Download - Network Reference Model
Network Reference ModelNetwork Reference Model
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Learning ObjectivesLearning Objectives 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 OSI
Proposed by the International Standards Organization (ISO)
Widely used Supplies important network terminology
IEEE 802 (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 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 Benefits of layered models
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 Transmission Control Protocol/Internet Protocol
(TCP/IP) Protocols adopted by Internet
Internetwork Packet eXchange/Sequenced Packet eXchange (IPX/SPX) Mainly used with NetWare
NetBIOS Enhanced User Interface (NetBEUI) Mainly for IBM and Microsoft networking environments
AppleTalk Developed by Apple for Macintosh computers
Systems Network Architecture (SNA) Developed by IBM for its mainframe computers
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OSI Reference Model 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 The model has 7
layers Each layer has its
own set of well-defined functions
Layers communicate and interact with the layers immediately above and below
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Relationships Among OSI Layers Each layer shields
the higher layer from the details of how its servicesare 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
For a simple animation of how the OSI model works, see http://www.mhhe.com/engcs/compsci/forouzan/dcn/graphics/animations/03_03.swf
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Application Layer (Layer 7) 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) 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) 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 half-duplex or full-duplex
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Transport Layer (Layer 4) 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)
More to follow …
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Transport Layer (Layer 4) 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) 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 follow …
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Network Layer (Layer 3) 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)
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) (1/2)
Transmits and receives raw signals over a communication channel
Typical questions to answer: 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 ) (2/2)
In brief, the physical layer 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 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: Logical Link Control (LLC) sublayer
• Error correction• Flow control
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 Two different, but complementary,
theoretical models for what networks are and how they work OSI model and its orderly, seven-
layered approach IEEE 802 model and its standards
How the two models fit together Where networking hardware plugs into
each model