Josh Bailey <josh@vandervecken.com> wrote:
>On Mon, 11 Dec 2000, Don Stokes wrote:
>
>> Thus normal modem transmission is eight bits per character, plus
>> framing and checksums per block, minus the savings from compression
>
>MNP3 and later let you be rid of the overhead of start/stop bits; you can
>have a 2400 bps modem, running (really - no compression) at 2900 bps.

I think that's what I said. You still need framing to synchronise the
two ends, but with synchronous comms you don't do it by adding two extra
bits every character; you typically add frame characters around each
block (with bit stuffing to stop other bit patterns from looking like
frame characters). Even if you toss in a checksum and sequence number
for error correction, that's a lot less extra bits than async comms.

>IOW, even if you enable error correction (whether V.42 or MNP4) on your
>modem, and *disable* compression, you'll get a win. That's why even when
>you're transferring something uncompressable, you can actually exceed the
>physical line rate.

Yup. Assuming of course you're counting ten bits per character rather
than eight. 9600 comes to 960 cps in normal async comms, but something
less than 1200 cps for sync comms.

>Much of xDSL is directly descended from so-called "PCM modem" technology;
>ironically PCM upstream in V.92 was descended from xDSL technology - the
>client modem has to assess the analog line's electrical characteristics,
>and then belt out a wavery, sloppy waveform that the analog line will
>contort by its very nature into a nice clean step-wave by the time it hits
>the codec in the exchange.

Hmmm, the way it was described when I first met ADSL in early 1993 (when
the total international deployment was a few thousand feet of cable on
a lab floors) was that ADSL borrowed the idea of QAM modulation from the
modem world..

(ADSL was being pushed to carry video via a 6Mbps downlink; nobody much
 was talking about other kinds of DSL. Of course I was thinking this
 would be a really good way to deliver Internet bandwidth, but the
 telcos were clearly thinking primarily in terms of using their existing
 copper to take the cable TV companies on. Using the technology for
 packet data (let alone IP packets) seemed to have escaped them; it was
 expected that the ADSL kit would then be attached through ATM, and
 circuit switched to whatever broadband service provider you wanted --
 one service at a time. Very typical of telco thinking at the time.
 <shudder>)

-- don

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Received on Mon Dec 11 14:16:10 2000