Monday, March 19, 2001		Article

The screeching before "handshaking"
By Chiranjeev Pal Singh

WHAT makes the Internet possible? Millions of Web sites, thousands of routers or the hundreds of Web servers — without the little hardware box that sits on your computer, cyberspace would be a ghost town, searching for people to populate it. The modem is your link to another computer, be it your neighbour’s PC or one of the millions in the Internet.

The basic function of a modem is to modulate and demodulate signals. It converts digital signals (0,1) that are generated by a computer into analog signals, which pass through a telephone line. This process is called modulation and the reverse is known as demodulation. Once the communication software has been loaded and the modem is switched ‘on’, the software sends a signal to the modem, along the line of serial port to which the modem is connected. This signal alerts the modem that the computer is ready to send data. At this stage, the LED (Light Emitting Diode), in front of the modem, lights up. At the same time, the modem responds to the signal and sends a voltage to the PC. This lets the computer knows that the modem is ready to send or receive data.

Once this initial exchange between the computer and the modem is over, the modem waits for a signal from the computer to dial a particular number, your Internet Service Provider’s, for instance.

To dial a number, the computer sends a signal to your modem, which asks the modem to open a connection with a phone line. The software follows up by giving another command asking the modem to start dialling the designated number and the modem responds to this message.

Next come the most interesting part, and the noisiest one too. A modem makes a noise while hooking up to another modem. Your modem calls out to the remote modem, letting it know you need to connect. This is when you hear a high-pitched screeching tone-which means that the two modems are connected. This marks the beginning of a virtual-handshaking.

Once the communication between both the modem has been established, your modem sends the compute a signal, which informs the communication software in the modem that it is receiving a carrier signal.

Once both modems acknowledge each other’s presence, they begin to exchange information on how to send data, at what speeds, composition of data packets and so on. After the ‘handshaking’ the modem decide on the composition of data packets that will be exchanged between them. It is imperative that both modem use the same speed settings for effective data transmission. If the settings do not match, the modem will simply hang-up and refuse to communicate at all.

When the communication software wants to send data to the remote modem, it communicates to your modem using line of serial port. This is the signal, which asks the modems if it is free to send data or carry out any other instructions, In case the modem is free, it acknowledges the signal by sending a signal to computer using serial port. Now your computer starts pumping in data to the modems for further transmission. If the data transmission rate of the modem is slower than the rate at which the computer is sending it, it drops the signal to tell the computer to stop sending any further data and resumes only after it receives another signal from it.

Meanwhile, the remote modem at the other end goes through a similar drill to prepare for reception of data. The data packets are received by the remote modem in the form of modulated frequencies, which are then demodulated and sent to the remote computer.

When you want to disconnect the call, the communication software signals the modem to stop communication with the other modem and hang up the line. In case, the connection is broken by the remote system, your modem will drop the carrier detect signal to the software and inform it that the connection has been broken.

Various techniques are used for modulation. These include carrier wave modulation, AM, FM, Phase Modulation, Complex Modulation. FM is the technique used by most modems.