TELECOM Digest OnLine - Sorted: Re: R-TEC Isolation Filter and Unknown Box

Re: R-TEC Isolation Filter and Unknown Box

William Warren (
Mon, 25 Jul 2005 13:12:46 -0400 wrote:

> I was in my basement, trying to remove the mess of wires, and putting
> them all on a punchdown block. I noticet two old grey bell co wires
> going to a box called an isolation filter. From there they went into a
> box called an "R-tec STU-7". Inside the r-tec box, there is a
> rechargable battery, and an instruction sheet. The sheet says:

> -Telephone must be wired for briged ringing
> -Use straight line ringers only with bias spring at minimum setting.

> The box is grey, says "Phone Co Property" and is about 9'' by 5'' by
> 2'' deep.

> What is this, and what does/did it do (it was disconected)

> Thanks,

> Warren


It's an "AML" box, which was used to provide additional dial tone to a
house that didn't have an extra pair available on the pole.

Asynchronous Multi-Line units use Frequency Division Multiplexing to
allow super-audible transmission of additional dial-tones, the same
way DSL uses super-audible frequencies to transmit data. They were
powered by a battery which was, in turn, trickle-charged by "bleeding"
the POTS line's 48 volts, and each required a matching unit in the CO.

AML units were pressed into service as a short-term solution during
the early days of the Internet explosion, when demand for extra dial
tones outstripped available plant. They tended to create more problems
than they solved, since users complained that dial tone delivered via
AML wouldn't work with the new, higher speed modems that also flooded
the market in the same time frame.

Ma Bell eventually abandoned them in most areas, prefering instead to
build new capacity. They're still available in the secondary market.



(Filter noise from my address for direct replies)

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