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Re: (ASCEND) Am I the only one ??

>>>The loop is part analog, and the conversion cannot be perfect, so of
>>>course your're going to lose some in the conversion. As far as I know,
>>>no one's about to connect 56K (or 53K or 54K or whatever the top
>>>speed is) if they're connecting to an ISP with CT1's which have only
>>>56K channels. Only if the ISP has 64K lines (ISDN) can you expect to
>>>reach such speeds.

>> I'll grant you that I don't know all the intimate details of
>> how the line cards in any particular switch work, but I don't
>> think that you can drive the transmitters higher than
>> the raw 56k provided over a CSV channel (Circuit-Switched-Voice),
>> so it shouldn't matter whether the line is ISDN or CT1,
>> a CSV is a CSV is a CSV...

> For some reason I was thinking that a voice call would be modulated over
> the full 64K channel whenever such was available end to end, but upon
> thinking about it, I guess that would require a different codec and
> they probably wouldn't have bothered.

The telcos wouldn't field 2 sets of codecs; it's not cost
effective nor manageable.

A voice channel is 8000 samples per second with 8 bits per
sample.  That's where the 64Kbps comes from.  Now if the
off-hook signalling is in-band, then the low order bit of
some octets is robbed to transmit this information.  Voice
calls with humans on both ends won't notice (at least, that
is the theory).

The low order bit is not robbed out of every octet, but the
devices using the link for data communications don't know
which bits will be "changed" and which will remain the same.
So if in-band signalling is used, the devices must limit
themselves to 7 bits per sample (i.e. ignore all low order
bits).  This gives 56Kbps.

Even if both end loops are clear channel 64Kbps, intermediate
segments may be 56Kbps.

128 discrete levels (7 bits) is also about all the resolution
that the client modem can expect to extract once a data octet
has passed through 1 stage of digital to analog conversion
(DAC) and then had phase and gain modifications as it passes
down the local loop to the customer's premises.  This also
nicely aligns with the voltage level jitter due to in-band
signalling (i.e. 56Kbps links).

						Lee Jones
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