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Why I can't get 56K - Credits to "Ed from Lucent" by GHTROUT
Posted: 19 Aug 06

Wait û credit where credit is due.  I am not the author of this.  However, there are still dial-up users and they often ask ôwhy canÆt I get 56Kö.  IÆve found this post to be timeless.

Repost of an article by ôEdö from Lucent
Forum: comp.dcom.modems   
Posted on: 1998/03/12
Message-ID: <6ea9cp$9km@bgtnsc03.worldnet.att.net>
Newsgroups: comp.dcom.modems
Organization: Lucent Technologies

SLC = Subscriber Loop Carrier, aka digital loop carrier system.  Instead of 1 call on 1 pair of wires, you put a SLC terminal on each end and carry 24 calls on 2 pairs.  ThatÆs why the telcos also call it ôpair gain,ö which is also the name of one company who makes this equipment.

Although the digital side of a SLC can now use fiber-optic technology, traditionally itÆs used 1.5 Mbit/s T1 on copper.  Western Electric, then  AT&T, now Lucent Technologies has had SLC-96 (96 channels on 5 T1 lines),

SLC Series 5, and now SLC-2000.  Successive versions have had higher density, more types of plug-in line cards, more flexible port concentration, better remote maintenance, etc.

When the central office has a digital switch, you can omit the SLC central-office terminal and run the T1 lines directly to the switch.   This is called ôintegratedö SLC.  In this configuration, thereÆs only the single analog/digital conversion at the SLC remote terminal in your neighborhood.  ItÆs digital all the way from the digital carrier to your digitally-connected Internet service provider.  Integrated SLC allows V.90 modems to work.

A non-integrated, or ôuniversalö SLC has an additional digital/analog conversion in the central office.  Your line hits the SLC in your neighborhood, is converted to a digital signal, goes to the central- office SLC terminal, is converted back to a voiceband signal, then connects to an analog line card on the switch, where itÆs probably digitized again.  V.90 modem operation is impossible in this configuration, so you are limited to V.34.  The multiple conversions, plus the robbed-bit signaling thatÆs typically used in SLC, limits V.34 rates to around 26.4 kbit/s, or 28.8 kbit/s if youÆre lucky (as I happen to be on this call).

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