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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>3.1</b></font></td>
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<td bgcolor="#336666" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Serial Point-to-Point Links</font></strong></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">3.1.2</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Time-division multiplexing</font></strong></td>
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<font face="Arial" size="2">Time-Division Multiplexing (TDM) is the
transmission of several sources of information using one common
channel, or signal, and then the reconstruction of the original
streams at the remote end.</font><p>
<font face="Arial" size="2"> In the example shown in the first figure,
there are three sources of information carried in turn down the output
channel. First, a chunk of information
is taken from each input channel. The size of this chunk may vary, but
typically it is either a bit or a byte at a time. Depending on whether
bits or bytes are used, this type of TDM is called bit-interleaving or
byte-interleaving.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<font face="Arial" size="2"> Each of the three input channels has its
own capacity. For the output channel to be able to accommodate all the
information from the three inputs, the capacity of the output channel
must be no less than the sum of the inputs.</font></p>
<p>
<font face="Arial" size="2">In TDM, the output timeslot is always
present whether or not the TDM input has any information to transmit.
TDM output can be compared to a train with 32 railroad cars. Each is
owned by a different freight company and every day the train leaves
with the 32 cars attached. If one of the companies has product to
send, the car is loaded. If the company has nothing to send, the car
remains empty, but it is still part of the train.</font></p>
<p>
<font face="Arial" size="2">TDM is a physical layer concept, it has no
regard for the nature of the information that is being multiplexed onto the output channel. TDM is independent of the Layer 2
protocol that has been used by the input channels.</font></p>
<p>
<font face="Arial" size="2"> One TDM example is Integrated Services
Digital Network (ISDN). ISDN basic rate (BRI) has three channels
consisting of two 64 kbps B-channels (B1 and B2), and a 16 kbps
D-channel. The TDM has nine timeslots, which are repeated.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> This
allows the telco to actively manage and troubleshoot the local loop as
the demarcation point occurs after the network terminating unit (NTU).</font></p>
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<p><font color="#808080">
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<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Time-Division Multiplexing<p>
<a target="_blank" href="http://whatis.techtarget.com/definition/0,,sid9_gci214174,00.html">
http://whatis.techtarget.com/
definition/0,,sid9_gci214174,00.html</a></font></p>
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