introduction to the osi

8/7/2019 Introduction to the OSI

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The OSI, or Open System Interconnection, model defines a networkingframework for implementing protocols in seven layers. Control is passed fromone layer to the next, starting at the application layer in one station, proceedingto the bottom layer, over the channel to the next station and back up thehierarchy.

Introduction to the ISO - OSI Model

The ISO (International Standards Organization) has created a layered model,called the OSI (Open Systems Interconnect) model, to describe defined layers ina network operating system. The purpose of the layers is to provide clearlydefined functions that can improve Internetwork connectivity between "computer"manufacturing companies. Each layer has a standard defined input and astandard defined output.

Understanding the function of each layer is instrumental in understanding data

communication within Local, Metropolitan or Wide networks.

OSI Model Explained

This is a top-down explanation of the OSI Model. It starts with the user's PC andit follows what happens to the user's file as it passes though the different OSIModel layers. The top-down approach was selected specifically (vs. starting atthe Physical Layer and working up to the Application Layer) for ease ofunderstanding. It is used here to show how the user's files are transformed(through the layers) into a bit stream for transmission on the network.

These are the 7 Layers of the OSI model:

7. Application Layer (Top Layer) 6. Presentation Layer 5. Session Layer 4. Transport Layer 3. Network Layer 2. Data Link Layer 1. Physical Layer (Bottom Layer)

Layer 7 - Application Layer


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Fig. 1. Basic PC Logical Flowchart

A basic PC logic flowchart is shown in Fig. 1. The Keyboard & Application areshown as inputs to the CPU (requesting access to the hard disk). The Keyboardrequests accesses through user inquiries (such as "DIR" commands) and theApplication seeks access through "File Openings" and "Saves". The CPU,through the Disk Operating System, sends and receives data from the local hard

disk ("C:" in this example).

Fig. 2. Simple Network Redirection

A PC setup as a network workstation has a software "Network Redirector" (theactual name depends on the network - we will use a generic term here) placedbetween the CPU and DOS (as shown in Fig 2.). The Network Redirector is a

TSR (Terminate and Stay Resident) program: it presents the network hard diskas another local hard disk ("G:" in this example) to the CPU. All CPU requestsare intercepted by the "Network Redirector". The Network Redirector checks tosee if either a local or a network drive is requested. If a local drive is requested,the request is passed on to DOS. However, if a network drive is requested, therequest is then passed on to the network operating system (NOS).


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Electronic mail (E-Mail), client-server databases, games played over the network,print and file servers, remote logons, and network management programs (or any"network aware" applications) are all aware of the network redirector. They havethe ability to communicate directly with other "network applications" on thenetwork. The "Network Aware Applications" and the "Network Redirector" make

up Layer 7 (the Application layer of the OSI Model, as shown in Fig. 3).

Fig. 3. PC Workstation with Network Aware Software

Layer 6 - Presentation LayerThe Network Redirector sends CPU operating system native code to the networkoperating system: the coding and format of the data is not recognizable by thenetwork operating system. The data consists of file transfers and network calls bynetwork aware programs.

For example, when a dumb terminal is used as a workstation (in a mainframe orminicomputer network), the network data is translated into (and from) the formatthat the terminal can use. The Presentation layer presents data to and from theterminal using special control characters to control the screen display (LF-line

feed, CR-carriage return, cursor movement, etc..). The presentation of data onthe screen would depend on the type of terminal that's used: VT100, VT52,VT420, etc.


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Similarly, the Presentation layer strips the pertinent file from the workstationoperating system's file envelope. The control characters, screen formatting, andworkstation operating system envelope are all stripped or added to the file (if theworkstation is receiving or transmitting data to the network). This could alsoinclude translating ASCII file characters from a PC world to EBCDIC in an IBM

Mainframe world.

The Presentation Layer also controls security at the file level: this provides bothfile locking and user security. The DOS Share program is often used for filelocking. When a file is in use, it is locked from other users to prevent 2 copies ofthe same file from being generated. If 2 users both modified the same file, andUser A saved it, then User B saved it, then User A's changes would be erased!

At this point, the data is contiguous and complete (i.e. one large data file). SeeFig. 4.

Layer 5 - Session LayerThe Session layer manages the communications between the workstation andthe network. The Session layer directs the information to the correct destination,and identifies the source to the destination. The Session layer identifies the typeof information as data or control. The Session layer manages the initial start-upof a session, and the orderly closing of a session. The Session layer alsomanages Log on procedures and Password recognition (See Fig. 5).


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Fig. 5. Session Layer

Layer 4 - Transport LayerIn order for the data to be sent across the network, the file must be broken up

into usable small data segments (typically 512 - 18K bytes). The Transport layerbreaks up the file into segments for transport to the network, and combinesincoming segments into a contiguous file. The Transport layer does this logically,not physically, and it is done in software as opposed to hardware.

The Transport layer provides error checking at the segment level (frame controlsequence). This makes sure that the datagrams are in the correct order: theTransport layer will correct out of order datagrams. The Transport layerguarantees an error-free host to host connection. It is not concerned with thepath between machines.

Layer 3 - Network LayerThe Network layer is concerned with the path through the network. It isresponsible for routing, switching, and controlling the flow of information betweenhosts. The Network layer converts the segments into smaller datagrams than thenetwork can handle: network hardware source and destination addresses arealso added. The Network layer does notguarantee that the datagram will reachits destination.


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Fig. 7. Network Layer

Layer 2 - Data Link LayerThe Data Link layer is a firmware layer of the network interface card. The Data

Link layer puts the datagrams into packets (frames of bits: 1s & 0s) fortransmission, and assembles received packets into datagrams. The Data Linklayer works at the bit level, and adds start / stop flags and bit error checking(CRC or parity) to the packet frame. Error checking is at the bit level only:packets with errors are discarded and a request for re-transmission is sent out.The Data Link layer is primarily concerned with bit sequence.

Fig. 8. Data Link Layer

Layer 1 - Physical LayerThe Physical layer concerns itself with the transmission of bits. It also managesthe network card's hardware interface to the network. The hardware interfaceinvolves the type of cabling (coax, twisted pair, etc.), frequency of operation (1Mbps, 10Mbps, etc.), voltage levels, cable terminations, topography (star, bus,ring, etc.), etc. Examples of Physical layer protocols are as follows: 10Base5 -Thicknet, 10Base2 - Thinnet, 10BaseT - twisted pair, ArcNet, FDDI, etc. (see Fig.9).

Fig. 9. Physical Layer


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Layer-Specific Communication

Each layer may add a Header and a Trailer to its Data (which consists of the nexthigher layer's Header, Trailer and Data as it moves through the layers). TheHeaders contain information that specifically addresses layer-to-layer

communication. For example, the Transport Header (TH) contains informationthat only the Transport layer sees. All other layers below the Transport layer passthe Transport Header as part of their Data.

PDU - Protocol Data Unit (a fancy name for Layer Frame)

OSI Model Functional Drawing


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Multiplexing is the transmission of multiple data communication sessions over acommon wire or medium. Multiplexing reduces the number of wires or cablerequired to connect multiple sessions. A session is considered to be datacommunication between two devices: computer to computer, terminal tocomputer, etc.

Individual lines running from 3 terminals to one mainframe is not a problem.However, when the number of terminals increases to 10 and up it becomes aproblem. Imagine a mainframe computer with 1200 terminals connected, andeach terminal running its own wire to the mainframe. If each wire was 1/4" in

diameter (typical Cat 5 cable), you would have a wiring bundle going into thecomputer that is roughly 2 feet in diameter.


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A multiplexer allows sharing of a common line to transmit the many terminalcommunications (as in the above example). The connection between themultiplexer and the mainframe is normally a high speed data link, and is notusually divided into separate lines.

The operation of multiplexers (abbreviated MUXs) is transparent to the sendingand receiving computers. Transparent means that, as far as everyone isconcerned, they appear to be directly connected to the mainframe with individualwires. The multiplexer does not interfere with the normal flow of data and it canallow a significant reduction in the overall cost of connecting to remote sites(through the reduced cost of cable and telephone line charges).

Multiplexers are used to connect terminals located throughout a building to acentral mainframe. They are also used to connect terminals located at remotelocations to a central mainframe through the phone lines.


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There are 3 basic techniques that are used for multiplexing:

a. Frequency Division Multiplexing (FDM)b. Time Division Multiplexing (TDM)c. Statistical Time Division Multiplexing (STDM)

FDM - Frequency Division Multiplexing

Frequency Division Multiplexing (FDM) is an analog technique where eachcommunications channel is assigned a carrier frequency. To separate the

channels, a guard-band would be used. This is to ensure that the channels donot interfere with each other.

For example, if we had our 3 terminals, each requiring a bandwidth of 3 kHz anda 300 Hz guard-band, Terminal 1 would be assigned the lowest frequencychannel 0 - 3 kHz, Terminal 2 would be assigned the next frequency channel 3.3kHz - 6.3 kHz and Terminal 3 would be assigned the final frequency channel 6.6kHz - 9.6 kHz.

The frequencies are stacked on top of each other and many frequencies can besent at once. The downside is that the overall line bandwidth increases.

Individual terminal requirement were 3 kHz bandwidth each, in the aboveexample: the bandwidth to transmit all 3 terminals is now 9.6 kHz.


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FDM does not require all channels to terminate at a single location. Channelscan be extracted using a multi-drop technique, terminals can be stationed atdifferent locations within a building or a city.

FDM is an analog--and slightly historical--multiplexing technique. It is prone tonoise problems, and has been overtaken by Time Division Multiplexing (bettersuited for digital data).

TDM - Time Division Multiplexing


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Time Division Multiplexing is a technique where a short time sample of eachchannel is inserted into the multiplexed data stream. Each channel is sampled inturn, and then the sequence is repeated. The sample period has to be fastenough to sample each channel according to the Nyquist Theory (2x highestfrequency), and to be able to sample all the other channels within that same time

period. It can be thought of as a very fast mechanical switch: selecting eachchannel for a very short time, then going on to the next channel.

Each channel has a time slice assigned to it (whether the terminal is being usedor not). Again, to the send and receiving stations, it appears as if there is a single

line connecting them. All lines originate in one location and end in one location.TDM is more efficient, easier to operate, less complex and less expensive thanFDM.


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STDM - Statistical Time Division Multiplexing

Statistical Time Division Multiplexing uses intelligent devices that are capable ofidentifying when a terminal is idle. They allocate time only to lines when required.This means that more lines can be connected to a transmission medium becausethis device statistically compensates for normal idle time (in data communicationlines). Newer STDM units provide additional capabilities: data compression, linepriority, mixed speed lines, host port sharing, network port control, automaticspeed detection and much more.

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