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LDP/howto/linuxdoc/Modem-HOWTO.sgml
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@ -3,10 +3,12 @@
<title> Modem-HOWTO </title>
<author>David S.Lawyer
<tt><url url="mailto:dave@lafn.org"></tt>
<date> v0.12, December 2000
<date> v0.13, February 2001
<!--
Change log: + => added more info ++ => added new topic
v0.13 serial-modem => digital-modem, fixed suse isdn link, antique
modems, typos fixed in "Getty", new section: "Ending a Dial-in Call"
v0.12 Dec. 2000 Zoltrix and Cirrus Logic linmodems; don't omit 0 in X0
(for some modems) X3 if no dial-tone; init string locations; more on
"Modem is busy"; PCI non-winmodems are OK; terminology: software-based
@ -91,12 +93,10 @@ maintainer. You may create a derivative work and distribute it
provided that you:
<enum>
<item> Send your derivative work to the LDP (Linux Documentation
Project) or the like for free distribution on the Internet in a
format they will accept. If not the LDP, then let the LDP know
where it is available. Except for a translation, send a copy to the
previous maintainer's url as shown in the latest version.
<item> If it's not a translation: Email a copy of your derivative work
to the LDP (Linux Documentation Project) for free distribution on the
Internet in a format LDP accepts. Also email such a copy to the
author(s) and maintainer (could be the same person).
<item>License the derivative work in the spirit of this license or use
GPL. Include a copyright notice and at least a pointer to the
license used.
@ -153,27 +153,24 @@ modem situation is rapidly changing (and since I'm still learning).
Your problem might be solved in the latest version. It will be
available to browse and/or download at LDP mirror sites. For a list
of such sites see: <url url="http://metalab.unc.edu/LDP/mirrors.html">
If you only want to quickly compare the date of this the version v0.12, December 2000
If you only want to quickly compare the date of this the version v0.13, February 2001
with the date of the latest version go to:
<url url="http://metalab.unc.edu/LDP/HOWTO/Modem-HOWTO.html">
<sect1> New in this Version
<p> v0.12 Zoltrix and Cirrus Logic linmodems; don't omit 0 in X0 (for
some modems) X3 if no dial-tone; init string locations; more on "Modem
is busy"; PCI non-winmodems are OK; terminology: software-based modem,
driver-based modem; Linmodem-HOWTO; types of software modems;
setserial irq5 to irq 5; Quick Install rewrite.
<p> v0.13 serial-modem => digital-modem, fixed suse isdn link, antique
modems, typos fixed in "Getty", new section: "Ending a Dial-in Call"
<sect1> What is a Modem ? <label id="what_is_modem">
<p> A modem is a device that lets one send digital signals over
ordinary telephone lines not designed for digital signals. If
<p> A modem is a device that lets one send digital signals over an
ordinary telephone line not designed for digital signals. If
telephone lines were all digital then you wouldn't need a modem. It
permits your computer to connect to and communicate with the rest of
the world. When you use a modem, you normally use a communication
program or web browser (which includes such a program) to utilize the
modem and dial-out on a telephone line. Advanced modem users can set
things up so that others may phone in to them and use their computer.
This is called "dial-in".
program or web browser to utilize the modem and dial-out on a
telephone line. Advanced modem users can set things up so that others
may phone in to them and use their computer. This is called
"dial-in".
There are two basic types of modems for a PC: external and internal.
The external sets on your desk outside the PC while the internal is
@ -668,7 +665,8 @@ the telephone line and don't use serial ports for the interface to the
computer. Instead, they interface directly to the PC bus via a
special card (which may also contain the "digital modems"). They are
able to send at near 56k, something no analog modem can do. They are
often a component of "remote access servers" or "digital modem pools"
often a component of "remote access servers" (RASs) or "digital modem
pools"
The cables from the phone company that carry digital signals have been
designed for high bandwidth so that the same cable carries multiple
@ -687,18 +685,18 @@ telephone line. Thus it only makes digital-to-digital conversions and
doesn't deal in analog at all. It thus is not really a modem at all
since it doesn't modulate any analog carrier. So the name "digital
modem" is a misnomer but it does do the job formerly done by modems.
Thus Some "serial modems" call themselves "digital signal processors",
"remote access servers", etc. and may not even mention the word
"modem". This is technically correct terminology.
Thus some "digital modems" call themselves "digital signal
processors", or "remote access servers", etc. and may not even mention
the word "modem". This is technically correct terminology.
Such a system may be a stand-alone proprietary server, a chassis
containing digital modems that connects to a PC via a special
interface card, or just a card itself. Digi calls one such card a
"remote access server concentrator adapter". One incomplete
description of what is needed to become an ISP is: See <htmlurl
url="/http://www.cyclades.com/sol_isp.html" name="What do I need to be
an ISP?">. Cyclades promotes their own products here so please do
comparison shopping before buying anything.
url="/http://www.cyclades.com/solutions/techtalk/techtalk0l.htm"
name="What do I need to be an ISP?">. Cyclades promotes their own
products here so please do comparison shopping before buying anything.
<sect> Serial Port and Modem Basics <label id="basics_">
<!-- basics.H begin <sect> Serial Port and Modem Basics
@ -1908,10 +1906,13 @@ modem.
Such command strings are either automatically sent to the modem by
communication programs or are manually typed in by you. Most
communication programs provide a screen where you may type such
commands. You may type in some commands to create the configuration
you want and then save this this configuration (profile) for later
use. It gets saved inside the modem itself.
communication programs provide a screen where you may change (edit)
and save the init string that the communication program will use.
The modem itself has a stored configuration (profile) which is like a
long init string. It represents the configuration of the modem when
it's first tuned on. You may change it to suit your taste. In most
cases there are a few different such configurations (profiles) and
there are ways to designate one of them to be active.
If you have a manual for your modem you can likely look up the AT
command set. Otherwise, you may try to find it on the Internet.
@ -1920,7 +1921,7 @@ search terms to avoid finding sites that just talk about such commands
but fail to list them. You might also try a few of the sites listed
in the subsection <ref id="web_sites" name="Web Sites">
<sect1> Init Strings: Saving and Recalling
<sect1> Init Strings: Saving and Recalling <label id="init_str">
<p> The examples given in this subsection are from the Hayes AT modem
command set. All command strings must be prefaced by the two letters
AT. For example: AT&amp;C1&amp;D3^M (^M is the return character).
@ -1952,11 +1953,10 @@ Modern modems have a few different stored profiles to choose from that
are stored in the modem's non-volatile memory (it's still there when
you turn it off). In my modem there are two factory profiles (0 and
1, neither of which you can change) and two user defined profiles (0
and 1) that the user may set and store. Your modem may have more.
To view some of these profiles send the command &amp;V.
At power-up one of the user-defined profiles is loaded. For example,
if you type the command &amp;Y0 then in the future profile 0 will be
used at power-on.
and 1) that the user may set and store. Your modem may have more. To
view some of these profiles send the command &amp;V. At power-up one
of the user-defined profiles is loaded. For example, if you type the
command &amp;Y0 then in the future profile 0 will be used at power-on.
There are also commands to load (activate) any of the stored profiles.
Such a load command may be put in an init string. Of course if it
@ -2004,7 +2004,8 @@ cases.
<itemize>
<item> Gnome: run pppsetup
<item> wvdial: edit /etc/wvdial.conf
<item> minicom: hit ^Ao, then select "Modem and Dialing"
<item> minicom: hit ^Ao (or possibly ALT-o), then select "Modem and
Dialing"
</itemize>
<sect1> Other Modem Commands <label id="modem_commands">
@ -2023,7 +2024,7 @@ S0=0 never answer (uugetty does this with the WAITFOR option)
Here's some more codes:
<itemize>
<item>&amp;C1 DCD is on only after connect
<item>&amp;C1 DCD is only on when you're connected
<item>&amp;S0 DSR is always on
<item>&amp;X3 Dial even if there is no dialtone (Use where
dial-tones don't exist).
@ -2665,30 +2666,30 @@ login:
<sect> Dial-In <label id="dialin_">
<sect1> Overview
<p> Dial-in is where you set up your PC so that others may dial in to
your phone number and use your PC. The "point of view" is your PC.
When you dial out from your PC you are also dialing in to another
computer (but not dialing in to your own computer)
your PC (at your phone number) and use your PC. The "point of view"
here is your PC. When you dial out from your PC you are also dialing
in to another computer (but not dialing in to your own computer)
Dial-in works like this. Someone with a modem dials your telephone
number. Your modem answers the call and connects. Once the caller is
connected, your PC (via the <tt/getty/ program) starts the login
connected the <tt/getty/ program is notified and starts the login
process for the caller. The original method was to send a login
prompt to the caller's screen (manual login). But a more modern
method (if you use <tt/mgetty/) is to start PPP (pppd) and let PPP
automatically login the caller (no need to manually type in a name or
password). See the PPP-HOWTO (new revision expected soon) and docs
for <tt/mgetty/ for more details.
prompt to the caller's screen (manual login). It's still used but if
you let callers use the PPP protocol then the login can take place
automatically after a PPP connection is started from both sides. See
the PPP-HOWTO for more details.
After the caller has logged in, the caller uses your PC. Using your
PC may mean that the caller has a shell account and can use your PC
just as if they logged in at the console (or from a text-terminal).
It could also mean that they get connected to the Internet thru your
PC (via PPP). The program that you use at your PC to handle dialin is
called <tt/getty/ or <tt/mgetty/. See <ref id="mgetty_" name="About
mgetty">.
After the caller has logged in, the caller then may use your PC.
Using your PC may mean that the caller has a shell account and can use
your PC just as if they logged in at the console (or from a
text-terminal). It could also mean that they get connected to the
Internet thru your PC (via PPP). The program that you use at your PC
to handle dialin is called <tt/getty/ or <tt/mgetty/. See <ref
id="getty_" name="Getty">
If you expect that people will be able to dial-in to you at 56k, it
can't be done unless:
<sect2> 56k doesn't work
<p>If you expect that people will be able to dial-in to you at 56k, it
can't be done unless you have all the following:
<enum>
<item> You have a digital connection to the telephone company such as a
trunkside-T1 or ISDN line
@ -2699,25 +2700,23 @@ can't be done unless:
</enum>
A "... concentrator" may be called a "modem concentrator"
or a "remote access concentrator" or it could be included in a "remote
access server" which includes the digital modems, etc. This type of
setup is used by ISPs (Internet Service Providers).
access server" (RAS) which includes the digital modems, etc. This
type of setup is used by ISPs (Internet Service Providers).
<sect1> Getty <label id="getty_">
<p> <tt/getty/ is the program you run for dialin. You don't need it
for dialout. In addition to presenting a login prompt, it also
for dialout. In addition to presenting a login prompt, it also helps
answers the telephone. Originally getty was used for logging in to a
computer from a dumb terminal. A major use of it today is for logging
in to a Linux console. There are a few different getty programs with
slightly different names. Only certain ones work with modems for
dialin. The getty program is usually started at boot-time. It must
be called from the /etc/inittab file. You may find an example in this
file of a call to getty which you will likely need to edit a bit. If
you use a different getty program than the one shown in such an
example, then you will need to edit it quite a bit since the options
will have a different format.
be called from the /etc/inittab file. In this file you may find some
examples which you will likely need to edit a bit. Hopefully these
examples will be for the flavor of getty installed on your PC.
<p> There are four different getty programs to choose from that may be
used with modems for dialin: <tt/mgetty/, <tt/uugetty/, <tt/getty_em/,
used with modems for dial-in: <tt/mgetty/, <tt/uugetty/, <tt/getty_em/,
and <tt/agetty/. A brief overview is given in the following
subsections. <tt/agetty/ is the simplest (and weakest) of the four
and some consider it mainly for use with directly connected
@ -2736,15 +2735,14 @@ your computer, use the "locate" command. Type: locate "*getty*"
call the program getty even though it may actually be agetty, uugetty,
etc. But if you read the man page (type: man getty), it might
disclose which getty it is. This should be the getty program with
path /sbin/getty.
path <tt>/sbin/getty.</tt>
<sect2>About mgetty <label id="mgetty_">
<p>
<tt/mgetty/ was written as a replacement for <tt/uugetty/ which was
in existence long before <tt/mgetty/. Both are for use with
modems. Although <tt/mgetty/ may be also used for directly connected
terminals the documentation for this is hard to pinpoint and mgetty
will not (as of mid 1999) support software flow control (used on many
in existence long before <tt/mgetty/. Both are for use with modems.
<tt/mgetty/ may be also used for directly connected terminals. Mgetty
doesn't (as of mid 1999) support software flow control (used on many
terminals) without recompiling. This defect is listed as a bug. In
addition to allowing dialup logins, <tt/mgetty/ also provides FAX
support and auto PPP detection. There is a supplemental program
@ -2760,7 +2758,7 @@ url="http://alpha.greenie.net/mgetty">
<p>
<tt/getty_ps/ contains two programs: <tt/getty/ is used for console
and terminal devices, and <tt/uugetty/ for modems. Greg Hankins
SUAL --(former author of Serial-HOWTO) used <tt/uugetty/ so his writings
(former author of Serial-HOWTO) used <tt/uugetty/ so his writings
about it are included here. See <ref id="uugetty_" name="Uugetty">.
The other gettys are well covered by the documentation that comes with
them.
@ -2777,17 +2775,18 @@ url="scicom.alphacdc.com/pub/linux">. The name of the collection is
you must use your full e-mail address as the password. For example:
greg.hankins@cc.gatech.edu
<sect2>About agetty and mingetty
<sect2>About agetty, mingetty, and fbgetty
<p>
<tt/agetty/ is a simple, completely functional implementation of
<tt/getty/ which is best suited for virtual consoles or terminals
rather than modems. But it works fine with modems under favorable
48%
Waiting for a call). <tt/agetty/ in the Debian distribution is just
named <tt/getty/.
conditions. <tt/agetty/ in the Debian distribution is just named
<tt/getty/.
<tt/mingetty/ is a small getty that will work only for consoles
(monitors) so you can't use it with modems for dialin.
<tt/mingetty/ is a small getty that will work only for monitors
(the usual console) so you can't use it with modems for dialin.
<tt>fbgetty</tt> is also only for monitors and supports framebuffers.
<sect1> What Happens when Someone Dials In ?
<p> The caller runs some sort of communication program that dials your
@ -2797,12 +2796,12 @@ automatically answer the call. The other way is for getty to sense
the ringing and send a command to the modem to answer the call. Once
the call is answered, your modem sends tones to the other modem (and
conversely). The two modems negotiate how they will communicate and
when this is done your modem sends a "CONNECTed" message (or the like)
to <tt/getty/. When <tt/getty/ gets this message, it sends a login
prompt out the serial port. Sometimes <tt/getty/ just calls on a
program named <tt/login/ to handle the logging in. <tt/getty/ usually
starts running at boot-time but it must wait until a connection is
made before sending out a "login" prompt.
when this is completed your modem sends a "CONNECT" message (or the
like) to <tt/getty/. When <tt/getty/ gets this message, it sends a
login prompt out the serial port. Sometimes <tt/getty/ just calls on
a program named <tt/login/ to handle the logging in. <tt/getty/
usually starts running at boot-time but it must wait until a
connection is made before sending out a "login" prompt.
Now for more details on the two methods of answering the call. By
setting the S0 register of the modem to 3, the modem will
@ -2822,12 +2821,12 @@ sends the modem an "ATA" command. The modem then makes a connection
and sends a "CONNECT ..." message to <tt/getty/ which then sends a login
prompt to the caller.
The automatic answer case uses the CD (Carrier Detect) wire from the
modem to the serial port to detect when a connection is made. It
works like this. At boot-time <tt/getty/ tries to open the serial
port but the attempt fails since there is normally no CD signal from
the modem. Then the <tt/getty/ program waits at the open statement in
the program until a CD signal appears. When a CD signal arrives
The automatic answer case uses the CD (Carrier Detect aka DCD) wire
from the modem to the serial port to detect when a connection is made.
It works like this. At boot-time <tt/getty/ tries to open the serial
port but the attempt fails since the modem has negated CD (the modem
is idle). Then the <tt/getty/ program waits at the open statement in
the program until a CD signal is asserted. When a CD signal arrives
(perhaps hours later) then the port is opened and <tt/getty/ sends the
login prompt. While <tt/getty/ is waiting (sleeping) at the open
statement, other processes can run since Linux is a multiprocessing
@ -2835,12 +2834,12 @@ operating system. What actually wakes <tt/getty/ up is an interrupt
which is issued when the CD line from the modem changes state to on.
You may wonder how getty is able to open the serial port in the
manual-answer case since there is no CD signal. Well, there's a way
to write a program to force the port to open even if there is no CD
signal present.
manual-answer case since CD is negated. Well, there's a way to write
a program to force the port to open even if there is no CD signal
asserted.
<sect1> Why Manual Answer is Best
<p> The difference between the two ways of answering will show itself
<p> The difference between the two ways of answering is exhibited
when the computer happens to be down but the modem is still working.
For the manual case, the "RING" message is sent to getty but since the
computer is down, getty isn't there and the phone never gets answered.
@ -2852,15 +2851,117 @@ much difference, although it may be frustrating waiting for a login
prompt that never arrives. <tt/mgetty/ uses manual answer. <tt/Uugetty/
can do this too using a configuration script.
<sect1> Ending a Dial-in Call
<sect2> Caller logs out
<p> When the call is over the normal way to end the connection is for
the user to log out. This will kill the user's shell and normally
will result in a hangup signal sent from the host's serial port (DTR
is negated) to the modem. This will happen if stty -a shows hupcl
(hang up on close) but this should be the default. The modem getting
the negated DTR signal will then will then hang up the phone line
(provided the modem has been configured to do this --see below). The
modem should then be ready to answer any new incoming calls. Killing
the user's shell causes getty to respawn to wait for the next call.
Besides using DTR to tell the modem to hang up the phone line, a
program (like mgetty) may send the unique escape code sequence to the
modem to put it into AT command mode. This is +++ with both an
initial and final time delay. Once in AT command mode, a hangup
command (H0) may be sent to the modem as well as other AT commands.
<sect2> When DTR drops (is negated)
<p>When DTR is negated (the "hang-up" signal), what the modem does
depends on the value of the &amp;D option in the modem's profile. If
it's &amp;D0 nothing at all happens (the modem ignores the negation of
DTR). In this section it's assumed that &amp;Q6 is set (in antique
modems it may not be).
&amp;D2: The modem will hang up and go into AT command mode (off-line)
to wait for the next call. Except that it will not automatically
answer the phone (if it should) until DTR is asserted again. But
since getty is set to respawn (in /etc/inittab) then getty will
immediately restart after a logout and this will assert DTR. So what
happens when someone logs out is that DTR only is negated for a
fraction of a second (winks) before it gets asserted again. The DTR
must be negated for at least the time specified by register S25.
&amp;D3: In this case the modem does a hard reset: It hangs up and
restores the saved profile . It should be now in the same state it
was in when first powered on and it's ready to answer incoming calls.
The S25 limit has no effect so even a very short "wink" is detected.
Thus this is a stronger "reset" than &amp;D2 which doesn't restore the
saved profile and requires a longer wink to work.
If you don't know which of the above two to use try using &amp;D3
first. Under favorable conditions, either one should work OK. If the
PC fails to successfully signal the modem when a logout happens, then
the modem is apt to remain in on-line mode and no incoming calls can
be received.
<sect2> Caller hangs up
<p> Instead of logging out the normal way a caller may simply hang up.
This results in a lost connection of course a loss of carrier. Other
problem could also cause a loss of carrier. As a result, the NO
CARRIER result is displayed (if this display is enabled). The modem
hangs up and waits for the next call. Except that there is no getty
running yet to start the login process.
Here's how getty gets started again: The loss of carrier should
negate the DCD signal sent by the modem to the serial port (provided
&amp;C1 has been set). When the the PC's serial port gets the negated
DCD signal it should kill the shell and then getty should respawn.
This paragraph is about other things that happen but do nothing. Only
the curious need read it. When the shell is killed a DTR wink is sent
to the modem but since the modem is not on-line anymore and has
already hung up, the modem ignores the negation of DTR (hang up). The
loss of carrier also negates the DSR signal sent by the modem to the
serial port (provided &amp;S1 or &amp;S2 is set) but this signal is
ignored (by Linux).
As an alternative to the above, getty might could watch for a "NO
CARRIER" message sent from the modem and take appropriate action.
Does it ??
<sect1> Dial-in Modem Configuration
<p> The getty programs have a provision for sending an init string to
the modem to configure it. But you may need to edit it. Another
method is to save a suitable init string inside the modem (see
<ref id="init_str" name="Init Strings: Saving and Recalling"> for how
to save it in the modem).
The configuration for dial-in depends both on the getty you use and
possibly on your modem. If you can't find suggested configurations in
your getty documentation here are some hints using Hayes AT commands:
<itemize>
<item>&amp;C1 Make the DCD line to the serial port track the actual
state of the carrier (DCD asserted only when there's carrier).
Getty_em requires &amp;C0 (DCD always asserted)
<item>&amp;D3 Do a hard reset of the modem when someone logs out
(or hangs up). For some modems it's reported that &amp;D2 is
required since they can't tolerate a hard reset ??
<item>&amp;K3 Use hardware flow control
<item>V1 Display results (such as CONNECT) in words (and not in code)
<item>Q0 Echo results words (such as CONNECT). Most gettys use them. But
it's reported an AT&amp;T version of uugetty requires Q2 (no result
words for dial-in)
<item>S0=? mgetty suggests S0=0 (manual answer). For other gettys if you
want auto-answer S0=4 will auto-answer on the 4th ring, etc.
<item>E0 Don't echo AT commands back to the serial port. mgetty suggest
E1 (echo AT commands) for some modems. For dial-out you want E1 so you
can see what was sent.
</itemize>
<sect1> Callback
<p> Callback is where someone first dials in to your modem. Then, you get a
little info from the caller and then call it right back. Why would
you want to do this? One reason is to save on telephone bills if you
can call the caller cheaper than the caller can call you. Another is
to make sure that the caller really is who it claims to be. If a
caller calls you and claims to be calling from its usual phone number,
then one way to verify this is to actually place a new call to that
number.
<p> Callback is where someone first dials in to your modem. Then, you
get a little info from the caller and then call it right back. Why
would you want to do this? One reason is to save on telephone bills
if you can call the caller cheaper than the caller can call you.
Another is to make sure that the caller really is who it claims to be.
If a caller calls you and claims to be calling from its usual phone
number, then one way to verify this is to actually place a new call to
that number.
There's a program for Linux called "callback" that works with mgetty.
It's at <url url="ftp://ftp.icce.rug.nl/pub/unix/">. Step-by-step
@ -2897,8 +2998,8 @@ who is familiar with vgetty please let me know its current status.
<sect> Uugetty for Dial-In (from the old Serial-HOWTO) <label id="uugetty_">
<p> Be aware that you could use mgetty as a (better?) alternative to
<tt/uugetty/. <tt/mgetty/ is newer and more popular than uugetty.
See <ref id="getty_" name="What is getty?"> for a brief comparison of
these 2 gettys.
See <ref id="getty_" name="Getty"> for a brief comparison of these 2
gettys.
<sect1>Installing getty_ps
<p> Since uugetty is part of getty_ps you'll first have to install
@ -3067,13 +3168,18 @@ ringback feature.
incorrectly calls it speed. For all modern modems you have no choice
of the speed that the modem uses on the telephone line since it will
automatically choose the highest possible speed that is feasible under
the circumstances. But you do have a choice as to what speed will be
used between your modem and your computer (PC-to-modem speed). This
is sometimes called "DTE speed" where "DTE" stands for Data Terminal
Equipment (Your computer is a DTE.) You need to set this speed high
enough so this part of the signal path will not be a bottleneck. The
setting for the DTE speed is the maximum speed of this link. Most of
the time it should operate at lower speeds.
the circumstances. If one modem is slower than the other, then the
faster modem will operate at the slower ones speed. On a noisy line,
this speed will drop still lower.
While the above speeds are selected automatically by the modems you do
have a choice as to what speed will be used between your modem and
your computer (PC-to-modem speed). This is sometimes called "DTE
speed" where "DTE" stands for Data Terminal Equipment (Your computer
is a DTE.) You need to set this speed high enough so this part of the
signal path will not be a bottleneck. The setting for the DTE speed
is the maximum speed of this link. Most of the time it should operate
at lower speeds.
For an external modem, DTE speed is the speed (in bits/sec) of the
flow over the cable between you modem and PC. For an internal modem,
@ -3200,17 +3306,25 @@ is:
<item> 33.6k (V.34): use 115.2 kbps
<item> 14400 bps (V.32bis), with V.42bis data compression: use 57600 bps
<item> 9600 bps (V.32), with V.42bis data compression: use 38400 bps
<item> slower than a 9600 bps (V.32) modem: set your speed to the
highest speed your modem supports.
<item> slower than a 9600 bps (V.32) modem: Set the speed to the
same speed as the modem.
</itemize>
<sect>Communications Programs And Utilities<label id="comms">
<p> While PPP is used for Internet access, there are various dialer
programs that dial a phone number, possibly login, and then start PPP.
Dialer programs include wvdial, chap scripts, kppp, and gnome-ppp.
There are various dialers which can be For dialing out to public
libraries, bulletin boards, etc. <tt/minicom/ is the most popular
followed by <tt/Seyon/ (X-Windows only) and <tt/Kermit/.
<p> While PPP is used for Internet access you also need a dialer
program that is designed to work with PPP. Such a dialer program will
dial a phone number. When the other side answers the phone then three
things happen: PPP is started at both ends and you get logged in
(often automatically). The exact sequence of these 3 events may vary.
Dialer programs for ppp include wvdial, chap scripts, kppp, and
gnome-ppp.
There are also other dialer programs which can dial out directly (thru
a modem) to public libraries, bulletin boards, etc. This isn't the
Internet. <tt/minicom/ is the most popular followed by <tt/Seyon/
(X-Windows only) and <tt/Kermit/. People have likely also used these
programs for dialing out with ppp for the Internet but it's not what
they were originally designed for.
<sect1> Minicom vs. Kermit
<p> Minicom is only a communications program while Kermit is both a
@ -3230,7 +3344,8 @@ Although all Minicom documentation is free, it's not as extensive as
Kermit's. Since permission is required to include Kermit in a
commercial distribution, and since the documentation is not entirely
free, some distributions don't include Kermit. In my opinion it's
easier to set up Minicom and there is less to learn.
easier to set up Minicom, there is less to learn, and you can still
use kermit from within Minicom.
<sect1> List of Communication Software
<p> Here is a list of some communication software you can choose from,
@ -3248,8 +3363,8 @@ Are the least popular ones obsolete?
<sect2> Most Popular Dialout
<p> <itemize>
<item> ppp dialers for getting on the internet: <tt/chat/, <tt/wvdial/
<item><tt/minicom/ - <tt/telix/-like communications program. Supports
scripts, zmodem, kermit
<item><tt/minicom/ - <tt/telix/-like communications program. Can work
with scripts, zmodem, kermit
<item><url url="http://www.columbia.edu/kermit/" name="C-Kermit"> -
portable, scriptable, serial and TCP/IP communications including file
transfer, character-set translation, and zmodem support
@ -3474,16 +3589,19 @@ Make sure you are using the correct syntax for your version of
syntax in the <tt>/etc/inittab</tt> file. Make sure you are using the
correct syntax for your version of <tt/getty/.
<sect1>I Keep Getting: ``Id "S3" respawning too fast: disabled for 5 minutes''
<p> Id "S3" is just an example. In this case look on the line which
starts with "S3" in <tt>/etc/inittab</tt>. This is causing the problem.
Make sure the syntax for this line is correct and that the device
(ttyS3) exists and can be found.
starts with "S3" in <tt>/etc/inittab</tt> and calls getty. This line
causes the problem. Make sure the syntax for this line is correct and
that the device (ttyS3) exists and can be found. If the modem has
negated DCD and getty opens the port, you'll get this error message
since negated DCD will kill getty. Then getty will respawn only to be
killed again, etc. Thus it respawns over and over (too fast). It
seems that if the cable to the modem is disconnected or you have the
wrong serial port, it's just like DCD is negated. All this can occur
when your modem is chatting with getty. Make sure your modem is
configured correctly. Look at AT commands <tt/E/ and <tt/Q/.
Make sure your modem is configured correctly. Look at registers
<tt/E/ and <tt/Q/.
This can occur when your modem is chatting with <tt/getty/.
<p>
If you use uugetty, verify that your <tt>/etc/gettydefs</tt> syntax is
correct by doing the following:
@ -3491,21 +3609,19 @@ correct by doing the following:
linux# getty -c /etc/gettydefs
</verb></tscreen>
<p>
This can also happen when the <tt/uugetty/ initialization is failing.
See section <REF id="nowork" name="uugetty Still Doesn't Work">.
See section <ref id="uu_nowork" name="uugetty Still Doesn't Work">.
<sect1>My Modem is Hosed after Someone Hangs Up, or uugetty doesn't respawn
<p>
This can happen when your modem doesn't reset when DTR is dropped.
Greg Hankins saw his RD and SD LEDs go crazy when this happened.
You need to have your modem reset. Most Hayes compatible modems
do this with <tt/&amp;D3/, but on his USR Courier, he had to set
<tt/&amp;D2/ and <tt/S13=1/. Check your modem manual (if you have
one).
You need to have your modem reset. Most Hayes compatible modems do
this with <tt/&amp;D3/, but for USR Courier, SupraFax, and other
modems, you must set <tt/&amp;D2/ (and <tt/S13=1/ for USR Courier).
Check your modem manual (if you have one).
<sect1> uugetty Still Doesn't Work <label id="nowork">
<sect1> uugetty Still Doesn't Work <label id="uu_nowork">
<p>
There is a <tt/DEBUG/ option that comes with <tt/getty_ps/. Edit your
config file
@ -3905,7 +4021,7 @@ may be found on the Internet.
<p> <itemize>
<item>Cable-Modem mini-howto
<item>ISDN Howto (not a LDP Howto) <url
url="http://www.suse.de/Support/sdb_e/isdn.html">: drivers for ISDN
url="http://sdb.suse.de/sdb/en/html/isdn.html">: drivers for ISDN
"Modems". Much related info on this is in German.
<item>Linux-Modem-Sharing mini-howto. Computers on a network share
a single modem for dial-out (like a shared printer).
@ -4046,11 +4162,12 @@ actually didn't.
To avoid this difficulty one may simultaneous change only the phase
and amplitude (with no change in frequency). This is called
phase-amplitude modulation (sometimes also called quadrature amplitude
modulation = QAM). This method is used today for the common modem
speeds of 14.4k, 28.8k, and 33.6k. The only significant case where
this modulation method is not used today is for 56k modems. But even
56k modems exclusively use QAM (phase-amplitude modulation) in the
phase-amplitude modulation. It is also called quadrature amplitude
modulation (= QAM) since there were only 4 possible phases in early
versions of it. This method is used today for the common modem speeds
of 14.4k, 28.8k, and 33.6k. The only significant case where this
modulation method is not used today is for 56k modems. But even 56k
modems exclusively use QAM (phase-amplitude modulation) in the
direction from your PC out the telephone line. Sometimes even the
other direction will also fall back to QAM when line conditions are
not good enough. Thus QAM (phase-amplitude modulation) still remains
@ -4143,10 +4260,11 @@ Two such circuits are used for a phone call, one in each direction.
The 56k signal is only used in one of these directions: from your ISP
to your PC. The other direction, from your home/office to the ISP,
uses the conventional phase-amplitude modulation scheme with a maximum
of 36.6kbps (and not 53.3kbps). Yet due to sophisticated cancellation
methods it's able to send simultaneously in both directions over the
analog portion of the telephone line as explained in the next
subsection.
of 36.6kbps (and not 53.3kbps). The analog portion of the circuit
from your home/office to the nearest telephone Co. office was never
intended to be bi-directional since it's only a single twisted pair.
But due to sophisticated cancellation methods it's able to convey data
simultaneously in both directions as explained in the next subsection.
<sect1> Full Duplex on One Circuit
<p> Modern modems are able to both send and receive signals
@ -4165,15 +4283,15 @@ the digital part of the phone system then bidirectional communication
(sending and receiving at the same time) would be no problem because
two channels would be available.
But the end portions of the signal path go over just one circuit. How
But the end portion of the signal path goes over just one circuit. How
can there be two-way communication on it simultaneously? It works
something like this. Suppose your modem is receiving a signal from
the other modem and is not transmitting. Then there's no problem.
But if it were to start transmitting (with the other received signal
still flowing into the modem) it would drown out the received signal.
If the transmitted signal was a "solid" voltage wave applied to the
end of the line then there is no way any received signal could be
present at that point.
But if your modem were to start transmitting (with the other received
signal still flowing into your modem) it would drown out the received
signal. If the transmitted signal was a "solid" voltage wave applied
to the end of the line then there is no way any received signal could
be present at that point.
But the transmitter has "internal impedance" and the transmitted
signal applied to the end of the line is not solid (or strong enough)
@ -4344,7 +4462,7 @@ next 3 sections: ISDN, DSL and 56k, concern digital-to-digital
<p> The "modem" is really a Terminal Adapter (TA). A Debian package
"isdnutils" is available. There is a ISDN Howto in German with an
English translation: <url
url="http://www.suse.de/Support/sdb_e/isdn.html">. It's put out by
url="http://sdb.suse.de/sdb/en/html/isdn.html">. It's put out by
the SuSE distribution of Linux and likely is about drivers available
in that distribution. There is an isdn4linux package and a newsgroup:
de.alt.comm.isdn4linux. Many of the postings are in German. You
@ -4394,7 +4512,7 @@ line modems are incompatible with other brands.
<p> Here's some info on the bloated bandwidth required for standard
fax including the dot density. You can of course send a fax via your
modem if you dial the real telephone number of the recipinet.
modem if you dial the real telephone number of the recipient.
<tscreen><verb>
A4 paper: 216mm (horizontal) * 297mm (vertical)
@ -4408,6 +4526,91 @@ paper using fine mode is (216*8) * (297*7.7) = about 4 million dots.
With a compression ratio of 8:1 it takes about 50 seconds at 9600bps
for transmission.
<sect> Appendix G: Antique Modems
<p> This section is about modems with modem-to-modem speeds of 9600
bps or less. Around 1980 many modems only had a speed of 300 bps
(which was also 300 baud). This is only 0.3kbps. Modern modems are
over 100 times faster. Many old-slow modems are still in use so they
are not really "antique" quite yet. This section compares the antique
modems with the modern ones. You should read it if you are interested
in modem history or are intending to actually use an antique modem.
<sect1> Autobauding <label id="autobaud_">
<sect2> Various meanings
<p>This term has a few different meanings. In general it means the
automatic adjustment of modem-to-modem speed or modem-to-serial_port
speed.
<sect2> Modem-to-modem speed
<p>Modern modems negotiate the modem-to-modem speed and protocol when
they first connect to each other. If one side can't negotiate, the
other side should accept whatever speed and protocol that the antique
modem has available. Sometimes this is called "autobauding". When
both modems automatically lower their speed due to a noisy line it's
called "fallback". Thus users (or computer programs in your PC) don't
need to deal with this (unless the S37 register has been set so as to
disable autobauding).
But many old modems didn't have such autobauding (although many had
fallback). If you have such a modem, it will likely work OK if the
other modem you connect to is a modern one that can adjust it's speed
and protocol to yours. But a problem arises if both modems which want
to communicate with each other are both antique and don't support
autobauding. How was this done?
In olden days, a computer dial-in site might have a number of phone
lines, each of which would have a specific speed modem on it. For
example, if you had a 1200 baud modem then you simply only dialed in
to certain telephone numbers that supported that speed. Once a site
obtained modems that could support various speeds on the same modem
and automatically detect the callers speed (do autobauding) then
people could call in using modems that didn't do this autobauding
(providing that their speed and protocol was supported).
<sect2> Modem-to-serial_port speed
<p> When a modem modem is sent an init string (or a dial command),
the modem detects the speed of the serial port and sets it's
modem-to-serial_port speed to this value. It does this by sensing the
speed of the "AT" at the beginning of the string.
Old modems couldn't do this and one would need to set the computer's
serial port speed (with stty or the like) to the same exact value as
the modem-to-modem speed (such as 1200 bps). If the modem had a
choice of speeds one could use the AT command F to select one. But
for dial-in when there was a choice of speeds (via autobauding), if a
connection was made at say 2400 baud, then the modem-to-serial_port
speed would change to 2400 baud. Then one would need to start getty
at 2400 baud. Thus getty needed to adjust to whatever speed was
coming from the modem. Today it's the other way around: the modem
adjusts its modem-to-serial_port speed to whatever the serial port is
using.
The original getty program configuration file /etc/gettydefs has an
optional autobauding feature (see "next-label" in the manual) which
will change the speed of the serial port. The agetty program has a
similar baud rate detection feature. This was used to adjust the speed
of the serial port to the modem-to-modem speed of an incoming dial-in
call.
Modern modems can use almost any serial port speed. To do this they
employ speed buffering and flow control. Speed buffering means that
modems have buffers so that there can be a difference between the
modem-to-modem speed and the modem-to-serial_port speed. If the flow
entering the modem is faster than the flow exiting it, the excess flow
is simply stored in a buffer in the modem. Then to prevent the buffer
from overflowing, the modem sends a flow control signal to stop the
input flow to the modem. See <ref id="flow_control" name="Flow
Control"> for more details.
<sect1> Before AT Commands
<p> Hayes introduced the AT command set and other modem manufacturers
adopted it as a standard. Before the AT commands, many modems used
dip switches to configure the modem. Another command set is the CCITT
V.25bis command set. Some modems supported both CCITT and AT
commands. The CCITT V.25bis also specifies how Synchronous
modem-to-serial_port communication is to take place using either the
ASCII or 8-bit EBCDIC character sets.
END OF Modem-HOWTO
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