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NETWORKING AND DATA COMMUNICATION
Definition of terms used in Networking
Network
A Network can be defined as a collection of independent entities that are arranged in such a manner as to exchange data, information or resources. Examples of networks: Road network: - this is the interconnection of roads in a country, continent or throughout the world. Road networks facilitate the transfer of goods & services from one area to another. Telephone network (voice networks): - it includes the many lines that criss-cross a country, and enables people to communicate. Railway network. Nervous system.
Computer Network
A computer network can be defined as a collection 2 or more computers connected together using transmission media (e.g., telephone cables, or Satellites) for the purpose of communication and sharing of resources. Usually there can be from 2 to hundreds or even thousands of computers on the network. Apart from computers, other devices such as Printers, plotters, fax machines, modems, etc can also be connected to the network. The term Transmission media refers to any physical or non-physical link between 2 or more computers, and in which a signal can be made to flow from source to destination. Network Server. Computer networks usually have one computer reserved as the “ Mother ” of all the other computers on the network. A Server is a powerful computer that provides services (shared resources) to the other computers on the network. It enables information, resources & network devices to be shared by users on a computer network. Network servers; i). Have a higher hard disk & main memory (RAM) capacity than the other computers on the network. ii). Store & run a special program called the server software (network operating system), which controls computers on the network. Clients (workstations) Clients (also referred to as Workstations ) are Personal Computers (PCs) attached to the network, on which the network users do their work. They are used by network users to send their requests to the server. Clients; i). Are usually less powerful than the server, and use the resources provided by the Server. ii). Have their own operating systems and files. The PCs can be IBM or compatible running MS-DOS, OS/2, Windows, etc.
The figure below shows a server on a network. DATA COMMUNICATION Data communication refers to the process of transmitting data signal from one place to another through a communication media. The basic components of a data transmission system are: (a) A central computer. (b) Terminal devices. (c) Telecommunications link between the central computer & the terminal devices.
Terms used in data communication
Data signal:
A data signal is a voltage level in the circuit which represents the flow of data. In data communication, there are 2 types of data signals; Digital and Analog. Analog data is made up of continuous waveforms, while digital data is made up of a non- continuous discrete waveform. Analog data signal Velocity (Sine wave) Time Digital data signal Velocity (Rectangular waved) Time
Broadband transmission:
This is where an analog signal is sent over the transmission medium using a particular frequency. This means that, several data signals can be sent at the same time through the same medium, but at different frequencies so as to prevent them from overlapping.
Attenuation:
Attenuation is the decrease in magnitude and energy of a signal as it progressively moves along a transmission medium. If the signal is not boosted, it will totally be lost along the way, and may never reach the destination. Attenuation (or signal loss) is usually corrected by placing signal amplifiers (also called repeater stations ) along the medium at appropriate distances in order to receive the weak signal, clean it, amplify it, then retransmit it. Modes of data communication There are 3 modes of data communication: (a). Simplex. (b). Half duplex. (c). Full duplex.
Simplex transmission:
This is where communication is only in one direction (as in radio or television broadcast). The listener or viewer cannot communicate back through the radio or television receiver back to the broadcaster.
Half duplex transmission:
This refers to communication in both directions, but one direction at a time. A sender must first send the data before the recipient can reply, e.g., if two police officers are communicating using a ‘walkie talkie’ radio, one has to say “ over ” to mark the end of every statement in order for the other to respond.
Full duplex transmission:
This is where communication occurs in both directions simultaneously (as in computers that are sending & receiving data on a network). Factors to consider when selecting a data transmission system
- Cost of each type of data transmission method.
- Distance between the computer & the terminal.
- Whether data should be transmitted direct to the computer online.
- Type of data transmission system to be used, i.e., whether the data transmission will be 1- way or 2-way.
- Volume of data to be processed; and whether it is batched at particular times, or whether it is collected individually and required to be processed immediately.
- Speed of transmission required. In many cases, it is acceptable to use the ordinary Postal service, Kenyan rail, or a private Courier service.
- Accuracy and reliability required.
PURPOSE OF NETWORKING
Some of the reasons for setting up computer networks include: 1). Resource sharing A Network resource refers to any component that can be attached to the network for access by users. Some of the shared resources include: i). Application programs. vii). Network Printers ii). Data and information. viii). Fax machines iii). Messages. ix). Modems iv). Graphics. x). Storage devices (optical drives). v). Files. xi). Communication ports. vi). Computer processing power. xii). Disk space Users whose computers are connected to a network can, for example, share their files, exchange mails, send faxes, schedule meetings, and print documents from any point on the network. This centralized access to data & information leads to less waste of time, and hence greater productivity. 2). Remote communications Remote communication refers to the transmission of data signals between two communication devices located at different geographical locations. E.g., using remote communication, one can work from home just as if he/she is in the office. It is mainly through remote communications that people can be able to share ideas, and pass messages over the Internet. A computer that tries to access resources from another computer on the network is called a remote client , while the computer being accessed is called a remote host. Remote communication has been made possible by use of wireless transmission media such as radio waves , microwave , and satellite. 3). Distributed processing facilities Distributed processing refers to the act of running the same programs or databases on different computers, which are on the same network but placed in separate locations. Each computer has its own local peripherals, e.g., disks, printers, terminals, etc. For example; In a large organization, each branch office has its own server that stores data, information, and other resources required for their daily operations. This implies that, files reside on the user’s computer rather than on a central computer, and are only transmitted periodically to update the central computer. Advantages of distributed processing.
- Failure of the central computer does not affect the operations of the other terminals.
- Processing load is shared equally; hence, no time wastage.
- There is faster access of data as each machine can process & store its data.
- It doesn’t need powerful and expensive servers for data storage.
- It can accommodate users with variety of needs. Disadvantages of distributed processing.
- It is more susceptible to virus, as any user could introduce an infected file and spread it throughout the network.
- Developing an effective back up plan is more difficult when users store data in their individual systems.
- File management (organization) is difficult as the files are stored in different locations.
(b). A Network server and a workstation. (c). Remote client and remote host. (d). Half duplex and full duplex transmissions.
4. State the factors to be considered while selecting a data transmission system. 5. Give four advantages and two disadvantages of networking. 6. (a) Explain the concept of distributed processing in networking. (b) State 3 advantages and 2 disadvantages of distributing processing. 7. Why is a network more reliable than stand alone computers? 8. What do you understand by the following terms in networking: (i). Baud. (ii). Baud rate. (iii). Bandwidth. (iv). Resource. 9. What name do we give to each of the following: (a). The computer that is dedicated to serving requests from other computers in a network. (b). The computers that sends requests.
TYPES OF COMPUTER NETWORKS
Computer networks are usually classified according to size. The three most common types of networks are:
- Local Area Network (LAN).
- Metropolitan Area Network (MAN).
- Wide Area Network (WAN).
Local Area Network (LAN).
This is a computer network that is formed whenever computers are connected together in a relatively small geographical area, e.g., in one building or a school. LAN is the smallest size of network & it normally covers an area within the radius of 10M – 3 Km. LAN is usually owned by one organization. However, one LAN can be connected to other LANs over any distance via data transmission lines or wireless media. A LAN connects several Personal Computers to a Server computer. The server computer makes available the resources requested by the other computers (workstations) on a network. In most LANs, each workstation has its own CPU which it uses to execute programs, but still the workstation user can also access data & devices anywhere on the network. Advantages of LANs. 1). They enable many users to share expensive devices such as Laser printers, as well as data. However, the no. of computers that can be connected & the distance to be covered is limited. 2). Have Low cost (requires less expensive equipment). 3). Enable users to communicate with each other, by sending messages or engaging in chat sessions. 4). LANs transmit data at very fast rates. They are much faster than data transmitted over telephone lines. 5). Small error counts (low error rates).
Metropolitan Area Network (MAN).
A MAN is made up of many LANs connected together. It covers a metropolitan (medium-sized geographical) area, e.g., a town or an entire city, within a radius of 5 – 50 Km.
Characteristics of MAN
- Larger than LAN. - Slower than LAN, but faster than WAN with data rates of 100MBps & above. - Are more expensive than LANs, since special equipment is needed to connect the different networks together. - Prone to few errors (moderate error rates).
Wide Area Network (WAN).
This is the largest size of network. A WAN covers a large geographical area such as an entire country, a continent, or even the whole world. It consists of many LANs and MANs connected together to form one large network such as the Internet. Characteristics of WAN They cover an unlimited (a very large) geographical area, e.g., can cover the whole world. They are expensive to build since it requires special equipment for connection. Their transmission links are also expensive. Long distance transmission. Have low data transfer rates compared to LANs (i.e., they are slower than LANs & MANs) More prone to errors (very high possible error rates compared to LANs and MANs). Differences between a Local Area Network and a Wide Area Network.
- LAN is limited to a small geographical distance.
- Data transmission speed in LANs is higher.
- Cost of data transmission in LANs is small.
- There are less transmission errors in LANs.
Review questions.
1. Describe THREE major data communication models. 2. Explain the three most common types of computer networks in use today. 3. Describe a Wide area network. 4. List THREE differences between Wide Area Network and Local Area Network. 5. Determine the type of a network characterized by: (a). connection between computers, printers and other resources using UTP cables. (b). over 250 computers connected to share resources in a city.
ELEMENTS (COMPONENTS) OF NETWORKING
A computer network is made up of several standard components, which can be classified into three (3) major categories, namely:
1. Data communication media. 2. Communication devices. 3. Networking software. Data communication (Transmission) media. A data communication medium is a physical pathway used for carrying data signals & information from one point to another. Data communication media can be divided into two: (a). Communication using cable (bound media). (b). Wireless communication (unbounded media).
Communication using cables (bounded media).
Noise may come from lightening sparks, radio signal, or radiations from spark plugs in motor vehicles. Shielded twisted pair (STP) cables. In STP cables, a braided shield is wrapped around the wires to protect them from noise. Twisted pair cables are grouped into 5 categories according to the type of data transmitted, and the maximum rate of transmission. Category Speed (max. limit) Suitable for transmitting 1 Less than 1 Mbps (i.e., Megabits per second) Voice 2 1 Mbps Data 3 16 Mbps Data 4 20 Mbps Data 5 100 Mbps Data Advantages of Twisted pair cables.
- Can support high data rates (bandwidth) of up to 100 Mbps.
- Telephone systems use UTP, which is present in most buildings. Therefore, it is easier to setup network media because; connection is readily available.
- Installation equipment is cheap & readily available.
- It is cheap because; of mass production for telephone use. Disadvantages of Twisted pair cables.
- They suffer from high attenuation. Therefore, for every cable length of 90m, a “ Repeater ” is needed to amplify (restore) the signal.
- It is sensitive to electromagnetic interference & eavesdropping.
- It has low data transmission rates as compared to other cables.
Coaxial cables.
A Coaxial cable resembles the cable that is used to connect television antenna to a television set. The cable has;
1. A central copper core (which is either solid or stranded wires). The diameter of the centre core determines the attenuation rate. If the core is thin, then the attenuation rate will be higher. 2. An insulator (a dielectric material) surrounding the copper core. 3. A hollow braid (mesh conductor) surrounding the insulator. The braid is made of copper or aluminium, and serves as the ground for the carrier wire. 4. A shield which covers the braid making the core more resistant to electromagnetic interference. The braid together with the insulator & the foil shield protects the carrier wire from Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI).
Coaxial cables have bandwidths of up to 1 Gbps (Gigabits per second). Hence, they can be used to link/connect different networks between buildings, and route trunk calls in telecommunication companies. The Two types of coaxial cables. (i). Thin coaxial cable (Thinnet) : - it has 1 dielectric insulator around the core. (ii). Thick coaxial cable (Thicknet) : - it has 2 dielectric insulators around the core, and is thicker than the thinnet. Advantages of coaxial cables.
- They are very stable even under high loads.
- They have a large bandwidth (up to 1Gbps) compared to twisted pair cables.
- They can carry voice, data and video signals simultaneously.
- They are more resistant to radio and electromagnetic interference than twisted pair cables. Disadvantages of coaxial cables.
- Thick coaxial cable is hard to work with.
- They are relatively expensive to buy & install compared to twisted pair cables.
Fibre optic cables.
A fibre optic cable uses light to transmit data signals from one point to another on the network. A Light Emitting Diode ( LED ) is used at the source/transmitter (sending computer) to convert electrical signals to light signals which are then send along the cable. At the receiving computer, a photosensitive device is then used to convert the light signals back to electric signals that can be processed by the computer. A fibre optic cable is made up of;
1. The Core.
It allows several light signals (rays) to be sent through the cable at the same time. Hence, there are high chances of the signal being distorted. It has a high attenuation rate, and is usually used for shorter distance transmission. Advantages of fibre optic cable.
- It is immune to electromagnetic interference, and eavesdropping.
- It is fast and supports high bandwidth.
- It has low attenuation; hence, a long distance can be covered.
- It does not generate electrical signals; hence can be used in dangerous (highly flammable) places.
- It is smaller & lighter than copper cables; hence, suitable for situations where space is limited. Disadvantages of fibre optic cable.
- Requires expensive connectivity devices and media.
- Installation is difficult because the cable must be handled carefully.
- It is relatively complex to configure.
- A broken fibre optic cable is difficult & expensive to repair.
Review questions.
1. Define the term Transmission media. 2. (a). Give two advantages of coaxial cables. (b). Explain the importance of the wire braid in coaxial cable. 3. Distinguish between Thinnet and Thicknet coaxial cables. 4. Define the term Pitch as used in twisted pair cabling. 5. (a). Give two advantages of fibre optic media. (b). Differentiate between single mode and multimode fibre optic cables.
Wireless communication (unbounded media)
Wireless ( unbounded ) media is a type of media that is used to transmit data from one point to another without using physical connections. In this case, a transmitting antenna & a receiver aerial are used to facilitate the communication. Examples of wireless communication media include:
1. Microwaves. 2. Radiowaves. 3. Infrared transmission. All these waves use different frequencies of the electromagnetic spectrum, and travel at the speed of light.
Below is a diagrammatic representation of the electromagnetic spectrum Radiowaves^ Microwaves^ Infra-red (IR)^ Visible light^ Ultra-violet (UV)^ X-Rays^ Gamma rays (HF)^ High Frequency^ Frequency (VHF)^ Very High^ Frequency (UHF)^ Ultra-High 10 (^6) Hz
(^7) Hz
(^8) Hz
10 Hz^
13 Hz^
15 Hz^
16 Hz^
20 Hz^
22 Hz^
Microwave transmission
Microwave frequencies have a small wavelength, and can easily release their energy in water as heat. This is why they are used in making domestic kitchen appliances, e.g., microwave ovens. In networking, microwaves are suitable for point-to-point transmissions, whereby a signal is directed through a focused beam from the transmitter to the receiver station. Line of sight
Satellite communication
A Satellite is a microwave relay station. The microwave earth stations have parabolic dishes with an antenna fixed on them in order to focus a narrow beam towards the satellite in space. A satellite transmission system has 3 main components:
1. Transmitter earth station - it sets up an uplink to the satellite in order to transmit data. 2. A Satellite that is somewhere in an orbit. It receives, amplifies, and retransmits the signal to a receiving earth station through a downlink frequency. The downlink & the uplink frequency are usually different. This is to prevent the downlink signal from interfering with the uplink signal. 3. Receiving earth station - receives the signal sent by the satellite on the other side of the globe.
Path Transmitting Receiving antenna antenna Power supply TRANSMITTER RECEIVER Fig.: A typical radio transmitter and receiver link Radio waves can be of: High frequency (HF). Very high frequency (VHF). Ultra-high frequency (UHF). High frequency (HF) radio waves The High frequency radio wave signal is transmitted by directing it to the ionosphere of the earth. The ionosphere reflects it back to the earth’s surface, and the receiver then picks the signal. Disadvantage of HF communication The signal can be intercepted by unauthorized parties. Very High frequency (VHF) radio waves They are transmitted along the earth’s surface. However, since the earth is somehow curved, the signal tends to attenuate at the horizons of mountains and buildings. This means that, repeater stations have to be built on raised areas in order to receive, amplify, and propagate the signal from one area to another. Note. The range of VHF is limited, however, it is preferred to HF because; it is possible to make a VHF wave follow a narrower & more direct path to the receiver. Ultra-High frequency (UHF) radio waves The UHF radiowaves use the line of sight principle used by the VHF waves. This means that, there should be no barrier between the sending & the receiving aerial. However, they require smaller aerials. For example; The Television aerial for VHF is bigger than the one for UHF radio waves. This is because; UHF radio waves can be made to follow a narrower & a more direct path to the receiver than VHF radio waves.
The Bluetooth technology
This is a worldwide and short range radio transmission technology that allows all personal, hand- held devices to be able to communicate with each other through wireless technology. It enables people to use hand-held communication devices such as mobile phones & Personal Digital Assistants (PDA’s) to access the Internet. The main component in Bluetooth is a small low power two-way radio transceiver, which can be inserted in small devices.
Bluetooth enabled devices use a network called the Wireless personal area network ( WPAN ) or piconet.
Infrared transmission
Communication through infrared waves (signals) is achieved by having infrared transmitters & receivers ( transceivers ) within a line of sight in the same room. This is because; infrared signals cannot penetrate obstacles like walls and ceilings. However, the signal can be reflected off these surfaces until they reach their destination. For example; Most mobile phones have an infrared transceiver. Once activated, two people in the same room can send messages to each other on their mobile phones without going through the mobile service provider; hence avoid being charged. In computer networking environment, infrared technology can be used to connect devices in the same room to each other without the need for cables, e.g., a computer and a printer. However, the computer’s infrared transceiver must maintain a line of sight with the one for the printer. Advantages of wireless communication.
- Wireless medium is flexible in operation, i.e., devices can be moved around without losing access to the network.
- Wireless networks can span large geographical areas easily.
- Wireless communication can take place via satellite even in very remote areas that do not have high cost physical infrastructure like telephone lines. Disadvantages of wireless communication.
- The initial cost is very high.
- It is relatively difficult to establish or configure.
Review questions.
1. Distinguish between radio and microwave transmission. 2. Describe an electromagnetic spectrum. 3. State two advantages of satellite communication. 4. Give one application area of Infrared transmission. 5. Describe the VSAT technology. 6. Explain the concept of a geostationary satellite. 7. Explain the line of sight principle in wireless communication. Communication devices For a network to be fully operational, communication devices are required, and act as interfaces between the Terminal devices. Terminal equipments are the devices at both ends of the communication link, e.g., computers. Some of the data communication devices are:
1. Network Interface cards (NIC)
A NIC acts as a physical connection (link/interface) between the computer & a properly terminated transmission cable. A NIC is plugged into an empty expansion slot on the motherboard, and has ports at the back in which the terminated end of a network cable can be plugged.
Purpose of using a Bridge a). It can divide a busy network into segments to reduce network traffic. b). To extend the length & number of workstations that a segment can support. c). To reduce overall traffic flow by allowing broadcasts only in the destination segment of the network. The bridge makes sure that packets that are not meant for a particular segment are not broadcast in that segment.
5. Repeater
A Repeater receives a signal from one segment of a network, cleans it to remove any distortion, boosts it, and then sends it to another segment. It therefore, enables the network to eliminate attenuation problems. Note. Repeaters can easily be used to expand a network. This is because; they broadcast the same message to other network segments.
6. Routers
A Router connects different networks, and directs the transfer of data packets from source to destination. Note. Routing depends on network addresses. Each network has a unique address (or identifier) called the IP address. The router will receive a packet of data from another router on the network, and check the network address of the destination. If the address is the same as the one on which the router is, the router will then read the address of the host and then pass the data packet to the destination, otherwise the packet will be routed to the next network address. NB : Network addressing has been made possible because of the use of a special interconnecting protocol called the Internet Protocol ( IP ).
7. Gateways
A Gateway is any device that can be configured to provide access to a Wide Area Network or the Internet. Note. A gateway may be a router, or a computer configured to provide access to the Internet.
Fig.: A gateway PC connecting a LAN to a WAN
8. Switches
Unlike a hub, a Switch forwards a data packet directly to the terminal equipment on the network without broadcasting. It does this by connecting the two nodes point-to-point as if they were linked by a direct cable. Note. Some hubs can also act as switches. Such a hub is referred to as a switching hub. Switches are more expensive than hubs. This means that, one switch may be used as a bridge to connect several hubs. This reduces collision problems caused by broadcasts. Fig. A switch on a Local area network Wireless communication devices For a long time, networks have been implemented using tangible transmission media like cables. However, since the cost of wireless technology has gone down & the quality of service increased, companies & individuals are now using wireless segments in their communications with the aim of extending the capability of wired networks. The most common devices (components) used in wireless communication are: Access Points (AP) , and Wireless antennae.
1. Access points (AP)
An Access point is an entry point into a bounded network. It is used by people who have wireless devices such as Personal Digital Assistants (PDA’s), Laptops, and computers with wireless links. Fig. 3.0: Wireless access point
2. Wireless antennae
The Access point should have antennae so as to detect wave signals in the surrounding. The waves may be Radio waves , microwaves or infrared waves. Most Access points have 2 antennae so that the one that receives the best signal at any particular time can be used.
