Chapter 1: Introduction 1) Data Communications: Deals with the - - PowerPoint PPT Presentation

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EE 4272 Spring, 2003

EE4272: Computer Networks EE4272: Computer Networks

Instructor: Tricia Chigan Dept.: Elec. & Comp. Eng. Spring, 2003

EE 4272 Spring, 2003

Chapter 1: Introduction

1) Data Communications: Deals with the transmission of signals in a

reliable & efficient manner. Topics: Signal Transmission; Transmission Media; Signal Encoding; Interfacing; Data Link Control; Multiplexing

2) Networking: Deals with the technology & architecture of the

communications networks used to interconnect communicating devices Topics: LANs; WANs;

3) Communications Protocols: Protocol Architectures; Analysis of

individual protocols at various layers of the architecture

EE 4272 Spring, 2003

Simplified Communications Model - Diagram

Purpose of Com. Sys.: Exchange of data between parties.

EE 4272 Spring, 2003

A Communications Model

• Source
• Generates data to be transmitted. E.g., Phone, computer
• Transmitter
• Converts data into transmittable signals. E.g., encode bit

stream into electromagnetic signals.

• Transmission System
• Carries data. E.g., transmission line, or a complex network
• Receiver
• Converts received signal into data readable to des. device.
• Destination
• Takes incoming data

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EE 4272 Spring, 2003

Key Communications Tasks

• Transmission System Utilization: efficient/fair facility sharing
• Signal Generation
• Synchronization
• Exchange Management
• Error detection and correction
• Addressing and routing
• Recovery
• Message formatting
• Security
• Network Management

Reading assignment: read the details of P6-p7

EE 4272 Spring, 2003

Simplified Data Communications Model

EE 4272 Spring, 2003

Networking

• Point-to-point communication not usually practical
• Devices are too far apart
• Large set of devices would need impractical number of connections
• Solution is a communications network

EE 4272 Spring, 2003

Wide Area Networks

• Large geographical area
• Crossing public rights of way
• Rely in part on common carrier circuits
• Alternative technologies: all about resource sharing

Circuit switching: Dedicated communications path established for the duration of the conversation. e.g. telephone network Packet switching: Date are sent out in a sequence of packets; Each packet passes through the network along some path; Used for terminal-to-computer, or computer-to-computer comm. Frame relay (up to 2 Mbps): reading assignment p10 Asynchronous Transfer Mode (ATM): reading assignment p11

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EE 4272 Spring, 2003

Local Area Networks

• Smaller scope
• Building or small campus
• Usually owned by same organization as

attached devices

• Data rates much higher
• Usually broadcast systems (e.g. Ethernet Bus)
• Now some switched systems (e.g. Gigabit

Ethernet Switch ) and ATM are being introduced

EE 4272 Spring, 2003

Protocols

• Used for communications between entities in a system
• Entities: User applications; e-mail facilities; terminals
• Systems: Computer; Terminal; Remote sensor
• Must speak the same language
• Key Elements of a Protocol:
• Syntax: Data formats; Signal levels
• Semantics: Control information; Error handling
• Timing: Speed matching; Sequencing

EE 4272 Spring, 2003

Protocol Architecture

• Framework about how a set of protocols work

together to finish the communication task between source and destination

• Three Protocol Architectures:
• Three-Layer Model
• TCP/IP Protocol Architecture
• OSI Model

EE 4272 Spring, 2003

TCP/IP Protocol Architecture

• Developed by the US Defense Advanced Research

Project Agency (DARPA) for its packet switched network (ARPANET)

• Used by the global Internet
• No official model but a working one.

Application layer: logic needed to support various user applications Host to host or transport layer: reliable end-to-end delivery

mechanisms, e.g. TCP

Internet layer: provide routing function across multiple networks Network access layer: concern the exchange of date between end

system & the network to which it is attached

Physical layer: Physical interface between a data trans. device & a

• trans. medium or network

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EE 4272 Spring, 2003

TCP/IP Protocol Architecture Model

EE 4272 Spring, 2003

OSI Model

• Open Systems Interconnection : A framework for

developing protocol standards.

• Developed by the International Standardization

Organization (ISO)

• Seven layers
• A theoretical system delivered too late!
• TCP/IP is the de facto standard

EE 4272 Spring, 2003

OSI Layers

• Application: e.g. web browser, email, file transfer
• Presentation: provides independence to the app. processes from

differences in data representation (syntax).

• Session: Provides control structure for comm. between applications;

establish, manages, and terminates connections (sessions) between cooperating applications

• Transport: end-to-end reliable delivery control
• Network: routing/switching; establish/maintain/terminate connections
• Data Link: reliable transfer of information across the physical links; send

frames with necessary synch., error control, and flow control

• Physical: concern with transmission of unstructured bit stream over

physical medium;

EE 4272 Spring, 2003

OSI v TCP/IP

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EE 4272 Spring, 2003

Standards

• Required to allow for interoperability between

equipments

• Advantages

Ensures a large market for equipment and software Allows products from different vendors to communicate

• Disadvantages

Freeze technology May be multiple standards for the same thing

EE 4272 Spring, 2003

Standards Organizations

• Internet Society
• ISO
• ITU-T (formally CCITT)
• ATM forum
• IETF

Further Reading Resource

• Web sites for IETF, IEEE, ITU-T, ISO
• Internet Requests for Comment (RFCs)