jacket.

Characteristics Have very high data transmission

rates. They are light in weight and not

affected by electromagnetic radiation

They are not affected by lightning or electronic surges

Light sources are Light Emitting Diodes (LED) and Injection Laser Diodes (ILD) and there is a photodiode to detect the light rays.

FIBER OPTIC CABLES(CONT)Advantages of Fiber Optic Very high transmission capacities (bandwidth) Smaller and lighter than copper wire Immune to cross talk /EMI Suitable in hostile environments (noisy environments) Less susceptible to spying (can not be easily tapped) Longer distances than copper wire Faster transmission rate

Disadvantages of Fiber Optic Repairing and installing is quite difficult and needs specialist

personnel and equipment. Limited physical arc of cable. Bend it too much and it will

break! Difficult to splice Optical transmission systems are unidirectional.

UNGUIDED TRANSMISSION MEDIA

Unguided transmission media is data signals that flow through the air.

They are not guided or bound to a channel to follow.

They are classified by the type of wave propagation.

RF Propagation There are three types of RF (radio

frequency) propagation: Ground Wave IonosphericLine of Sight (LOS)

1. RF PROPAGATION

Ground wave propagation:

follows the curvature of the Earth.

Ground waves have carrier frequencies up to 2 MHz.

AM radio is an example of ground wave propagation.

RF PROPAGATION(CONT) Ionospheric

propagation: bounces off of the Earth's

ionospheric layer in the upper atmosphere.

It is sometimes called double hop propagation.

It operates in the frequency range of 30 - 85 MHz.

Because it depends on the Earth's ionosphere, it changes with the weather and time of day.

The signal bounces off of the ionosphere and back to earth.

Ham radios operate in this range.

RF PROPAGATION(CONT)

Line of sight propagation:

transmits exactly in the line of sight.

The receive station must be in the view of the transmit station.

It is sometimes called space waves or tropospheric propagation.

It is limited by the curvature of the Earth for ground-based stations (100 km, from horizon to horizon).

Reflected waves can cause problems.

Examples of line of sight propagation are: FM radio, microwave and satellite.

MICROWAVE Microwave transmission is line of

sight transmission. The transmit station must be in

visible contact with the receive station.

Microwave antennas are usually put/placed on top of buildings, towers, hills and maintain peaks.

The antennas house radio transmitters and receivers (transceivers) known as repeaters.

This sets a limit on the distance between stations depending on the local geography.

Typically the line of sight due to the Earth's curvature is only 50 km to the horizon!

Repeater stations must be placed so the data signal can hop, skip and jump across the country.

Microwaves operate at high operating frequencies of 3 to 10 GHz.

This allows them to carry large quantities of data due to their large bandwidth.

Advantages: They require no right of way acquisition between towers. They can carry high quantities of information due to their

high operating frequencies. Low cost land purchase: each tower occupies only a small

area. High frequency/short wavelength signals require small

antennae.Disadvantages: Attenuation by solid objects: birds, rain, snow and fog. Reflected from flat surfaces like water and metal. Diffracted (split) around solid objects. Refracted by atmosphere, thus causing beam to be

projected away from receiver.

SATELLITE

SATELLITE Satellites are transponders (units that receive on one frequency and

retransmit on another) that are set in geostationary orbits directly over the equator.

These geostationary orbits are 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of

Earth's rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to

fling it out into space. Quite similar to radio systems. Their difference with radio systems is that

their intermediate link stations are in orbit around the earth (geosynchronous satellites) and can be as far as 30 000 km. Geosynchronous means the satellite maintains a fixed position relative to the earth, and revolve at the same as the earth under them.

The satellite overcomes the problem/weakness of the microwave for their demand for line- of – sight position, since the horizon intervenes even between high points at distance exceeding a few tens miles.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called

Earth stations because they are located on the Earth. The footprint is the "shadow" that the satellite can transmit to, the shadow

being the area that can receive the satellite's transmitted signal.

Advantages of satellites Reliability of satellites is high Band width reasonably large Suitable for intercontinental communications  Disadvantages of Satellites Huge delay – the long distance between terrestrial

stations via the satellite causes a significant delay of about 240 ms.

It is expensive to launch a satellite Big total attenuations because of the large

distance Data security is low. It is easy to intercept the

transmissions as it travels through the air. Bad weather can severely affect the quality of

satellite transmissions

ASSIGNMENT 4 DUE DATE: 24 – 10 - 2011

1. In a form of a table compare and contrast the use of database with traditional filing system. [10]

2. Write short note on followinga.  Print server

[3]b.  File server [3]

3. Explain the role of a router in context of other networking devices. [4]

ASSIGNMENT 5 DUE DATE: 31 – 10 - 2011

1. Explain ring topology. [5]2. List the different types of guided media. [3]3. Mention any three advantages and three

disadvantages of Internet Shopping.[6]

4. Explain any three ways in which viruses may be spread in computer systems. [6]

5. Explain how you can increase security on an information system against hacking.

[5]