Re: full duplex or half duplex

Grant Edwards <grante@xxxxxxxx> wrote:
On 2006-05-18, Eugene Blanchard <blanchae@xxxxxxxxx> wrote:
mike7411@xxxxxxxxx wrote:

Are old analog phones full duplex or half duplex?

They are full duplex, both parties can talk at the same time, data is
transmitted in both directions. Modems are full duplex,

Most are, some aren't. Mass-market consumer modems have pretty
always been full-duplex. But, there are still a lot (as in
millions) of half-duplex modems used in industrial
process-control settings.

Are that many left that are actually half duplex? I was never
involved in "industrial process-control settings", so I don't
have a good feel for how many used to exist, much less how many
are still being used.

The telecommunications industry used to see relatively few to
begin with, and that quickly diminished to virtually none when
asymmetrical full duplex modems became available.

they transmit data in both directions but at different
frequencies.

The full duplex ones do. The half-duplex ones both transmit
using the same carrier frequency and therefore have to take
turns.

Most full duplex modems in use today use the full VF spectrum in
both directions, and separate the two using echo canceling
circuits. V.32 and v.34, for example, utilize all of the
bandwidth in both directions. Earlier modems using 2400 bps or
less generally use the different transmit frequencies for the
two directions in order to get full duplex.

The original problem was that technology was not available to
obtain more than about 1800 bps in one direction using the full
bandwidth. The initial answer to that was to run a low speed
channel in one direction (e.g., 75 bps) and a high speed channel
in the other direction, which worked quite well for a terminal
used for manual data entry from a keyboard... but wasn't worth
much for anything else.

Some manufacturers then came up with modems that would switch
the high speed channel from one direction to the other as needed.
That worked very well for file transfers. In fact Telebit modems
emulated certain protocols in order to reduce the number of times
that a channel switch was necessary, thus making for some relatively
very high speed data transfers if those particular protocols were
used.

When technology made v.32 (and later, v.34) modems available at
prices anyone could afford, the use of anything else rapidly
diminished.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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