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Re: And we thought the text part of the Starr Report would be bad

  • From: David Stoddard
  • Date: Tue Sep 22 11:43:41 1998

Miquel van Smoorenburg writes:
> In article <199809182139.RAA10174@us.net>, David Stoddard  <dgs@us.net> wrote:
> >	There is more to this than meets the eye -- 28.8K is asynchronous
> >	and has start and stop bits for every byte, so there are a maximum
> >	of 2880 bytes/sec available over 28.8K.  Then there is the issue
> 
> Almost every modem supports V42 error correction, which makes the modems
> speak a sort of synchronous with each other (actually data is transmitted
> in blocks with a start-of-block and end-of-block marker, and a checksum).
> That gets you 8 bits in a bith minus some negligeble V42 overhead.
> So an 28k8 modem can actually transfer almost 3.6 Kbytes/sec.
> Because of the block-oriented approach you do get a bit higher latency
> on interactive connections, which is why gamers often turn of V42.

	As long as we are being specific about this, I omitted data
	compression too, which has a real effect on the data transfer rate
	as well.  I also omitted 56K modems, ISDN dial-up, and other aspects
	of dial-up communications (like MPP).  The point I was trying to
	make was not to disassemble 28.8K modem protocols, but to point
	out the fallacy in believing that a T1 line can only support
	53 modems.

	As far as the start and stop bit issues go, they are controlled in
	the UART on the PC serial port, not the modem.  Regardless of how
	the modem encodes and transmits data over the wire, the start and
	stop bits remain in the asynchronous serial protocol on PCs -- years
	ago, PC terminal programs used to let you control the number of start
	and stop bits you used to communicate with a device.  The fact that
	DCE/DTE rates are usually higher that the actual speed of the modem
	connection negates some of the effect of start and stop bits, but
	they do exist.

	For modem to modem communications, start and stop bits were used
	up through 2400 baud when one bit = one tone.  The method of one
	bit = one tone was dropped for other forms of encoding when modems
	started supporting connection speeds in excess of 2400 baud.  While
	the 8000 bit/sec sampling rate of POTS lines can theoretically
	support up to 4000 Hz bandwidth, it is technically impossible to
	get this (reliably) out of the analog phone system.  28.8K modems
	need at least 3200 Hz of bandwidth on a POTS line to support 28.8K.

	All said, I still stand by my comment that 53 modems per T1 is
	ludicrous.

	Dave Stoddard
	dgs@us.net




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