Data and Computer Communications (Chapter 1)

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William Stallings Data and Computer Communications Chapter 1 Introduction 

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A Communications Model I Source I generates data to be transmitted 0 Transmitter ! Converts data into transmittable signals 0 Transmission System ! Carries data 0 Receiver | Converts received signal into data 0 Destination I Takes incoming data 

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Simplified Communications Model - Diagram 

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Key Communications Tasks I Transmission System Utilization Interfacing Signal Generation Synchronization Exchange Management Error detection and correction Addressing and routing Recovery Message formatting Security Network Management Speeeeseeeooe 

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Simplified Data Communications Model Digital bit Analog Analog Digital bit stream signal signal stream ‘Text 7 an Antu SI ‘Text Source ® 9 © 9 © © Input Input data ‏من‎ Received Output data Output information 0 signal gna #0 information 2 0 3 7 

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Networking I Point to point communication not usually practical I Devices are too far apart I Large set of devices would need impractical number of connections 1 Solution is a communications network 

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Simplified Network Model =e network 

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Wide Area Networks 0 Large geographical area 0 Crossing public rights of way 0 Rely in part on common carrier circuits 0 Alternative technologies I Circuit switching I Packet switching I Frame relay I Asynchronous Transfer Mode (ATM) 

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Circuit Switching Dedicated communications path established for the duration of the conversation 0 e.g. telephone network ص 

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Packet Switching 0 Data sent out of sequence 0 Small chunks (packets) of data at a time 0 Packets passed from node to node between source and destination 0 Used for terminal to computer and computer to computer communications 

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Frame Relay 0 Packet switching systems have large overheads to compensate for errors 0 Modern systems are more reliable 0 Errors can be caught in end system 0 Most overhead for error control is stripped out 

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Asynchronous Transfer Mode ATM Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10Mbps to Gbps Constant data rate using packet switching technique ‎eas‏ ابحم زاو ‎Sy‏ اس 

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Integrated Services Digital Network 0 ISDN 0 Designed to replace public telecom system 0 Wide variety of services 0 Entirely digital domain 

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Local Area Networks Smaller scope I Building or small campus Usually owned by same organization as attached devices Data rates much higher Usually broadcast systems Now some switched systems and ATM are being introduced ص ص ‎oe‏ از تس 

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Protocols I Used for communications between entities in a system Must speak the same language Entities 1١ User applications 1 e-mail facilities 1 terminals 0 Systems 1 Computer 1١ Terminal 1 Remote sensor 

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Key Elements of a Protocol 0 Syntax ! Data formats I Signal levels 0 Semantics ۱ Control information | Error handling 0 Timing I Speed matching I Sequencing 

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Protocol Architecture 0 Task of communication broken up into modules For example file transfer could use three modules I File transfer application ! Communication service module I Network access module 

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Simplified File Transfer Architecture Computer ¥ Computer X ‘Communications network modi Network interfac Logie Network interface} logic 

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A Three Layer Model 0 Network Access Layer 0 Transport Layer 0 Application Layer 

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Network Access Layer 0 Exchange of data between the computer and the network Sending computer provides address of destination May invoke levels of service Dependent on type of network used (LAN, packet switched etc.) ص oo 

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Transport Layer 0 Reliable data exchange 0 Independent of network being used 0 Independent of application 

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Application Layer 0 Support for different user applications 0 e.g. e-mail, file transfer 

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Addressing Requirements 0 Two levels of addressing required 0 Each computer needs unique network address 0 Each application on a (multi-tasking) computer needs a unique address within the computer I The service access point or SAP 

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Protocol Architectures and Application ‘Transport Network acess Networks 

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Protocols in Simplified Architecture Computer ¥ Computer X Application Protocol Application Transport | سس ند ‘Communications Neiwork access Network access network Network access protocol protocol 

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Protocol Data Units (PDU) I At each layer, protocols are used to communicate 0 Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added I Destination SAP I Sequence number I Error detection code ! This gives a transport protocol data unit 2 o o 

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Network PDU 0 Adds network header I network address for destination computer I Facilities requests 

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Operation of a Protocol Architecture Destination ¥ 

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TCP/IP Protocol Architecture 0 Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) 0 Used by the global Internet 0 No official model but a working one. I Application layer I Host to host or transport layer I Internet layer 1 Network access layer ۱ Physical layer 

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Physical Layer 0 Physical interface between data transmission device (e.g. computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates etc. جح حر حر ا 

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Network Access Layer 0 Exchange of data between end system and network 0 Destination address provision 0 Invoking services like priority 

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Internet Layer (IP) 0 Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers ص ص 

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Transport Layer (TCP) 0 Reliable delivery of data 0 Ordering of delivery 

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Application Layer 0 Support for user applications 0 e.g. http, SMPT 

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TCP/IP Protocol Architecture Model Destis Application }---- Application TCP ‘TCP IP ‏صر‎ Network Access} --~ Network Access} Physical Physical Source System Destination System 

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OSI Model 0 Open Systems Interconnection 0 Developed by the International Organization for Standardization (ISO) 0 Seven layers 0 A theoretical system delivered too late! 0 TCP/IP is the de facto standard 

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OSI Layers Application Presentation Session Transport Network Data Link Physical SS) Sh ey or ea 

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OSI v TCP/IP ‎TCPAP‏ اوه ‎Application‏ ‎Application‏ |__ ‎Presentation User‏ ‎Space‏ ‎Session‏ ‎Software‏ ‎‘Transport‏ ‎Transport _|(host-to-host)‏ ‎Internet‏ ‎Network Firmware‏ ‎Network Operating‏ ‎DataLink | Access Hardware‏ ‎System‏ ‎Physical |‏ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ 

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Standards 0 Required to allow for interoperability between equipment 0 Advantages I Ensures a large market for equipment and software I Allows products from different vendors to communicate 0 Disadvantages I Freeze technology ۱ May be multiple standards for the same thing 

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Standards Organizations 0 Internet Society 0 ISO 0 ITU-T (formally CCITT) 0 ATM forum 

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Further Reading I Stallings, W. Data and Computer Communications (6th edition), Prentice Hall 1999 chapter 1 1 Web site for Stallings book ! www.shore.net/~ws/DCC6e.html 0 Web sites for IETF, IEEE, ITU-T, ISO J Internet Requests for Comment (RFCs) 0 Usenet News groups 1 comp.dcom.* 1 comp.protocols.tcp-ip 

William Stallings Data and Computer Communications Chapter 1 Introduction A Communications Model  Source  generates data to be transmitted  Transmitter  Converts data into transmittable signals  Transmission System  Carries data  Receiver  Converts received signal into data  Destination  Takes incoming data Simplified Communications Model - Diagram Key Communications Tasks  Transmission System Utilization  Interfacing  Signal Generation  Synchronization  Exchange Management  Error detection and correction  Addressing and routing  Recovery  Message formatting  Security  Network Management Simplified Data Communications Model 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 Simplified Network Model Wide Area Networks     Large geographical area Crossing public rights of way Rely in part on common carrier circuits Alternative technologies     Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM) Circuit Switching  Dedicated communications path established for the duration of the conversation  e.g. telephone network Packet Switching  Data sent out of sequence  Small chunks (packets) of data at a time  Packets passed from node to node between source and destination  Used for terminal to computer and computer to computer communications Frame Relay  Packet switching systems have large overheads to compensate for errors  Modern systems are more reliable  Errors can be caught in end system  Most overhead for error control is stripped out Asynchronous Transfer Mode       ATM Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10Mbps to Gbps Constant data rate using packet switching technique Integrated Services Digital Network  ISDN  Designed to replace public telecom system  Wide variety of services  Entirely digital domain Local Area Networks  Smaller scope  Building or small campus  Usually owned by same organization as attached devices  Data rates much higher  Usually broadcast systems  Now some switched systems and ATM are being introduced Protocols  Used for communications between entities in a system  Must speak the same language  Entities  User applications  e-mail facilities  terminals  Systems  Computer  Terminal  Remote sensor Key Elements of a Protocol  Syntax  Data formats  Signal levels  Semantics  Control information  Error handling  Timing  Speed matching  Sequencing Protocol Architecture  Task of communication broken up into modules  For example file transfer could use three modules  File transfer application  Communication service module  Network access module Simplified File Transfer Architecture A Three Layer Model  Network Access Layer  Transport Layer  Application Layer Network Access Layer  Exchange of data between the computer and the network  Sending computer provides address of destination  May invoke levels of service  Dependent on type of network used (LAN, packet switched etc.) Transport Layer  Reliable data exchange  Independent of network being used  Independent of application Application Layer  Support for different user applications  e.g. e-mail, file transfer Addressing Requirements  Two levels of addressing required  Each computer needs unique network address  Each application on a (multi-tasking) computer needs a unique address within the computer  The service access point or SAP Protocol Architectures and Networks Protocols in Simplified Architecture Protocol Data Units (PDU)  At each layer, protocols are used to communicate  Control information is added to user data at each layer  Transport layer may fragment user data  Each fragment has a transport header added  Destination SAP  Sequence number  Error detection code  This gives a transport protocol data unit Network PDU  Adds network header  network address for destination computer  Facilities requests Operation of a Protocol Architecture 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 Host to host or transport layer Internet layer Network access layer Physical layer Physical Layer  Physical interface between data transmission device (e.g. computer) and transmission medium or network  Characteristics of transmission medium  Signal levels  Data rates  etc. Network Access Layer  Exchange of data between end system and network  Destination address provision  Invoking services like priority Internet Layer (IP)  Systems may be attached to different networks  Routing functions across multiple networks  Implemented in end systems and routers Transport Layer (TCP)  Reliable delivery of data  Ordering of delivery Application Layer  Support for user applications  e.g. http, SMPT TCP/IP Protocol Architecture Model OSI Model  Open Systems Interconnection  Developed by the International Organization for Standardization (ISO)  Seven layers  A theoretical system delivered too late!  TCP/IP is the de facto standard OSI Layers        Application Presentation Session Transport Network Data Link Physical OSI v TCP/IP Standards  Required to allow for interoperability between equipment  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 Standards Organizations     Internet Society ISO ITU-T (formally CCITT) ATM forum Further Reading  Stallings, W. Data and Computer Communications (6th edition), Prentice Hall 1999 chapter 1  Web site for Stallings book  www.shore.net/~ws/DCC6e.html  Web sites for IETF, IEEE, ITU-T, ISO  Internet Requests for Comment (RFCs)  Usenet News groups  comp.dcom.*  comp.protocols.tcp-ip