Transcript The Layers of OSI Model

Chapter 2
Network
Models
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Figure 2.1
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Sending a letter
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Layered Protocols
 Communication tasks are divided into series of
layers or levels
 Each layer is responsible for particular task and act
on them by using one or more protocols
 Each layer is built upon one bellow it
 The number and name of the layers differ from
network to network
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Figure 2.17
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The OSI seven layer model
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The Layers of OSI Model
End
System
R
Application
Presentation
Intermediate
System
Application
Presentation
Session
Session
Transport
Network
Transport
Network
Data Link
Physical
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Network
Data Link
Physical
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Data Link
Physical
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Summary of OSI Layer Functions
Application
To translate, encrypt and
compress data
Presentation
Session
To provide reliable end-toend message delivery and
error recovery
To organize bits into
frames, to provide nodeto-node delivery
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To allow access to network
resources
Transport
Network
Data Link
Physical
To establish, manage and
terminate sessions
To move packets from source
to destination; to provide
internetworking
To transmit bits over a
medium; to provide
mechanical and electrical
specifications
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Open System Interconnection (OSI)
 Developed by International Standard Organization
(ISO) as a first step towards international
standardization
 De jure protocol
 Deals with interconnecting systems that are open for
communication with other systems
 Open protocol suite
 Good as theoretical model, but not widely
implemented in practice
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The OSI layers
 Session layer
 Provides the control structure for communication
between applications (dialog control)
 Establishes, manages and terminate connections (sessions)
between cooperating applications
 Presentation layer
 Provides independence to the application processes from
differences in data representation
 Application layer
 Provides access to the OSI environment for users and
provides distributed information services
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The OSI layers
 Physical layer
 Transmission of unstructured bit stream
 Deals with the mechanical, electrical, functional and procedural
characteristics to access the physical medium
 Data link layer
 Provides reliable transfer across the physical link between
two ends connected via single link
 Sends blocks of data (frames) with the necessary synchronization,
error control and flow control
 Can add header and trailer
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The OSI layers
 Network layer
 Provides upper layers with independence from the data
transmission and switching technologies accross
internetwork
 Responsible for source-to-destination delivery, addressing and
routing in the internetwork
 Transport layer
 Provides transparent transport of data between end points
that might not be connected via single link
 Provides source-to-destination connection, error recovery and
flow control
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Protocol Suites
 Open System Interconnection (OSI)
 Today used mostly as a reference model
 Prevously used in X.25 based protocols
 Internet (TCP/IP)
 Most popular suite today
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Xerox Networking Sysytems (XNS)
System Network Architecture (SNA – IBM)
Digital Network Architecture (DNA – DEC)
NetBIOS (Software interface)
AppleTalk
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The TCP/IP five layer model
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TCP/IP-modellen
Exempel:
SMTP, HTTP
TCP, UDP
IP
Ethernet
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TCP/IP Protocol Suite
 De facto (and after that de jure) standards
 Open (All modification and newly proposed
protocols are published in a form of RFC (Request
for Comments)
 RFC as well as drafts are published on the Internet
 can be found on many URL (one is www.rfceditor.org)
 RFC becomes a standard when it is:
 Stable and well understood
 Technically competent
 Implemented on multiple independent places
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The TCP/IP Protocol Suite (Cont.)
 Allows computers of many sizes, vendors and
operating systems to communicate with each other
 History:
 Developed as de facto standard before OSI
 1960’s: started as goverment financed research project
 1990’s: most widely used form of networking
 Forms the basis for the Internet (capital “I”)
(a WAN that spans the globe)
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Protocols
Construction versus Reduction
DATA
Construction
Layer 5
H5 DATA
Layer 5
Layer 4
Layer 3
H4 DATA UNIT
Layer 4
Layer 3
Layer 2
Physical
H3
H2
DATA UNIT
DATA UNIT
BITS
T2
Layer 2
Physical
Reduction
H – header (pakethuvud): control data added at the front end of the
data unit
T – trailer (svans): control data added at the back end of the data unit
Trailers are usually added only at layer 2
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Illustration of the Construction and
Reduction Process
 Observe how headers and trailer are added at the
sender and removed at the receiver
Animation of Figure 2.4 in the book
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Figure 2.4
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An exchange using the Internet model
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Figure 2.3
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Peer-to-peer processes
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An Example of Five Layers Network
Machine 1
Machine 2
Layer 5 protocol
Layer 5
Layer 4/5 interface
Layer 4
Layer 3/4 interface
Layer 3
Layer 2/3 interface
Layer 5
Layer 4 protocol
Layer 3 protocol
Layer 2
Layer 2 protocol
Layer 1/2 interface
Layer 1
Layer 1 protocol
Layer 4
Layer 3
Layer 2
Layer 1
Physical medium
The path through which the actual transmission take place
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The Concept of Layers
 Layer n on one machine communicates with layer n on
the other machine via layer n protocol.
 The communication is virtual
 Peers are entities comprising the corresponding layers
on different machines.
 There is an interface between each pair of adjacent
layers for communication with the layer above and the
layer below.
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Peer-to-peer Processes
 The processes on the two machines that
communicate at a given layer are called peer-to-peer
processes
 At the physical layer communication is direct
 At the upper layers the communication has to go
down through the layers on the sender machine,
than to be transmited through the physical layer and
than to go back up to the same layer at the receiving
machine
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Messages and Protocol Stacks
 On the sender machine, each layer:
 Accepts an outgoing message from the layer above
 Adds a header and does other processing
 Passes resulting message to next lower layer
 On the receiver, each layer:
 Receives an incoming message from the layer below
 Removes the header for that layer and performs other
processing
 Passes the resulting message to the next higher layer
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Illustration of the Source-to-end Delivery at
the Network Layer
 Observe how data are sent from node to node to
reach the final destination.
Animation of Figure 2.11 in the book
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Figure 2.5
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Physical layer
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Note:
The physical layer is responsible for
transmitting individual bits from one
node to the next.
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Figure 2.6
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Data link layer
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Note:
The data link layer is responsible for
transmitting frames from
one node to the next.
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Figure 2.7
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Node-to-node delivery
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Example 1
In Figure 2.8 a node with physical address 10 sends a
frame to a node with physical address 87. The two nodes
are connected by a link. At the data link level this frame
contains physical addresses in the header. These are the
only addresses needed. The rest of the header contains
other information needed at this level. The trailer usually
contains extra bits needed for error detection
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Figure 2.8
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Example 1
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Figure 2.9
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Network layer
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Note:
The network layer is responsible for
the delivery of packets from the
original source to the
final destination.
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Figure 2.10
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Source-to-destination delivery
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Example 2
In Figure 2.11 we want to send data from a node with
network address A and physical address 10, located on
one LAN, to a node with a network address P and
physical address 95, located on another LAN. Because
the two devices are located on different networks, we
cannot use physical addresses only; the physical
addresses only have local jurisdiction. What we need here
are universal addresses that can pass through the LAN
boundaries. The network (logical) addresses have this
characteristic.
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Figure 2.11 Example 2
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Figure 2.12
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Transport layer
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Note:
The transport layer is responsible for
delivery of a message from one process
to another.
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Figure 2.12
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Reliable process-to-process delivery of a message
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Example 3
Figure 2.14 shows an example of transport layer
communication. Data coming from the upper layers have
port addresses j and k (j is the address of the sending
process, and k is the address of the receiving process).
Since the data size is larger than the network layer can
handle, the data are split into two packets, each packet
retaining the port addresses (j and k). Then in the network
layer, network addresses (A and P) are added to each
packet.
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Figure 2.14
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Example 3
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Figure 2.15
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Application layer
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Note:
The application layer is responsible for
providing services to the user.
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Figure 2.16
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Summary of duties
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