Computer network
A computer network refers to a collection of computers linked together using transmission media for the purpose of communication and resource sharing. Some of shared resources include application programs, printers, fax machines, modems, storage devices etc.
Data communication
Data communication refers to the process of transmitting data signals from one point to another through the network. Some of terms used in data communication include:
Data signal
A data signal is a voltage level in the circuit which represents the flow of data. Data signal can either be analog or digital.
Signal modulation
Signal modulation refers to the process of converting data signal from digital form to analog form for the purpose of transmitting it over analog telephone lines.
Signal demodulation
Signal demodulation refers to the process of converting the analog signal into digital form.
Multiplexing
Multiplexing is the process of sending multiple data signals over the same medium.
Demultiplexing
Demultiplexing is the process of separating the multiplexed signals at the receiving end.
Bandwidth
A bandwidth is the maximum amount of data that a transmission medium can carry at any time.
Baseband signal
Baseband signal is a digital signal that is generated and applied to the transmission medium directly without modulation.
Broadband transmission
In broadband transmission an analog signal is sent over the transmission medium using a particular frequency. This means that several data signals can simultaneously be sent through the same medium but at different frequencies.
Attenuation
Attenuation refers to the decrease in magnitude and energy as a signal progressively moves along a transmission medium.
Modes of data transmission
Simplex
Simplex transmission refers to communication in only one direction. An example of simplex communication is a radio broadcast.
Half duplex
Half duplex refers to communication in both directions but one direction at a time.
Full duplex
Full duplex transmission communication occurs in both directions simultaneously.
Types of Computer Networks
Computer networks can be classified using several aspects but the most common classification is according to size.
Local Area Network (LAN)
Metropolitan Area Network (MAN)
Wide Area Network (WAN)
Local Area Network (LAN)
LAN is a computer networks that covers a relatively small geographical area like one building or a school. A LAN is usually owned by one organization. However, one LAN can be connected to other local area networks over any distance via data transmission lines or electromagnetic waves.
Metropolitan Area Network (MAN)
MAN is a network that covers a metropolitan area like a town or city. The MAN size typically covers a radius of between 5 to 50 kilometers. It is made up of many LANs in a metropolitan area. The MAN infrastructure may be owned by a single company that has offices across a metropolitan area or by a service provider who provides network services to subscribers in the area.
Wide Area Networks (WAN)
WAN is larger than a MAN and covers a large geographical area such as a country, a continent or the whole world. It consists of many LANs and MANs connected together to form one large network such as the Internet.
Purpose of networking
Some of the reasons for setting up computer networks include:-
Resource sharing
A resource means data/information, files, printers, modems, communication links, storage devices, fax machines, application programs and any other component that can be attached to the network for access by users.
Remote communications
Remote communication refers to the transmission of data signals between two communication devices located at different locations.
Cost effectiveness
Networks are cost effective in the organization. Network increases the efficient use of scarce resources.
Computer networks have enhanced daily communication by providing paperless communication. Users can send electronic messages and mail to each other instead of having to bear the cost of stamp duty or delivery charges.
Company executives may not need to travel across continents to hold meetings, they can hold video conferences instead and save on travelling expenses and terms.
Reliability
A computer network is reliable in two ways especially when communicating or accessing information.
Data can be transferred with minimum error from source to destination
In case one computer breaks down, a user can still access data and information from the other computers using other computer on the network
Limitations of networking
Networking has many challenges that are associated with implementation and human factors. Some of the limitations are:-
Security issues
Data and information held on network is prone to more illegal access threats. When information is sent over the network from source to destination, it can be tapped or listed to by unauthorized parties.
High initial cost
The initial cost of network hardware and software is very high.
Moral and cultural effects
Large networks like the Internet have chat rooms and messaging services that enable under age children to meet peers and adults on the net some of whom may have bad intentions. Access to pornographic and other negative materials has made the fight against HIV/AIDS, bad sexual behaviour, drugs and substance abuse more complicated.
Spread of terrorism and drug trafficking
Terrorists and drug traffickers communicate easily because internet provides easy flow of information from one point to another.
Over-reliance on networks
Modern organizations today have phased out most manual operations and depend on computer networks. If by any chance the network fails or go down, then all systems in the organization will be brought to a halt.
Elements of Networking
A computer network is made up of several standards elements (components) which can be classified into the following categories:
Data communication media
Communication devices
Networking software
Data Signal
Data Communication Media
A data communication medium is a pathway used for carrying data and information from one point to another
Types of data communication media
Bounded media
Unbounded media
Bounded Media
In bounded media data signals are transmitted from the source to destination through a restricted pathway such as through cable.
Types of Bounded Media
Two wire open lines cables
Two wire open lines cables are made up of parallel copper wires separated by a plastic insulator. They are used in telecommunication network to transmit voice signal.
The plastic insulator is meant to reduce inter-line interference called cross-talks. However their linear nature allows an electromagnetic field to build around them during heavy data transmission which may cause interference to the signals. The wires also capture environmental frequencies such as radio waves hence causing noise in the transmission channel. Noise refers to random unwanted signals picked up by the channel.
Twisted Pair Cables
A twisted pair cable is made up of two solid copper wire strands wound around each other in a double helix manner. The winding of the wires is meant to reduce the development of an electromagnetic field around the two wires as they transmit data. These cables are mostly used to transmit both voice and data signals.
Types of twisted cables
Unshielded Twisted Pair (UTP)
UTP cables do not have a shield that prevents Electromagnetic interference (EMI) from the environment. Noise may come from lightening sparks, radio signal and even the radiations from spark plugs in motor vehicles. Therefore UTP is not suitable for environments that are electrically noisy.
Shielded Twisted Pair (STP)
STP is similar to UTP except that a braided is wrapped around the wires to shield or protect them from noise.
Twisted pair cables can also be categorized into five groups according to the type of data transmitted and maximum rate of transmission
Most organizations use category 5 twisted pair cables to set up their local area networks.
Advantages of Twisted Pair Cables
Telephone systems use UTP which is present in most buildings. It is easier to set up network media because connection is readily available.
Installation equipment is cheap and readily available
It is cheap because of mass production for telephone use.
Disadvantages of Twisted Pair Cables
It suffers high attenuation. For every cable length of 90 meters, a device for restoring the signals called a repeater is needed to amplify the signal.
It is sensitive to electromagnetic interference and 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. It has a central copper core which may be of solid or stranded wires surrounded by a dielectric material (insulator). The dielectric material is then surrounded by a hollow mesh conductor which is covered by a shield. Data is carried on this cable using direct current (DC).
Coaxial cables have bandwidth of up to 1 Gbps, hence they are installed in a network to form the network backbone.
Types of coaxial cables
Thin coaxial cable (Thinnet) It has one dielectric insulator.
Thick coaxial cable (Thicknet)
It has two 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 large bandwidth (up to 1Gbps) compared to twisted pair.
They can carry voice, data and video signal 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.
Coaxial cables are relatively expensive to buy and install as compared to twisted pair.
Fibre optic cables
Fibre optic cable is the one of the latest types of bounded transmission media to be developed. The fibre optic cable utilizes light to transmit data from one point to another on the network. The light signal travels through the core through the process referred to as total internal reflection. The process that causes total internal reflection is called refraction. Refraction is the bending of light when it crosses the boundary of two mediums that have different density.
The fibre optic cable is made up of the core, cladding, buffer, strength members and the jacket.
The core:- It is the central part of the cable and is made of a hollow transparent plastic or glass.
Cladding:-It is a single protective layer surrounding the core. It has some light bending characteristics in that, when the light tries to travel from the core to the cladding it is redirected back to the core.
Buffer:- The buffer surrounds the cladding and its main function is to strengthen the cable.
The jacket:- It is outer covering of the cable.
Types of fibre optic cables
Single mode fibre
Single mode fibre has a very narrow centre core. The light in the cable can therefore take only one path through it. Due to this, it has very low attenuation rate and is preferred for long distance transmission.
It has a bandwidth of 50Gbps. Single mode fibre is very expensive and requires very carefully handling during installation.
Multimode fibre
It has a thicker core than the single mode. It allows several rays to be fed in the cable at an angle. Because of multiple light signals navigating the cable at the same time, distortion of the signal is possible. Multimode fibre optic cables have high attenuation rate and are usually used for shorter distances than single mode.
Advantages of fibre optic cables
It is immune to interference and eavesdropping.
It is fast and support high bandwidth.
Large distance can be covered because it has low attenuation especially single mode fibre.
It can be used in hazardous places (highly flammable) because they do not generate electrical signal.
It is smaller and lighter than copper cable hence ideal for space limited situations.
Disadvantages of fibre optic cables
Connectivity devices and the media are expensive.
Installation is difficult because the cable must be carefully handled.
It is relatively complex to configure.
Unbounded Media (Wireless communication)
Wireless or unbounded media, is a type of media that is used to transmit data from one point to another without using physical connections. Examples of wireless transmission media include microwaves, satellite, radio waves, and infrared transmission all which use different frequencies of the electromagnetic spectrum.
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Microwave transmission
Microwave frequencies range from about 3GHz to 40GHz on the electromagnetic spectrum. In networking microwaves are very suitable for point to point transmission. This means that a signal is directed through a focused beam from transmitter to the receiver station.
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 three main components
Transmitter earth station
It sets up an uplink to the satellite in order to transmit data. The uplink will have a unique frequency.
Satellite
It is somewhere in an orbit that receives, amplifies and retransmits the signal to receiving earth station via a downlink frequency that is different from that of the uplink to avoid interference with the uplink signal.
Receiving earth station
It receives the sent signal on the other side of the globe
Radio communication
Radio waves travel just like water waves, they are omnidirectional. They start from the central point and spread outwards in all directions. The waves are radiated into the atmosphere by a radio frequency antenna at constant velocity.
Radio waves are not visible to the human eyes. Radio waves are used in radio and television broadcasts. Data can also be transmitted over radio waves communication channels. Radio waves can be of High Frequency, Very High Frequency or Ultra-High Frequency.
High Frequency (HF)
HF radio waves signal is propagated by directing it to ionosphere of the earth. The ionosphere will reflect it back to the earth’s surface and the receiver will pick the signal. The biggest challenge of HF communication is the danger of signal interception by unauthorized parties.
Very High Frequency (VHF)
VHF radio waves are transmitted along the earth’s surface. Due to the curvature of the earth, the signal will most likely attenuate at the horizon. This means that repeater stations have to be placed strategically to maintain a line of sight in order to receive, amplify and propagate the signal from one area to another. This technology is popular for the hand held radio device like “walkie-talkie” radios.
Ultra-High Frequency (UHF)
UHF radio waves are like VHF when it comes to the line of sight principle. This means that there should be no barrier between the sending and the receiving aerial. However they require smaller aerials.
UHF radio waves can be made to follow even narrower and direct path to the receiver than VHF radio waves. Therefore UHF is popular for horizon limited broadcasts.
Bluetooth technology
Bluetooth is the one of the latest radio transmission technologies. It is a worldwide and short range radio technology that enables people to use handheld communication devices such as cell phones and Personal Digital Assistants to access the Internet.
The main component in Bluetooth is a small low power two-way radio Trans-receiver, small enough to be inserted in small devices. A network of Bluetooth enabled devices is called a Wireless Personal Area Network (WPAN) or piconet. Bluetooth networks are best suited for personal or handheld devices. This has made radio transmission become very popular in mobile communication and Internet connectivity.
Infrared transmission
Just like the radio waves, infrared waves are not visible to the human eye. Communication through this medium is achieved by having infrared transmitters and receivers (transceivers). Transceivers of infrared signals must be within a line of sight in the same room. This is because unlike radio signals, infrared signals cannot penetrate obstacles like walls. However the signal can be reflected off surfaces like walls and ceiling until they reach their destination.
An example of an infrared device is the infrared transceiver on most mobile phones. Once activated, two people in the same room can send messages to each other using infrared technology on their mobiles without going through the mobile service provider hence avoid being charged.
Advantages of wireless communication
Wireless medium is flexible in operation. Devices can be moved around without losing access to the network.
Wireless network 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
It is relatively difficult to establish or configure.
The initial cost is very high.
Communication devices
Computers and communication media require communication devices for the network to be fully operational. These devices are used as interfaces or junctions between the terminal devices. Some examples of data communication device include
Network Interface Card (NIC)
Modems and codecs
Hubs
Bridges
Repeaters
Routers
Gateways
Switches
Access points
Network Interface Card (NIC)
NIC creates a physical link between the computer and the transmission media. A Network Interface Card is plugged into an empty expansion slot on the motherboard. However, most computer motherboards today come ready with an onboard network interface controller.
Modems and codecs
A modem is a device that converts a signal from digital to analog for the purpose of transmission over the analog media while a codec converts an analog signal to digital form for transmission via a digital medium. A modem can be external, an add-on card or built on the motherboard.
Hubs
A hub (concentrator) is a device that connects networks that have a common architecture. Network that has the same set of communication software usually called protocol. Protocols are a set of rules that govern the communication between devices on a network.
A hub transmits signals by broadcasting them to all the computers on the network. The computer whose address is on the message picks the message from the network that is part of the broadcast domain. Several hubs can be connected together one after another to expand a network.
Bridges
A bridge is a network device that selectively determines the appropriate network segment for which a message is meant for delivery through address filtering. Hence a bridge can divide a busy network into segments to reduce traffic. A bridge makes sure that packets that are not meant for a particular segment are not broadcasted in that segment.
Repeaters
A repeater is a device that receives signal from one segment of a network, cleans it to remove any distortion, boots it and then sends it to another segment. Repeaters are the simplest way to expand a network because they broadcast the same message to other network segments. They enable the network to eliminate attenuation problems.
Routers
A router is a network intermediary device that interconnects different networks and directs the transfer of data packets from source to destination. Some modern routing devices combine the functionality of a bridge and a router, such a device is called a brouter.
Gateways
A gateway is any device that can be configured to provide access to wide area networks or Internet. One such device is the router in which the gateway software is installed. A gateway is the most powerful network and internetwork connectivity device because of its ability to convert data across different network architectures and protocols.
Switches
A switch is a device that forwards a packet directly to the address node without broadcasting. A node refers to a data terminal equipment such as a workstation or a computer on the network. The switch connects two nodes point to point as if they were linked by a direct cable between them. This reduces the broadcast problems on the network. Some hubs also incorporate the switching mechanisms. Such a hub is referred to as switching hub. Typically switches are more expensive than hubs.
Wireless communication devices.
Access Points (AP)
The Access Point is one of the most common wireless network components. It is an entry point to a bounded network for people who have wireless devices such as Personal Digital Assistants (PDAs), laptops and computers with wireless links.
Wireless Antennae
An antennae is a device that detects signals in the surrounding. The Access Point needs to have antennae in order to detect signals in the surrounding. The waves may be radio waves, microwaves or infrared waves in nature. In most cases, Access Points will have two antennae so that the one that receives the best signal at any particular time can be used.
Personal Computer Memory Card International Association (PCMCIA) cards
A PCMCIA is an add-on card inserted into a device such as PDA or a laptop in order to enable wireless communication between the devices and a wired network server.
Network software
Network software can be classified into two main groups:-
Network Operating System (NOS)
Network protocols
Network Operating Systems
These are operating systems specifically designed to optimize the networked computers ability to respond to service requests. Servers run on a network operating system. In addition to the normal operating system, this software performs the following network related functions
Provide access to network resources e.g. printers and folders.
Enables nodes on the network to communicate with each other more efficiently.
Supports inter-process communication i.e. enables the various processes on the network to communicate with one another.
Respond to requests from application programs running on the network.
Supporting network services like network card drivers and protocols.
Implementing network security features
Network Operating Systems are designed as multiuser operating systems that run the network server program. Once installed on the right hardware platform and configured as a server, the OS will provide network management tools to the network administrator. The administrator can use the tools to do the following:
Secure the network against unauthorized access.
Track network usage and keep a log of all people who have used the network.
Ensure inter-operatability between various systems on the network.
Performance monitoring to ensure maximum throughput on the network.
Examples of network operating systems are Windows NT/2000/2003, UNIX, Linux and Novell Netware.
Network protocols
Protocols are set rules and procedures that govern communication between two different devices or people. In computer networking, protocols refer to the rules and technical procedures that govern communication between different computers.
How protocols work
Data transmission process over the network has to be broken down into discrete systematic steps. At each step, a certain action takes place. Each step has its own rules and procedures as defined by the network protocols.
The work of these protocols must be coordinated so that there are no conflicts or incomplete operations. This co-ordination is achieved through protocol layering. Network protocols are designed after the Open Systems Interconnections (OSI) model. The OSI model has seven layers, each performing distinct function.
Application layer
Application protocols work at the highest layer of the OSI model. They provide services to application programs. An example of an application protocol is e-mail program that enables composing or reading of e-mail messages. Other examples include:-
Simple Mail Transfer Protocol (SMTP) – An Internet protocol for transferring emails.
File Transfer Protocol (FTP) – An Internet protocol for file transfer.
Apple talk and apple share – Apple computers networking protocol suit.
Transport layer
Transport protocols ensure that data is passed between computers more reliably. Some examples include:-
Transmission Control Protocol (TCP) – This is responsible for delivery of sequenced data over the network.
Sequential Packet Exchange (SPX) – This part of the Novell’s Internetwork Packet Exchange/Sequential Packet Exchange (IPX/SPX) for sequenced data.
NetBEUI – a local area network protocol for Microsoft and IBM networks that establishes communication sessions between computers.
Apple Transaction Protocol (ATP) – Apple computer’s communication session and data transport protocol.
Network layer
Network protocols provide link services. They handle addressing and routing information, error checking and retransmission of requests. Some examples of network layer protocols include:-
Internet Protocol (IP) – It does packet forwarding and routing.
Internetwork Packets Exchange – Netwares protocol for packet forwarding and routing.
NetBEUI – Provides data transport services NetBius Sessions and application.
Data signal
All messages that are sent and received through the network must be represented using a data signal. Metallic media would like an electrical signal; wireless media need electromagnetic signals, while fibre optic cables need light signals. A signal can either be analog or digital.
Network topologies
Network topology refers to the way in which computers and other devices have been arranged or how data is passed from one computer to another in the network. Therefore network topology can be viewed in two ways namely logical and physical topology.
Logical topology
Logical topology/signal topology deals with the way data passes from one device to the next on the network. Examples of logical topologies are Ethernet and token ring topology.
Ethernet topology
In Ethernet topology all computers listen to the network media and can only send data when none of the others is sending
Token ring topology
In Token ring topology, a special package for data called token goes around the network and only the computer whose address is on the data held in the token will take up the token to read the data then release the token. The token can then be captured by another computer which needs to transmit data.
Physical topology
Physical topology refers to the physical layout or arrangement of computers and other components on the network. Examples of physical topologies include star, bus, ring, mesh and tree/hierarchical topology.
Star topology
In star topology, all devices are connected to a central hub. Nodes communicate across the network by passing data through the hub. When the hub receives data from transmitting computer, it broadcasts the message to all the other nodes on the network
Advantages of star topology
It allows centralization of key networking resources like concentrators and servers.
It gives the network administrator a focal point for network management.
Star network are easy to configure.
Disadvantages of star topology
The star-based network is costly because it requires one complete cable per computer.
If the central hub fails, the entire network will be down.
Installation time consuming because each node forms a segment of its own.
Bus topology
In bus topology, all devices are connected to a central cable called the bus or backbone.
A terminator is attached to each end of the cable to avoid signals from bouncing back and forth on the cable causing signal distortion. As the data passes along the cable, each station checks whether the data is addressed to it. If the address matches the machines address, it receives the data otherwise it rejects it. The network address of computers on a network is called the Medium Access Control (MAC) address.
Advantages of the bus topology
It is easy to install.
It is less costly. Does not require a complete cable length per computer.
Disadvantages of the bus topology
A cable break in any section brings down the whole networks.
Troubleshooting a cable fault is difficult because the fault could be anywhere on the cable.
The bus topology limits the number of computers that can be connected to the cable because each computer is listening to the cable in order to transmit. This means that an increase in the number of computers result in an increased collision as machines compete for transmission.
Ring topology
In a ring topology, all computers are connected to one another in the shape of a closed loop.
Each station is responsible for regenerating and retransmitting signals around the network to its neighbor. A token is used to exchange data from one station to another. A token is like an envelope or a bag where data is placed for transmission and carried around the network.
Advantages of ring topology
They use a short length cable.
Ring topology is simple to install.
Disadvantages of ring topology
Modification may be difficult because adding or removing a device can disrupt the entire network.
Troubleshooting can be difficult.
One device or media breakdown may affect the entire network.
Mesh topology
This is the most type of topology used in wide area network where there are many paths between different locations.
Devices are connected with many redundant interconnections between the nodes. In a true mesh topology every node has a connection to every other node in the network.
Advantages of mesh topology
The network can still operate even when a node breaks down or a connection breaks.
The network is reliable.
Point to point connections optimize throughput of data.
Disadvantages of mesh topology
It is expensive on cable due to redundant links.
Administration of the network is difficult because of the peer to peer connections.
Tree or Hierarchical topology
This is a hybrid topology. Groups of star-configured networks are connected to a linear bus backbone.
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