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Getting started

The 'modem' word comes from "MOdulator-DEModulator", special kind of devices which were designed to transfer data over the telephone lines first. Today there are many types of modems which support many types of communication protocols and speeds. Here is the short guide about modem connection efficiency increasing presented.

AT-commands

There are only few modems which can be configured directly from internal keyboard and display, and much less modems that are not "Hayes-compatible" (comes from the name of this basic command set and first modem developer, Pedro Hayes), so you'll configure your modem using "AT-commands". The AT-prefix must be defined once for the whole command line at the begin. If your modem responces OK or 0 (if it is configured for numerical responces instead of verbal ones), the last instructions you enter was applied. If the modem reports ERROR (4), last entered command line contains some errors. The modem usually stops to apply entered commands at the first error in line.

Any modem which even supports AT-commands should response OK(0) for the AT with no instructions followed. If modem doesn't response, there are possible conflicts in hardware and/or software.

Command Set

Usually every modem is supplied with the modem guide, which contains a few or (sometimes) even a lot of words related to its command set. Moreover, many modems has built-in HELP. That is why I won't describe AT-commands here, but only tell you some important ones:
ATIn - Get some information about modem (n = 0...9). Once ATI7 told me internal help :-/
AT$ - Get an internal help in ZyXEL, USR modems (and probably some others)
AT&Hn - Get an internal help in some GVC modems (and also probably some others)
AT&Vn - Display an active and/or stored modem profile (in the most cases). U.S. Robotics modems use ATI4(ATI5) for this purpose and one strange modem I know used AT*O instead.
AT&Wn - Write current settings to the profile number n. Some modems have only one profile and support only AT&W command with no digits followed, otherwise they're reported ERROR.
ATZn - Recall profile number n.
AT&Fn - Recall factory-defined profile number n.

Needed Settings

There are some settings which are usual for the most hardware and software configurations when operating in the modern communication programs. Some of them are usually set by the modem factory defaults, other you'll need to set by hand. They are:
8 data bits, no parity, 1 stop bit. These settings are written sometimes as 8N1. Less common combination is 7E1 (7 data bits, even parity, 1 stop bit)
DCE-DCE flow control off - this setting probably would be better to keep turned on but the today's software doesn't support it usually
DTE baudrate adjustment off - some data compression protocols (such as v.42bis) can increase line performance up to 4:1, so the DTE baud rate must be higher than DCE rate. Use 57600 or 38400 DTE speed fixed for v.34 (28800) and v.34+, v.evr, v.FC (33600) modems, 38400 for v.32terbo and ZyX modems (19200) and 38400 or 19200 DTE speed for v.32bis modems (14400).
Asynchronous mode - only the special software works in synchronous connection, usual dial-up clients all work in asynchronous connection.

Preferable Settings

There are some settings which are preferable if your hardware/software allows you to use variants. These settings are:
Hardware (RTS/CTS) flow control instead of software (Xon/Xoff) or none
MNP or V42(LAP-M) data link instead of normal or direct link. It is recommended often to noisy lines to disable normal link at all and establish only reliable (with error correction) one
MNP-4 (with no data compression) instead of MNP-5 protocol. The MNP data compression is not intelligent and even decreases connection efficiency when the connected computers are exchanging with compressed data, such as ZIP archives. Sometimes MNP-5 is called MNP COMPRESSED
v.42bis (with data compression) instead of v.42 protocol. The v.42bis data compression is intelligent and automatically turns off when transferring compressed data. Sometimes v.42 and v.42bis are called LAP-M and LAPM COMPRESSED respectively.
DTR off action - modem goes on-hook (disconnects) or reset from defined profile. Some hardware and/or software knows nothing about Escape sequence (+++) and DTR off-on is the one way to end the session. Usual command for the DTR off action "disconnect" - &D2, "modem reset" - &D3
DCD normal operations - the one way for the common software to detect established connection is CD signal up and/or CONNECT message. Usual command for the normal DCD operations is &C1
RTS follows CTS - this option required when the Hardware Flow Control is used. Usually it is defined with &R(&R0) and sometimes &R2
Break option expedited, non-destructive or queued - see your modem guide for details
Timeout before disconnection 5-15 minutes - unfortunately some modems can hang during connection or some software may turn into never-ending cycle with no data transferred. In this case it would be better to break the line after some timeout with no data transmission. If it is possible to set such timeout for your modem, it probably will be the \T command or some S-register.
Handshake timeout time extended to 90 or even 120 seconds - usually it is defined with S7 register (in seconds) and set by default to 45 or 60 seconds.
Carrier loss time before retrain/disconnect extended to 3 seconds - usually it is defined with S10 register (in tenth of seconds) and set by default to 0.7 or 1.4 seconds.

V.34 Specials

The v.34 protocol and its extensions become the most popular for the today's dial-up connections. It has some features, which can be completely used only in some advanced modems which allow you to configure them. For instance, V.34 protocol allows you to use different baud rates, which are 2400, 2743, 3000, 3200 and 3429 Hz. The highest baud rate allows the highest speed. Some telephone lines won't let you establish connection at the baud rate 3429, but will work well at the 2743 for example.

The other feature which can be very useful is the Power Back-off request. If your modem allows to control this option, compare the efficiency of your connection with this one turned on and off. Some PTUs with auto-controlled volume level can play the bad trick with your connection when this option is turned on.

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