A. Zahid, EE450 1

Network Layered ArchitectureOSI Reference Model & TCP/IP Protocol Suite

Professor A. ZahidEE450: Computer Networks

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ProtocolsHuman Protocols:• what’s the time?• I have a question• Introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

Network Protocols:• Machines rather than

humans• All communication

activity in Internet governed by protocols

Protocols define format, order of msgs sent and received

among network entities, and actions taken on msg transmission, receipt

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Get http://www.cnn.com/politics.html

Human v.s. Network Protocols

Hi

Hi

Got thetime?

2:00

TCP connectionrequest

TCP connectionreply

<file>time

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Key Elements of a Protocol

• Syntax– Data formats, compression, encryption, etc..– Signal levels

• Semantics– Control information such as flow & congestion– Error detection and control mechanisms

• Timing– Speed matching– Sequencing

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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

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OSI Reference Model

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Data Transmission in OSI Model

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Summary of Layer Functions

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The Internet is based on TCP/IP

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TCP/IP Protocol Suite• Developed by the US Defence Advanced Research

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

Application Layer

Transport (TCP) Layer

Network (IP) Layer

Network Access (Data Link) Layer

Physical Layer

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TCP/IP Protocol Suite & OSI

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Layer Communications

• Peer layers communicate via protocols• Adjacent layers communicate via service

interfaces• Protocol control information (PCI) is added

to Service data unit (SDU) at each layer to form a Protocol data unit (PDU)– (SDU)N = (PDU)N+1

– (PDU)N = (SDU)N +(PCI)N

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Protocol Data Unit (PDU)User/Application Data

Transport layer header

Transport layer PDU

Network layer headerNetwork layer PDU

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Layering: Logical CommunicationsEach layer:• distributed• “entities”

implement layer functions at each node

• entities perform actions, exchange messages with peers

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

data

transport

transport

ack

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Layering: Physical Communicationsapplicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

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Layered Architecture & Networks

• Each host needs a unique global address referred to as the IP address

• Each application on a multitaskingcomputer needs a unique address, referred to as the Port number, within the computer