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G Lesson 1 – Protocols and OSI

What is a network protocolDescription of the OSI model

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G Introduction

Protocols

Are sets of rules that govern how two devices communicate and exchange data when connected over a network. Examples are TCP/IP, HTTP, FTP, SMTP etc.

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G The Open Systems Connection (OSI) model

The OSI is a theoretical model that standardises network architecture and protocols within a seven layer hierarchy.

Physical

Data Link (MAC)

Network

Transport

Session

Presentation

ApplicationLayer 7

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

“Application layers”

“Transport layers”

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G The Open Systems Connection (OSI) model

• Top level layer where users actually interact with applications that create the data that will be sent over the network.

e.g. when using a web browser the browser requests a web page from a server

• Examples of application-layer programs include web browsers, ftp programs, telnet, email, and so forth.

• The user interacts with these directly.

7 Application layer

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G The Open Systems Connection (OSI) model

• This layer is concerned with the actual format that the data is presented in.

• It is home to a number of data presentation standards including JPG, GIF, and MPEG.

• This layer is also usually responsible for data encryption and compression.

6 Presentation layer

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G The Open Systems Connection (OSI) model

• The responsibility of this layer is to create, manage, and terminate communication sessions between systems.

• Think of the session layer as a coordinator of communication between systems

• A variety of protocols exist at the session layer, including Remote Procedure Calls (RPCs), the Network File System (NFS), SQL, and the AppleTalk Zone Information Protocol (ZIP).

5 Session layer

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G The Open Systems Connection (OSI) model

Its responsibilities include :

• segmenting data that comes from the upper layers

• establishing a connection between end-to-end systems that defines the parameters of the connection

• and ensuring that data actually gets to its destination in the correct order and error free

• Examples of protocols found at this layer include TCP and UDP.

4 Transport layer

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G The Open Systems Connection (OSI) model

• This layer is responsible for two main functions – network addressing and routing.

• It identifies systems uniquely on an internetwork, and also handles how to get from one part of the network to another.

• Most of the network configuration takes place, since protocols such as IP, IPX, and equipment such as routers work here

• Its concerned with ‘where am I going (the address), and how do I get there (the routing).

3 Network layer

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G The Open Systems Connection (OSI) model

• is concerned with how upper level data is formatted for transmission over a given network technology.

• For example, if your network is running Ethernet, this layer will format data packets accordingly, and follow the rules of Ethernet transmission on the network.

• Examples of Data Link protocols include Ethernet, Token Ring and FDDI

• At this layer, systems are generally identified by what is referred to as their hardware or MAC address.

2 Data Link (MAC) layer

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G The Open Systems Connection (OSI) model

• it defines the actual media and characteristics used to send the 1’s and 0’s over the network.

• This includes the actual wiring itself, the connectors used, voltage levels, maximum media distances and so forth.

• For example, a packet might start on twisted-pair wiring, move onto a wireless network, and then ultimately end up on fibre – each of which using different media and methods during the transfer process. In this case, copper wiring, then air, and then fibre optic cable.

1 Physical layer

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G The Open Systems Connection (OSI) model

So, in summary…

Physical

Data Link (MAC)

Network

Transport

Session

Presentation

ApplicationLayer 7

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Type of communications e.g. email, ftp

Encryption, data conversion

Starts, stops session

Ensures delivery of entire file or message

Routes data over LANs or WANs via IP address

Transmits packets via MAC address

Electrical signals and cabling