How Regular Modems Work

In order to understand modems you need to know how they work. Once you know this, picking one will be easier. Let’s take this subject step-by-step.

 

·        Modem Introduction: A modem is a device that uses frequency shift keying (FSK). This is the method modems use to transmit digital information over phone lines. FSK is a process where each tone (frequency) is used for the different bits. When the originating modem transmits, it sends a signal of 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The receiving modem can transmit back using 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the tones are different, they both can transmit at the same time. This is known as full-duplex.

·        Transmitting speeds: If two modems are connected, the sender sends information to another computer. There is a device inside the modem called a UART (universal asynchronous receiver/transmitter). This device takes the signal and converts it to a bit. It then sends a series of bits, one at a time, to the modem by way of the RS-232 port. This is a port that has a cable connect to it on one end and a modem on the other. The modem receives these bits, one at a time, and converts them to analog. Today, modems are internal so the UART is embedded in the modem. Everything happens internally. Then the analog signal is transmitted out the modem through the phone line to its destination.

·        Making modems faster: The above is the way the first modem was designed. Since then faster modems were created. To make this happen, modem designers had to redesign modems to handle faster speeds. They did this using phase-shift keying (PSK). This didn’t work well, so they changed to quadrature amplitude modulation (QAM). This allowed a lot of information to get stored in 3000 hertz of bandwidth with no problem. The 56K modem was the last modem to work with this scheme. 

·        Point-to-Point Transmission: Today people connect to the Internet by way of an Internet service provider (ISP). They in turn connect you to the Internet. To do this your modem does not send characters as older modems use to do. Instead, new modems send or routes TCP/IP packets to your ISP. This routing is referred to as Point-to-Point Protocol (PPP).