Network Reference Model

Learning 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 2004/05 Network reference models 2

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 2004/05 Network reference models 3

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 2004/05 Network reference models 4

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 2004/05 Network reference models 5

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 2004/05 Network reference models 6

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 2004/05 Network reference models 7

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 2004/05 Network reference models 8

Relationships Among OSI Layers

  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

) 2004/05 Network reference models 9

Data Exchanges Between Devices Using OSI Model

 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)

  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 2004/05 Network reference models 11

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 2004/05 Network reference models 12

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 2004/05 Network reference models 13

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) Network reference models 14

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 80

port number

,e.g., FTP uses port number 21 whereas HTTP uses port number 2004/05 Network reference models 15

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 Network reference models 16

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 2004/05 Network reference models 17

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 2004/05 Network reference models 18

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 2004/05 Network reference models 20

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 2004/05 Network reference models 22

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 2004/05 Network reference models 24

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 2004/05 Network reference models 27