Network Reference Model

Network Reference ModelNetwork Reference Model

2004/05 Network reference models 2

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


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


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


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


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


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


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


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)


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


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


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


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


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 …


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


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 …


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


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


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.


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


OSI Reference Model Summary


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


IEEE 802 Standards


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


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

Network Reference Model

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