7. High Speed Modems
High speed modems today would be 33.6 kbps or 56K. Some of the
following would apply to any modem with a speed of 9600 bps or higher.
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 33.6 bps and faster
modems, you would normally set the interface to either 57600 or
115k. 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.
These are the things you need to do to set all this up:
• Set ZTerm to a fast data rate in the Connection settings (57600 or
higher for a 56K bps 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
Note: CCITT is the former name of the ITU (International
Telecommunications Union).
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.
V.90 is the ITU standard for 56K modulation.