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Chapter 2

NetworkModels

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Figure 2.1 Sending a letter

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Layered Protocols

Communication tasks are divided into series of layers or levels Each layer is responsible for particular

task and act on them by using one or more protocols

Each layer is built upon one bellow it The number and name of the layers

differ from network to network

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Figure 2.17 The OSI seven layer model

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The Layers of OSI Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network

Data Link

Physical

IntermediateSystem

EndSystem

Application

Presentation

Session

Transport

Network

Data Link

Physical

R

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Summary of OSI Layer Functions

Application

Presentation

Session

Transport

Network

Data Link

Physical

To allow access to network resources

To establish, manage and terminate sessions

To move packets from source to destination; to provide internetworking

To transmit bits over a medium; to provide mechanical and electrical specifications

To translate, encrypt and compress data

To provide reliable end-to-end message delivery and error recovery

To organize bits into frames, to provide node-to-node delivery

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Open System Interconnection (OSI)

Developed by International Standard Organization (ISO) as a first step towards international standardization De jure protocol

Deals with interconnecting systems that are open for communication with other systems Open protocol suite

Good as theoretical model, but not widely implemented in practice

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The OSI layers

Session layer Provides the control structure for communication

between applications (dialog control) Establishes, manages and terminate connections

(sessions) between cooperating applications

Presentation layer Provides independence to the application

processes from differences in data representation Application layer

Provides access to the OSI environment for users and provides distributed information services

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The OSI layers

Physical layer Transmission of unstructured bit stream

Deals with the mechanical, electrical, functional and procedural characteristics to access the physical medium

Data link layer Provides reliable transfer across the physical

link between two ends connected via single link Sends blocks of data (frames) with the necessary

synchronization, error control and flow control Can add header and trailer

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The OSI layers

Network layer Provides upper layers with independence from

the data transmission and switching technologies accross internetwork Responsible for source-to-destination delivery,

addressing and routing in the internetwork

Transport layer Provides transparent transport of data

between end points that might not be connected via single link Provides source-to-destination connection, error

recovery and flow control

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Protocol Suites

Open System Interconnection (OSI) Today used mostly as a reference model Prevously used in X.25 based protocols

Internet (TCP/IP) Most popular suite today

Xerox Networking Sysytems (XNS) System Network Architecture (SNA – IBM) Digital Network Architecture (DNA – DEC) NetBIOS (Software interface) AppleTalk

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The TCP/IP five layer model

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TCP/IP-modellen

TCP, UDPIP

Ethernet

SMTP, HTTP

Exempel:

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TCP/IP Protocol Suite

De facto (and after that de jure) standards Open (All modification and newly proposed

protocols are published in a form of RFC (Request for Comments)

RFC as well as drafts are published on the Internet can be found on many URL (one is

www.rfc-editor.org) RFC becomes a standard when it is:

Stable and well understood Technically competent Implemented on multiple independent places

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The TCP/IP Protocol Suite (Cont.)

Allows computers of many sizes, vendors and operating systems to communicate with each other

History: Developed as de facto standard before OSI 1960’s: started as goverment financed research

project 1990’s: most widely used form of networking

Forms the basis for the Internet (capital “I”)(a WAN that spans the globe)