diagnosing network speed bottlenecks [SOLVED]

[Jerry Feldman]

On 10/01/2009 04:53 PM, Ben Scott wrote:
>
>   I didn't think this was the case, but I just checked the V.32
> standard, and it does indeed say that the signaling on the telephone
> line is synchronous.  Now, I think every modem I've encountered only
> implemented the RS-232 lines needed for async.  So the modem must be
> buffering synchronous data to/from the telephone line internally, so
> it can go back and forth to the host asynchronously.  The host then
> has to buffer the UART for the same reasons.  How's that for
> backwards?  :)
>
>   But it's still serial, and still bit oriented.  Synchronous
> transmission doesn't necessarily mean you have to have only one
> particular framing discipline.
>
>   
The consumer dialup modems we used to buy in the store are and were
aynchroinous, but before the Internet was widespread, there were
synchronous modems. I used them at various work locations through the
70s and 80s. While I was working for a company in Texas, they were
designing a point of sale device for their fast food stores, and the
modem vendor was going to charge us $50 per unit to make the modem
synchronous. The POS designer said it wasn't worth it to spend $50,000
($1000 stores) for a 20% savings (2 bits per byte). But they didn't take
phone line costs into consideration. I was able to successfully argue
that synchronous (actually in this case a variant of IBM's bisynch
protocol) would actually save $50,000/year in phone costs. Another thing
they did not take into account is that the data they were sending was
binary so it had to be converted to 7-bit ASCII where bisync simply
escaped the 10 or so control characters. While IBM's bisynch and similar
protocols were not good by todays standards, it worked well. Basically
it was not a windowing protocol.  A bit later X.25 and IBM's SNA
protocols emerged as did the various IP protocols essentially obsoleting
communications over dialup and direct phone grade lines. 

-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix
PGP key id: 537C5846
PGP Key fingerprint: 3D1B 8377 A3C0 A5F2 ECBB  CA3B 4607 4319 537C 5846


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