High speed modems would include 9600 bps or higher modems and also 2400 bps modems with compression and error control protocols like MNP, V.42 and V.42bis.
Maximizing the Efficiency
To get the most out of a high speed modem, you may need to change the configuration some from the factory settings. Because the compression and error correction features can make the expected throughput somewhat variable, it is best to set the data rate between your Mac and the modem to a higher speed. This is often called “locking the port” at a higher speed. For 9600 bps and faster modems, you would normally set the interface to either 19200 or 38400. In addition, you should use a “Hardware Handshaking” cable so the Mac and the modem can handle the variable throughput. All Macintosh models since the Mac Plus can take advantage of hardware handshaking. The Mac 128, Mac 512K and Mac 512KE cannot use hardware handshake because the serial ports have only one of the two necessary control signals available.
These are the things you need to do to set all this up:
• Set ZTerm to a fast data rate in the Connection settings (38400 for a 9600bps modem).
• Set ZTerm to use Hardware Handshaking in the Connection settings.
• Connect the Mac to the modem with a Hardware Handshake cable (see Appendix).
• Set the modem to use a fixed DTE speed.
• Set the modem to ignore DTR (&D0 ).
• Set the modem to use CTS transmit flow control.
• Set the modem to use RTS receive flow control.
Modem Standards
V.32 is the CCITT (international) standard for modulation (the way a modem converts bits into sounds that can be carried by the voice telephone system) at 4800 and 9600 bps, full-duplex on voice-grade two-wire dial-up and leased telephone lines. It was adopted in 1984.
V.32bis, which was adopted in early 1991, is 100% backward compatible with V.32. In addition, it supports rates of 7200, 12000 and 14400 bps. It also supports a “rapid rate renegotiation” capability, which allows the modems to change speeds up or down in as little as 1/10th of a second, for example, to handle changes in line conditions (V.32 requires full retrain to change speeds, which can take 5 to 10 seconds).
V.34, adopted in 1994, is a modulation standard that supports speeds up to 28800 bps.
V.42 is the CCITT standard for error control in modems. It defines two protocols: the primary protocol, LAPM, on which all future extensions to the standard will be based, and an Alternative protocol, defined in an annex, which provides for backward compatibility with modems that implement MNP classes 2-4. V.42 can be used with any full-duplex modem that uses async-to-sync conversion (e.g. V.22, V.22bis, V.26ter, V.32, V.32bis).
V.42bis is the CCITT standard for data compression in modems. It is based on the Lempel-Ziv-Welch technique, with extensions from IBM, British Telecom, and Hayes, and can provide up to 4-to-1 compression on certain types of data. It is currently standardized for use only in conjunction with v.42 LAPM. V.42bis doesn’t imply the implementation of any particular modulation scheme; specifically, you can’t assume that a V.42bis modem also supports V.32 or V.32bis.
