Transcript William Stallings Data and Computer Communications

EE4272: Computer Networks
Instructor: Tricia Chigan
Dept.: Elec. & Comp. Eng.
Spring, 2003
Spring, 2003
EE 4272
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
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Simplified Communications Model Diagram
Purpose of Com. Sys.: Exchange of data between parties.
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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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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
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Simplified Data Communications Model
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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
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Wide Area Networks
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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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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
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Protocols
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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
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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
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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.
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Application layer: logic needed to support various user applications
Host to host or transport layer: reliable end-to-end delivery
mechanisms, e.g. TCP
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Internet layer: provide routing function across multiple networks
Network access layer: concern the exchange of date between end
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Physical layer: Physical interface between a data trans. device & a
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system & the network to which it is attached
trans. medium or network
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TCP/IP Protocol Architecture Model
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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
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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;
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OSI v TCP/IP
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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
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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)
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