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7/29/2019 Computer Networks OSI LAYER
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Networking Standards
and the OSI Model
Free Notes For All Students By: ABURAFEY
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Network Communication
s Recognize data
s Divide data into manageable chunks
s
Add information to each chunk to:Determine where the data is coming from
Identify where the data is going to
s
Add timing and error checking informations Put the data on the network and send it on
its way
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Standard Protocols Needed
sNetwork operating systems follow strict
rules (protocols) to control how each of the
previous tasks are accomplished.
sNeed for standard protocols so that
software/hardware from different vendors
could communicate
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Standards Organizations
s ANSIAmerican National Standards InstituteDetermine standards for electronics industry
Represent US in setting international standards
s EIAElectronics Industry AllianceTrade organization representing electronic firms across
US
s IEEEInstitute of Electrical and Electronic
Engineers International society of engineering professionals
Promote development and education in engineering andcomputer science fields
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Standards Organizations
s ISOInternational Organization for
StandardizationCollection of organizations from 130 countries
Establish international technological standards to
facilitate global exchange of data
s ITUInternational Telecommunication Union
United Nations agency that regulates internationaltelecommunications including radio and TV
frequencies, satellite and telephone specifications, and
networking infrastructure
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OSI and 802 Models
s Two primary sets of standards
s Define rules for:How network devices communicate
Methods used to determine when to send data
Methods to ensure that data is received correctly
How the network is cabled
How the network maintains the flow of dataHow bits of data are represented
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Open System Interconnection (OSI)
s 1977-78 International Organization for
Standardization (ISO) began developing
specifications for network communicationss 1984 OSI model was released
s International standard
s Best known and most widely used guide tounderstanding network communications
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802 Model
s Model published by the Institute for Electrical and
Electronic Engineers, Inc. (IEEE)
s Project began in February 1980 (802)
s Predates OSI standard but developed in
cooperation with OSI
s Defines aspects of networking related to physical
cabling, connectivity, error checking, datatransmission, encryption, and emerging
technologies
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The seven-layer OSI model
s To help you remember
the order of the layersAll people seem to
need data processing
Please do not throw
sausage pizza away
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Relationship of OSI Layers
s Each layer of the OSI model must
communicate with the layer above and
below itFor example, the Presentation layer must
communicate with the Application layer (one
above) and the Session layer (one below)
s As data passes down through the OSI
layers, each layer (except Physical) adds
some information to the
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Relationship of OSI Layers
s When data reaches the receiving computer,
the information added by each layer of the
OSI model is read and processed by thecorresponding layer on the receiving
computer
s This is referred to as peer-layer
communications
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Relationships among OSI layersComputer A Computer B
Virtual
Communication
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
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Packet Creation ProcessData Data
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Physical Layer
s Responsible for transmitting bits from onecomputer to another
s Physical topologys
Network connection typesNumber of pins on the network connector Passive and active hubs, terminators, cables, repeaters,
transceivers, etc.
s
Defines the electrical detailsBit encodingwhat represents 0 and 1How data will be synchronizedBaseband/broadband transmission
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Data Link Layer
s Accepts packets of data from Network layer andpackages the data into data units called frames
s Adds information such as frame type and
physical addresss Responsible for providing error-free transfer of
data framesFrame Check Sequenceerror checking algorithm is
added at the end of each frame so that receivingcomputer can detect damaged frames and request thatframes be resents Cyclic Redundancy Check (CRC)most commonly used
algorithm
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Data Link Layer
s Intelligent hubs, bridges, and switches are
addressed by the Data Link Layer
s
Defines how media will be accesseds Divided by 802 model into two sub-layers:
Logical Link Control (LLC)
Media Access Control (MAC)
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Project 802 Logical Link Control and
Media Access Control sub-layers
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Data Link Layer
s Logical Link ControlUses error check algorithm (FCS) to verify that
data is received correctly
Ensures that the rate of transmission is appropriatefor the receiving computer
Provides the link between the Data Link layer and
the Network layer
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Data Link Layer
s Media Access ControlControls the way multiple devices share the
same media channelsDefines the method the NAC will use to determine
when to send data
Communicates directly with the networkadapter card
MAC (physical) address for network adaptercards provide unique identifier for each NAC
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Network Layer
s Responsible for addressing messages acrossnetworks
s Translates network logical addresses into
physical machine addressess Breaks large segments into smaller packetss Determines routing across networkss Supports communications between logically
separate networksRouters and gateways (devices that pass data from
one network to another) operate in the Network layer
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Transport Layer
s Ensures data is delivered error-free in sequence
and without duplication or loss
s Breaks large packets from the Session layer into
segments to be sent to the receiving computer
s Adds sequencing information to segments
s Reassembles the segments into messages
s Sends acknowledgement to sending computer
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Session Layer
s Responsible for establishing and maintaining
communications between two nodes on the
network
s Allows applications on separate computers toshare a connection (called a session)
s Provides data synchronization and checkpointing
so that if there is a network failure only the data
sent after the checkpoint must be resent
s Provides name recognition and security needed to
allow two applications to communicate
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Presentation Layer
s Translates data between the format the
network requires and the format the
application expects
s Responsible for encrypting data,
changing or converting the character set
and interpreting graphic commands
s Manages data compression to reduce
number of bits to be transmitted
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Application Layer
s Provides services to support user
applications, such as file transfer, database
access, and e-mailNot a particular applicationbut services thatare provided to applications
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OSI Process
P h y s i c a l L aP l a c e s b i t s o n t o
D a t a L i n k L aP a c k a g e s d a t a
A d d s F C S ; a d d sp a s s e s t o P h y
N e t w o r k l a yA d d n e t w o r k a
p a s s e s d a t a t o
T r a n s p o r t l aS u b d i v i d e s d
a d d s s e q u e n cp a s s e s d a t a t o
S e s s i o n l a y e r A d d s a c o n t r o l f r a m
i n d i c a t e s t h a t y o u h a vp a s s e s d a t a t o T r a
P r e s e n t a t i o n l a ye n c y r p t s r e q u e s t ( i
a d d s a n y c o d e s r e q u i r p a s s e s r e q u e s t t o
A p p l i c a t i o n l e v e lf o r m u l a t e s r e q u e s tf o r d a t a a n d s e n d s
r e q u e s t t o P r e s e n t a t i o n l a y e
R e q u e s t f o r e - m a i l i s r e c e i v e d b y
o n r e c e i v i n g c o m p u t e r R e q u e s t i s p a s s e d u p t h e l a y e r s oE a c h l a y e r r e a d , p r o c e s s e s a n d r e m o v e s i n f
U s e r R e q u e s t s M a i l
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Network Packets (Frames)
s Small chunks of data and other information
s Packet StructureHeader
s Source address
sDestination address
s Instructions that tell the computer how to pass the
data along (routing)
sReassemble information
Data
Trailer containing error-check information
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A simple data packet
Destination ID Control CRC
Sender IDData
HeaderTrailer
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Two Major Frame Types
s EthernetNetworking technology developed in 1970
Most commonly used todayUsed in bus and star topologies
s Token Ring
Developed by IBM in 1980sUsed in ring topology
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Addressing
s MAC Addressphysical address burned
onto NIC card
Unique address for each NIC card produced inthe world
Consists of a Block ID and a Device ID
sEach manufacturer has one or more Block IDs
Added to frame by Data Link LayerMAC
sublayer
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Addressing
s Network addresslogical address assigned to anetwork device that identifies the network that adevice belongs
Can be assigned automatically when a computer isturned ons DHCP server provides IP (network) addresses to
computers
Can be assigned manually so that address remainsthe same each time computer is turned ons Addresses for servers and printers are assigned manually
so that other devices can always locate them
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By: Deepak Kumar