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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
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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