Transcript Module 1.4 OSI Model.pptx

NETWORKING CONCEPTS
OSI MODEL
 Established in 1947, the International Standards
Organization (ISO) is a multinational body
dedicated to worldwide agreement on
international standards.
 An ISO standard that covers all aspects of network
communications is the Open Systems
Interconnection (OSI) model.
 It was first introduced in the late 1970s.
OSI MODEL
 Open System:
 Set of protocols that allow only 2 different systems to
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communicate regardless of underlying architecture
OSI model is not a protocol,but a model for designing
n/w architecture
Purpose is to show how to facilitate communication b/w
different systems without requiring changes to the logic
of the underlying h/w or s/w
OSI model is a layered framework for design of network
systems
Consist of 7 layers
OSI MODEL LAYERS
The interaction between layers in the OSI model
OSI MODEL
 Fig shows the layers involved when a message is sent
from A to B
 As message travels from A to B it may pass through
many nodes and intermediate nodes usually involve
only first 3 layers of OSI model
 With a single machine each layer calls on services of
layer just below it. Ie layer 3 provides services to layer
4 and uses services of layer2
OSI MODEL
 Between m/c layer x on one m/c communicates
with layer x on other m/c and that communication
is governed by agreed rules called protocols
 Processes on each machine that communicate at a
given layer are called peer to peer processes
OSI MODEL
 Peer to Peer Process
 At physical layer communication is direct
 In fig device A sends stream of bits to device B(through
intermediate nodes)
 At higher layer communication must move down
through the layers through the layer on device A over to
device B and then back up through layers
 Each layer in sending device adds its own information to
message it receives from the layer just above it and
passes whole package to layer below it
OSI MODEL
 Peer to Peer Process
 At layer 1 the entire package is converted to a
form that can be transmitted to the receiving
device
 At the receiving device msg is unwrapped layer
by layer with each process receiving and
removing the data meant for it
 Eg. Layer 2 removes the data meant for it and
then passes the rest to layer 3.Layer 3 removes
the data meant for it and passes the rest to layer
4 and so on
OSI MODEL
 Interface between layers
 Defines the information and services a layer
must provide for the layer above it
 Organization of Layers
 7 layers belong to 3 subgroups
 Layer 1,2,3 are n/w support layer deal with
physical aspects of moving data from one device
to other
 Eg.electrical specifications,physical connector
etc
OSI MODEL
 Organization of Layers
 Layer 5,6,7.ie session presentation,application
are user support layers and allow
interoperability among unrelated s/w systems
 Layer 4-Transport layer
 This links two subgroups and ensures that lower
layer transmission is in a form that the upper
layer can use
OSI MODEL
 Organization of Layers
 Upper OSI layers are commonly implemented in
s/w but lower layers are combination of h/w and
s/w except physical which is mostly h/w
EXCHANGE USING OSI MODEL
EXCHANGE USING OSI MODEL
 D7 means data unit at layer 7
 The process starts at application layer and moves
from layer to layer in descending sequential order
 At each layer header or trailer is added to data unit
 Commonly trailer is added at layer 2
 After data unit passes through layer 1 and it
changed to some electromagnetic signal and is
transported along a physical link
EXCHANGE USING OSI MODEL
 On destination side signal passes to layer 1 and is
transformed back into digital form
 The data units then move backup through the OSI
layers
 As each block reaches next high layer, header and
trailer attached to it at the corresponding sending
layer are removed actions appropriate to the layer
are taken
EXCHANGE USING OSI MODEL
 When it reaches layer 7 the message is again in a
form appropriate to the application and is made
available to the recipient
EXCHANGE USING OSI MODEL
 Encapsulation
 A packet (header+data) at level 7 is encapsulated
at level6
 ie data portion at level N-1 carries whole packet
from level N.This is encapsulation
EXCHANGE USING OSI MODEL
 Encapsulation
 A packet (header+data) at level 7 is encapsulated
at level6
 ie data portion at level N-1 carries whole packet
from level N.This is encapsulation
LAYERS OF OSI MODEL
 Physical Layer
LAYERS OF OSI MODEL
 Physical Layer
 Deals with mechanical and electrical
specifications of interface and transmission
medium
 Responsible for movements of individual bits
from one node to next
Physical Layer
 Physical Layer
 Concerned with
 Physical characteristics of interfaces and medium
It defines type of medium
 Representation of bits
 Physical layer data consist of stream of bits
 For transmission these must be converted to
electrical or optical signal
 Physical layer defines type of encoding(how bits
are changed to signal)
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Physical Layer
 Data rate
 No of bits /sec is also defined by this layer
 Synchronization of bits
 Sender and receiver must use same bit rate
 Line configuration
 Physical layer concerned with connection of
devices to media
 Point-point configuration:2 devices connected
through dedicated line
 Multipoint configuration:a link shared among
multiple devices
Physical Layer
 Physical topology
 This defines how devices are connected to make a
n/w
 These may be mesh,star,ring,bus etc
 Transmission mode
 Physical layer defines direction of transmission
between 2 devices
 that may be
 Simplex
 Half duplex
 Full duplex
Data Link Layer
Data Link Layer
 DLL
 Responsible for moving frames between one hop to
next
 Responsibilities are
 1.Framing
 DLL divides stream of bits received from n/w layer
to manageable data units called frames
 2.Physical addressing
 If frame are to be distributed to different systems
on n/w the data link layer adds a header to frame
to define the sender and /or receiver of the frame
Data Link Layer
 2.Physical addressing
if the frame is intended for a system outside
the sender’s network , the receiver address is
the address of the device that connects to the
n/w to the next one
 3. Flow control
 If rate at which the data is absorbed by the
receiver is less than rate at which it is
produced at sender the dll imposes a flow
control mechanism to avoid overwhelming the
receiver
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Data Link Layer
 4.Error Control
Dll adds mechanism for to detect and
retransmit damaged frame or lost frame
 Also adds one mechanism to recognize
duplicate frames
 Error control is achieved by adding trailer to
end of fame
 5. Access control
When 2 or more devices are connected to
same link dll protocols determine which
device has control over link at any time
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Data Link Layer
 4.Error Control
Dll adds mechanism for to detect and
retransmit damaged frame or lost frame
 Also adds one mechanism to recognize
duplicate frames
 Error control is achieved by adding trailer to
end of fame
 5. Access control
When 2 or more devices are connected to
same link dll protocols determine which
device has control over link at any time
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Hop-to-hop delivery
Hop-to-hop delivery
 Communication at all layer occurs between 2 adjacent
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nodes
To send data from A-F , 3 partial deliveries are made
1.DLL layer at A send frame to DLL layer at B
2.DLL layer at B send frame to DLL layer at E
3.DLL layer at E send frame to DLL layer at F
 Frame from A-B has A is source address B is destination
address
 Frame from B-E has B is source address E is destination
address
 Frame from E-F has E is source address F is destination
address