TOPIC 10: DATA COMMUNICATION & NETWORKING. Sub-topic 10.2: Iintroduction to Computer Networks


INTRODUCTION TO COMPUTER NETWORKS

Definition of a computer Network

A computer network is defined as a collection of computers linked together using transmission media for the purpose of communication and resource sharing.

Some of the shared resources include internet connectivity, printers, fax machines, modems, storage devices, networked software programs etc.

COMPONENTS OF A NET WORK

There are several components that make up computer network. These include the following:

(1) Network operating system:

This is a master control program that manages basic network operations like data and file transmissions, etc. e.g.

-Windows Server 2008

-Ubuntu Server

-Windows MultiPoint Server 2011

-IBMs LAN. Etc.

(2) Network Adaptor/Network Interface Card (NIC)

Hardware devices which make it possible for all computers on the network to communicate with others on the network.

(3)  Hubs and Repeaters

Are devices which accept transmitted signals, amplifies them, and put them back on the network media rejuvenated. Hubs are typical of LANs whereas repeaters are typical of WANs.

NB. The most common network faults necessitating hubs and repeaters include;

(i)  Distortion: – where information is confused at the source.

(2) Attenuation; i.e. Loss of signal strength.

Hubs can take any of the following forms:

  • Active Hub: – Rejuvenates data signals
  • Passive Hub
  • Managed Hub
  • Unmanaged Hub
  •  

(4) Shared peripherals

E.g. Network printers, scanners, fax machines etc.

(5) Client computers:

I.e.  Computers sharing resources and capable of sending and receiving data signals to and from the server.

(6) Transmissions/Communication Media:

       These can also be referred to as communication lines or links.

Coaxial cables 

These are metallic connecting wires. They are typical of the following characteristics;

                        –   They are shielded

  • They have a lot of signal noise
  • Provide minimum data strength.
  • Used on Network antennas.

Fiber optic cables 

These are non-metallic transmission media carrying signals in form of light beans.

Twisted pair cables:

These are typical of the following characteristics;

  • Two or more twisted wires.
  • Twisting conceal out or eliminates signal noise.
  • Usually shielded but unshielded at times.

Wireless media

-Radio waves.

-Infra-red waves.

– Micro waves. These transmit voice and data through the atmosphere as super high frequency radio waves. They cannot bend around corners.– use microwave Dishes/Antennas.

-satellite systems. I.e. microwave wave relay stations in orbit around the earth.

-pagers. Simple radio receivers that receive data sent from a special radio transmitter.

(7)  Server:

Very fast processor computer dedicated to providing specific services for other computers connected on the network. A server can be in form of:-

  • A file server.
  • Application server.
  • Communication server, etc.

Prerequisites for a Server

  • Very strong processor e.g. Motorola G4 and Pentium V, IV, III, II, etc, Power Pc, etc.
  • High RAM capability – DIMMs for RAM 500 + mBs.
  • Very powerful storage abilities.
  • Enhanced Network through-put.
  • Expansion abilities.
  •  

Functions of a Server                                                              

  • Administration of client computers.
  • Security of files and applications.
  • Managing printer Jobs.
  • Data bank.
  • Software and Applications handling.

(8) MODEM

A hardware device which transforms digital signals into wave form (analog – Modulation) to facilitates their transmission into air space, and vise versa (demodulation)

(9) Multiplexer

A hardware device which merges several low speed transmissions into one high-speed transmission.10. concentrator

A network device which enables several client computers to share a single line.

Factors to consider for a Network choice

  • Size of the organization. A peer-to-peer network configuration is ideal for less than ten users.
  • Needs of the Network e.g. network administrator, hardware and software, etc.
  • Level of data security needed. A peer-to-peer network configuration is not very secure.
  • Amount of network traffic expected (band width)
  • Network budget/cost of the network configuration and installation.
  • Maintenance requirements.
  • Level of administrative support available.
  • Types of business or organization.

Advantages of computer Network facilities/Rationale of networking

  • They facilitatesharing of peripherals devices.
  • Facilitate Sharing of programs and data files (resources).
  • Better and enhanced communication and collaboration (networking).
  • Better access to data bases and files for sharing through Uploading and Downloading receipt of files from host computer
  • Downsizing and reduced operational cost. Flatter organizational structures.
  • Offers more Data security. Information on networks (internet) cannot be easily altered.
  • Better management control through shortening the decision making process.
  • Data consistence. Necessary changes can easily be made.
  • Down sizing
  • Large data transfers.

          ATM = Asynchronous Transfer Mode.

          ISDN = Integrated Service Digital Network.

          ISO = Integrated service organization.

NETWORK TOPOLOGIES

Is the pattern by which the signaling and cabling medium is laid to interconnect the various computers which form the network. Include:-

(1) Bus or Linear Bus Topology

This is a topology in which all client computers connected to main coble – common bus or back born server or peer – to – peer network.

Advantages of bus topology;

  • It is cheaper as it does not involve expensive hubs and servers.
  • Break down of one work station does not affect others.
  • Easily organized into client server.
  • Communication is very direct and faster.

Problems of bus topology;

Data is not very secure.

(2) Star Topology

Is a network cabling configuration that uses a central connection point called a hub, through which all communication is directed.                                                                    

Advantages of bus topology;

  • Life continuous even when connection is broken
  • Data flow is faster with an active hub. Hub minimizes collisions between messages.
  • Easily organized into client server.

Problems of bus topology;

  • It is a bit expensive.
  • If the hub goes down the entire network stops.

(3) Ring Topology

Is a topology in which the networked computers are connected in a series forming a closed loop.

Advantages of ring topology;

  • Signal flow is one direction which avoids the danger of data collision.
  • Where a double ring (FDDI) is involved data flow is continuous even in cases where one ring brakes down.
  • Having no central server makes it cheaper.

Disadvantages of ring topology

  • If a connection in a single ring is broken the entire network stops working.
  • Having no central server makes data very insecure.                                                                                                                
  1. Mesh Topology:

Is a topology in which every client computer (WS) has got its own link to every other client computer on the network.  This allows every client work station to communicate freely at any time.                                                                 

(5) FDDI = Fiber Distributed Data interface.

FDDI network uses fiber-optic cables with an adaptation of a two token-ring configuration topology.

Advantages of FDDI

  • Data flow is very fast. It can transmit 100 – 200mbs
  • Where one ring fails another ring can continue with data supply.
  • It can transmit 100 – 200mbs

Disadvantages of FDDI

  • It is relatively expensive.
  • A bit time consuming to construct.

(6) Hybrid/Tree Topology

Is a network configuration formed as a result of combining two or more of the above topologies interconnected by hubs. It can help in linking different types of LANs

NB;

A network can either stand a lone (i.e. PBX configuration) or be connected to other different network configurations which can be either similar or not, hence need for the following devices:

(a) A BRIDGE:

Is a device used link two similar networks to communicate to one another e.g. Bus – 2 – Bus or Star 2 star, etc.

(b) A GATEWAY

Is a device (interface) used link two different networks to communicate with one another. E.g. Bus to star, LAN to MAN, etc.

(c) ROUTER

Is a device used to link more than two different network configurations to communication to one another?

Factors affecting communication among devices

  1. Transmission rate – frequency and Bandwidth. High frequency, wider bandwidth means more data.

2. Line configurations – point-to-point Vs multipoint. In a P-2-P a single line the sending and receiving devices are directly connected to one another. Whereas, in a multi-point configuration, a single line interconnects communication devices to one computer.

3. Serial (bits are transmitted sequentially one after the other) Vs parallel transmission (bits are transmitted through separate lines simultaneously).

4. Direction of transmission – Simplex (one direction flow), half – duplex (two but one at a time) and full – duplex (back and forth at the same time).

5. Transmission mode – asynchronous (one byte at a time) Vs synchronous (in blocks).

6. Protocol – Set of convention governing the exchange of data amongst devices. Protocols are built into the hardware and software one is using.

7. Type of material one is trying to access. Pictures and other complex graphics take long to load because of larger files.

8. Speed of your modem, and modem at the other end.

9. Amount of other traffic on the Network.

10. Transmission media

Factors/Parameters of a good network

Consistence/reliability parameter. This describes the predictability of the network.

Flexibility. This defines networks ability to grow or change with minimal disturbances to the users and applications.

Availability  at anytime one wishes to access it.

Recovery parameter. It should be very easy to restore the network to its operational level following it failure.

Performance. This is measured in terms of network throughput and response time. Throughput = work load per unit time ratio whereas response time = the rate at which the CPU interprets commands to the user.

Security parameter. This describes the restrictions to access information from the network, and limitations to network spasm plus hackers/crackers.

Network Protocol
This refers to a set of rules and procedures governing transmission between components in a computer network.

The role played by networking protocols as used in Networking

  • Identifying each device in the communication path;
  • Securing the attention of the other device;
  • Verifying correct receipt of the transmitted message;
  • Determining that a message requires retransmission if it is incomplete or has errors;
  • Performing recovery when errors occur.

Common protocols as used as in networking

Simple Mail Transfer Protocol (SMTP) – an internet protocol for transferring of e-mails.
File Transfer Protocol (FTP): It allows files containing text, programs, graphics, numerical data, and so on to be downloaded off or uploaded onto a network.
Internet Protocol (IP) – does the packet forwarding and routing.
Transmission Control Protocol/Internet Protocol (TCP/IP) is a network standard that defines how messages (data) are routed from one end of a network to the other, ensuring the data arrives correctly.
Transmission Control Protocol (TCP) :responsible for delivery of data over the network.
Hypertext Transfer Protocol (HTTP): It allows Web browsers and servers to send and receive Web pages.
Simple Network Management Protocol (SNMP): It allows the management of networked nodes to be managed from a single point.Telnet Protocol: It provides terminal emulation that allows a personal computer or workstation to act as a terminal, or access device, for a server.
Sequential Packet Exchange (IPX/SPX):works with the Novell’s internet work’ packet / sequential exchange; responsible for delivery of sequential data over the network

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