Modem selection considerations

Some of the more important modem features that need to be considered when selecting a modem are as follows:

• Automatic smart features: Modems normally use the Hayes AT command set, which automates most modem features. If compatibility problems arise some of the features can be manually overridden.

• Data rate: This is normally one of the first features chosen. It is important to distinguish between the effective data rate and the baud rate. Note that more reliable performance is obtained at lower data rates because of the correspondingly lower baud rates.

• Asynchronous/synchronous: Higher speed data rates are generally achieved using synchronous transmission, however if the line is noisy then asynchronous transmission can achieve greater throughput.

• Full-duplex/half-duplex: Full-duplex transmission is normally more efficient than half-duplex where the line turnaround time reduces efficiency.

• Modem standards: Most modems support a range of ITU standards such as V.22 bis, which supports 1200 and 2400 bps transmission, V.32 bis which supports 4800/9600 and 14 400 bps transmission, V.34 which supports 28 800 bps and V.90 which supports downloads at 56 kbps. In addition many of the Bell series of modem standards are also supported.

• Data compression: The data compression algorithm is selected by the MNP class, and may be overridden by modem command.

• Error correction/detection: The error detection and correction mechanisms are implicit in the MNP class and the chosen modem standard. These are normally auto-negotiated but may be overridden if problems are occurring.

• Mounting: Modems are available in various forms of mounting such as PC internal, external or rack mounted.

• Power supplies: Modems can have individual power supplies or can be supplied from a common rack supply.

• Testing: Modems have varying degrees of self testing available, and local and remote loopback tests are normally available using AT&Tn commands for localizing faults.

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