COMP2221 Networks in Organisations

COMP2221COMP2221

Networks Networks in in

OrganisationsOrganisations

Richard HensonRichard Henson

February 2014February 2014

Week 2: Standards and Week 2: Standards and Computer NetworksComputer Networks

ObjectivesObjectives– Explain functions of client-server networks Explain functions of client-server networks

and network servicesand network services– Define networking standardsDefine networking standards– Explain how digital networks standards Explain how digital networks standards

have helped make world-wide digital have helped make world-wide digital communications more effectivecommunications more effective

Requirements of Requirements of organisational networksorganisational networks

The server would be expected to offer The server would be expected to offer the following to its users:the following to its users:– Network AccessNetwork Access– Access to “restricted” filesAccess to “restricted” files

» users with permission directly access files on users with permission directly access files on the serverthe server

– ApplicationsApplications– PrintingPrinting– Access to email & The InternetAccess to email & The Internet

The Client-Server ModelThe Client-Server Model

Centralisation of organisational resourcesCentralisation of organisational resources– client can still hold resourcesclient can still hold resources

» a lot (fat client)a lot (fat client)

» Not much (thin client)Not much (thin client)

Microsoft model: called a domainMicrosoft model: called a domain

Request and responseRequest and response1. All network users use clients

2. Client requests information…

2. Server processes the request, sends a response back to the client

CLIENT SERVER

RESPONSE

Send RequestRead Results

Process RequestSend Back Results

ClientProgram

REQUEST

ServerProgram

Servers in Larger NetworksServers in Larger Networks

Larger networks have MANY serversLarger networks have MANY servers– University ITS network: at least 50University ITS network: at least 50

Functions can be distributed around Functions can be distributed around different individual servers. Examples:different individual servers. Examples:– Login ServerLogin Server– File and Print serverFile and Print server– Applications ServerApplications Server– Internet GatewayInternet Gateway

Login Servers Login Servers (the most crucial!)(the most crucial!)

Dedicated to logging on usersDedicated to logging on users– database of usernames/passwordsdatabase of usernames/passwords

Only allows a potential user to access the Only allows a potential user to access the network if both username and password network if both username and password exactly correspond with entries in the exactly correspond with entries in the databasedatabase– In Windows networks known as In Windows networks known as Domain Domain

ControllersControllers

Problems with this Scenario?Problems with this Scenario?

Discussion in Groups…Discussion in Groups…

Possible Solutions?Possible Solutions?

Further Discussion…Further Discussion…

Peer-Peer networksPeer-Peer networks

Also known as Also known as workgroupsworkgroups No central serverNo central server Computer nodes can act as both clients Computer nodes can act as both clients

and serversand servers No expensive powerful machine No expensive powerful machine

dedicated to providing servicesdedicated to providing services

Peer-Peer networksPeer-Peer networks

All users have their own local storage All users have their own local storage capacitycapacity

Bears the following responsibilities…Bears the following responsibilities…– local security & network administrationlocal security & network administration– granting access to their computer’s services granting access to their computer’s services

and resources via the networkand resources via the network

Advantages & disadvantages Advantages & disadvantages of Client-Server, compared to of Client-Server, compared to

Peer-peerPeer-peer

In groups…In groups…

Don’t look at next Don’t look at next slides!slides!

Advantages of a client-server Advantages of a client-server network, compared to a network, compared to a

workgroupworkgroup Centralised securityCentralised security Centralised access to resourcesCentralised access to resources Centralised network administrationCentralised network administration With more than about 10 users, much With more than about 10 users, much

easier to manage than a workgroup. easier to manage than a workgroup. Can handle up to thousands of usersCan handle up to thousands of users

Disadvantages of client-server, Disadvantages of client-server, compared to a workgroupcompared to a workgroup

Expensive dedicated computer(s) not Expensive dedicated computer(s) not accessible to usersaccessible to users

Expensive server operating system Expensive server operating system neededneeded

Network management requiredNetwork management required Reduces user autonomyReduces user autonomy If one server, and it goes down, the If one server, and it goes down, the

network ceases to function!!!network ceases to function!!!

Windows NetworksWindows Networks

Peer-peer networks:Peer-peer networks:– workgroupsworkgroups– limited resource sharing abilitylimited resource sharing ability

Client-server networks:Client-server networks:– domainsdomains– access to domain via domain controller(s)access to domain via domain controller(s)

Enterprise networksEnterprise networks– multiple domains logically linked in a hierarchymultiple domains logically linked in a hierarchy

Virtual (client) and Cloud Virtual (client) and Cloud (server) Networks(server) Networks

Extension of client-server model…Extension of client-server model… Client-end less resource intensiveClient-end less resource intensive Most of resources & processing at Most of resources & processing at

server endserver end Popular because clients need less Popular because clients need less

CPU power & less maintenanceCPU power & less maintenance– therefore lower cost…therefore lower cost…

Thin Client/CloudThin Client/Cloud

Advantages and disadvantages?Advantages and disadvantages?

Another 3 minutes….Another 3 minutes….

More about StandardsMore about Standards

Definition:

– “A standard is an established or accepted model”

Communication protocols…

– “Elements of a communication system that are

defined by an agreed set of rules, conditions,

parameters or methods”

Type of StandardsType of Standards

De FactoDe Facto– A product or service that is a standard by A product or service that is a standard by

virtue of its widespread use by interested virtue of its widespread use by interested usersusers

De JureDe Jure– The standard devised by a committee of the The standard devised by a committee of the

organisation or, a working group of a organisation or, a working group of a subcommittee of a committee of the subcommittee of a committee of the organisationorganisation

Communication Protocols Communication Protocols in 1977in 1977

Lots of “proprietary standards” had arisenLots of “proprietary standards” had arisen– IBM IBM – HoneywellHoneywell– ICL (UK)ICL (UK)– Bull (France)Bull (France)– DECDEC

Each corporation thought theirs was best…Each corporation thought theirs was best…

Historic Geneva Meeting (1978)Historic Geneva Meeting (1978)

All stakeholders in International All stakeholders in International communications protocols invited to communications protocols invited to conference by the Lake…conference by the Lake…

Had to agree to a hypothetical International Had to agree to a hypothetical International communications protocolcommunications protocol

No expectation that it would be No expectation that it would be implemented….implemented….

Open Systems Interconnect (OSI)

Dilemma: all manufacturers wished to have their own communication models represented

Principle: – “A new layer must be created for each

new level of abstraction” Result: they agreed only by developing

a model based on… 7 software layers!

OSI Model – Benefits All manufacturers…

– target to aspire towards Benefits of OSI compatible products:

– other manufacturers products would be able to communicate with their own

– consumer would no longer be “locked in” to specific vendor products

– vendors would be able to produce products that work at specific layers only

» specialise and hence produce better products

Layer Communication (Sending)

Each layer in the OSI model considers itself to be talking to a peer layer in another computer– adds/removes its own “header” (formatting info)

e.g. application layer– adds a header to the user data on screen– passed to the presentation layer as a single block

e.g. presentation layer– adds its header to the block of data – passed on to session layer as a single block…

and so on…

The OSI reference model

Physical Physical LayerLayer

Data link Data link LayerLayer

Network Network LayerLayer

Transport Transport LayerLayer

SessionSessionLayerLayer

Presentation Presentation LayerLayer

Application Application LayerLayer








TransmitTransmitStationStation

ReceiveReceiveStationStation

AH DATA

PH AH DATA

SH PH AH DATA

TH SH PH AH DATA

NH TH SH PH AH DATA

LH NH TH SH PH AH DATA LT

DATA AH

DATA AH PH

DATA AH PH SH

DATA AH PH SH TH

DATA AH PH SH TH NH

LT DATA AH PH SH TH NH LH

LinkLink

Layer Communication (Receiving)

Each layer in the OSI model strips away its own header

e.g. physical layer – removes header from data block– passed to the data link layer

e.g. data link layer– removes header to the block of data – passed on to network layer

and so on…

The OSI reference model















TransmitTransmitStationStation

ReceiveReceiveStationStation

AH DATA

PH AH DATA

SH PH AH DATA

TH SH PH AH DATA

NH TH SH PH AH DATA

LH NH TH SH PH AH DATA LT

DATA AH

DATA AH PH

DATA AH PH SH

DATA AH PH SH TH

DATA AH PH SH TH NH

LT DATA AH PH SH TH NH LH

LinkLink

Simplifying The OSI model

Layers can be sub-divided into two groups

– The top 3 layers (interworking layers)» user applications and support services

– The lower 4 layers (interconnection layers)» the network (and navigation of packets)

Memory aids: – PDNTSPA – Please Do Not Throw Sausage Pizza Away!

Interconnection Layers

Concerned with packets of data– and navigating them through the network

Transport Network Data Link Physical

The Four Layers Model

Introduced with Unix (mid-1970s, pre-OSI)– based on Internet protocols…

“application”

“transport”

“network”

“physical”

TCP/IP

Evolved with the Unix four layers…

TCP

Application, presentation, session

IP

Connecting with physical medium

Transport Layer (from Unix)

Manages the transmission of level 4 data from sender to corresponding layer in receiver– segments data streams into chunks of a given

packet size for the medium being used– checks for errors due to corruption, requests

retransmission etc.

Gateways can operate at this layer

Transport Layer (from Unix)

Other roles:– managing flow control– providing acknowledgement of successful

transmission of chunks of data– software multiplexing – routing in an Internetwork

Manages OSI levels 1-4 so messages travel between network nodes via pairs of “sockets”

socket A (sender)

socket B (receiver)

Transport layer





UpperUpperOSI OSI

LayersLayers







End UserEnd User





UpperUpperOSI OSI

LayersLayers

End UserEnd User

Peer-to-Peer communicationsPeer-to-Peer communications

Network ANetwork A Network BNetwork B

SocketA

SocketB

End-end v logical neighbour communications

Top four OSI layers communicate logically with remote peer… – regardless of topology or distance

The lower layers all communicate physically with their nearest neighbour in a network– dependent on topology and routing to get the

packets through


Network Layer


User SpecifiesUser SpecifiesServiceService

NetworkNetworkServiceService

Network providesNetwork providesServiceService

Network layer service definitions

Data Link Layer Responsible for error free transmission, using data

frames A frame is a basic unit for network traffic, and has a

highly structured format Mechanism:

– data from the upper layers (ie the network layer) is converted by the data link layer into frames

– groups raw data bits received via the physical layer into frames, for passing on to the upper layers

– may include an error recovery mechanism and also a flow control mechanism, although this may be done at the transport layer

Bridges operate up to this level

Physical Layer

Responsible for communicating with the network media

Bits are converted into electrical signals and vice versa

Issues include modulation of signals and timing Manages the interface between a computer and the

network medium, but cable type and speeds of transmission are deliberately omitted to allow future technology to be easily included

Repeaters work only at this level

Standards and the OSI reference model

OSI designed to promote the development of protocols…– that support open systems interconnection

Become an agreed standard in 1984– ISO 7498 (the International Standard)

– BSI 6568 (the identical British Standard)

– CCITT recommendation X.200

How Apple complied with OSIHow Apple complied with OSILevel 7…. AppleShare (files, printers, PCs)

AppleTalk Filing Protocol (AFP), Printer Access Protocol (PAP)

AppleTalk Session Protocol (ASP), Zone Protocol (ZIP), AppleTalk Data Stream Protocol (ADSP)

AppleTalk Transport Protocol (ATP), AppleTalk Echo Protocol (AEP), Name-Binding Protocol (NBP), Routing Table Maintenance Protocol (RTMP)

Level 3…. Datagram Delivery Protocol (DDP)

LocalTalk Link-Access Protocol (LLAP), Ethernet Link-Access Protocol (ELAP), Token ring Link-Access Protocol (TLAP), Other Link-Access Protocols

LocalTalk, EtherTalk, TokenTalk, or other Network Topologies

Level 1…. STP, UTP, Coax, Fiber

IEEE 802 Specifications and Layers 1&2 of the OSI model

Emerged from IEEE/OSI meeting: February ‘80– applied mainly to lower level OSI layers (1/2)– found it necessary to extend the data link layer into

two parts– Essential for development of LANs

Definitions used by manufacturers for hardware and software of network interface cards– origin of the MAC address

Effect of IEEE 802 on the OSI model

To cover engineering issues, IEEE divided the Data Link Layer into two sub-layers:– Layer 2 (upper): Logical Link Control –

IEE 802.1 & 802.2– Layer 2 (lower): Media Access Control –

IEEE 802.3, 4, 5, 11, 12, etc.

Layer 2 (upper) Logical Link Control

Focuses on IEEE 802.1 & 802.2

Controls transfer of data to the network layer

Uses logical interface points called SAPs (service access points)

Layer 2 (lower) Media Access Control

Direct communication with the network card– provides packets with MAC address

Focuses on IEEE 802.3, 4, 5, 11, 12… Provides shared access for multiple network

interface cards to the physical layer Responsible for ensuring error-free

communication across the network

OSI layer software and Network cards

Layer 1 and 2 software supplied with the network card – card itself should contain software (on ROM)

that conforms to one of the sixteen IEEE 802 specifications

Cards for wired connections have connectors for cables:– usually IEEE 802.3– more rarely… IEEE 802.5

Wireless Cards– usually based on IEEE802.11

“Binding” Network Card Software

Data received by the network card needs to be passed on to level 3 software

Normally held on the computer hard disk

Configuration:– level 2 software needs to

combine with level 3– achieved through “binding”

OSI Level 1/2

software

OSI Level 3 software

binding

Now for the practical…

Group A: after break (10.25)Group B: in one hour (11.25)

Self-Study: check out all the IEEE 802.x standards and decide which of these are most important in 2014… feedback next

session so you’d better do this!

Documents

COMP2221 Networks in Organisations