Download William Stallings Data and Computer Communications

Computer Networks
Chapter 1
Introduction
A Communications Model
Ñ Source
Ñ generates data to be transmitted
Ñ Transmitter
Ñ Converts data into transmittable signals
Ñ Transmission System
Ñ Carries data
Ñ Receiver
Ñ Converts received signal into data
Ñ Destination
Ñ Takes incoming data
Simplified Communications
Model - Diagram
Key Communications Tasks
Ñ Transmission System Utilization
Ñ Interfacing
Ñ Signal Generation
Ñ Synchronization
Ñ Exchange Management
Ñ Error detection and correction
Ñ Addressing and routing
Ñ Recovery
Ñ Message formatting
Ñ Security
Ñ Network Management
Simplified Data
Communications Model
Networking
Ñ Point to point communication not usually
practical
Ñ Devices are too far apart
Ñ Large set of devices would need impractical number
of connections
Ñ Solution is a communications network
Simplified Network Model
Wide Area Networks
Ñ Large geographical area
Ñ Crossing public rights of way
Ñ Rely in part on common carrier circuits
Ñ Alternative technologies
Ñ Circuit switching
Ñ Packet switching
Ñ Frame relay
Ñ Asynchronous Transfer Mode (ATM)
Circuit Switching
Ñ Dedicated communications path established for
the duration of the conversation
Ñ e.g. telephone network
Packet Switching
Ñ Data sent out of sequence
Ñ Small chunks (packets) of data at a time
Ñ Packets passed from node to node between
source and destination
Ñ Used for terminal to computer and computer to
computer communications
Frame Relay
Ñ Packet switching systems have large overheads
to compensate for errors
Ñ Modern systems are more reliable
Ñ Errors can be caught in end system
Ñ Most overhead for error control is stripped out
Asynchronous Transfer Mode
Ñ ATM
Ñ Evolution of frame relay
Ñ Little overhead for error control
Ñ Fixed packet (called cell) length
Ñ Anything from 10Mbps to Gbps
Ñ Constant data rate using packet switching
technique
Integrated Services Digital
Network
Ñ ISDN
Ñ Designed to replace public telecom system
Ñ Wide variety of services
Ñ Entirely digital domain
Local Area Networks
Ñ Smaller scope
Ñ Building or small campus
Ñ Usually owned by same organization as
attached devices
Ñ Data rates much higher
Ñ Usually broadcast systems
Ñ Now some switched systems and ATM are being
introduced
Protocols
Ñ Used for communications between entities in a
system
Ñ Must speak the same language
Ñ Entities
Ñ User applications
Ñ e-mail facilities
Ñ terminals
Ñ Systems
Ñ Computer
Ñ Terminal
Ñ Remote sensor
Key Elements of a Protocol
Ñ Syntax
Ñ Data formats
Ñ Signal levels
Ñ Semantics
Ñ Control information
Ñ Error handling
Ñ Timing
Ñ Speed matching
Ñ Sequencing
Protocol Architecture
Ñ Task of communication broken up into modules
Ñ For example file transfer could use three
modules
Ñ File transfer application
Ñ Communication service module
Ñ Network access module
Simplified File Transfer
Architecture
A Three Layer Model
Ñ Network Access Layer
Ñ Transport Layer
Ñ Application Layer
Network Access Layer
Ñ Exchange of data between the computer and
the network
Ñ Sending computer provides address of
destination
Ñ May invoke levels of service
Ñ Dependent on type of network used (LAN,
packet switched etc.)
Transport Layer
Ñ Reliable data exchange
Ñ Independent of network being used
Ñ Independent of application
Application Layer
Ñ Support for different user applications
Ñ e.g. e-mail, file transfer
Addressing Requirements
Ñ Two levels of addressing required
Ñ Each computer needs unique network address
Ñ Each application on a (multi-tasking) computer
needs a unique address within the computer
Ñ The service access point or SAP
Protocol Architectures and
Networks
Protocols in Simplified
Architecture
Protocol Data Units (PDU)
Ñ At each layer, protocols are used to
communicate
Ñ Control information is added to user data at
each layer
Ñ Transport layer may fragment user data
Ñ Each fragment has a transport header added
Ñ Destination SAP
Ñ Sequence number
Ñ Error detection code
Ñ This gives a transport protocol data unit
Network PDU
Ñ Adds network header
Ñ network address for destination computer
Ñ Facilities requests
Operation of a Protocol
Architecture
TCP/IP Protocol Architecture
Ñ Developed by the US Defense Advanced
Research Project Agency (DARPA) for its packet
switched network (ARPANET)
Ñ Used by the global Internet
Ñ No official model but a working one.
Ñ Application layer
Ñ Host to host or transport layer
Ñ Internet layer
Ñ Network access layer
Ñ Physical layer
Physical Layer
Ñ Physical interface between data transmission
device (e.g. computer) and transmission
medium or network
Ñ Characteristics of transmission medium
Ñ Signal levels
Ñ Data rates
Ñ etc.
Network Access Layer
Ñ Exchange of data between end system and
network
Ñ Destination address provision
Ñ Invoking services like priority
Internet Layer (IP)
Ñ Systems may be attached to different networks
Ñ Routing functions across multiple networks
Ñ Implemented in end systems and routers
Transport Layer (TCP)
Ñ Reliable delivery of data
Ñ Ordering of delivery
Application Layer
Ñ Support for user applications
Ñ e.g. http, SMPT
TCP/IP Protocol Architecture
Model
OSI Model
Ñ Open Systems Interconnection
Ñ Developed by the International Organization for
Standardization (ISO)
Ñ Seven layers
Ñ A theoretical system delivered too late!
Ñ TCP/IP is the de facto standard
OSI Layers
Ñ Application
Ñ Presentation
Ñ Session
Ñ Transport
Ñ Network
Ñ Data Link
Ñ Physical
OSI v TCP/IP
Standards
Ñ Required to allow for interoperability between
equipment
Ñ Advantages
Ñ Ensures a large market for equipment and software
Ñ Allows products from different vendors to
communicate
Ñ Disadvantages
Ñ Freeze technology
Ñ May be multiple standards for the same thing
Standards Organizations
Ñ Internet Society
Ñ ISO
Ñ ITU-T (formally CCITT)
Ñ ATM forum
Further Reading
Ñ Stallings, W. Data and Computer
Communications (6th edition), Prentice Hall
1999 chapter 1
Ñ Web site for Stallings book
Ñ www.shore.net/~ws/DCC6e.html
Ñ Web sites for IETF, IEEE, ITU-T, ISO
Ñ Internet Requests for Comment (RFCs)
Ñ Usenet News groups
Ñ comp.dcom.*
Ñ comp.protocols.tcp-ip