The word “modem” is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line.
Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. A typical arrangement is shown below:
Most modem standards are referred to by a code assigned by the Consultative Committee for International Telephony and Telegraphy (CCITT).
The standards fall into three categories:
- Modulation (speed)
- Error correction
- Data compression
1. Modulation (speed) standards
Modulation (or speed) standards involve the rates and ways modems communicate with each other, and how they negotiate the best communication speed they can both use for the connection. These are
common modulation standards:
V.22. The CCITT standard for data transmission at speeds up to 2400 bps.
HST The USR proprietary standard for data transmission at speeds from 9600 to 16800 bps, depending on the model.
The reverse channel is much slower. If you are connecting to another type of modem, the best speed you can expect is 2400 bps.
Hayes V-series The Hayes proprietary standard for data transmission at speeds up to 9600 bps. The reverse channel is much slower. If you are connecting to another type of modem, the best speed you can expect is 2400 bps.
V.32 The CCITT standard for data transmission at speeds up to 9600 bps.
V.32bis The CCITT standard for data transmission at speeds up to 14400 bps.
V.32terbo The AT&T proprietary standard for data transmission at speeds up to 19200 bps.
V.fc The Rockwell chip set proprietary standard for data transmission at speeds up to 28800 bps.
V.34 The CCITT standard for data transmission at speeds up to 28800 bps.
The Lucent and Rockwell proprietary standard for 56K modems.
V.90 The CCITT standard for 56K modems.
2. Error correction standards
Error correction standards provide a way of correcting errors that result from outside interference, such as noise on the phone line. Error correction ensures that data coming out of the receiving modem is exactly the same as data going into the sending modem.
Error correction standards correct only those errors occurring between the two modems. They cannot correct errors occurring between the modem and the computer (a connection which is considerably more reliable when proper cables are used and connections secured).
3. Data compression standards
Data compression standards provide a way of compressing data at the sending modem, transmitting it across the modem link in compressed form, and then expanding it at the receiving modem.
If the data can be compressed, data compression increases the effective throughput. If the data cannot be compressed (for example, if it has already been compressed with a utility such as Stuffit or PKZip), then modems with data compression give little benefit.
The measure of how fast a modem transfers data. The faster the baud rate, the faster the data transfer. If you divide the baud rate by 10, you get a rough estimate of the number of bytes (or characters) transferred per second, and this provides an idea of how long a file transfer will take. For example, a 2400 bps modem can transfer about 240 bytes per second (that is, about 1Kb every four seconds). Therefore, a 25Kb file would take about 100 (25×4) seconds to transfer.
Flow control and handshaking
Most high speed modems can still connect to older, lower speed modems. When they do, they receive information from the compute at high speed, and send the data out at a lower speed. Therefore, they may have to instruct the computer to slow down periodically.
There are two widely recognized standard methods for flow control. They are:
- Software handshaking (using XON/XOFF)
- Hardware handshaking (using RTS/CTS).
Installation of a Dial-up Modem
- Turn System Off and unplug it.
- If your modem is a conventional type that uses jumpers to set the COM port and IRQ number, you need to configure the jumpers according to the instructions in your modem manual. If your modem is plug and play (most newer ones are) then you do not need to configure any jumpers on it.
- Select an open expansion slot (shown above), preferably away from as many other components as possible.
- Unscrew and remove the metal insert on the back of the system case that corresponds to this expansion slot.
- Insert the card into the expansion slot carefully. You may have to rock it back and forth from front to back to get it to go in. Figure below shows a modem card and insertion of the card into the slot.
- Secure the card by screwing it into place.
- Connect one end of the phone cord to the back of the modem (in the “Wall” or “Line” jack) and the other end to the wall socket.
- Included with your new modem should be a CD (or perhaps a floppy diskette) that contains the software the computer needs to recognize and work with the modem. This small utility program is called a device driver. Most computers come with preinstalled device drivers for a selection of the most common modems, but it’s best to work with the software the modem manufacturer provides, as it is specifically created to match your modem. (It also may be more up-to-date than that supplied with your computer’s operating system.)
- When your computer starts up after the new modem has been installed, you should see the Found New Hardware Wizard. Follow the instructions on your screen, and the wizard will configure the computer to work with the new modem.
- Insert the disc into the CD (or floppy) drive on your computer when the wizard prompts you to do so. When the installation is complete, the wizard will ask you to restart your computer.
- If the Found New Hardware Wizard doesn’t start up, Follow the steps given below (For Windows XP)
a) Click Start -> Control Panel
b) Double click on the Printers and Other Hardware icon
c) Double click on Phone and modem Options
d) To install a new modem, click on the Modems tab and press the Add button.
e) Click Next
f) Windows will now detect your modem.
g) Press Finish to complete the installation procedure.
Note: By following the instructions above, you can install the majority of the modems that can be found in the market. However, if for any reason the installation of your modem fails, you will have to consult the manual that came with the modem for further instructions. There is also a possibility that your modem gets installed automatically by your O/S, since Windows XP uses the Plug and Play technology.
11. Set Up Your Modem
a) Click Start, Settings, and Control Panel and select Dial-Up Networking.