Networks 2 Network Layer Models Dr John Cowell phones off (please) 1.
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Transcript of Networks 2 Network Layer Models Dr John Cowell phones off (please) 1.
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
4
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
7
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”
8
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
27
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