Re: Phone lines

[pausch@saaf.se (Paul Schlyter)]

In article <Dtpn6.53764$o85.305060@news-server.bigpond.net.au>,
Simon Biber <simon@basilisk2.cjb.net> wrote:
 
> I don't know of any modems capable of 27 kb/sec. You probably have what's
> called a 28.8k modem, which can transfer 28800 bits per second, which is
> only 2.88 kb/sec.
 
FYI:  kb can refer to kilo-bits as well as kilo-bytes.  And since modem
speeds usually are given in bits/s rather than bytes/s, it's reasonable to
interpret 27 kb/s as 27 kilobits/s -- which is fairly clsoe to 28.8 kbits/s.

Regarding possible 27 kbyte/s modems: ADSL modems can reach up to
800 kbytes/s !!!!!  You connect them to your computer through an
Ethernet cable rather than an RS232 port though.  But ADSL runs on normal
phone lines, but requires special equipment at the nearest phone
station, so it's not available everywhere.
 
> This is because there are 8 bits in a byte
 
Depends on the byte size -- in various situations, bytes can be between
5 bits and 12 bits in length.
 
> plus two which are used by the modem for control purposes.
 
...or three if you configure your modem for two stop bits instead of one.
 
However, such a bit stream was transferred over the modem only on the old
lower speeds such as 300 b/s, 600 b/s or 75/1200 b/s.  Already at 1200/1200
and 2400/2400 bps two bits were sent at the same time by special modulation
techniques - the actual carrier on the phone line changed state only 600
times per second (= 600 baud -- note that here baud and bps are no longer
the same thing).
 
Modern high-speed modens have a carrier which uses the entire 3 kHz bandwidth
of the phone line - that carrier is transmitting at 2400 baud (you can't get
much faster than that within 3000 Hz bandwidth).  However, by special
modulation techniques made possible by digital signal processors, the
modem can squeeze up to 57.6 kbits/second on that 2400 baud carrier.
 
> The Apple II "Super Serial Card" is limited to 19200 bps, however, and there
> is no point getting a faster modem if you're using an Apple II.
 
A very good point.
 
I remember back in 1986 when I permanently strapped my old Comm. Card
for 1200/4800 bps -- the I thought: "I'll never get 4800 bps on modem
on a phone line!".  Boy was I wrong!  But then I had a 300 bps
acoustic modem, and I knew about 1200 bps modems and had heard about
2400 bps modems, but that was about it.  And since the bandwidth of
the phone line only is 3000 Hz, it appeared reasonable that tbe bit
rate couldn't go above 3000 bps.
 
-- 
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