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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
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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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