Wow  - I'm impressed to see at least 2 other people on this list who do
have some pretty good idea of how the phone stuff works.  I also have
been a designer or phone equipment - T1's for TR-8 and GR303 up to
10Gbps Sonet stuff.

Thank you to Nate for the wonderfully full description!

I disagree on the uses of the AC signal for powering the repeaters, as
everything I have see has network power of up to 130V DC power on a
separate pair of wires to the repeater.

I'll also disagree with Dave's comment that it is "older" equipment that
robs the least significant bit on every 64Kbps line.  Every GR303 is
pretty much as modern as you can get for digital lines out towards the
subscriber and it uses ESF which robs 1 least significant bit out of
every 6 frames.  Granted GR-303 has been in use for 15+ years (can't
find my copy right now) but there isn't anything newer.  He is right
though - a lot of equipment is not multi-framed aligned - partially
because trying to do it would add a lot of latency to the connection,
and partially because equipment manufacturers sometimes take the easy
way out and just don't worry about it.

Those "disagreements" about some pretty trivial matters aside thank you
to both of you for some pretty wonderful write-ups.

Dan

Dave Tweed wrote:

>"M. Adam Davis" <adampic@UBASICS.COM> wrote:
>
>
>>To save on the cost of having an extra clock wire for certian portions
>>of the digital transmission, the last bit of each byte is toggled,
>>regardless of the actual value of that bit from the A/D conversion.
>>This enables easy clock recovery for the phone company, and one less
>>dedicated wire for the clocking circuit.
>>
>>
>
>
>
>>Of course, the 56k calculation and bit about the phone companies
>>throwing away the last bit for clocking purposes is out of a DSP book.
>>
>>
>
>Unfortunately, it's dead wrong. Telcos have *never* used this method of
>clocking data over digital lines at any rate.
>
>In North America and other countries using the T1 standard, each channel
>really has 64 kbps of data (8 ksps x 8 bits per sample) devoted to it.
>
>However, older equipment using in-band signaling will "rob" (overwrite)
>bit 7 once every 6 or 12 samples, in order to indicate ringing / off-hook
>status at each end. A signal that passes through multiple such links in
>tandem may lose bit 7 in additional samples, because tandem links do not
>necessarily synchronize at the "multiframe" level, which would also
>synchronize the bit-robbing. As a result, you can only assume that you've
>got only 7 usable bits per sample, or 56 kbps.
>
>Nate Duehr <nate@NATETECH.COM> wrote:
>
>
>>Ain't all this stuff FUN?  ;-)
>>
>>Telco geek for many years turned Unix geek, but still love telco as
>>it's such a cool "natural" progression of technology for 30 years...
>>
>>
>
>Yes, it is. While the general concepts of your narrative were mostly
>accurate, the details were wrong in almost every respect. I'm speaking
>as a recent (up until 2002) designer of T1/E1 terminal multiplexer
>equipment.
>
>For example, repeaters were never "powered by the signal". The
>ones density requirement is related entirely to maintaining clock
>synchronization. AMI doesn't help create ones density, but bit-8
>stuffing and B8ZS do.
>
>And so on. I don't have time to address all of your points.
>
>-- Dave Tweed
>
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>
>
>

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