numerous modem questions including terminology
Joe Wells
I just spent the last several hours reading through the manual that came
with a "Pegasus 14,400efx" modem from Nuvo, and it has inspired a bunch of
questions. A *BUNCH* of questions. :-) If any of you would be so kind
as to answer any of these questions, I would be very grateful.
Are MNP and V.42 just standard sliding window protocols? Is there
anything unusual about them?
Why can't LAP-M be used at less than 1200 bps?
Why was "EIA-232-D" the successor to "EIA RS-232C"? In other words, where
did the "RS" go?
What in particular does a modem do when it "retrains"?
Have any of you had any interesting troubles with the "Pegasus 14,400efx"
or "Nuvotel 14,400ifx" modems from Nuvo? How about troubles with the
"Rockwell RC144DP modem data pump" that these modems use?
There are quite a few words in the manual for which I do not know the
meaning. I would be interested in finding out the meaning and/or
derivation for any of the following terms:
"ter" as in "V.27ter"?
"bis" as in "V.32bis"?
"RS" as in "RS-232C"?
"line quality receive level monitoring"?
"hybrid circuitry" as in "the modem connects to the line interface
through external hybrid circuitry"?
"external echo canceller RAM"?
"plastic leaded chip carrier (PLCC)"?
"plastic quad flat pack (PQFP)"?
"bandsplit filtering"?
"lowpass filtering"?
"guard tone"?
"transmit level attenuation"?
"automatic gain control (AGC)"?
"OEM"?
"sleep mode entry"?
"ringback" as in "ringback detection"?
"answer tone"?
"debounced" as in "these signals are debounced for 10 milliseconds"?
"Eye Quality Monitor (EQM)"?
"GSTN Cleardown"?
"TSTL"? "RLSD"? "RLSDL"? "TM"? "TML"? "CI"? "AAL"?
"ARQ" as in "CONNECT ####/ARQ"?
Thanks for any answers!
--
Joe Wells <j...@cs.bu.edu>
Member of the League for Programming Freedom --- send e-mail for details
Lawrence Kirby
>Hi, exalted modem gurus,
>
>I just spent the last several hours reading through the manual that came
>with a "Pegasus 14,400efx" modem from Nuvo, and it has inspired a bunch of
>questions. A *BUNCH* of questions. :-) If any of you would be so kind
>as to answer any of these questions, I would be very grateful.
>
>Are MNP and V.42 just standard sliding window protocols? Is there
>anything unusual about them?
Essentially, yes. They assemble the data sent to the modem by the DTE into
packets with sequence and checksum information. They transmit up to a window
size and retransmit corrupted packets. MNP 3 (and higher levels based on it)
and V.42(LAP-M) make use of synchronous carriers to allow the packets to
be assembled without start/stop bits. This gives a 25% speed boost so that,
for example, a 9600bps modem can transmit 1200cps (slightly less in real terms
due to the packet overhead).
>
>Why can't LAP-M be used at less than 1200 bps?
>
Typical modulations below 1200bps (e.g. V.21) are not synchronous are not
synchronous so cannot support 'synchronous' protocols such as V.42 or MNP3.
MNP2 would work though.
>Why was "EIA-232-D" the successor to "EIA RS-232C"? In other words, where
>did the "RS" go?
I believe the maintenance of the standard was taken over by a different
standards body.
>
>What in particular does a modem do when it "retrains"?
>
The higher speed modems have equalisers and echo cancellers which must be
'trained' to the characteristics of a particular connection. If a modem is
receiving a low quality signal it may retrain to try to improve matters.
The training sequence consists of various tones and signals which allow
the modems to set themselves up. They can't transmit data while retraining.
>Have any of you had any interesting troubles with the "Pegasus 14,400efx"
>or "Nuvotel 14,400ifx" modems from Nuvo? How about troubles with the
>"Rockwell RC144DP modem data pump" that these modems use?
Not used it.
>There are quite a few words in the manual for which I do not know the
>meaning. I would be interested in finding out the meaning and/or
>derivation for any of the following terms:
>
> "ter" as in "V.27ter"?
> "bis" as in "V.32bis"?
bis and ter roughly mean 'second' and 'third'. e.g. V.32bis is a standard
which is likely to be based on or related to V.32.
> "RS" as in "RS-232C"?
I think it means something like 'Radio Standard'. Perhaps somebody could
confirm/correct this?
> "line quality receive level monitoring"?
Pretty much what it says really. The receiver monitors the quality of the
signal it receives and probably uses it to initiate various things like
fall forward/back and retrains.
> "hybrid circuitry" as in "the modem connects to the line interface
> through external hybrid circuitry"?
Hybrid means a mix of ifferent types of componoent. I've no idea if it means
anything more specific here (probably not).
> "external echo canceller RAM"?
V.32/V.32bis use an echo canceller to transmit and receive in the same
frequency band. To do this they need to subtract the output signal and any
echoes of it down the line from the input signal. 'external echo canceller
RAM' would simply be some RAM external to the main chipset which is used in
the echo cancellation calculations.
> "plastic leaded chip carrier (PLCC)"?
> "plastic quad flat pack (PQFP)"?
Just the type of chip packaging.
> "bandsplit filtering"?
> "lowpass filtering"?
Some signal filtering to reject any signals outside the frequency band the
modem is interested in.
> "guard tone"?
Some telephone exchanges (generally older ones) use certain tones for
internal signalling. A modem might occasionally generate tones which confuse
the exchange. If it also send the 'guard tone' the exchange can recognise
this and ignore the other tones.
> "transmit level attenuation"?
Attenuation is a reduction in signal strength. You may get an attenuation
between transmitter and receiver caused by the telephone network. Alternatively
the transmitting modem may attenuate its output level to bring it down to an
acceptable level.
> "automatic gain control (AGC)"?
As part of the receiver this controls how much the modem amplifies the
input signal internally. The idea is to bring the input signal to a standard
level which the modem can best work with.
> "OEM"?
Original Equipment Manufacturer.
> "sleep mode entry"?
Sounds like some feature/terminology specific to your modem.
> "ringback" as in "ringback detection"?
Ringback used to be popular for sharing a line between modem and voice. If
somebody wanted to dial the modem they call the number, let it ring a couple
of times and then let it drop. They ring again immediately and the modem will
then answer.
> "answer tone"?
The first tone generated by an answering modem.
> "debounced" as in "these signals are debounced for 10 milliseconds"?
Debouncing is a term normally applied to switches. When you press a switch
(such as on a keyboard) the contacts generally 'bounce' i.e. the contact is
made and broken several times. This will appear to a digital system as
several keypresses. Debouncing is what you do to eliminate this i.e. only
register a single keypress. One way to do this is with a timeout - i.e.
ignore any further 'keypresses' for a period of time after the first.
The modem manual is referring to a similar operation on the modem signals.
It may just mean that if the signal changes state for less than 10ms the
change is ignored (in which case they are not using the term 'debounced'
correctly).
> "Eye Quality Monitor (EQM)"?
Tests the quality of your eyes? No idea. Maybe it is some visual indicator
of line quality.
> "GSTN Cleardown"?
> "TSTL"? "RLSD"? "RLSDL"? "TM"? "TML"? "CI"? "AAL"?
Don't know.
> "ARQ" as in "CONNECT ####/ARQ"?
Automatic Retry reQuest (or something similar). It just means that V.42 or
MNP has been established on the link.
-----------------------------------------
Lawrence Kirby | fr...@genesis.demon.co.uk
Wilts, England | 7073...@compuserve.com
-----------------------------------------
Christian Weisgerber
> Typical modulations below 1200bps (e.g. V.21) are not synchronous are not
> synchronous so cannot support 'synchronous' protocols such as V.42 or MNP3.
> MNP2 would work though.
Hmmm. I think there *are* modems around that will run LAPM over V.21.
(SupraModem 9600, SupraModem 2400plus, for instance). Could somebody
check?
> > "hybrid circuitry" as in "the modem connects to the line interface
> > through external hybrid circuitry"?
> Hybrid means a mix of ifferent types of componoent. I've no idea if it means
> anything more specific here (probably not).
In modem chipset data sheets it usually refers to the 4W<->2W conversion
circuit, I gather.
> > "Eye Quality Monitor (EQM)"?
> Tests the quality of your eyes? No idea. Maybe it is some visual indicator
> of line quality.
Some modems offer EQM ports (Rockwell data pump, ZyXEL). With some
additional circuitry it allows you to visualize modem signals on an
oscilloscope.
> > "GSTN Cleardown"?
The V.32(bis) shutdown sequence, I think.
> > "TSTL"? "RLSD"? "RLSDL"? "TM"? "TML"? "CI"? "AAL"?
> Don't know.
These are signal designators out of Rockwell's OEM schematics.
--
Christian 'naddy' Weisgerber, Germany na...@ruessel.sub.org
"Sex is friction. Preferably, friction with a friend. And preferably
well-lubed. After that, it's all a matter of taste." --Solomon Short
ITO Takayuki
j...@bigbird.bu.edu (Joe Wells) writes:
> "RS" as in "RS-232C"?
Recommended Standard.
--
ITO, Takayuki <yu...@is.s.u-tokyo.ac.jp>
Senior, Department of Information Science,
Faculty of Science, University of Tokyo, JAPAN.
Toby Nixon
> I just spent the last several hours reading through the manual that came
> with a "Pegasus 14,400efx" modem from Nuvo, and it has inspired a bunch of
> questions. A *BUNCH* of questions. :-) If any of you would be so kind
> as to answer any of these questions, I would be very grateful.
Most of your questions have been answered in previous followups. I'll just
take a crack at the ones that haven't yet been answered or in which I think
the previous answers need clarification.
> Why was "EIA-232-D" the successor to "EIA RS-232C"? In other words, where
> did the "RS" go?
The American National Standards Institute (ANSI) uses a particular naming
convention for its standards and those from ANSI-accredited organizations.
Part of that is the the initials of the accredited organization should be
the first portion of the standard name, then the number, then the version,
then the year. Prior to EIA-232-D, EIA was wholly independent of ANSI;
when the Electronic Industries Association became accredited by ANSI, they
had to change the naming convention for their standards. "RS" (Recommended
Standard) as a prefix went away, and "EIA" had to be used. Actually, now
it is EIA/TIA-232-E, "TIA" being the Telecommunications Industry Association,
a merger of the United States Telecommunications Association and the
telecom-related standardization parts of EIA.
> "ter" as in "V.27ter"?
> "bis" as in "V.32bis"?
These are French terms (the CCITT is based in Geneva, in the French-speaking
portion of Switzerland). In this context, "bis" means the second in a set of
related standards or extensions to a standard, and "ter" means the third.
"Quatre" would mean the fourth, but I don't recall having seen one of those yet.
> "RS" as in "RS-232C"?
Answered above.
> "hybrid circuitry" as in "the modem connects to the line interface
> through external hybrid circuitry"?
In telephony, a "hybrid" is the part of the modem's circuitry which combines
the "four wire" internal signal (separate transmit and receive circuits) into
the external "two wire" connection used by the phone network. A well-designed
hybrid puts the transmitted signal onto the two-wire line without permitting
it to leak back into the reciever of the modem (especially important in an
echo-cancelling modem, because it reduces the work the AGC and echo canceller
have to do).
> "bandsplit filtering"?
Full duplex modems which don't use echo cancellation accomplish simultaneous
transmission and reception by allocating one portion of the frequency band to
transmission by one modem, and another portion to transmission by the other
modem (usually, the modem that called uses the low band, and the modem that
answered uses the high band). A bandsplit filter is used in such modems so
that the receiver of the modem hears only the portion of the band in which
the other modem is transmitting.
> "lowpass filtering"?
A lowpass filter is one which permits signals below a certain frequency to
pass through, rejecting higher-frequency signals. This is "half" of a
bandsplit filter. It is also used in echo cancelling modems to reject
high-frequency harmonics of the "real" signals, since those harmonics can
interfere with the demodulation process. One of the tougher problems in
modem receiver design is dealing with the harmonics which fall within the
signal band (e.g., a 600 Hz signal will have harmonics at 1200 and 2400,
which are also in the passband of a V.32 modem).
> "transmit level attenuation"?
Normally, a modem will transmit at the maximum level permitted by the
regulatory agency for the country in which it is being used (e.g., the FCC,
which permits a maximum -9dB transmit level at the modem which gets
attenuated to -12dB at the switch). Sometimes, though, on systems with
low loss such as cellular, this level is too high, causing distortion in
the analog-to-digital converters in the network. It can sometimes be
advantageous to reduce the signal level to reduce this distortion (you
can hear the same thing by turning up the volume too high on a stereo
with cheap speakers). MNP10, which is designed to maximize performance
on cellular systems, includes transmit level attenuation capability.
> "sleep mode entry"?
"sleep mode" refers to a low-power idle state. It is frequently used in
battery-powered modems, internal modems in notebook PCs, and PCMCIA card
modems, to reduce the power consumption when the modem is not in active use.
Without this, a modem could run down your batteries pretty quickly, since
many of the components potentially draw a lot of current.
> "ringback" as in "ringback detection"?
In this context, ringback refers to the sound the remote telephone switch
makes when you place a call and the switch is ringing the phone on the
other end. Typically, it is 2 seconds on, and four seconds off, just like
the ring signal. Busy signals, at least in North America, are normally
one second on, one second off. The modem must differentiate between
ringback and busy to accurately report call progress information. Some
modems will give a result code (e.g., REMOTE RING or RINGBACK) when they
hear the ringback signal, to let you know that ring voltage is being sent
to the line which you are calling; this is especially helpful when you're
using a low-power modem (e.g., PCMCIA) which doesn't have a speaker.
> "Eye Quality Monitor (EQM)"?
When you see "eye" or "eye pattern" referred to in a modem, it is talking
about when the modem signal looks like on an oscilloscope which is hooked
up to a particular test point in the modem. The X and Y coordinates are
linked to the amplitude and phase of the signal. Some people think it
looks like an "eye"; I suppose it did in some only modems, but in newer
modems it is much more of a dense matrix (but the term sticks anyway).
"Eye quality" refers to the average distance of each demodulated point
from where it ideally would be located on this display. Because V.32 and
V.32bis modems don't technically have a "carrier detect" signal (you
really can't when you're transmitting and receiving in the same frequency
band), the status of the DCD/RLSD signal must be synthesized from the
signals being received. If the remote modem gets disconnected without
going through the prescribed disconnection sequence (say, if the phone
network drops the call, or power is shut off on the remote modem, or
something like that), then the local modem will not have seen the sequence
of signals indicating that DCD/RLSD should be turned off and the call
terminated. Many modems will, however, monitor the receive signal
quality (with an "eye quality monitor") and, if the signal goes completely
to hell, assume that this was cause by the remote modem prematurely
disconnecting, and drop DCD/RLSD anyway.
> "GSTN Cleardown"?
This is one of the prescribed shutdown sequences for a V.32 or V.32bis modem
which wants to terminate a call. It is normally used if the modem
is operating in non-error-control mode, particularly in synchronous mode.
In non-error-control async mode, you can use long-break disconnect; in
error control mode, you can use the "disconnect" frame defined in the
protocol. In some cases, both the protocol and the GSTN cleardown are
used, to be sure the other end "got it". The GSTN cleardown sequence
consists of a retrain request with all of the "permitted speed" bits
turned off.
-- Toby