SOME BITS ABOUT TELECOMMUNICATIONS WITH MODEMS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ With the huge number of Communications Protocols, Baud rates, brands of modems and computers and claims made by advertisers, the beginner can feel very confused. All he wants to do is get onto a Bulletin Board or a Database. I'll try to clear up a little of that confusion here, and review a couple of packages which should help. OVERVIEW Modems are needed to communicate with your computer over the phone lines, simply because computers emit digital pulses (bits and bytes in bursts) while phone lines transmit in the form of a frequency. This will change as voice telephone networks slowly change to digital transmission, when it should be possible to connect a computer directly into the network - but not yet. Modulator/Demodulator Keeping it simple, modems take the digital output of a computer and changes these signals into an audio frequency, a process known as MODULATION. The reverse process, going the other way, is called DEMODULATION, hence the name MODEM. You can get internal, external or cellular modems. To date, for the Amiga, stand-alone external modems are available, and it is possible to get an ACOUSTIC COUPLER, a simpler type of modem which is a linked pair of insulated acoustic sockets that you push your handset into. These are fairly rare these days as modem technology speeds along. Smart & Dumb As Modems go, there are "dumb" modems and "smart" modems. A dumb one requires that you do everything for it, and will probably handle just one speed or protocol, and is fine if you have specific needs covered by its capabilities - an acoustic coupler is an example of a dumb modem. Smart modems have some fancy frills, such as doing things like calling and answering automatically, and are faster, easier to use and have a bit more style. Some may monitor the signal level and take action to compensate for fading lines, or suspend communications if the line is suspect. A really smart modem can choose between tone or pulse dialling or even mix the two for dialling overseas from a pulse-dial PABX, and it will have an error checking system to put intelligible messages up on the screen if there are problems. The standard by which modems are judged is the Hayes standard, developed as a result of intelligent design and good marketing. There is a set of commands devised by Hayes, each of which begins with the prefix AT, hence you'll often see an ad for a modem claiming "Hayes compatibility" or "AT compatibility". Bauds & Bps Modem speeds are measured in "bauds", which is essentially the same as "bits per second" or bps. When a computer sends a character out through the serial port, it uses the conventional ASCII code of 8 bits for each character or byte, to which are added a "start" bit (to say that a character is coming) and a "stop" bit (to indicate the end of that character byte). There are variations on this, but that's the basic idea. So a 300 bps transmission translates to about 30 characters/sec (say about 5 words), thus taking about a minute to transmit a full A4 page of characters. So from the point of view of saving time on the phone lines, 1200 bps will be more economical. A smart modem will have a selection of baud rates, and if it's really smart will be able to detect the rate of an incoming call and set itself to match. Telecom's Viatel service uses 1200/75 bps: the host Viatel computer downloads to you at 1200 bps, while you communicate with it at 75, about the speed of a fast typist. If you're interested in Viatel, you'll need a modem and corresponding software to handle these rates. There are Viatel packages for the Amiga: SuperTex in the Public Domain, for example. Synchronous/Asynchronous Below 2400 bps, asynchronous communications is used - ie, there is no overall timing control of the communications system. High speed modems are usually synchronous, ie, a crystal-clock timing system keeps both ends in sync to help preserve data integrity. If you want to get really flash, and have a dedicated line, you can move into speeds of 9600 bps and up. At present, however the upper limit for personal computers is 2400, and that is rare still. Standards & Protocols CCITT, an agency of the International Telecommunications Union, has set speed standards, giving each method the letter V prefix. The common ones are: V21 - mostly used for Bulletin Boards, 300 bps each way. V22 - 1200 bps each way. V22 bis - doubles the speed of V22 to 2400 V23 - transmits at 1200 bps, receives at 75 (Videotex services) Naturally, the Americans have their own set of standards called Bell standards, requiring a form of analogue modulation, and incompatible with our standards (just as they have NTSC video signals while we have PAL). Note that it's unnecessary to buy a modem that claims to adhere also to Bell standards - if you're going to call the US, OTC's MIDAS overseas packet-switching network will handle it using the normal CCITT standards. RS-232 About the nearest thing to a universal standard, but with a lot of unstandardised aspects. The RS-232 port on the back of your Amiga can feed (serially, rather than in parallel) your serial printer or modem or numerous other peripherals. The actual physical standard of the connector itself has developed into use of the 25-pin connector known as DB-25. Not all these pins will be connected, and you may find as few as 3, or up to 10 or so. The standard contains both receive and transmission lines and also a number of status lines that advise the computer or the modem of the condition of the other. There is some confusion about the gender of cables at times - the Amiga has a female port, and you may find you need a "gender-bender" if your modem's cable is the wrong sex. Also read carefully the manual for which pins should or should not be connected. For the enthusiast, investment in a "break-out" box will let you cross-wire plugs at will. The average user should make sure that the modem bought comes with the right cabling - demand it from your dealer.  ~~~~~~~~~~~~~~~~~~~~~~~~~~END OF ABOUTMODEMS~~~~~~~~~~~~~~~~~~~~~~~~~~~~