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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">3</font></b><font face="Arial" size="2" color="#FFFFFF"><b>.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">3.1.1</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Introduction to serial
communication</font></strong></td>
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<font FACE="Arial" SIZE="2">WAN technologies are based on serial
transmission at the physical layer. This means that the bits of a
frame are transmitted one at a time over the physical medium.</font><p>
<font FACE="Arial" SIZE="2">The bits
that make up the Layer 2 frame are signaled one at a time by physical
layer processes onto the physical medium.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> The signaling methods
include Nonreturn to Zero Level (NRZ-L), High Density Binary 3,
(HDB3), and Alternative Mark Inversion (AMI). These are examples of
physical layer encoding standards, similar to Manchester encoding for
Ethernet. Among other things, these
signaling methods differentiate between one serial communication
method and another. Some of the many different serial communications
standards are the following:</font></p>
<ul>
<li>
<font FACE="Arial" SIZE="2">RS-232-E</font></li>
<li>
<font FACE="Arial" SIZE="2">V.35</font></li>
<li>
<font FACE="Arial" SIZE="2">High Speed Serial Interface (HSSI)</font></li>
</ul>
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