Network Reference Model
Learning Objectives
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Understand and explain the OSI reference
model
Understand and explain the IEEE 802
networking model and related standards
Explain the OSI reference model’s layers and
their relationship to networking hardware
and software
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OSI and 802
Networking Models
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OSI
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Proposed by the International Standards
Organization (ISO)
Widely used
Supplies important network terminology
IEEE 802
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(http://www.ieee802.org/802%20overview.pdf)
Influential set of networking standards
Encompasses most types of networking
Open-ended; allows addition of new types of
networks
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Role of a Reference Model

Provides a common frame of reference that
breaks networking functions into a series of
interconnected, but discrete, topical areas in
a layered model
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Adjacent layers will communicate and upper
layer will use the services provided by the
lower layer
Non-adjacent layers do not communicate
directly
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Benefits of layered models
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Benefits of layered models
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Provide a reference framework
Developing networking technology in a
specific layer does not need to know the
services to be provided by non-adjacent
layers
Allow specialization as companies can focus
their development effort in selected
networking layer(s); this gives more rooms
for small and medium size companies to
survive
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Most Common Protocol Suite
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Transmission Control Protocol/Internet Protocol
(TCP/IP)
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Internetwork Packet eXchange/Sequenced Packet
eXchange (IPX/SPX)
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Mainly for IBM and Microsoft networking
environments
AppleTalk
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Mainly used with NetWare
NetBIOS Enhanced User Interface (NetBEUI)
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Protocols adopted by Internet
Developed by Apple for Macintosh computers
Systems Network Architecture (SNA)
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Developed by IBM for its mainframe computers
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OSI Reference Model
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Breaks networking communications into a
series of interconnected tasks and activities
(layers)
The idea of layered model creates a method
to solve big problems by deconstructing
them into a series of smaller problems with
individual solutions
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OSI Reference Model Structure
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The model has 7
layers
Each layer has its
own set of welldefined functions
Layers communicate
and interact with
the layers
immediately above
and below
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Relationships Among OSI
Layers
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Each layer shields
the higher layer
from the details of
how its services
are implemented
Virtual
communication is
done in peer layers
(http://www2.themanualpage.org/
networks/networks_osi.php3)
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Data Exchanges Between
Devices Using OSI Model
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For a simple animation of how the OSI
model works, see
http://www.mhhe.com/engcs/compsci/forouz
an/dcn/graphics/animations/03_03.swf
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Application Layer (Layer 7)
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Provides interfaces to permit applications to
request and receive network services with
the use of application programming
interfaces (APIs)
Handles general network access, flow control
and error recovery at the application level,
e.g., dialogue design and incorrect data input
handling
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Presentation Layer (Layer 6)
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Handles data format information for
networked communications so that it can be
understood by systems across the network
Handles protocol conversion, data encryption
or decryption, character set issues, and
graphics commands
May compress data to reduce volume
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Session Layer (Layer 5)
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Sets up, maintains, and ends ongoing
sequences of communications (sessions)
across networks
Provides synchronization services between
tasks on both ends of a connection, e.g., if a
client requests for a web page from a web
server, the web browser at the client side will
wait until the requested page is received (or
a timeout is detected)
Describe whether message is to be sent halfduplex or full-duplex
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Transport Layer (Layer 4)
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Manages conveyance of data from sender to
receiver across a network by fragmenting
large PDUs (protocol data units) from the
session layer into segments
Inserts sufficient integrity controls and
manages delivery mechanisms to allow for
their error-free reassembly on the receiving
end of a network transmission, e.g., backup
copies of data (i.e., PDU) being kept
Handles flow control and error handling
(using checksum, for example)
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Transport Layer (Layer 4)
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Transport layer provides the last chance for
error recovery regarding any data error due
to networking problem
May be responsible for creating several
logical connections over the same network
connection by multiplexing, e.g.,

In the TCP protocol, each software element
with the transport layer is given a transport
address, usually is a combination of the
network layer address and a 16-bit numeric
value called port number,e.g., FTP uses port
number 21 whereas HTTP uses port number
80
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Network Layer (Layer 3)
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Breaks segment into packets
Handles addressing messages for delivery
Translates logical network addresses (e.g.,
IP address) and names into their physical
counterparts (e.g., network card’s MAC
address) and vice versa
Decides how to route transmissions from
sender to receiver, based on network
conditions, quality of service (QoS)
information, cost of alternative routes, and
delivery priorities
More to follow2004/05
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Network Layer (Layer 3)
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Handles packet switching, data routing
(with the use of a router) and congestion
control
Permits packets from one kind of medium
to traverse another kind of medium
Copy of segments may be stored
temporarily for error recovery purpose
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Data Link Layer (Layer 2)
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Manages access to the
networking medium
Ensures error-free delivery
of data frames from sender
to receiver; a data frame is
the basic unit of network
traffic that can be sent
across a networking media
Functions implemented in
NIC
Copy of each packet is temporarily kept for error
recovery purpose
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Physical Layer (Layer 1)
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(1/2)
Transmits and receives raw signals over a
communication channel
Typical questions to answer:
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how many volts should be used to represent a
1 and how many for a 0
how many microseconds a bit lasts
would duplex transmission be support
how the initial connection is established and
how it is torn down when both sides are
finished, and
how many pins the network connector has
and what each pin is used for.
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Physical Layer (Layer 1 )
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(2/2)
In brief, the physical layer
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Manages computer’s interface to the network
medium and instructs driver software and
network interface what to send across the
medium
Specifies physical details of cables, adapter
cards, connectors, and hardware behavior
(but does not include any hardware)
 Physical layer  physical media
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OSI Reference Model
Summary
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IEEE 802 Networking
Specifications

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Define a set of LAN standards to ensure that network
interfaces and cabling from multiple manufacturers
would be compatible
Focus on standards that describe physical elements
of a network
Describe how NICs may access and transfer data
across a variety of networking media
Describe what is involved in attaching, managing,
and detaching devices in a networked environment
The IEEE 802 standards predate the OSI model but
they are essentially compatible with one another
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IEEE 802 Standards
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IEEE 802 Extensions to the
OSI Reference Model

Break the Data Link layer into two
sublayers:
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Logical Link Control (LLC) sublayer
• Error correction
• Flow control
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Media Access Control (MAC) sublayer
• Access control
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IEEE 802 Extensions to the
OSI Reference Model
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IEEE 802 Extensions to the
OSI Reference Model
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Chapter Summary
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Two different, but complementary,
theoretical models for what networks
are and how they work
OSI model and its orderly, sevenlayered approach
 IEEE 802 model and its standards
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How the two models fit together
Where networking hardware plugs into
each model
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