CSCI1412Lecture 16

Networks 2Network Layer Models

Dr John Cowell

phones off (please)

2

OverviewProtocol layer models

what is a layer model and why use it?standards and standards agencies

The OSI seven layer modeloverview of the layer modelthe seven layers explained

The real worldhow the theoretical model applies to the Internet

Layer Models

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Protocol Layer ModelsHead of State

Formulate messagein country’s interest

Ambassador/DiplomatsRephrase in

diplomatic language

TranslatorTranslate to

intermediate language Communicate, but no concept of meaning

Translate into national language

Rephrase usingappropriate language

Receive message and decide upon

response

Communicate ideasno generation or

change of meaning

High-level communicationno common language

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Protocol StandardsA protocol is a set of rules governing

communication issues - a standarddiplomats have protocolsso do networksso do computer networks!

Two types of standardsde-facto - devolved standards

e.g. many IBM products, Microsoft Office, TCP/IPdesigned - written standards

submitted to a standards agency for approval agreed by a group of companies or organisations

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AgenciesProtocols often written/regulated by agencies, e.g.

ANSI American National Standards Institute

FDDI, ASCIICCITT

Comité Consultatif Internationale de Télégraphic et Téléphonique V series for telephone communications (modems) X series for network interfaces & public Networks

IEEE Institute of Electrical & Electronic Engineers

Project 802 - LAN standardsIAB

Internet Activities Board set and document internet policies and standards, e.g. TCP/IP

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Who is ISO?International Organisation for Standardisation

a worldwide federation of national standards bodies from 130 countries, one from each country a non-governmental organization established in 1947 to promote the development of standardization

ISO’s work results in international agreements which are published as International Standards

ISO’s name shouldn’t the acronym be “IOS”?

yes, if it were an acronym; but it is not“ISO” is a word, from the Greek isos, meaning “equal”

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ISO’s OSI ModelOpen Systems Interconnection

a standard description or “reference model” for how messages should be transmitted between any two points in a telecommunication network any two computers can communicate when connected regardless of underlying architecture

The seven layer modelthe process of communication between two users in a

comms. network can be divided into layers, with each layer adding its own set of special, related functions written standard not implemented

The OSI 7 Layer Model

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Origins of the ModelLate 1970’s 2 separate projects byISOCCITT To standardise a framework for emerging

network technologies.1984 published as ISO 7498 and CCITT as X.200

The Internet and growth of TCP/IP (Transmission Control Protocol/ Internet Protocol) became the de-facto standard.

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The Seven LayersApplication layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

User services, application, activities

Actual data transmissioncommunications subnet

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What They DoThe layers are in two groups

the upper four layers are used whenever a message passes from or to a user within the same computer or to another computer

lower three layers used when any message passes through the host computer to another messages destined for some other host are not passed up

to the upper layers but are forwarded to another host

Purpose is to separate specific functionsmakes implementation transparent to other

componentsallows independent testing & maintenance of each

layer

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Information Processing

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data Link layer

Physical layer

Process sending information

Process receiving information

Logicalcommunication

betweenlayers

same computer

other computers

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Data Unit at Different LayersEach layer deals with different grouping of data

Data - Application, Presentation, SessionSegment – TransportPacket – Network Frame – Data LinkBit - Physical

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Application (Layer 7)The application layer

interacts with applications that offer some form of communication component not an application program

Provides services such asidentifying communicating partners

determining the identity and availability of partnersdetermining resource availability

decide whether sufficient networking resources are availablesynchronising communication

all communication between applications requires cooperation

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Presentation (Layer 6)The presentation layer

presents data in user understandable format allows user to see information, not data translates between formats

Provides a variety of coding and conversion functions that are applied to application layer datadata representation

e.g. JPEG, GIF, TIFF formats for graphicscharacter representation

e.g. ASCII, EBCDIC encodings for charactersdata compression schemesdata encryption schemes

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Session (Layer 5)The session layer

responsible for managing connections between two machines during the course of communication

determines whether all pertinent information for the session has been transmitted and/or received

The layerestablishes logical connection

controls synchronisation between two endsmanages acknowledgements and retransmissionsreconnects on premature termination (at lower level)releases logical connection

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Transport (Layer 4)The transport layer

end-to-end communications decides upon network to use

according to data type, e.g. packets or continuous bit stream

Transport layer functions typically includeflow control

ensuring there is enough ‘space’ to send/receive messagesmultiplexing

combining more than one message on the same connectionpacketing

breaking larger units down into small pieces to be transferrederror checking and recovery

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Network (Layer 3)The network layer

provides routing and addressing functions that enable multiple data links to be combined into a network sending it in the right direction to the right destination on

outgoing transmissions receiving incoming transmissions at the packet level

It provides functions such asrouting strategies

deciding which route to use and whyforwarding

sending on packets destined for other hostscontrolling communications subnet

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Data Link (Layer 2)The data link layer

provides the first level of organization of the data bits into a rudimentary structure called a datalink frame

this is organized into fields of information that convey the beginning and ending of the frame the address of the sender the address of the receiver a method to ensure that the frame does not contain errors an area to provide some basic administrative function

It is sometimes broken down in two sub layerslogical link control layer (LLC)media access control layer (MAC)

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Physical (Layer 1)The physical layer

defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems

transmits/receives bits as electrical impulsesThis layer describes

the network topology (shape, structure)the transmission media (cables, broadcasts)the transmission devices (network cards, modems)the electrical or optical signalling characteristics

The Real World

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The Real World!Loosely, a five layer model

application layer

presentation layer

session layer

transport layer

network layer

data link layer

physical layer

OSI

7

6

5

4

3

2

1

world wide web (HTML/HTTP)email (POP / SMTP)file transfers (FTP)

remote sessions (TELNET)

transmission control protocol (TCP)

internet protocol (IP)

(physical level)

ethernet

The Real World

5

4

3

2

1

PPP

modem

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Application LayerTop layer (5) comprises other protocols such as

HTML / HTTP (the World Wide Web)POP / SMTP (email)FTP (file transfer)TELNET (remote sessions)DNS (Domain Name Server)

maps domain names to IP addresses 146.227.102.2 http://www.cms.dmu.ac.uk/

This application layer roughly corresponds to layers 5, 6, and 7 of the OSI model

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TCP/IPTCP/IP is the basic communication protocol

(language) of the InternetTransmission Control Protocol

manages assembling a message or file into smaller parts (packets) that will be transmitted over a network

reassembles packets when received TCP OSI layer 4 (transport)

Internet Protocolhandles the address part of each packet to ensure that

it gets to the correct destination IP OSI layer 3 (network)

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Lower LevelsThree of the most common methods of connecting to the

Internet areEthernet

LAN protocol developed by Xerox, DEC & Intel in 1976 specifies the way data is transmitted as well as the physical

connection options incorporates layers 1 and 2 of the OSI model

PPP over a modem link point-to-point protocol controls how the data is sent (level 2)

now included in Windows and other operating systems modem handles transmission of data over telephone line (level 1)

DSL (Broadband) utilises unused bandwidth on standard telephone wires need frequency divider (DSL ‘modem’) to separate voice call from

data signal

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SummaryProtocol layer models

what is a layer model and why use it?standards and standards agencies

The OSI seven layer modeloverview of the layer modelthe seven layers explained

The real worldhow the theoretical model applies to the Internet