Computer Networks

Chapter 1 - Introduction

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Data Communication

Exchange of any type of information (data) via some medium Its predecessor is telecommunication

where human voice was transmitted Three fundamental characteristics

Delivery Accuracy Timeliness

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Bits and Bytes

Computers use binary digits or bits (0s and 1s) to represent data BIT = BInary digiT The common code for encoding letters is ASCII

(American Standard Code for Information Interchange) 7 bits are used to encode a single character

To accomodate larger number of characters Extended ASCII code with 8 bits per character was introduced

8 bits = 1 byte (a unit for measuring amount of data)

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Data Representation

Text – using different codes Each character is represented by certain

number of bits The number of bits in the code determins the

number of different characters ASCII (7 bits), Extended ASCII (8 bits), Unicode

(16 bits), ISO (32 bits) Numbers – Binary number system Images – A matrix of pixels represented by

bit patterns Video – A combination of images Audio – Digitized voice and music

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Bits and electrical signals

Data communication uses corresponding electrical signals to represent bits

Different encodings are possible The simplest way is to represent 1 with

negative and 0 with positive voltage

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transmitterreceiver

MEDIUMOriginal message

Noise + attenuation Transformed message

Humans can recognized the transformed message

Machines(Not necessarily)Therefore, a strict set of rules (protocols) are required.

Humans vs. Machines

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Components of Data Communication System

Step 1:Step 2:Step 3:::::::::::

Message

Step 1:Step 2:Step 3:::::::::::

MediumSender Reciever

Protocol Protocol

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Direction of Data Flow

Simplex channel The transmission is only in one direction

Half-duplex channel The transmission is in both directions, but

only one at a time (both directions cannot be used at the same time)

Duplex channel The transmission is in both directions

without limitation

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Different Line Configurations

Point-to-point Two devices on a

single channel (dedicated channel)

Multipoint Many devices on a

single channel (shared channel)

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Topology of Networks

TopologyTopology

BusBusStarStarFull MeshFull Mesh RingRingPartial MeshPartial Mesh

Topology defines the arrangement of links in a network

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Computer Networks Classification

0.1 m1 m

10 m100 m

1 km

10 km

Circuit board

SystemRoom

Building

Campus

Local area Network (LAN)

Data flow machine

Multicomputer

CityCountry

Continent

Planet

Metropolitan Area Network (MAN)

Wide Area Network (WAN)

The Internet

100 km

1000 km

10,000 km

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LAN – Local Area Network

Single building LAN

backbone

Multiple building LAN

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MAN - Metropolitan Area Network

Public city network

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WAN – Wide Area Network

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Internetworking Concept and Model

The goal is to build a unified, cooperative interconnection of networks that supports a universal communication service

Detaches the notions of communication from the details of network technologies, and hides low level details from the user

Provides a mechanism that delivers packet from their source to their ultimate destination in real time

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The Internet Today

Communication is possible by using a common Internet protocol that glues different networks.

Internet emerged from the academic community and therefore has no central governance so far.

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The internet versus the Internet

Internetowork or internet (small ”i”) – generic term to mean an interconnection of networks

Internet (Uppercase I) – the specific worldwide network that uses the IP protocol (Internet protocol)

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Protocols

A set of rules or conventions agreed by communicating parties.

Protocols allow one to specify/understand communication without knowing details of a particular vendor’s hardware

Complex data communication systems do not use a single protocol to handle all transmission tasks.

They require a set of cooperative protocols: protocol family or protocol suite

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The Key Elements of a Protocol

Syntax referes to the structure of data, meaning

the order in which they are presented

Semantics refferes to the meaning of each section of

bits, how a particular pattern to be interpreted and which action should be taken based on the interpretation

Timing refferes to when data should be sent and

how fast they can be sent

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Standards

Standards provide guidelines to the manufacturers, vendors, goverment agencies and other service providers to ensure connectivity between different entities Development of standrds is a very slow process

Two types of standards: De jure (by law) -legislated by an officially

recognized body De facto (by fact) – that are actually implemented

into the products) propriatory (closed) nonpropriatory (open)