The term “modem” refers to communications equipment that enables the transmission of digital information using audio channels. The term itself refers to the act of “modulation” and “demodulation”. At the sending end, digital information is used to modulate a signal that can be carried over a communications channel. Once the modulated signal arrives at its destination, it is demodulated in order to recover the digital information it is carrying.
Conventional voice-band modems typically operate in the audio band of a telecommunications voice channel. Because of the limited bandwidth available over these voice channels, sophisticated modulation techniques were developed in attempts to drive more and more data through the line. As modem technology evolved, the bandwidth of traditional voice channels was exploited to a theoretical maximum limit. This limit then needed to be overcome.
Digital subscriber line (DSL) is one technology that has been developed in efforts to overcome the bandwidth limitations of voice-band modems. Different DSL technologies use different modulation techniques. The modulation techniques most commonly used for DSL are pulse amplitude modulation (PAM), quadrature amplitude modulation (QAM), carrierless amplitude modulation (CAP), and discrete multi-tone (DMT). PAM modulation varies the amplitude of a base-band signal to convey information to the remote modem. QAM and CAP modulations modulate the phase and amplitude of a carrier signal to convey information to a remote modem. DMT uses QAM modulation to modulate a large number of individual carrier signals. As the data carrying capabilities of a communications line vary, the DMT modulation strategy can vary the amount of data that is carried by each individual QAM modulated carrier. This may be accomplished by changing the modulation density for each individual carrier.
A communications channel may not always be able to carry a maximum amount of data. This is because the communications channel may not be able to propagate a modulated signal with sufficient fidelity so that it can be demodulated at a receiving modem. The condition of a communications channel dictates just how much data can be carried by the channel; and the condition of a channel may actually vary with time.
The condition of a communication channel may be described in terms of noise. Noise comprises both naturally occurring and externally induced radiation that may be coupled into the communications medium. A communication channel may also be characterized in terms of its transfer function. The transfer function describes the attenuation of signals carried on the line as a function of signal frequency. Channel condition may also be defined in terms of an echo characteristic. The echo condition of a channel describes the amount of signal reflected back to the transmitter and is typically defined in terms of a reflection factor versus signal frequency.
In order to maximize data throughput, modern modems monitor the condition of the communications channel and use sophisticated methods to compensate for temporal variations in the communications channel. Through the use of digital signal processing, modems can minimize the effects that noise may impose on the communications channel. Other signal processing techniques allow modems to compensate for any echoed signal that might be received on the line. Still other techniques enable modems to equalize the spectral response of a communications channel. All of these technique may be used to improve the ability of a modem to recover a modulated signal.
Typically, modems engage in a startup sequence that comprises various forms of communications channel characterization. Such startup sequences are executed whenever a communications link is established. These startup sequences are costly in terms of the amount of time that is required to characterize the channel. It goes without saying that while modems are engaged in communications channel characterization, virtually no data can be transmitted or received.
Many types of modems also require significant power because of the amount of energy needed to drive a signal on to the line from a subscriber facility to a central office. Because of this, it becomes advantageous at the systems level to quiesce data channels when they are not needed. This may reduce overall power consumption and heating internal to a modem. By severing a modem connection when it is not needed, telecommunications networks may also experience a reduced risk of crosstalk. From the perspective of any individual subscriber, though, shutting down a modem connection could reduce the data throughput that would otherwise be available. Every time the modem connection needs to be re-established, some amount of delay would be incurred due to execution of a modem startup sequence.
A typical startup sequence comprises a channel characterization phase and a coefficient exchange phase. During channel characterization, the modem may use several techniques to determine the condition of the communications channel. Among these, the modem may monitor channel noise, determine the echo-path-model for the line, and it may determine the spectral response of the channel. In any of these techniques, the modem typically develops a series of coefficients that describe and/or quantify the corresponding line attributes. In the coefficient exchange phase, the modem may need to communicate some or all of the line attribute coefficients that were developed during the characterization phase to the corresponding modem receiving the modulated carrier signal.