why ISDN is slower than ADSL?

Posted on 2003-03-16
Medium Priority
Last Modified: 2013-12-14
what's the reason why the ISDN is slower in transmission rate than ADSL?
Thanks for help.

Question by:hewu
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Expert Comment

ID: 8149827
ISDN is 2/24 of a T1 line.  A T1 is 24, 64k logical lines, basicaly the same rate it takes to digitally trasmit a phone call without compression.  An ISDN line is 2 of those logical 16k lines, plus a 16k signaling line that you can sometimes use for data, putting an ISDN line at either 128k or 144k.  ISDN service to the public dates back to the 70's, but generally used to connect two offices for data or for general digital phone lines.  Well, enough history on ISDN, but it's locked in the specification at 128-144k and should be available anywhere your local phone company can deliver a dialtone.

Xdsl was concieved in the late 90's as a last mile solution for internet access using existing phone lines.  ADSL can only be delivered within a certain radius of the carrier office (technically wire length.)  ADSL can only travel over lines that do not have multiplexers, choke filter coils, or plain bad wire.  ADSL is transmitted on the same lines as voice lines and can be run simultaniously as a voice call by using a very high frequency transmission.  Those high frequencies allow for up to 7Mb transmission downstream to the customer and 1Mb back to the carrier office as long as the signal is clear enough.  Available speed goes down as noise increases.

So, ISDN is an old technology that just plain isn't as fast as the newer DSL technologies, but you can get ISDN services just about anywhere.

Expert Comment

ID: 8149837
oops, I should have checked before posting.  The third sentence is wrong.  It should read:

"An ISDN line is 2 of those logical 64k lines, plus a 16k signaling line that you can sometimes use for data, putting an ISDN line at either 128k or 144k."

Expert Comment

ID: 8149912
there are different implentations of isdn, so it's a little tough to answer your question definitively. basically, the difference in performance has to do with the difference in their respective methods of data transmission. but here are some key differences. number one, isdn is a switched "dialup" type of connection, whereas dsl is mostly "always on." two, isdn uses two combined voice/data channels, whereas dsl is predominantly a data only service.
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Expert Comment

ID: 8266741
i thought ADSL was multiplexed ran throught a t1 and then demultiplexed at teh vendor.  just a thought, i am probably wrong.

Expert Comment

ID: 8266746
DSLAM = digital subscriber line access multiplexer...i think

Accepted Solution

matguy earned 315 total points
ID: 8266876
ADSL is only the "last mile" of a particular connection.  What the providers do is install a DSLAM in to a local switch which switches calls to local homes and is the last link in the loop closest to the home.  From this local switch physical copper lines go out to homes and businesses, across these copper lines Voice calls, ADSL, ISDN, or DS1(T1) connections can be transmitted (within distance and line quality constraints.)  Feeding the DSLAM with data can be any type of data carying connection, generally big enough to cary a percentage of the potential throughput of the DSLAM if it was fully occupied (general ISP customer/data ratios apply here.)  This has to be done at every local switch in an area a vendor wants to service, but often is done by the local phone company and ports leased to vendors.

So, really your ADSL line you may have is only a physical and logical connection to your local switch, from there you generally dump in to a Frame Relay type pool to your vendor which may have any kind of backbone connection, but hopefully more than a T1 (as a T1 is 1.54Mb connection, which would probably only serve 5-20 ADSL connections depending on what speed connections and the expected performance.)  At the local switch the DSLAM generally handles connection requests (low volume being ADSL is an always on type connection) that repeat to a RAS server for authentication at the vendor, and govern connection speed and transfer data.

So, really the DSLAM is kind of like the receiving modem at a traditional ISP, but just a lot closer to you to attain a physical copper connection to make the ADSL possible.

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