1. Introduction (8%)

Computer Networks

Introduction

• Interconnection of number of computers

• Interconnection through• copper wire

• fiber optics

• microwaves

• infrared

• communication satellite

Uses of Networks

• Distributed Systems– Software system built on top of computer networks

– Primary objective of Computer Networks:

• Transfer data from machine A to machine B• Facilitate access to remote information• Facilitate sharing of data• Facilitates person to person communication• Facilitate Interactive Entertainment

Social Issues

• The topics for bulletin boards, news groups etc should be kept under watch for sensitive topics

• Hacking

• Virus spreading

• Anonymous messages can be sent

Transmission Technology

• Transmission Technology– Broadcast Technology

– Point-to-Point Technology

• Broadcast Technology– Single Communication Channel

– Data sent by sender will be received & processed by all

– Multicasting: Transmission to a subset of machines

– Used for small networks

Transmission Technology

• Point to Point Technology– Many connections between individual pairs of

machines – Used for big networks

• Unicasting: Point to point transmission with one sender and one receiver.

Local Area Networks

• 3 Characteristics:1. Size

• Restricted to limited size

• Worst case transmission time is bounded and known in advance

2. Transmission Technology• Connected through cable

• LANs run at 10 to 100 Mbps or now Gbps

3. Topology– Bus & Ring

Two Examples of Broadcast NWs

BusRing

The Broadcasting in LAN• At any instance only one machine is allowed to send • Arbitration mechanism is required (Common Bus)

– Static• divide time into discrete intervals and use a round-robin algorithm • Waste of time

– Dynamic• Decentralized

– Sender check the channel before sending

• Centralized– Bus arbitration unit decides who goes next

• Ethernet (also known by no 802.3)– bus-based broadcast network with decentralized control

Metropolitan Area Network• It is a bigger version of LAN and may

encompass the whole city– Ex Cable Television

• In MANs we use DQDB and also known by no 802.6– DQDB= Dual Queue Dual Bus– DQDB contains two unidirectional busses to

which all the computers are connected

• Upper bus for right, lower bus for left

Architecture of DQDB MAN

Wide Area Networks

• It spans a large geographical area, usually a country or continent

• The hosts are connected by a communication subnet, or just subnet for short

• Subnet is operated by Telephony Company or ISP• The job of the subnet is to carry messages from

host to host

Relation Between Hosts and SubnetSubnet = Transmission lines + Switching Element

Transmission Line – copper wires,optical fiber, radio links

Switching Element - Router

Internetwork

• A collection of interconnected networks is called an internetwork or internet.

• Many networks exist in the world, often with different hardware and software.

• To fulfill this Gateway is used– Also known as Protocol converters

• Internet is a collection of LANs connected by WAN– If system contains only routers => subnet– If system contains routers & hosts => WAN

Wireless Networks• Categories

1. System Interconnection– Short-range radio signals– bluetooth– Keyboard, mouse connection

2. Wireless LANs– Require radio modem and antenna– Less range

3. Wireless MANs– Radio networks– Wide range– Used in cellular telephones– 1G – analog and voice– 2G – digital and voice– 3G – digital voice and data

Figure 2-19Internetwork

(Internet)

Need For Protocol Architecture

• Reduce Complexity• Data exchange can involve complex procedures like

file transfer• Better if task broken into subtasks• Implemented separately in layers in stack

– each layer provides functions needed to perform communication for layers above

• Peer layers communicate with a protocol

Design Issues in layers• Identification of Host• Rules for data transfer

– In one direction or both• Error Control

– Error detecting and correcting codes• Sequencing of data• Flow Control • Inability of accepting long messages

– Disassembling, transmitting and reassembling messages• Avoid separate connection because of inconveniency and expensive,

– Multiplexing and Demultiplexing• Multiple paths between source and destination, a route must be chosen

– Routing

Connection Oriented & Connectionless Services

• CO – Establish connection, use the connection and

releases the connection– Eg: Telephone system

• CL – Eg: Postal service

Addressing Requirements

• Two levels of addressing required

• Each host on a subnet needs a unique global network address– its IP address

• Each application on a (multi-tasking) host needs a unique address within the host– known as a port

Reference Models

There are two very famous models– OSI Reference model

• OSI = Open Systems Interconnection

• Developed by ISO (International Standard Organization)

• Protocols are not used but the model is valid

– TCP/IP Reference model• TCP/IP = Transport Control Protocol/Internet

Protocol

• Here protocols are widely used

The TCP/IP Reference

Model

The OSI Reference Model

Communication between End-Systems

The Physical Layer• It is concerned with transmitting raw bits

• 1 should be received as 1 and not zero!

• It deals with mechanical, electrical, timing interfaces & physical transmission

Figure 3-4

Physical Layer

The Data Link Layer

• It’s job is to provide error-free transmission between two adjacent computers

• It breaks the sender data in data frames• Assign sequence no. to each frame • If the service is reliable, the receiver confirms

correct receipt of each frame by sending back an acknowledgement frame.

• Checks for the buffer space at the receiver end

The Data Link Layer

• Flow control– Sender and Receiver may not match in speed

• MAC (Medium access control)– For Broadcast Networks Only– Decides “ Who will access the Medium “

• Medium = Ethernet (802.11)

Figure 3-5

Data Link Layer

Figure 3-6

Data Link Layer Example

The Network Layer• Controls the operation of subnet• To route the packets from ultimate source to

ultimate destination• Congestion control

– When many no. of computers want to send data destined to another network simultaneously

– Each data has to pass from same router

• Problems with heterogeneous NWs like addressing, packet size etc are important

Routing

• Directing packets over number of networks

Computer A Computer B

Figure 3-8

Network Layer Example

Figure 3-8-continued

Network Layer Example

The Transport Layer• It accepts data from the upper layer, split it

into smaller units if needed, pass to NWL, and ensure it arrives intact at the other end

• Every process send data to network with the help of port

• So transport layer adds port nos to the data received from upper layer (i.e. Session Layer)

• Hence it’s first true end-to-end layer

Transport Protocols

Session Layer• To establish a session betn 2 processes• It establishes, manages and terminates the

connections between the local and remote application.

• The Session layer is typically completely unused

• Dialogue control – To decide whose turn to speak (send data)

• Synchronization– To add checkpoints to the stream of data for

roll backing

Presentation Layer

• Presentation of the data to be sent

• Translation– Translate data to be sent in bit streams

• Encryption– Transform the original info to another form

• Compression– Reduce the no of bits to be transmitted

The Application Layer

• Enable us to access network

• User friendly software application

• Examples– Network virtual terminal (eg: telnet)– Mail services (eg: Eudora)– Directory services

Figure 3-14

Summary of Layer Functions

OSI Model

  Data unit Layer Function

Hostlayers

Data

7. Application Network process to application

6. Presentation Data representation and encryption

5. Session Interhost communication

Segment 4. Transport End-to-end connections and reliability (TCP)

Medialayers

Packet/Datagram 3. Network Path determination and logical addressing (IP)

Frame 2. Data link Physical addressing (MAC & LLC)

Bit 1. Physical Signal and binary transmission

The TCP/IP Reference Model

• Used in the Internet

• History– First used by ARPANET (Advanced Research

Projects Agency Network)– ARPANET was a research network

Protocols and Networks in TCP/IP Model

Internet Layer

• Packet switching network based on connectionless internetwork layer.

• Packets travel independently in the network

• Order can be different– It’s the job of higher levels to rearrange this

packet.

A Comparison of OSI and TCP/IP• TCP/IP model is widely used compared to

OSI model

• OSI model explicitly defines services, interfaces and protocols (Well organized)

• OSI Model designed first and then protocols

• TCP/IP Protocols designed first and then model

• No of layers are 4 compared to 7