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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">2.2</font></b></td>
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<strong><font face="Arial" size="2" color="#FFFFFF">WAN Technologies</font></strong></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
2.2.7</font></b></td>
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<strong><font face="Arial" size="2" color="#FFFFFF">DSL</font></strong></td>
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<font face="Arial" size="2">Digital Subscriber Line (DSL) technology
is a broadband technology that uses existing twisted-pair telephone
lines to transport high-bandwidth data to service subscribers. DSL
service is considered broadband, as opposed to the baseband service
for typical LANs. Broadband refers to a technique which uses multiple
frequencies within the same physical medium to transmit data. The term
xDSL covers a number of similar yet competing forms of DSL
technologies:
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font><ul>
<li><font face="Arial" size="2">Asymmetric DSL (ADSL)</font></li>
<li><font face="Arial" size="2">Symmetric DSL (SDSL)</font></li>
<li><font face="Arial" size="2">High Bit Rate DSL (HDSL)</font></li>
<li><font face="Arial" size="2">ISDN (like) DSL (IDSL)</font></li>
<li><font face="Arial" size="2">Rate Adaptive DSL (RADSL)</font></li>
<li><font face="Arial" size="2">Consumer DSL (CDSL), also called
DSL-lite or G.lite</font></li>
</ul>
<p>
<font face="Arial" size="2">DSL technology
allows the service provider to offer high-speed network services to
customers, utilizing installed local loop copper lines. DSL
technology allows the local loop line to be used for normal
telephone voice connection and an always-on
connection for instant network connectivity. Multiple
DSL subscriber lines are multiplexed into a single, high capacity link
by the use of a DSL Access Multiplexer (DSLAM) at the provider
location. DSLAMs incorporate TDM
technology to aggregate many subscriber lines into a less cumbersome
single medium, generally a T3/DS3 connection. Current DSL technologies
are using sophisticated coding and modulation techniques to achieve
data rates up to 8.192 Mbps. </font></p>
<p><font face="Arial" size="2">The voice channel of a standard
consumer telephone covers the frequency range of 330 Hz to 3.3 KHz. A
frequency range, or window, of 4 KHz is regarded as the requirements
for any voice transmission on the local loop. DSL technologies place
upload (upstream) and download (downstream) data transmissions at
frequencies above this 4 KHz window. This technique is what allows
both voice and data transmissions to occur simultaneously on a DSL
service.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font></p>
<p><font face="Arial" size="2">The two basic types of DSL technologies
are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL service
are categorized as ADSL or SDSL and there are several varieties of
each type. Asymmetric service provides
higher download or downstream bandwidth to the user than upload
bandwidth. Symmetric service provides the same capacity in both
directions.</font></p>
<p><font face="Arial" size="2">The different varieties of DSL provide
different bandwidths, with capabilities exceeding those of a T1 or E1
leased line. The transfer rates are dependent on the actual length of
the local loop and the type and condition of its cabling. For
satisfactory service, the loop must be less than 5.5 kilometers (3.5
miles). DSL availability is far from universal, and there are a wide
variety of types, standards, and emerging standards. It is not a
popular choice for enterprise computer departments to support home
workers. Generally, a subscriber cannot choose to connect to the
enterprise network directly, but must first connect to an Internet
service provider (ISP). From
here, an IP connection is made through the Internet to the enterprise.
Thus, security risks are incurred. To address security concerns, DSL
services provide capabilities for using Virtual Private Network (VPN)
connections to a VPN server, which is typically located at the
corporate site. </font></p>
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<p>Checkbox: WAN Technologies Digital Subscriber Line (DSL)</p>
<p>Upon completing this activity, the student
will be able to identify the characteristics associated with DSL
connections.</p>
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