CCM 4300 Lecture 3 Computer Networks: Wireless and Mobile - - PowerPoint PPT Presentation

CCM 4300 Lecture 3

Computer Networks: Wireless and Mobile Communication

• and Mobile Communication

Systems

• introduced a classification of computer networks

Introduced circuit and packet switching

• described networks with different scale

introduced the various network standards, how they

are created and by whom

• Lesson Objectives

Network Topologies Introduction – an example of human-to-human interaction What is a protocol?

• What is a protocol?

ISO OSI Reference Model

• TCP/IP protocol

• At the completion of this lesson you should be able to

understand network logical and physical topologies understand the concept of layering and structure of the ISO Reference Model describe the functionality of each layer in the ISO

• describe the functionality of each layer in the ISO

Reference Model understand the basic concepts of the client server architecture

• ! "

#

$%

"

&!

'

&!

"

(!

• "

Bus

)

Each computer is connected to a single cable which connects all of the computers. This is the cheapest network topology as the smallest amount of cabling is required. If the network cable breaks anywhere then none of the computers can access the network.

Ring

*

Each computer is connected to the two computers on either side of it. The last computer is linked to the first to form a ring. If the network cable breaks anywhere then none of the computers can access the network. Long cable length

Mesh

+

Any computer can be connected to any other computer. There are multiple routes. If one link fails messages can go another way so this topology is very reliable. The Internet has a mesh topology.

Star

,

All communication takes place via a central computer. If the central computer fails the whole network will break down. If one of the network cables breaks only the computer connected to that cable is affected.

Extended Star

An extended star topology is like a star topology except that each device that links to the centre device is also the centre of another star. Each of these devices acts as the centre of another star.

• another star.

An apparent advantage of this is to extend the length and the size of the network. Limits the number of devices and the numbers of cables needed to interconnect to the central device. Logically, the information in an extended star topology stays local

(!.$

• /.

.

• .
• .

!1.

• 2!3

• Represent the “rules” for communication:

how do the computers initiate communication? what features can be negotiated at the start? the communication vocabulary

• i.e. what requests (commands) can be given

and what are the valid responses? what kinds of data can be exchanged ? how the communication can be closed in an

• rderly way ?

A human protocol and a computer network protocol:

• !!"""#$%##&'
• $

( ))

• *#
• #

+, $

$

4 Header Data

• Header

Data Control Information Message

For two networked devices to communicate they

must speak the same language (i.e. use the same protocol)

The protocols must be able to:

• indicate who (what address) they want to talk to

'

• indicate who (what address) they want to talk to
• provide any required delivery assurances /

recovery

• control the flow of information

Must specify how to initiate, maintain and conclude

the communications exchange

Several protocols are involved in a network Operate together in a layered manner

• each layer builds upon the services of its lower

layer(s)

File Transfer

)

Workstation Remote Server WAN End-to-end integrity Internetworking Router LAN

A protocol (or more typically a protocol suite) is

needed for communications

• some organisations may use multiple suites

(e.g. Novell and TCP/IP protocols)

*

Protocols are specified in the form of documents

and usually implemented in software

A separate software package is needed for each

different protocol suite to be supported

• ..-/

Open Systems Interconnection Developed by International Standards

Organisation (ISO) - 1981

• revised in 1994

+

Described in ISO-7498 standard Proposes 7 layers Provides:

• a common terminology
• a framework for networking

.-

A layer should be created where a different level of

abstraction is needed

Each layer should perform a well-defined function The function of each layer should be chosen with a

view to defining internationally standardised protocols The layer boundaries should be chosen to minimise

, The layer boundaries should be chosen to minimise

the information flow across the interfaces

The number of layers should be:

• large enough that distinct functions are not

thrown together

• small enough that the architecture is not

unwieldy

.-/

5--6-+

• Source :

Computer Networks 1996

Application

• contains a number of standard protocols of

general use

• e.g. file transfer protocols, email, virtual

.-

• e.g. file transfer protocols, email, virtual

terminals, telnet, etc...

• any service program may define an application

level protocol that clients must use.

Presentation

• concerned with the representation of data (e.g.

between different hardware, Operating Systems etc.)

Session

• allows clients of an OS on one machine

to

.-

• establish and use sessions with clients
• f an

OS on another machine

• Controls the dialogue between applications in

end systems.

• Controls the direction of traffic.
• Other functions: recovery, synchronisation, etc..

.-

• Transport
• concerned with transmission from end system to end

system

• transparent – data format might be different at end

system, so it makes sure the correct delivery of data.

• Determines what type of services to provide the session
• Determines what type of services to provide the session

layer.

Network

• concerned with transmitting data from a source to

destination across networks

• must determine route for data packets and attempt to

avoid congestion by controlling the number transmitted

.-

Data Link

• Concerned with taking a raw

transmission facility and turning it into a link that appears to be free from errors

Physical

• Concerned with transmitting
• Concerned with transmitting

uninterrupted bits from on computer to another and managing the connection

A common mnemonic device for remembering the

layers in the right order is

All People Seem To Need Data Processing

.-

Layer Number Layer Name Layer Services 7 Application Support for e-mail, file transfer etc. 6 Presentation Data representation 5 Session Control the dialog 4 Transport End-to-end data integrity

'

4 Transport End-to-end data integrity 3 Network Internet addressing, routing and segmentation 2 Data Link Package bits into frames and control their delivery 1 Physical Adapt bits for transmission over the medium

0&

If OSI is a Reference Model then what is a

network architecture? 1 "'&#-

)

1 "'&#- $$&2$$2 *&#

.-

Service – what is done

• defines what the layer does (but not how

entities above access it or how it works) Service vs. Interface Vs. Protocol

*

entities above access it or how it works)

Interface – how it is called above

• tells the processes above how to access it
• specifies what the parameters are and the

results to expect

Protocol – how it “talks” to its peer layer

• how a layer works (i.e. provides the offered

services)

1

Standard interfaces between layers

• allows internal developments within a particular

layer to evolve

Alternative services may be offered at a given layer +

• via different options or routes through the layer

Internal mechanisms of each layer are invisible to the

• ther layers

Layers may be completely removed if not required, or

substituted by simpler versions

$.-

The following factors limited adoption of OSI in practice:

Timing

• TCP/IP protocols were already in widespread use

,

• TCP/IP protocols were already in widespread use

Technology

• the 7 layers are not optimal (e.g. Session and

Presentation layers hardly perform any function)

$.-

Implementation

• initial implementations were unwieldy and slow -

led to a lasting bad reputation

Politics

• perceived as bureaucratic organisations

attempting to impose inferior standard BUT

valuable as a conceptual architecture

• a ‘reference model’ for comparison purposes

/.-

• Source :

Computer Networks 2003

!-

• Source :

Computer Networks 2003

OSI

• model useful as a means of discussing

computer networks and educational purposes

• protocols have not become popular yet

.-3!-

• protocols have not become popular yet

TCP/IP

• model practically non-existent !!
• protocols very widely used

• A. application, transport, internet, host-to-network

0&

• B. application, internet, transport, host-to-network
• C. application, presentation, session, network, transport, data

link, physical

• D. application, presentation, session, transport, network, data

link, physical

!$

• A client server system is more structured than

general distributed computing

• A client sends request to servers to execute tasks

The tasks may be just to provide information, or to

'

• The tasks may be just to provide information, or to

perform a complex computation (perhaps returning data, results, etc)

• A client and servers are asymmetric
• A server may be a client of another server

!

• Clients and servers are separate processes
• They may run on the same or different machines
• Each process can hide internal information

)

• Each process can implement its own set of

business rules (integrity)

• They communicate by peer-to-peer protocols

Lesson Objectives Network Topologies Introduction – an example of human-to-human interaction

Summary!

*

What is a protocol? ISO OSI Reference Model

• TCP/IP protocol

Are there any questions?