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In an effort to distinguish theirs from others on the market, modem manufacturers are continually redesigning their products to incorporate the latest standards, enhancing existing features, and adding new ones. Advancements in modulation techniques, error correction, data compression, and diagnostics are among the continuing efforts of modem manufacturers.
The modulation technique has a lot to do with the speed and reliability of data transmission. Modems convey information by exchanging analog symbols, each of which represents multiple bits.
The symbol rate for modems operating over ordinary phone lines is limited to about 3,400 bps. When a modem has data to transmit, a bit sequence is selected from a pool of available symbols to represent that particular sequence. By packing more data bits into one symbol, modems can achieve higher bit rates. For example, the V.34+ specification squeezes up to 9.8 bits per symbol, vs. only 8.4 bits for V.34. Under ideal line conditions, this equates to 33.6K bps for V.34+ modems and 28.8K bps for V.34 modems.
Increasing the size of the symbol pool allows the modem to adjust to a range of noise conditions, which results in an overall higher speed. The V.34+ offers 1,664 symbols, whereas V.34 offers only 960. The higher number of symbols makes it easier for the receiving modem to differentiate data from noise, which also results in more reliable transmission.
Both V.34+ and V.34 modems use adaptive techniques that enable them to learn about the quality of the line and make adjustments. For example, the sending and receiving modems exchange a set of signals to determine the maximum transmission rate on a particular circuit before user data is actually sent. They also compensate for signal loss detected on a line. If the line exhibits signal loss, the modem can guess how a signal is likely to be degraded across a circuit and boost the signal accordingly to offset the impairment.
These techniques do not guarantee higher speeds and error-free transmission. They only optimize performance on a line-by-line basis.
Regardless of manufacturer or standards, the advertised data rate of most modems does not always coincide with the actual data rate. This is because the quality of the connection has a lot to do with the speed of the modem.
If the connection is noisy, for example, a 28.8K-bps modem may have to step down to 24K bps (i.e., fall back) to continue transmitting data. Likewise, a 19.2K-bps modem more frequently operates at 9.6K bps when the line gets too noisy.
Although some modems are able to sense improvements in line quality and automatically step up to higher data rates (i.e., fall forward), consistently noisy lines may mean that the user is not getting the return on investment that was anticipated at the time of purchase. If the 19.2K-bps modem works at only half the data rate most of the time, then the user has paid double the price for what is essentially a 9.6K-bps modem.
Networks and telecommunications carriers often contain disturbances with which modems must deal or, in some cases, overcome. These disturbances include attenuation distortion, envelope delay distortion, phase jitter, impulse noise, background noise, and harmonic distortion all of which negatively affect data transmission. To alleviate the disturbances encountered with transferring data over leased lines (without line conditioning) and dialup lines, most products include an error-correction technique in which a processor puts a bit stream through a series of complex algorithms before data transmission.
The most prominent error-correction technique has been the Microcom Networking Protocol (MNP), which uses the cyclic redundancy check (CRC) method for detecting packet errors, and requests retransmissions when necessary.
Link access procedure B (LAP-B), a similar technique, is a member of the high level data link control (HDLC) protocol family, the error-correcting protocol in X.25 for packet-switched networks. LAP-M is an extension to that standard for modem use and is the core of the International Telecommunications Union (ITU) V.42 error-correcting standard. This standard also supports MNP Classes 1 through 4. Full conformance with the V.42 standard requires that both LAP-M and MNP Classes 1 through 4 are supported by the modem. Virtually all modems currently made by major manufacturers conform with the V.42 standard.
The MNP is divided into nine classes. Only the first four deal with error recovery, which is why only those four are referenced in V.42. The other five classes deal with data compression. The MNP error recovery classes perform the following functions:
With the adoption of the V.42bis recommendation by the ITU in 1988, there is a single data compression standard Lempel-Ziv. This algorithm compresses most data types, including executable programs, graphics, numerics, ASCII text, or binary data streams. Compression ratios of 4:1 can be achieved, although actual throughput gains from data compression depend on the types of data being compressed. Text files are the most likely to yield performance gains, followed by spreadsheet and database files. Executable files are most resistant to compression algorithms because of the random nature of the data.
Most modems perform a series of diagnostic tests to identify internal and transmission line problems. Most modems also offer standard loopback tests, such as local analog, local digital, and remote digital loopback. Once a modem is set in test mode, characters entered on the keyboard are looped back to the screen for verification.
Most modems also include standard calling features such as automatic dial, answer, redial, fall back, and call-progress monitoring. Calling features simplify the chore of establishing and maintaining a communications connection by automating the dialing process. Telephone numbers can be stored in nonvolatile memory.
Other standard modem features commonly offered include fall back and remote operation. Fall back allows a modem to automatically drop, or fall back, to a lower speed in the event of line noise, and then revert to the original transmission speed after line conditions improve. Remote operation, as the name implies, allows users to activate and configure a modem from a remote terminal.
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