20902881 the Seven Layers of the OSI Model
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Transcript of 20902881 the Seven Layers of the OSI Model
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OSI Layers
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ISO-OSI ISO-International Standards Organisation
Multinational body dedicated to worldwide agreementon international standards
An ISO standard that covers all aspects of network
communication is the OSI OSI-Open System Interconnection
OSI is a model and not a protocol
Vendor specific protocols close off communication
between unrelated systems OSI model is to open communication between different
systems without requiring changes to the logic of theunderlying hardware and software
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Layered Communication
Person-A
(English)
Person-B
(French)
I like rabbits
Jaime les lapins
Ik hou van konijnen
Translator-B(French-Dutch)
Translator-A(English-Dutch)
Ik hou van konijnen
Secretary-A
(FAX)
Ik hou van konijnen Secretary-A
(FAX)Ik hou van konijnen
FAX
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Layered Communication Each layer has a distinct identity and a specific set
of functions so that any change in theimplementation does not affect other layers.
Each layer has an active element to carry out thelayer functions
Each layer provides certain services to the nexthigher layer which uses these services to carry out
its assigned functions. Layers must follow some meaningful sequence
Number of layers should be minimum as the eachadded layer adds to the cost and delay
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OSI Layers
The OSI model is built of seven ordered
layers:
Layer-1: Physical
Layer-2: Data Link
Layer-3: Network
Layer-4: Transport
Layer-5: Session
Layer-6: Presentation
Layer-7: Application
ApplicationPresentation
Session
Transport
Network
Data Link
Physical
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OSI Layers The seven layers can be thought of as
belonging to three sub groups
Network Support Layers (Layers 1-3) Deal with the physical aspects of moving data from
one device to another
User Support Layers (Layers 5-7)
Allow interoperability among unrelated softwaresystems
Layer-4 ensures end to end reliable data
transmission
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OSI LayersEnd System
Network Network Network Network
Data Link Data Link Data Link Data Link
Application
Presentation
SessionTransport
Physical PhysicalPhysical Physical
PSPDN PSTN Dedicated CSPDN
User Support Layers
Network Support Layers
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Peer-to-Peer communication Between machines, layer-n on one machine
communicates with layer-n on another machine
This communication is governed by an agreed-upon
series of rules and conventions called protocols The processes on each machine that communicates
at a given layer are called peer-to-peer processes
At the physical layer, communication is direct
At higher layers, communication moves down
through the layers on the transmitting machine and
back up through the layers at receiving machine
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Hierarchical & Peer-to-Peer Communications
Peer-to-Peer
PCI
Interface Control Information (ICI)
Protocol Control Information (PCI)
Interface
N+1-Layer
N-Layer
Interface
N+1-Layer
N-Layer
Hierarchical
ICI
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(N+1)-Layer
(N)-Layer
Data Units in the OSI Model
(N+1)-SDU(N+1)-PCI
(N+1)-PDU
(N+1)-SDU(N+1)-PCI (N)-ICI
(N)-IDU
(N)-PCI
(N)-PDU
(N+1)-SDU(N+1)-PCI
(N)-PCI
(N)-ICI
(N+1)-SDU(N+1)-PCI(N)-SDU
(N)-ICI
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Data Units Protocol Control Information (PCI)
(N)-PCI is the protocol control informationexchanged between the (N)-entities tocoordinate their functions
Service Data Units (SDU) (N)-SDU is the data unit transferred between
the ends of a (N)-connection whose identity ispreserved during the transfer
Protocol Data Unit (PDU) (N)-PDU is the combination of (N)-PCI and
(N)-SDU
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Data Units Interface Control Information (ICI)
(N)-ICI is the information exchanged between
(N+1)-entity and (N)-entity to coordinate their
functions Interface Data Unit (IDU)
(N)-IDU is the total data unit transferred across
the SAP between (N+1)-entity and (N)-entity
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OSI Layers
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
7-Application
6-Presentation
5-Session
4-Transport
3-Network
2-Data Link
1-Physical
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
7-Application
6-Presentation
5-Session
4-Transport
3-Network
2-Data Link
1-Physical
3-2 Interface
2-1 Interface
3-Network
2-Data Link
1-Physical
Intermediate
Node
Link Link
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Layer-1(Physical)
First of three network support layers Concerned with physical transmission of data
bits and ensures that a bit entering at one end of
the transmission media reaches the other end Deals with the mechanical and electrical
specifications of the interface and transmission
medium e.g. Optical, coax, RF, twisted pair etc.
Defines the type of encoding i.e. how 0s and 1sare changed to signals
Defines data rate / transmission rate i.e. defines
the duration of a bit
A
P
S
T
N
D
P
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Layer-1(Physical)
Responsible for synchronisation of sender
and the receiver clocks Concerned with the connection of the
devices to the medium Point-to-point configuration Multipoint configuration
Physical topology Mesh; Star; Ring; Bus
Transmission Mode Simplex; Half-Duplex; Full-Duplex
A
P
S
T
N
D
P
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Layer-1(Physical)
L2 Data
L1 -1010100000
L2 Data
L1 -1010100000
Transmission Medium
Physical Layer
From Data Link Layer To Data Link Layer
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Layer-2 (Data Link) Second of three network support layers Divides the bit stream received from network
layer into manageable data units called frames
Transforms the physical layer to a reliable link by adding mechanism to detect and retransmit
damaged frames Responsible for physical addressing of the
devices Responsible for link-by-link flow control and
error free delivery of data Responsible for Media Access Control
A
P
S
T
N
D
P
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Layer-2 (Data Link)
Data Link
LayerH2T2 H2T2
L3 Data L3 Data
L1- 00101111000001 L1- 00101111000001
From Network Layer To Network Layer
To Physical Layer From Physical Layer
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Layer-3 (Network) Last of the three network support layers Responsible for Source-to-Destination
delivery of individual packets across
multiple links If two systems are connected to the same
link there is usually no need for a network
layer Responsible for the unique logical
addressing of the sender and the receiver Responsible for routing of packets
A
P
S
T
N
D
P
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Layer-3(Network)
Network LayerH3 H3
L4 Data L4 Data
L3 Data L3 Data
From Transport Layer To Transport Layer
To Data Link Layer From Data Link Layer
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Layer-4 (Transport) Responsible for Source-to-Destination
delivery of the entire message Uses service-point address (port address)
for end-to-end delivery Network layer gets each packet to correctcomputer, transport layer gets the entiremessage to the correct process
Responsible for segmenting a messageinto transmittable segments
At the destination the message is correctlyreassembled
A
P
S
T
N
D
P
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Segmentation and Reassembly
Transport LayerH4
L3Data
L5 Data
H4
L3Data
H4
L3Data
L5 Data
H4
L3Data
From Session Layer To Session Layer
To Network Layer From Network Layer
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Layer-4 (Transport) Utilises network layer to ensure reliable,
sequenced data exchange Transport layer can be connectionless or
connection oriented A connectionless transport layer treats each
segment as an independent packet A connection oriented transport layer makes
a connection with the transport layer at thedestination machine before delivering the
packets After all the data is transmitted, the
connection is terminated
A
P
S
T
N
D
P
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Layer-4 (Transport) Responsible for end-to-end flow control
of data Responsible for end-to-end error control
of data Error correction is usually achieved through
retransmission
A
P
S
T
N
D
P
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Layer-5(Session) First of the three user support layers It is the network dialog controller It establishes, maintains, and synchronises
the interaction between communicatingsystems It allows the communication between two
processes to take place either in half-
duplex or full-duplex Allows a process to add checkpoints
(synchronisation points) into a stream ofdata
A
P
S
T
N
D
P
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Layer-5(Session)
SYN
H5
L5 Data
L6 Data
H5
L5 Data
L6 Data
Session
Layer
To Transport Layer From Transport Layer
From Presentation Layer To Presentation Layer
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Layer-6 (Presentation) Second of the three user support layers Concerned with the syntax and semantics
of the information exchanged between
two systems At sender end, changes the information
from sender dependent format into acommon format
At the receiving end, changes theinformation from common format into itsreceiver dependent format
A
P
S
T
N
D
P
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Layer-6 (Presentation) Responsible for encryption and decryption
of sensitive information Responsible for data compression of the
data to be transmitted
A
P
S
T
N
D
P
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Layer-6(Presentation)
Presentation
LayerH6 H6
L7 Data L7 Data
L6 Data L6 Data
Encoded, Encrypted
and Compressed data
Decoded, Decrypted and
Decompressed data
To Session Layer From Session Layer
From Application Layer To Application Layer
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Layer-7(Application) Top of the three user support layers Enables the user, human or software, to
access the network
It provides user interfaces and support forservices e.g. electronic mail, remote file
access and transfer, shared database
management and other types of distributedinformation services
No headers or trailers are added by this
layer
A
P
S
T
N
D
P
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MTP:MESSAGE TRANSFER PART: RESPONSIBLE FOR TRANFERING MESSAGE TAKING MESSAGE FROM ONE NETWORK TO ANOTHER NETWORK CONSIST OF THREE LAYERS
TELEPHONE USER PART
RESPONSIBLE FOR CALL STTING, CALL SUPERVISING,CLEARING
ITU ALLOWED THE VARIATION IN ONE MESSAGES DEPENDENT UPONCOUNTRY TO COUNTRY THAT IS KNOWN AS NATIONAL USERPART NUP
ON ACCOUNT OF INTRODUCTION OF ISDN SOME EXTRA SET OFMESSAGES ARE REQUIRED THAT IS KNOWN AS A ISUP THAT ISHEPING FOR SETTING OF CALL
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SCCP
SIGNALLING CONNECTION AND CINTROL PART
AS THE SIGNALLING REACH THROUGH DIFFERENT PART.
DUE TO THIS SEQUENCE OF ORDER OF MESSAGE ATRECEIVING END MAY DIFFER HENCE THERE IS A NEED
FOR ESTABLISHING A VIRTUAL CONNECTION THAT IS A
CONNECTION ORIENTED PROTOCOL WHICH ENABLE A
CORRECT SEQUENCING.
ANOTHER ISSUE THAT MTP IS CAPABLE OF ROUTING AMESSAGE WITH IN ONE NETWORK ONLY. IN CASE OF
ABSENCE OF A CALL ACROSS MULIPLE NETWORK MTP
CAN NOT SEND A SIGNALLING ACROSS MULTIPLE
NETWORK
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EXTRA SIGNALLING REQUIREMENT IN GSM DUE TODIFFERENT ARCHITECTURE BECAUSE IN THIS CASE
GREATER PART IS REQUIRED FOR NON CALL RELATED
DUE TO MOBILITY. FOR THAT ADDITIONAL PROTOCOL
LAYERS ARE.
o BASE STATION SUBSYSTEM APPLICATION PART: THIS
LAYER IS USED WHEN MSC COMMUNICATES WITH BSC
AND MOBIKE STATION.o AUTHENTICATION VERIFICATION AND ALLOCATION OF
TMSI WILL TAKE PLACE THROUGH BSSAP. IT SERVE THE
o MSC----- BSC SIGNALLINGo MSC----- MS SIGNALLING
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MOBILE APPLICATION PART : IN CASE OF MOBILETERMINATED CALL DURING INTERNATIONAL ROAMING
MSRN IS REQUESTED FROM HLR WITHOUT ROUTING A
CALL, FOR THAT ANOTHER LAYER IS REQUIRED THAT IS
A MOBILE APPLICATION PART FOR SIGNLLING BETWEEN
NSS ELEMENTS
TRANSACTION CAPABILITIES APPLICATON PART: WHEN HLR
SENDS SOME RESULTS TO MSC. IT MIGHT BE IN PARTS;
FOR THAT MAP REQUIRES CERTAIN ASSISTANCE TO
TRANSACT THAT MESSAGES BETWEEN MULTIPLEELEMENTS; THIS IS CALLED A TCAP
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EXTRA SIGNALLING REQUIREMENT IN GSM DUE TODIFFERENT ARCHITECTURE BECAUSE IN THIS CASE
GREATER PART IS REQUIRED FOR NON CALL RELATED
DUE TO MOBILITY. FOR THAT ADDITIONAL PROTOCOL
LAYERS ARE.
o BASE STATION SUBSYSTEM APPLICATION PART: THIS
LAYER IS USED WHEN MSC COMMUNICATES WITH BSC
AND MOBIKE STATION.o AUTHENTICATION VERIFICATION AND ALLOCATION OF
TMSI WILL TAKE PLACE THROUGH BSSAP. IT SERVE THE
o MSC----- BSC SIGNALLINGo MSC----- MS SIGNALLING
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User plan : A+P+S+T
TRANSPORT PLANE: P+D+N
CONTROL PLAN: A+P+S+TWhen the bearer passes through some
reference plan than the defination of thethese reference points
when the defined bearer is used for
traffic , the control of this traffic in
the bearer in the network is user
plane signalling.
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TRANSPORT PLAN PROVIDE THE MEANS HOW
THE PHYSICAL CONNECTION IS ESTABLISHED
BETWEEN THE MOBILE TERMINAL AND THE
NETWORK.
IN UMTS WE CAN USE DIFFERENT PHYSICAL
CONNECTIONS. HOWEVER THE SPECIFICATION
IS MAINLY BASED ON THE ATM.
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PACKETS CONTAINING THE TRANSFORMED
INFORMATION IS KNOWN AS ATM CELLATM CELL CONSIST OF TWO PARTS
PAYLOAD INFORMATION 48 BYTES
HEADER INFORMATION 5 BYTES
TWO TYPES OF ATM CELLS AREUNI USER NETWORK INTERFACE: ATM SW-
ATM END POINTS
NNI NETWORK NODE INTERFACE : ATM SW-
ATMSW
ATM TRANSMISSION PATHS : VIRTUAL PATHS
VIRTUAL CCTS
ATM LAYERS: PHYSICAL LAYER, ATM
LAYER,ATM ADAPTION LAYER
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ATM ADAPTION LAYERS:
AAL1: SYNCHRONOUS MODE------ CONNECTION
ORIENTED CONSTANT BIT RATE
AAL2: SYNCHRONOUS MODE ----CONNECTION
ORIENTED VARIABLE CONSTANT BIT RATEAAL3/4: CONNECTION LESS CONSTANT BIT RATE
AAL5: CONNECTION LESS VARIABLE CONSTANT
BIT RATE
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An exchange using OSI model
L6-Data
L5-Data
L4-Data
L3-Data
L2-Data
0100011110001001000111111000011101010
H6
H5
H4
H3
H2T4
A-7
P-6
S-5
T-4
N-3
D-2
P-1
L7-Data
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An exchange using OSI model
L7-Data
L6-Data H6
L5-Data H5
L4-Data H4
L3-Data H3
L2-Data H2T4
A-7
P-6
S-5
T-4
N-3
D-2
P-10100011110001001000111111000011101010
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Summary of OSI Layers Functions
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
Application
Presentation
Session
Transport
Network
Data Link
Physical
Allow access to network
resources
Translate, encrypt and
compress data
Establish, manage and
terminate sessions
Reliable end to end delivery
& error recovery
Movement of packets;
Provide internetworking
Transmit bits; Mechanical
and electrical specifications
Organise bits into streams;Node to node delivery