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

[David Stoddard <dgs () us net>]

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