– Shelly –
If you have Internet access – and you probably do, otherwise you probably wouldn’t be reading this – chances are that you use a modem and a phone line to go online. But what exactly is a modem? How does it work? How do you make sure you get the best performance out of your modem? Read on for the answers to these and more questions!
What is a Modem?
The word modem is derived from MOdulator-DEModulator and that pretty well describes their function. Standard telephone circuits, known as POTS lines for Plain Old Telephone Service, are designed to carry analog information like the sound waves of your voice. That is not compatible with information between computers which is a digital stream composed of ones and zeros. The computers digital signal must be modulated or converted to analog sound waves to travel through the phone lines, and demodulated or converted back into digital pulses that can be understood by the receiving computer.
Modems need to have a set of instructions that tell them how to accomplish various communication tasks. An early pioneering modem manufacturer, Hayes, developed a set of instructions called the Hayes Command Set, which has been modified and expanded as the technology advanced. Other manufacturers adopted these commands so that modems made by different companies would be able to talk to each other. Thus the Hayes set became a de facto standard that is usually referred to as the AT command set because commands are prefaced with AT to let the modem know that an instruction is to follow. A group of instructions is called a Command String.
The original modems operated at the blazing speed 300 data bits per second (bps). Today the fastest analog modems are rated at 56,000 bps, unfortunately the Federal communications Commission (FCC) limits them to a maximum speed of 53 Kbps in order to protect the telephone network from interference that could affect normal voice communication. If you ever see a 56K modems speed reported over 53K, it is reporting the port speed of your computer (DTE), NOT the connection speed (DCE). This can occasionally happen when any modem is initialized, it is in error, your 56K modem will not connect at 115.2K. A modem can be forced to report port speed with an AT command so if you consistently see unrealistically high speeds reported you can correct your command string to fix it, Unfortunately, not all modems use the same command code so consult your modem manual or manufacturers web site.
Modern modems are designed to adapt to phone line and noise conditions, and dynamically increase or decrease in speed as needed. When you sign on and the computer tells you that you connected at e.g. 48,000, don’t worry about reconnecting to get a higher speed, that was just the speed at the moment you connected and it normally changes constantly as conditions dictate.
There are other things to be aware of with the 56K modems that most of us use today. In order to deliver speeds above 28K there can only be one digital conversion between your computer and the telephone companies switch to which it connects. Since there is one in your home (your modem), that means that the phone company must have a digital rather than an analog switch. Unless you are really out in the sticks, nearly all TelCo switches are digital today. There is, unfortunately, another limitation, you must be located within about 3½ miles (5.6 Kilometers) of the TelCo switch, or line loss and noise will make high speeds impossible.
Two different types of 56K modems originally were sold, the X2 modem developed by U.S. Robotics, and the Flex developed by Rockwell and Lucent. The two different types were unable to speak with each other, and it was important to use an Internet service provider (ISP) that used the same type of modem that you had. Eventually saner heads prevailed and the two camps came up with a system that combined the best features of each. This new system was adopted by the International Telecommunication Union (ITU) and is now universal under the ITU V.90 Standard.
There are still different types of V.90 modems. External modems are self contained in their own box and have their own power supply that must be plugged into a power outlet. They are preferred by some users because they are easy to set up, just connect them to an available serial port, or USB port, and they have status lights that can give you information about their operation and performance. They can be moved easily between different computers, and can be turned off when not in use. Internal type modems are expansion cards that plug into either ISA or PCI slots inside the computer. They cost less than comparable external models since they need neither cabinets or power supplies. They, of course, require you to go inside the computer to be installed and can’t have status lights. They are their own serial port and so do not use up one of the ports on your computer that may be needed for something else. Most internal modems today are Plug and Play (PnP) and are recognized and assigned by a PnP operating system. Make sure that your BIOS is NOT configured to set up PnP devices, Windows will handle the PnP setup. They require installation of drivers supplied by the manufacturer on a diskette or CD ROM.
Some internal modems are Winmodems, they off load some functions to the computer CPU that are normally handled by onboard processors on a traditional modem. This obviously uses some of the computational power otherwise available to run applications. Manufacturers are not always up front in identifying winmodems, so look for the fine print on the box. A winmodem will always specify a minimum speed requirement for the computer’s CPU. A winmodem also must be used only with the Windows operating system