Networking: Basic Concepts

Computer Network

•  A communication system for connecting computers/hosts

Why?

• Better connectivity

• Better communication

• Better sharing of resources

• Bring people together

Types of Computer Networks

•  Local Area Network(LAN)

                 @ Connects hosts within a relatively small geographical area

• Wide Area Network(WAN)

               @ Hosts may be widely dispersed

LAN and WAN: Comparison



* LAN
  @Typical speeds: 10Mbps to 10Gbps
  @Typical cost: 1 crore for a hundred node LAN(one-time cost)

* WAN
  @Typical speeds: 64Kbps to 8Mbps
  @Typical cost: 30 lakhs(recurring cost)

Circuit Switching
* A dedicated communication path is required between two stations.
    @ The path follows a fixed sequence of intermediate links.
    @ A logical channel gets defined on each physical link.
* In ciruit switching, three steps are required for communication:
    1 - Connection establishment
    2 - Data transfer
    3 - Connection Termination

*Drawbacks:
    @ Channel capacity gets dedicated during the entire duration of communication.
    @ There is an initial delay.

Packet Switching
* Modern form of long-distance data communication.
    @ Network resources are not dedicated.
    @ A link can be shared.
* The basic technology has evolved over time.
* Data are transmitted in short packets(~Kbytes).
    @ A longer message is broken up into smallerns.
    @ The chunks are caleld packets.
    @ Every packet contains a header.
*Packet switching is based on store-and-forward concept.
    @ Each intermediate network node receives a whole packet.
    @ Decides the route.
    @ Forwards the packet along the selected route.
*Advantages:
    @ Links can be shared; so link utilization is better.
    @ Suitable for computer-generated traffic.
    @ Buffering and data rate conversion can be performed easily.
    @ Some packets may be given priority over others, if desired.

How are the packets transmitted?
    * By using two alternative approches:
        1 - Virtual circuits
        2 - Datagram.

1- Virtual circuit approach
    @ Similar in concept to circuit switcing. It's analogy is like a telephone system.

   How it works?
    1. Route is established a priori.
    2. Packet forwarded from one node to the next using store-and-forward scheme.
    3. Only the virtual circuit number need to be carried by a packet.
    4. No dynamic routing decision is taken by the intermediate nodes.


2- Datagram Approach
    @ Basic concept:
        - No route is established beforehand.
        - Each packet is transmitted as an independent entity.
        - Does not maintain any history.
    @ Analogy: Postal System.
    @ Every intermediate node has to take routing decisions dynamically.
    @ Problems:
        1. Packets may be delivered out of order.
        2. If a node crashes momentarily, all of its queued packets are lost.
        3. Duplicate packets may also be generated.
    @ Advantages:
        1. Faster than virtual circuit for smaller number of packets.
        2. More flexible.
        3. Packets between two hosts may follow different paths.

7 Layers of the OSI(Open Systems Interconnection) Model and it's Function:

1- Application - Interface point for user applications.
2- Presentation - Provides data independence.
3- Session - Manages sessions.
4- Transport - End-to-end reliable data transfer, with error recovery and flow control.
5- Network - us to establish, maintain, and terminate connections.
6- Datalink - Reliable transfer of frames over a point-to-point link.
7- Physical - Transmit raw bit stream over a physical medium.

3 Internetworking Devices:
1 - Hub
    * Extends the span of a single LAN.
2 - Bridge/Layer-2 Switch
    * Connects two or more LAN's together.
3 - Router/Layer-3 Switch
    * Connects any combination of LAN's and WAN's
    * Works at network layer level.