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                            <td class="rlohdr">The OSI Reference Model</td>
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                            <td class="riohdr">Encapsulation</td>
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                                                                <td valign="middle"><img height="22" width="22" border="0" src="../../../../../images/ccna/common/inotes.gif"></td><td valign="middle"><span class="cstitle">Instructor Note</span></td>
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                                                                <td valign="top">&nbsp;</td><td valign="top"><span class="cstext">
                                                                        <p>The purpose of this target indicator is to again introduce a crucial piece of terminology. Have the students repeat the word out loud -- we believe this helps empower the students to use the vocabulary, of which there is a tremendous amount in semester 1.</p>
                                                                        <p>A useful activity for this term requires the following materials: writing paper, small envelopes, larger envelopes or Federal Express envelopes. Have the students choose an idea (Layer 7), represent that idea on paper (Layer 6), decide how to send the letter (Layers 4 and 5), add general addressing information (Layer 3), add specific addressing information (Layer 2), and mail the letter (via courier, in the classroom), to someone else. Then pose the question -- why are all the envelopes and addresses necessary? This will help emphasize that data, like their letters, must be encapsulated in order to be delivered.</p>
                                                                        <p>This TI relates to CCNA Certification Exam Objective # 5.</p>
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<p>All communications on a network originate at a source, and are sent to a destination. The information sent on a network is referred to as data or data packets. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged by a process called encapsulation.</p>
<p>Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information. </p>
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                                                                <td><span class="note"><b>NOTE:</b>
<p>The word "header" means that address information has been added.</p>
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<p>To see how encapsulation occurs, lets examine the manner in which data travels through the layers as illustrated in Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47502/RLOID=knet-v214aRLO47528/RIOID=knet-v214aRIO120888/knet/v214adataimage1/1.gif" width="12" height="12">. Once the data is sent from the source, as depicted in the figure, it travels through the application layer down through the other layers. As you can see, the packaging and flow of the data that is exchanged goes through changes as the layers perform their services for end users. As illustrated in the figures, networks must perform the following five conversion steps in order to encapsulate data:</p>
<p>Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47502/RLOID=knet-v214aRLO47528/RIOID=knet-v214aRIO120888/knet/v214adataimage2/2.gif" width="12" height="12">:</p>
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                                        <b>Build the data.</b>
<p>As a user sends an e-mail message, its alphanumeric characters are converted to data that can travel across the internetwork.</p>
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                                        <b>Package the data for end-to-end transport.</b>
<p>The data is packaged for internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.</p>
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                                        <b>Add the network IP address to the header.</b>
<p>The data is put into a packet or datagram that contains a network header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.</p>
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                                        <b>Add the data link layer header and trailer.</b>
<p>Each network device must put the packet into a frame. The frame allows connection to the next directly-connected network device on the link. Each device in the chosen network path requires framing in order for it to connect to the next device.</p>
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                                        <b>Convert to bits for transmission.</b>
<p>The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN.</p>
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                                        <td valign="middle" width="8%"><img height="23" width="23" src="../../../../../images/ccna/common/icon2.gif"></td><td valign="middle" width="92%"><span class="cstitle">Web Links</span></td>
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                                        <td valign="middle" width="8%">&nbsp;</td><td valign="middle" width="92%"><span class="smtext"><a target="_blank" href="http://www.rad.com/networks/1997/nettut/protocols.html#OSI7">The OSI Seven Layers Model</a></span></td>
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