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
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
Physical Physical LayerLayer
Data link Data link LayerLayer
Network Network LayerLayer
Transport Transport LayerLayer
SessionSessionLayerLayer
Presentation Presentation LayerLayer
Application Application LayerLayer
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
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
Physical Physical LayerLayer
Data link Data link LayerLayer
Network Network LayerLayer
Transport Transport LayerLayer
UpperUpperOSI OSI
LayersLayers
Physical Physical LayerLayer
Data link Data link LayerLayer
Network Network LayerLayer
Physical Physical LayerLayer
Data link Data link LayerLayer
Network Network LayerLayer
End UserEnd User
Physical Physical LayerLayer
Data link Data link LayerLayer
Network Network LayerLayer
Transport Transport LayerLayer
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 Network LayerLayer
Network Layer
Transport Transport LayerLayer
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!