i realy need to know about it, what is it, differences, why of work and
any other information

10Q

certain modems support this feature, generally for multidrop networks
you'd have the server working with a constant carrier and the clients
on switched/controlled. On point to point connections you'd have both
on constant.

Constant carrier means that RTS is always on so that the modem can
always send information. Switched means that the RTS switches on and
off depending on whether or not the modem needs to send data, and so is
typically used for half duplex modems or multidrop terminals.

Perdition wrote:
where switched carrier is/was common. Modems such as the Bell 202, Bell
208, and V.27 could not split the 2-wire channel into separate,
simultaneous send and receive paths. Therefore they switched which end
was transmitting carrier into the circuit under control of the
terminal's RTS signal. Other modems (Bell 103, Bell 212, V.32, V.90)
have a mechanism to split the channel.

--reed

Reed <reedh@rmi.net> wrote:
Those are 4-wire leased line protocols, and do not do dial-up.
I've never heard of using one of them on a 2-wire dial-up. 202
modems could in fact be used on a 2-wire leased line in half
duplex mode. I never saw anyone use them that way, but it could
be done...

Bell 103 and 212 modems split the channel. V.32 and .v90 use
echo cancellation to allow each direction full access to the
entire channel bandwidth.

Typically switched carrier is used with a single master and
multiple drops. Examples that were in common use for many years
were the airline reservation networks and the FAA's weather
circuits. A single master would be transmitting carrier all of
the time to all receiving stations. It would poll each one in
turn, sending a code that would be recognized as an "Okay to
send" by one station only, and doing all stations round robin.
When each station received the code it recognized, it would
switch on its carrier and transmit whatever data it had ready to
go.

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

snip

Floyd,
I know you have probably forgotten more than I have ever known about
data communications overall, but:

Bell 208A = LL 208B = dial
Bell 202T = LL 202S = dial
V.27ter = dial
also
Bell 201C = dial 201B = LL

Yes, I should have explained this better; split ch (FDM) vs EC.

FWIW, I was with Motorola Codex for 19 years.

Best regards,
Reed

Reed <reedh@rmi.net> wrote:
Could be... but more likely Reed has forgotten more about
modems than I ever knew.

For example, I neve knew about the models he lists below!

A short timer... :-) But I do appreciate the correction!

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