COM111 Lecture Schedule 7

COM111 Lecture Schedule 7

Introduction to Computer Networks, Network Topologies, Communication Protocol, Network Devices

Computer Network

Computer network is a collection of two or more computers, which are connected together to share information and resources.

Computers are powerful tools that can process and manipulate large amounts of data quickly, but they do not allow users to share that data efficiently.  Before networks, users either prints documents or copy files from one disk to another. The changes made by another user in the document cannot be merged and that type of working environment is called stand alone environment.

When two computers are linked by using cable, that allows sharing data, it is known as network environment. The computers and other devices (printers) are connected through a cable, called as network. Connecting two or more computers and sharing resources is called networking.

            Operating system such as DOS is designed for single user to control one computer; Network Operating System (NOS) is an interconnected system of computing devices that provides shared and economical access to computer services. NOS coordinate the activities of multiple computers across a network.

NOS are broadly divided into

1.      Client/Server networks

2.      Peer – to –Peer (P2P) networks.

Client/Server Network

Each computer is either a client or a server to complete a particular task. Centralised host computer is known as server and the user’s individual workstation is known as client. Client request a service from the server and the server response by providing service.  Server provides access to resources, while the clients have access to the resources available on server. Server software runs on a computer, for hosting the software and supports its services. Client software runs on personal computers or workstations. In Client/Server relationship, both client and server carry out data processing in their ends.

 

P2P Network

P2P network does not rely on dedicated servers for communication; instead it uses direct connections between clients. A pure P2P network does not have the notion of clients or servers, but only equal peer nodes that simultaneously function as both clients and servers to other nodes on the network. The number of nodes that can function as both clients and servers on a P2P network is between 10 and 25. If there are more nodes, then P2P machine can be used as dedicated servers. P2P networks have been designed primarily for small to medium LANs. Many sharing services such as Morpheus and Kaaza are used extensively to find files on the network. Most of these file sharing services actually integrate both P2P and client/server networking design. These file sharing applications are known as hybrid networks.

Generations in P2P Networks

P2P file sharing networks can be classified according to their generations based on Internet based file sharing networks.

First Generations – this generation has centralised file list like Napster. Centralized file is responsible for infringement of copyright or other illegal activities, which might occur while transferring files.

Second Generation – This generation had decentralized file list such as Gnutella and Fast track. This is necessary for network creators, as a central authority will be held liable for copyright infringement.

Third Generation – This generation is an improvement upon previous generation. This network has   inbuilt features such as efficiency, reliability and anonymity. Examples are freenet,  GNUnet etc.

Difference between P2P and Client/Server Network

Basics	P2P Network	Client/Server Network
Centralized	No central repository for files and applications	Resources and data security are controlled through the server.
Maintenance	Low maintenance cost file	A large network requires extra staff to ensure efficient operation
Installation	Can be easily installed	Requires experts for proper installation of the network
Expensive	No need for a dedicated server, thus not much expensive	Expensive, as it requires a dedicated server
Security	Lack of proper security policies is the biggest drawback	Provides high level of security
Dependence	All nodes are independent of each other. Failure occurring in one node does not affect the functioning of other nodes in the network.	When server goes down, it affects the functioning of entire network.

Types of computer networks

A network can be as few several personal computers on a small network or a large as internet, a worldwide network of computers. The types are networks are

1.      LAN(Local Area Network)

2.      MAN(Metropolitan Area Network)

3.      WAN (Wide Area Network)

LAN (Local Area Network)

LAN is a computer network that covers only a small geographical area such as office, house or building. LAN is connected to computers which have NOS installed. Computers designed as server, which controls the network.  It contains software that can be shared by computers attached to network. Other computers connected to the server are called workstations. The work stations are not powerful as server. LAN cables are used to connect the computers. LAN offers a bandwidth of 10-100 Mbps.

MAN (Metropolitan Area Network)

MAN is a network of computers spread over metropolitan area such as city and its suburbs.   In MAN city bridges its LANs with series of backbones, making one large network for the entire city. It may be a single network such as cable television network or connecting number of LANs.

WAN (Wide Area Network)

WAN is a system of interconnecting many computers over a large geographical are such as cities, states, countries or even whole world. This kind of network uses telephone lines, satellite links and other range communications technologies to connect.

WAN offers many advantages to business organizations. They are as follows

·         It offers flexibility of location because not all the people using the same data have to work at the same site

·         Communication between branch offices can be improved using e-mail and file sharing.

·         It facilitates a centralized company wide data backup system.

·         Companies located in number of small, interrelated offices can store files centrally and access each other’s information.

Network topologies

Topology refers to the way a network is laid out, either physically or logically. Topology is considered as a network shape. The five basic topologies are

1.      Bus topology

2.      Ring topology

3.      Star topology

4.      Tree topology

5.      Mesh topology

Bus topology

Bus topology uses a common bus or backbone to connect all devices with terminators at both ends. The backbone acts shared communication medium and each node is attached to it with an interface connector. When a message is to be transmitted on the network, it is passed back and forth long the cable, past the stations and between the two terminators from one end of the network to the other. 

As the message passes each station, the station checks the message’s destination address. If the address in the message matches, the bus carries the message to the next station receives the messages. If the addresses do not match, the bus carries the message to next station and so on.

Advantages of Bus Topology

1.      Connecting a computer or peripheral to linear bus is easy.

2.      This topology requires least amount of cabling to connect the computers and therefore, less expensive than other cabling arrangement.

3.      It is easy to extend a bus since two cables can be joined into one longer cable with connector.

Disadvantages of Bus Topology

1.      Entire network shuts down if there is a failure in the backbone.

2.      Heavy traffic can slow down a bus because computers on such networks do not coordinate with each other to reserve time to transmit.

 Ring Topology

In ring topology, computers are placed on a circle of cable without any terminated ends since there are no unconnected ends. Every node has exactly two neighbours for communication purposes. All messages travel through a ring in the same direction until it reaches its destination. Each node in the ring incorporates a repeater.

Advantages of Ring Topology

1.      Ring topology is easy to install and reconfigure.

2.      Every computer is given equal access to the ring. Hence no single computer can monopolize the network.

Disadvantages of Ring Topology

1.      Failure in any cable or node breaks the loop and can take down the entire network.

2.      Maximum ring length and number of nodes are limited.

Star topology

In star topology, devices are not directly linked to each other, but they are connected via a centralized network component known as hub or concentrator. The hub acts as central controller and if a node wants to send data to another node, it boosts up the message and sends the message to the intended node. This topology commonly uses twisted pair cables; however, coaxial cable or fibre optic cable can also be used.

Advantages of Star topology

1.      Star topology is easy to install and wire

2.      The network is not disrupted even if the node fails or is removed from the network.

3.      Fault detection and removal of faulty parts is easier in the star topology.

Disadvantages of Star topology

1.      It requires a longer length of cable.

2.      If the hub fails, nodes attached to it are disabled.

3.      The cost of the hub makes the network expensive as compared to bus and ring topologies.

Tree Topology

            A tree topology combines the characteristics of linear bus and star topologies.  It consists of groups of star-configured workstations connected to a bus backbone cable.  Not every node plugs directly to the central hub. The majority of nodes connect to a secondary hub that, in turn, is connected to the central hub.  Each secondary hub in this topology functions as the originating point of a branch to which other nodes connect.

Advantages of Tree Topology

The advantages of tree topology are:

            The distance to which a signal can travel increase as the signal passes through a chain of hubs.

            Tree topology allows isolating and prioritizing communications from different nodes.

            Tree topology allows for easy expansion of an existing network which enables organizations to configure a network to meet their needs.

Disadvantages of Tree Topology

 The disadvantages of tree topology are

            If the backbone line breaks, the entire segment goes down.

            It is more difficult to configure and wire than other topologies.

Mesh Topology

            In a mesh topology, every node has a dedicated point-to-point link to every other node.  Messages sent on a mesh network can take any of several possible paths from source to destination.

Advantages of Mesh Topology

 The advantages of mesh topology are

The use of large number of links eliminates network congestion.

If one link becomes unusable, it does not disable the entire system.

Disadvantages of Mesh Topology

The disadvantages of mesh topology are

As every node is connected to the other, installation and reconfiguration is very difficult

The amount of hardware required in this type of topology makes it expensive to implement.

Communication Protocol

A communication protocol is a set of rules that coordinates the exchange of information. If one computer is sending information to another and they both follow the same protocol, the message gets through, regardless of what types of machines they are and on what operating systems they are running.

OSI Model

Open System Interconnection (OSI) is standard reference model for communication between two end users in a network. The basic reference model for open system interconnection, which visualize network protocols as a seven layered model. The layers are physical, data link, network, transport, session, presentation and application layers.

            A layer in the OSI model communicates with two other OSI layers, the layer directly above it and the layer directly below it.

Physical Layer

The physical layer defined the physical and electrical characteristics of the network. The layer acts as a conduit between computers’s networking hardware and its networking software. It handles the transfer of bits from one computer to another. This is where the bits are actually converted into electrical signals that travel across the physical circuit. Physical layer communication media include various types of copper or fibre optic cable, as well as many different wireless Medias.

Data Link Layer

The function of data link layer is to provide service interface to the network layer and is responsible for error free transmission and preventing the slow receivers from being overflow by the fast senders. To achieve this, the data link layer divides the packets received from the network layer into manageable form known as frames. These data frames are then transmitted sequentially to the receiver.

Network Layer

The network layer is responsible for transporting traffic between devices that are not locally attached. Router provides the routing services in the network. When a packet is received on a router interface, the destination IP address is checked. If the packet is not designated for the router, then the router will look up the destination network address in the routing table. Once the exit interface is chosen the packet will be sent to the interface to be framed and sent to the interface to be framed and sent out on the local network.

Transport Layer

The basic function of the transport layer is to handle error recognition and recovery of the data packets. This layer establishes, maintains and terminates communications between the sender and receiver. At the receiving end, transport layer rebuilds packets into the original message and to ensure that the packets arrived correctly, the receiving transport layer sends acknowledgements.

Session Layer

The session layer comes into play primarily at the beginning and the end of the transmission. At the beginning of the transmission, it lets the receiver know its intent to start transmission. At the end of transmission, the session layer determines if the transmission was successful.

Presentation Layer

The function of presentation layer is to ensure that information sent from the application layer of one system would be readably by the application layers of another system. This is the place where application data is packed or unpacked and is made ready to use by the running application.

Application Layer

The application layer is the entrance point that program use to access the OSI model and utilize network resources. This layer represents the services that directly support applications. OSI layer is closest to the end-user

Network Devices

NIC (Network Interface Card)

NIC is a hardware device that connects clients, server and peripherals to the network through a port. Most network interface come as a small circuit board that can be inserted onto one of the computer motherboard’s slots. In modern computers NIC is a part of circuit board (mother boards). Each networks \ interface as associated with a unique address called its media access control (MAC) address. The MAC address helps I sending information to its intended destination.

            NICs are a major factor in determining the speed and performance of a network. It is a good idea to use the fastest network card available for the type of workstation one is using.

Repeater

            A repeater is the most basic device on a network.  Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data.  A repeater installed on the link receives signal, regenerates it, and sends the refreshed copy back to the link.  Doing this means that the new signal is clean, free from any background noise introduced while travelling down the wire.

            Repeaters are most commonly used to extend a network cable.  All network cable standards have maximum cable length specification.   If the distance between two network devices is longer than this specification, a repeater is needed to regenerate the signal.  Without the repeater, the signal will be too weak for the computes on each end to reliably understand.  A good example of the use of repeaters would be in a LAN using a star topology with unshielded twisted-pair cabling.  The length limit for unshielded twisted-pair cable is 100 m. The repeater amplifies all the 100m limit.

Hub

A hub is a small box that connects individual devices on a network so that they can communicate with one another.  The hub operates by gathering the signals from individual’s network devices, optionally amplifying the signals, and then sending them onto all other connected devices.  Amplification f the signal ensures that devices on the network receive reliable information.  A hub can be thought of as the centre of a bicycle wheel, where the spokes (individual computers) meet.

The terms repeater and hub are used synonymously, but they are actually not the same.  Although at its very basic level, a hub can be thought of as a multi-port repeater.  Typically, hubs have anywhere from 4 to over 400 ports.

Bridge

 A bridge is a device that allows division of a large network into two or more smaller and efficient network.  It monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges can “listen” to the network and automatically figure out the address of each computer on both sides of the bridge.  It examines each packet as it enters though one of the ports.  A bridge first looks at the MAC address of the sender and creates a mapping between the port and the sender’s MAC address.  It then looks at the address of the recipient, comparing the MAC address to the list of all learned MAC addresses. If the address is in the list, the bridge looks up the port number and forwards the packet to the port where it thinks the recipient is connected.  If the recipient’s MAC address is not in the list, the bridge then does a flood; it sends the signal to all the ports except the one from where it was received.  As a result, a bridge reduces the amount of traffic on a LAN by dividing it into two segments.

Bridges can be used to connect networks with different types of cabling or physical topologies.  They must, however, be used between networks with the same protocol.  Since a bridge examiners the packet to record the sender and lookups the recipient, there is overhead in sending a packet through a bridge.  On a modern bridge, this overhead is miniscule and does not affect network performance.

Switch

 A switch is a multi-port bridge.  It connects individual devices on a network so that they can communicate with one another.  The behaviour of a switch is the same as that of a bridge.  It is capable of inspecting the data packets as they are received, determining the source and destination device of that packet, and forwarding that packet appropriately.  The difference is that most switches implement these functions in hardware using a dedicated processor.  This makes them much faster than traditional software-based bridges.

Router

A router is an essential network device for interconnecting two or more networks. Router’s sole aim is to trace the best route for information to travel.  As network traffic changes during the day, routers can redirect information to take less congested routes.  A router creates and/or maintains a table, called a routing table that stores the best routes to certain network destinations.  While bridges know the address of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network.  Routers can even ‘liston’ to the entire network to determine which sections are the busiest-they can then redirect data around those section until they clear up.

Gateway

A gateway is an internetworking device, which joins networks operating on different protocols together.  It is also known as protocol converter.  A gateway accepts the packet formatted for one protocol and converts the formatted packet into another protocol.  It can be implemented completely in software, hardware, or as a combination of both.  For example, a gateway can receive e-mail message in one format and convert it into another format.  One can connect systems with different protocols, languages, and architecture using a gateway.