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> If DSL means Digital Subscriber Lines, and there is no digital, why wouldn't
> it be called ASL? There must be Digital in there somewhere. My apologies
> for not quite understanding.
>
It's not that there is no digital, it's - what part is digital?
DSL provides you a Digital service, it doesn't provide you the service 100%
digitally. The difference is largely semantic, or point of view. Once the
service is up and running, it is a digital stream from your box to your ISP.
But there are probably several different transport media involved between
you two. For instance, I think that virtually all of the local CO to ISP
connections are Frame Relay connections.
The service provided is Digital from the modem to your box, but the modem
to modem connection is analog -- that's why your regular telephone
continues to operate underneath without modification or a separate modem to
connect it to. (And why your phone line continues to work if the xDSL
service dies, or when you power off the modem.)
This is unlike an ISDN line, where you cannot connect a standard analog
phone directly to the ISDN line and have it work. You must connect the
analog phone to the ISDN modem because the ISDN line doesn't pass the
analog frequencies.
DSL is a frequency multiplexing arrangement, running data over voice. The
filters carve out a 0-5Khz(or10K) voice channel and leave the rest of the
frequency spectrum to the DSL two modems to chatter on. Try using your
phone without the mini-filters or line-splitter in place. It works, but you
get the "DSL hash" on top.
There are actually several different ways of providing DSL service,
depending upon the vendor's CO equipment and configurations. Just as an
example, Connective Communications does NOT offer voice service on a DSL
circuit while Bell Atlantic does. Off hand, I don't remember if COVAD
offers voice on their DSL circuits or not.
As another exercise, consider the difference between a terminal emulator
connection to a host, and a PPP connection to a host. Are both digital
connections? Neither? Either?
--
===<Tru64 UNIX-SIG Chair>===
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T.T.F.N.
William H. Magill Senior Systems Administrator
Information Services and Computing (ISC) University of Pennsylvania
Internet: magill@isc.upenn.edu magill@acm.org
magill@upenn.edu http://pobox.upenn.edu/~magill/
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