Re: Phone lines

["Simon Biber" <simon@basilisk2.cjb.net>]

> 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.

You're right, kb _correctly_ means kilobits. Exactly 1000 bits.

But it was clear that the original poster assumed he could get 27 kibibytes
per second out of his "27 kb/sec" modem.

All this confusion could be avoided if people stuck to the case-sensitive
NIST/IEC approved forms:

b is bits, B is bytes.
k (kilo) is 10^3=1000, M (mega) is 10^6=1000000, G (giga) is
10^9=1000000000, etc.
Ki (kibi) is 2^10=1024, Mi (mebi) is 2^20=1048576, Gi (gibi) is
2^30=1073741824, etc.

His modem does 28.8 kb s^-1. My hard disk is 45 GB. A HD floppy is 1440 KiB
(but neither 1.44 MB nor 1.44 MiB -- the latter isn't even a whole number!).

> 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.

Yes, I have a cable modem which can do 30 Mib s^-1 but is limited by its
ethernet port to 10 Mib s^-1 and further limited by our ISP to 512 Kib s^-1
downstream and 128 Kib s^-1 upstream.

> > 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.

Bytes have been 8 bits on almost all computer equipment for the last 10+
years. I know the unambiguous term is octet.

> > 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.

But everyone uses standard settings of N,8,1 (no parity, 8 data bits and 1
stop bit) for modems connecting to the internet.

> 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.

Yes.

> > 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.

People will always find ingenious solutions to technical problems :-)

Simon.