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Frequency Modulation (FM)
Frequency Modulation (FM), also known as Frequency Shift Keying (FSK), is the sole technique used in low-speed Hayes-compatible modems. FSK involves the modulation of the frequency of the analog sine wave (Figure 7.2). Using a single bit FM technique, 1 bits are transmitted as relatively low frequency signals, and 0 bits as high frequency signals. Again, the benefits of dibit transmission can be realized by defining four levels of frequency, with each sine wave representing a 2-bit pattern (00, 01, 10, 11).
Figure 7.2 Frequency Modulation.
Phase Modulation (PM)
Phase Modulation (PM), or Phase Shift Keying (PSK), involves the carefully synchronized shifting of the position of the sine wave (Figure 7.3). Using a single-bit technique, the continuous sine wave pattern is interrupted and restarted at the baseline to indicate a change in value (e.g., from a 1 bit to a 0 bit). Once again, the advantages of dibit transmission can be achieved by defining four degrees of phase shift. Through the definition of eight degrees of phase shift, contemporary modems can affect tribit transmission, achieving 3 bits of data per signal.
Figure 7.3 Phase Modulation.
AM and PSK
AM and PSK often are used in combination. The use of multiple levels of amplitude and phase shift yield transmission rates of as much as 28.8 Kbps over voice-grade analog circuits, using conventional high-speed modems. For example, Quadrature Amplitude Modulation (QAM), combines differential phase and amplitude shifts to achieve 16 distinct states; thereby four bits can be represented with a single signal. At 2400 baud, a transmission rate of 9600 bps can be achieved. The ITU-TS V.29 recommendation specifies two possible amplitude values for each of eight phase angles.
Conventional modems can be external or internal. They can also be in the form of a PCMCIA (Personal Computer Memory Card Industry Association) card, which fits into a slot on a laptop computer. Multiplexing modems perform some of the functions of a multiplexer, but at a more limited level. Multiple data streams can be transmitted over a single circuit. Voice/data modems allow voice and data transmission over the same circuit, with data being transmitted during lulls in the voice conversation. Data rates up to 28.8 Kbps can be achieved, although the quality of the voice signal can suffer considerably with such a high data rate coexisting on the same circuit.
Conventional modems can be characterized along a number of dimensions including asynchronous versus synchronous, compression, diagnostics, equalization, gain control and band limitation.
Asynchronous versus Synchronous
Modems can be either asynchronous or synchronous. Asynchronous modems transmit a character at a time, with the receiving device relying on start and stop bits to separate transmitted characters. Synchronous modems are much faster, as the signal is synchronized (timed) by a transmit clock (TC) in either the transmit modem or the transmit terminal. The paired modems synchronize on that clocking pulse, in order to distinguish between blocks of data being transmitted, rather than identifying each individual character in a transmission. When large amounts of data are being transmitted, synchronous modems increase the efficiency of data transfer, resulting in increased speed of transfer and lower associated transmission cost [7-3].
Diagnostics
Diagnostics is a characteristic of higher-speed and more expensive modems. Such modems can test their internal clock, transmitter, and receive circuits. Additionally, they may have the capability to monitor their performance and even diagnose certain conditions contributing to performance degradation. Further, they are manageable through higher-level Element Management Systems, which typically are located remotely and which manage large numbers of modems and modem pools (groups of modems to which access is shared amongst multiple users).
Error Correction
Error Correction capabilities are included in some modems. Among the first of which was a modem protocol known as MNP (Microcom Networking Protocol). While proprietary error correction software still is embedded in certain modems, the ITU-T V.42 modems have standardized this function.
Compression
Compression is a characteristic of high-speed modems, allowing the transmission of multiple bits with a single signal (sine wave) or change in signal. Compression makes use of digital shorthand to represent a large number of bits in a specific sequence through a much lesser set of analog signals or signal changes in a specific sequence. Compression rates of 4:1 are routine in many contemporary modems. In addition to numerous proprietary compression schemes, there are standard techniques such as those embedded in modems designed in compliance with ITU-T V.42bis recommendations. The term bis is from Latin, meaning second; in other words, the second and enhanced release of the standard. Third releases are designated ter, translated from Latin as third.
Equalizers
Equalizers compensate for channel distortion, thereby improving transmission rate and error performance.
AGC (Automatic Gain Control)
AGC (Automatic Gain Control) amplifiers are included to adjust for amplitude variations and to ensure that the incoming signal is of a constant strength.
Band-Limiting
Band-Limiting filters improve error performance by managing the frequencies of the incoming signal, filtering out any extraneous frequencies.
The reverse conversion of analog to digital is necessary in situations where it is advantageous to send analog information across a digital circuit. Certainly, this is often the case in carrier networks, where huge volumes of analog voice are digitized and sent across high capacity, digital circuits. This requirement also exists where high capacity digital circuits connect premise-based, analog PBX or KTS voice systems to Central Office Exchanges or to other PBXs or KTSs.
The device which accomplishes the A-to-D conversion is known as a codec. Codecs code an analog input into a digital (data) format on the transmit side of the connection, reversing the process, or decoding the information on the receive side, in order to reconstitute the analog signal. Codecs are widely used to convert analog voice and video to digital format, and to reverse the process on the receiving end.
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