Modem Q&A

Initialization Strings

Q: What is an initialization string?

A: An initialization string is a command sent to a modem that affects its Active Profile.

Q: What is an Active Profile?

A: An Active Profile is a list of settings that the modem is currently using. These settings range from the mundane, like Tone or Pulse dial, to the arcane, like Enable Trellis Encoding.

Q: What's a good Initialization String?

A: Here are a few:

       "ATZ" - equivilant to turning the modem off, and back on again. Reloads the Active Profile from the modem's RAM.

       "AT&F" - (sometimes AT&F1) reinstalls the factory default settings from the modem's ROM and writes it to the modem's RAM. This is a stronger reset than "ATZ." Sometimes a modem's RAM can become scrambled or changed, either on purpose or by accident. "ATZ" will load the profile from this hypothetically misconfigured RAM. "AT&F" will load the profile from the modem's ROM, so the modem will be configured as the manufacturer intended for a default.

Click here for some modem specific init strings.

Communication Port Speeds

Q: What should I set the speed at in my communications program?

A: Most modern modems use what's called Data Compression, either MNP5 or V.42bis. This is a style of compression which takes less time to transmit information over the phone line. The modem handles this automatically. Therefore, it's a good idea to set your speed as high as possible.

Q: More specifically, what speed should I set my modem to?

A: If you have an external modem, you need to check something first.

PC:

       At DOS type "msd" and hit enter.

       You are now in Microsoft Diagnostics. Press "c" for Communications. You should see a list of your comm ports.

       Check the comm port you are using for your modem.

       Check what type of UART you have.

Use these guidelines:

       8250 - the slowest level of UARTs. 19200 is the highest speed you can use. Anything above that will cause data problems.

       16450 - the next level of UARTs, was commonly found in many PC's. 57600 is its upper speed, but sometimes errors occur at this speed. To be safe, we suggest you use 38400.

       16550 - the faster level of UARTs, it supports speeds up to 115200. A multitasking operating system like Windows, OS/2, or UNIX, will usually need a 16550 UART.

       16650 - the fastest level of UARTs, it is similar to the 16550, but with a 32-character FIFO instead of a 16-character FIFO.

MACINTOSH:(Serial Ports)

Old (SE, Classic, etc. . .)	19200
Mac II, 68020 Macs:	38400
68030 Macs:	38400 (some 57600)
68040 Macs Performa 5200-6200 series:	57600
Power Macs:	115200 (some 230400)
Powerbooks:	19200 external (see below for internal modems)

Q: What if I have an internal modem?

A: You should be able to set your port speed for 57600 if it's a 14.4K modem, or 115200 if it's a 28.8K modem. But it's always best to check with the modem manufacturer.

Misc. Information

Q: What is a UART?

A: An Acronym, (stands for) Universal Asynchronous Receiver Transmitter. Handles the transfer of information to and from your serial port.

Q: What is FIFO?

A: Stands for First In First Out. Think of the line at the grocery store. The first person in line gets helped (and thus gets out) first. Same idea. Bigger FIFO buffers hold more and greatly extend the time available for responding to incoming characters.

Q: What's 'v.something'? What does it mean?

A: An international organization, CCITT, now known as ITU created international standards for data communications. All these 'v-dot' things are standards. Here are some:

       v.32 - the standard for 9600 full duplex Data Communications

       v.32bis - the standard for 14.4K Data Comm.

       v.32ter - the standard for 19.2K

       v.34 - the standard for 28.8K

Most of these standards include intermediate speeds as well. That's why you might get a 24K connection with a 28.8K modem. They connect at the highest speed the phone line will allow. Aditionally, the ITU sets standards for other aspects besides data speeds. For example:

       v.42 - a standard for error correction

       v.42bis - a standard for data compression or

       v.17 - a standard for half-duplex 14.4K fax comm.

So you might have a modem that's a V.34 with V.42bis. Confused? If it's a fax/data modem it would support V.34 with V.42bis data communications, and V.17 Group III Class 2 fax.