William StallingsData and Computer Communications

Conceptos Básicos

Protocols

Used for communications between entities in a system

Must speak the same languageEntities

User applications e-mail facilities terminals

Systems Computer Terminal Remote sensor

Key Elements of a ProtocolSyntax

Data formats Signal levels

Semantics Control information Error handling

Timing Speed matching Sequencing

Protocol ArchitectureTask of communication broken up into

modulesFor example file transfer could use three

modules File transfer application Communication service module Network access module

Simplified File Transfer Architecture

A Three Layer ModelNetwork Access LayerTransport LayerApplication Layer

Network Access LayerExchange of data between the computer

and the networkSending computer provides address of

destinationMay invoke levels of serviceDependent on type of network used (LAN,

packet switched etc.)

Transport LayerReliable data exchangeIndependent of network being usedIndependent of application

Application LayerSupport for different user applicationse.g. e-mail, file transfer

Addressing RequirementsTwo levels of addressing requiredEach computer needs unique network

addressEach 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

communicateControl information is added to user data

at each layerTransport layer may fragment user dataEach fragment has a transport header

added Destination SAP Sequence number Error detection code

This gives a transport protocol data unit

Network PDUAdds network header

network address for destination computer Facilities requests

Operation of a Protocol Architecture

F I N A L