OSI Reference Model 1

Protocols and Reference Models

Faisal Karim Shaikhfaisal.shaikh@faculty.muet.edu.pk

DEWSNet GroupDependable Embedded Wired/Wireless

Networkswww.fkshaikh.com/dewsnet

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Protocols

Cooperative action is necessary computer networking is not only to exchange bytes huge system with several utilities and functions.

For proper communication, entities in different systems must speak the same language

Those conventions and associated rules are referred as “PROTOCOLS”

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Need For Protocol Architecture

data exchange can involve complex procedures, cf. file transfer example

better if task broken into subtasks implemented separately in layers in stack

each layer provides functions needed to perform comms for layers above

using functions provided by layers below peer layers communicate with a protocol

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Key Elements of a Protocol

syntax - data format semantics - control info & error handling timing - speed matching & sequencing

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A Real World Example to Protocol Architecture

Issues: peer-to-peer protocols are independent of each otherfor example, secretaries may change the comm. medium to emailor the translators may agree on using another common language

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LAYERED TASKS: Tasks involved in sending a letter

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Simplified Network Architecture

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Standard Protocol Architectures

Two approaches (standard) OSI Reference model

•• never used widelynever used widely•• but well knownbut well known

TCP/IP protocol suite•• Most widely usedMost widely used

Another approach (proprietary) IBM’s Systems Network Architecture (SNA)

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OSI Reference Model

Open Systems Interconnection Reference model

provides a general framework for standardization defines a set of layers and services provided by each layer one or more protocols can be developed for each layer

Developed by the International Organization for Standardization (ISO) also published by ITU-T (International Telecommunications

Union)

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OSI Reference Model

A layered model Seven layers – seven has been presented as the optimal number

of layer Delivered too late (published in 1984)!

by that time TCP/IP started to became the de facto standard Although no OSI-based protocol survived, the model is

still valid

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Standardized Protocol Architectures

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Seven layers of the OSI model

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Interaction b/w layers in the OSI model

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Exchange of data using the OSI model

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Physical layer

Physical interface between devices Characteristics

•• Mechanical Mechanical -- interface specsinterface specs•• Electrical Electrical -- voltage levels for bits, transmission ratevoltage levels for bits, transmission rate

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Data link layer

Principle service: error detection and control Higher layers may assume error free transmission link to link (point to point) Later a sublayer is added to Data Link Layer

•• MAC (Medium Access Control) MAC (Medium Access Control) sublayersublayer•• to deal with broadcast networksto deal with broadcast networks

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Hop-to-hop delivery

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Network layer Transfer of information through comm. network Higher layers do not need to know about underlying networking

technology Not needed on direct links If needed network nodes (relays) should perform switching and routing

functions QoS (Quality of Service) and congestion control are also addresses in

this layer Several other internetworking issues

•• e.g. differences in addressing, max. data length, etc.e.g. differences in addressing, max. data length, etc.

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Diff b/w InterServ and DiffServ Model for ensuring QoS for Networks

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Source-to-destination delivery

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Transport layer

Exchange of data end to end If needed, upper layer data are split into smaller units In sequence, no losses, no duplicates

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Reliable process-to-process delivery of a message

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Session layer Control of dialogues

•• whose turn to talk?whose turn to talk? Dialogue discipline (full-duplex, half-duplex) Checkpointing and recovery

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Presentation layer

Data formats and coding Data compression Encryption

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Application layer

Support for various protocols

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Summary of layers

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www.muet.edu.pk/~shaikh/courses