Computer and Data CommunicationsComputer and Data Communications

Read Chapters 1 & 2

A Communications ModelA Communications Model

output information

Input device Transmiter Transmission

medium Receiver Output Devices

input information

input data

transmitted signal

receivedsignal

output data

data :

A representation of facts ,concepts in a formalized manner suitable for communication

information :

The meaning thatis currently assignedto data by means of the conventions applied to those data

Data Communication NetworkData Communication Network

Communication Network

Communication Network Node

Network Station

Data communication networksData communication networks Public Switched Telephone Network (PSTN) Local Area Network (LAN) Leased lines Public switched data network (PSDN) Integrated services digital network (ISDN)

PSTNPSTN

modem modem

modem link via PSTN

PSTN

LANLANhubhub

hubhub

hubhub

hubhub

1st floor1st floor

2nd floor2nd floor

3rd floor3rd floor

4th floor4th floor

routerrouter

confined geographical areaconfined geographical area under single managementunder single management high data ratehigh data rate

serverserver

stationstation

stationstation

stationstation

Leased linesLeased lines• Lease pubic circuits to connect different sites• Communication channel is privated

LAN

Voice

LAN

Voice

LAN

Voice

leased circuit

PSDNPSDN

PSDN= Public Switched Data Network

=

LAN

LAN

LAN

PSDN

• PSDN is designed specifically for the transmission of data rather than voice• Communication is shared

ISDNISDN

ISDN

NTE

NTE NTE

NTE

NTE = Networktermination equipment

ISDN = Integrated servicesdigital network

Using ISDN to carry both voice and data

Computer Communication Computer Communication ArchitectureArchitecture

Communication-related messages

Network interface

File and file transfer command

Communicationnetwork

File transferapplication

Communicationsservice module

Network accessmodule

Computer A

File transferapplication

Communicationsservice module

Network accessmodule

Computer B

Three-layer's modelThree-layer's model

Applications

Transport

Network access

Communication architectures Communication architectures and networksand networks

Applications

Transport

Network access

Applications

Transport

Network access

Applications

Transport

Network accessCommunicationnetwork

Service Access Point

Network Address

Protocol layersProtocol layers

Transport protocol

Network Access Protocol

Application protocolApplication

Transport

Network access

Computer A

Application

Transport

Network access

Computer B

Protocol data unitProtocol data unit

Application data

Applicationdata

Transportheader

Transportheader

Transportheader

Transportheader

Networkheader

Networkheader

Transport protocoldata unit (TPDU)

Network protocoldata unit (NPDU)

Applicationdata

Applicationdata

Applicationdata

control info + data in each layer = protocol data unit (PDU)

OSI reference ModelOSI reference Model

Layer 1Physical link

Layer 2Data link layer

Layer 3Network layer

Layer 4Transport layer

Layer 5Session layer

Layer 6Presentation layer

Layer 7Application layer • developed by ISO

(International Standards Organization)