What is a Modem?
Modem is abbreviation for Modulator – De-modulator. Modems are used for data transfer from one computer network to another computer network through telephone lines. The computer network works in digital mode, while analog technology is used for carrying massages across phone lines.
Modulator converts information from digital mode to analog mode at the transmitting end and de-modulator converts the same from analog to digital at receiving end. The process of converting analog signals of one computer network into digital signals of another computer network so they can be processed by a receiving computer is referred to as digitizing.
When an analog facility is used for data communication between two digital devices called Data Terminal Equipment (DTE), modems are used at each end. DTE can be a terminal or a computer.
The modem at the transmitting end converts the digital signal generated by DTE into an analog signal by modulating a carrier. This modem at the receiving end demodulates the carrier and hand over the demodulated digital signal to the DTE.
The transmission medium between the two modems can be dedicated circuit or a switched telephone circuit. If a switched telephone circuit is used, then the modems are connected to the local telephone exchanges. Whenever data transmission is required connection between the modems is established through telephone exchanges.
To begin with the Data Terminal Equipment or DTE (better known as a computer) sends a Ready To Send or RTS signal to the Data Communication Equipment or DCE (better known as a modem). This is sometimes known as a wakeup call and results in the modem sending a Data Carrier Detect or DCD signal to the receiving modem. There then follows a series of signals passed between the two until the communication channel has been established. This process is known as handshaking and helps to explain why, even now, some companies like CompuServe use the symbol of two hands grasping each other to mean being on-line. Of course, after that all it takes is for the second modem to send a Data Set Ready or DSR signal to its computer and wait for the Data Terminal Ready or DTR reply. When that happens the first modem sends a Clear To Send or CTS signal to the computer that started the whole process off and data can then be transmitted. It is as simple as that.
Alternatively, for anyone confused by what the entire Internet industry dubs TLA’s which means Three Letter Acronyms, the following diagram should help.
It only looks confusing. Take a second look and everything will soon become obvious.
By way of completeness, these signals are all sent through different pins in the plug which is why the handbooks for all modems and printers carry a pin diagram somewhere in the section on troubleshooting. They are also standardized after the industry leaders met to agree standards for a whole range of peripheral equipment. The Recommended Standard for cable was number 232 which explains that one technical term probably everybody has heard of: RS 232.
Of course, that still leaves the question of exactly how data is transferred from one computer to another; something that is more of a problem than might first appear mainly because the phone lines are analogue while computers are digital. In simple terms this means a telephone signal is constantly changing. To understand that just think of a sine wave as produced on an oscilloscope. The signal might be constant, but it is constantly changing from positive to negative and back again in a series of smooth curves. Computers, on the other hand, can only understand information when it is presented as a string of binary digits so the idea is to map digital output onto an analogue signal.
Without going into technical details this is done by superimposing different frequencies onto the analogue signal (which then becomes known as the carrier wave). Different frequencies can then represent different groups of binary digits in a process which is known as modulation when it is being transmitted and demodulation when it is decoded at the receiving end. Naturally two way communication is achieved by having a single device being capable of both modulation and demodulation, from which the unit takes its name: the modem.
From this it becomes obvious that the more frequencies that can be superimposed on the carrier wave the faster data can be transmitted. Alternatively, to take a different point of view, the more data there is to be transmitted so the more frequencies are needed.
Unfortunately it is only possible to send a limited number of frequencies at the same time, known as the bandwidth, which means communication takes that much longer as the size of the signals steadily increases. Now that pictures, sound and even video sequences are transmitted over the Internet on a regular basis, and as these all call for massive data files, the amount of available bandwidth is likely to be a problem for some time.
Finally, as the whole process comes down to sending binary digits or bits over a phone line the speed of the system is expressed as Bits Per Second or BPS which is a figure quoted by all the modem manufacturers.
Unfortunately when it comes to data communications there is a lot more involved than just how fast bits can be sent down a phone line. There is also the problem of what those bits mean and how they can be assembled into something intelligible at the far end. Here a whole range of issues need to be addressed and so it might be a good idea to briefly look at the first of these which are the transmission protocols.