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Introduction to Computer Networks INTRODUCTION TO COMPUTER NETWORKS

Computer Networks Computer network connects two or more autonomous computers. The computers can be geographically located anywhere. Introduction to Computer Networks

LAN, MAN & WAN Introduction to Computer Networks Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network) Network in a City is call MAN (Metropolitan Area Network) Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

Applications of Networks Introduction to Computer Networks Resource Sharing Hardware (computing resources, disks, printers) Software (application software) Information Sharing Easy accessibility from anywhere (files, databases) Search Capability (WWW) Communication Message broadcast Remote computing Distributed processing (GRID Computing)

Network Topology The network topology defines the way in which computers, printers, and other devices are connected. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions. Introduction to Computer Networks

Bus Topology Commonly referred to as a linear bus, all the devices on a bus topology are connected by one single cable. Introduction to Computer Networks

Star & Tree Topology Introduction to Computer Networks The star topology is the most commonly used architecture in Ethernet LANs. When installed, the star topology resembles spokes in a bicycle wheel. Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.

Ring Topology Introduction to Computer Networks A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame. The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. Single ring – All the devices on the network share a single cable Dual ring – The dual ring topology allows data to be sent in both directions.

Mesh Topology The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance. It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions. Implementing the mesh topology is expensive and difficult. Introduction to Computer Networks

Network Components Introduction to Computer Networks Physical Media Interconnecting Devices Computers Networking Software Applications

Networking Media Networking media can be defined simply as the means by which signals (data) are sent from one computer to another (either by cable or wireless means). Introduction to Computer Networks

Networking Devices Introduction to Computer Networks HUB, Switches, Routers, Wireless Access Points, Modems etc.

Computers: Clients and Servers In a client/server network arrangement, network services are located in a dedicated computer whose only function is to respond to the requests of clients. The server contains the file, print, application, security, and other services in a central computer that is continuously available to respond to client requests. Introduction to Computer Networks

Networking Protocol: TCP/IP Introduction to Computer Networks

Models of the Internet OSI/ISO Reference Model TCP/IP Reference Model Open Systems Interconnection (International Standards Office) Application Presentation Session Transport Network Data Link Physical Model Layers Application Transport Internet Host-to-network Transmission Control Protocol/ Internet Protocol

1) Physical layer Purpose: Necessary infrastructure. Think "wires in the ground and switches connecting them". This is the physical hardware of the internet. Wires/optical cables/wireless links and other technologies provide a way for transmission of raw bits (0s and 1s). Routers and switches connect these cables and direct the traffic.

2) Data link layer Purpose: Provides basic connection between two logically connected machines. Think: “I stuff packets down a wire to my neighbour” Send raw packets between hosts. Basic error checking for lost data. In TCP/IP the "Physical layer" and the "Data Link" layer are grouped together and called the host-to-network layer.

3) Network Layer/Internet Layer Purpose: Provide end-to-end communication between any two machines. Think: “I try to get a packet to its destination” Tells data which link to travel down. Addresses the problem known as routing. Deals with the question "where do I go next to get to my destination?" Ensures packets get from source A to destination B.

4) Transport Layer Purpose: Ensure that data gets between A and B. Think: “From the source and destination, I make sure that the data gets there”. Ensures a data gets between source and destination. If necessary ensure that connection is lossless (resend missing data). Provides flow control if necessary (send data faster or slower depending on the network conditions).

5) Session Layer (not TCP/IP) Purpose: Provides a single connection for one application. Think: “I am in charge of the entire message.” This connection may be two way or may be synchronised. Not discussed much as it is never implemented.

6) Presentation Layer (Not TCP/IP) Purpose: Provides commonly used functions for applications. Think: “I meet internationalisation standards”. The main job of the presentation layer is to ensure that character sets match – e.g. that Chinese characters are correctly received by the sends. Again not discussed much as it is never implemented.

7) Application layer Purpose: The computer programs which actually do things with the network. Think: “I deliver the mail, browse the web etc.” For example, your client program which will talk to the server at the other end. At this layer, we have many protocols (http, snmp, smtp, ftp, telnet) which different bits of software use. We often talk in terms of client and server architecture for the software.

TCP/IP model in summary

Internet (IP) addresses ( ) (www) ftp://ftp.uk.debian.org (file transfer) telnet://towel.blinkenlights.nl (telnet) These are the “real” IP addresses of the above sites. IP addresses are 32 bits grouped into 4 octets. (Octet = 8 bits – a number from 0-255)

IP Networks(1) IP addresses use less significant bits first to indicate sub-networks. IP address: Netmask: (no holes allowed) If two IP addresses are the same when bitwise AND’d against the netmask then they are on the same subnet ?? is always on the same subnet in the above example.

IP Networks(2) IP networks were originally subdivided into class A, B, C, D and E networks. StartEndNetworksHosts/network A million B ,38264K C million254 D Multicast E Reserved

Hub

Hubs A hub is a physical layer device. When it receives a bit from one of its interface, it simply repeats it on all its outgoing interfaces. It does not provide multiple access control (e.g. no collision detection) Connecting LAN segments using hubs has its limitations: (1) it merges the LAN segments into one collision domain, (2) it only connects LAN using the same Ethernet technology, and (3) there is a limit to number of nodes and maximum distance between nodes.

Switch

Switches A switch is a link-layer devices. It operates on Ethernet frames. When a frame is received, it checks the destination address and either forwards it to correct interface or drops (filters) it. Switches understands CSMA/CD and therefore separates the LAN segment into different collision domain. A switching table on a switch contains mapping from MAC addresses and its corresponding interfaces. An entry (X, Y) is created automatically when the switch received a frame from MAC address X on interface Y. Entries are deleted after they expires (have not heard from X for some time).

Switch? Router?

Switches vs Routers Switches does not modify MAC address of frames, but routers do. Adapters on switches do not have MAC addresses. Switches support plug-and-play, while routers needs their interfaces’ IP address to be configured Topology using switch is restricted to a tree, while routers (due to shortest path routing and TTL) allows loops. Switch is suitable for smaller networks, while router should be used for larger networks.

Basic Definitions: Host, Router, Switch, Source, Destination Host: A machine which is a point on a network which packets travel through – a node in a graph. Router: A host which finds a route for packets to travel down – an intermediate point in a journey. Switch: Often used interchangeably with router but implies that the routes are “fixed”. Source: Where data is coming from. Destination: (or sink) Where data is going to.

Applications Searchable Data (Web Sites) E-Commerce News Groups Internet Telephony (VoIP) Video Conferencing Chat Groups Instant Messengers Internet Radio Introduction to Computer Networks