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A modem allows digital information to be passed through an analog transmission facility by modulating carrier signals within the network passband. This can be done by using amplitude, frequency, and phase modulation or some combination. The device at which the digital information begins and ends its journey is called a DTE.. The modem is generically referred to as the DCE..

Modems can be characterized by their speed, modulation technique or transmission mode. The four basic transmission modes are:

  • Simplex: able to transmit or receive
  • Half duplex: able to transmit and receive but not simultaneously
  • Full duplex: able to transmit and receive simultaneously over a single loop
  • Full-full duplex: able to transmit and receive simultaneously over two loops

Basic Modem Block Diagram

Most modems transmit data asynchronously. That is they add start and stop bits to each data byte. Some more expensive modems transmit information synchronously.

At one time, AT&Tís Bell Labs dominated the communications industry. As a result, the equipment it created had a tendency to establish de facto standards. Bell modem numbers are still used to classify many types of DCE.

Modulation Techniques

All forms of modulation create sidebands comprised of sum and difference frequencies of the data signal and carrier. It is necessary that the resultant spectrum stay within the bandwidth confines of the transmission channel. In the case of the switched PSTN the bandwidth channel is 300 Hz to 3400 Hz. In spite of this severe limitation, it is possible to up to 56 Kbps though the network.

Newer line technologies such as DSL are rapidly making the conventional 'dial up' or PSTN modem obsolete.

This brings up the subject of bauds. The baud is defined as the shortest signaling element on the loop, and the baud rate is its reciprocal. It may be that the bit rate and baud rates are the same, but in order to achieve high throughput, they are not. The bit rate is inevitably higher than the baud rate. The highest baud rate possible on a standard phone line is about 2400 baud. By encoding 2 bits in every baud, 4800 bps is obtained. By encoding 4 bits in every baud, 9600 bps is obtained, and so on.

The baud is also called a symbol.

SystemVue has a token which can be used to convert bits to symbols.

The bits must be resampled to provide one sample per bit. The Bits to Symbol Token will then groups the bits (a byte) and forms a symbol. The symbol is simply the numeric value of the digital byte. A Polynomial or Xtrnl Token can be used to convert this value to a level more appropreate to the model.

A PN Token can also be used as a symbol source.


Bandwidth Efficiency

One of the main goals of digital modulation is to place as many bits as possible into the smallest bandwidth. The following table is the theoretical limit assuming perfect techniques and no noise.




For Further Research

AN-1298 An Introduction to Digital Modulation by hp