should use a cable with hardware handshaking, as shown in the diagram above.
√π See the Serial Ports chapter for more about modem connections
Simplex and Duplex
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  Modems can operate in different modes, depending on the application and the
nature of the link:-
Simplex
Messages are sent in one direction only — commonly used in old teleprinter systems.
Half Duplex
Provides either-way communication over a common circuit in which each device takes it in turn to transmit or receive data. This switching process can waste a lot of time. It’s rarely appears in CCIT standards (see below) but is used in V.29 for a Group 3 fax machine.
Full Duplex
Most CCIT standards (see below) operate in this mode, permitting either device to send a message at any time. In low-speed systems a different carrier frequency is assigned to each modem whilst at high speeds a technique called echo-cancelling is employed.
Full simultaneous communication in both directions requires a secondary carrier on the line. This back channel or secondary channel is usually owned by the recipient of a call and often uses a lower bit rate than the main channel that’s owned by the caller. The RS-232C interface includes duplicate data, handshake and other circuits for a secondary channel.
Modem Speed
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  A fast modem is convenient — and it reduces your phone bill! In most instances
the bit rate or speed used in both directions is identical — typically 1.2, 2.4, 9.6,
14.4 and 28.8 kilobits per second (kbps or kbit/s). For the Internet you’ll need a
modem that runs at 28.8 kbit/s (rarely used by providers) or 14.4 kbit/s.
Many modern modems can operate at 33.6 or even 115 kbit/s. The common rates and protocols defined by the International Telegraph and Telephone Committee (CCITT) are:-
CCIT standard Speed (kbit/s) Notes
V.21 0.3 Incompatible with Bell 103 standard used in USA
V.22 1.2 Incompatible with Bell 212A standard used in USA
V.22bis 2.4
V.26ter 2.4 For noisy telephone lines (rarely used)
V.29 9.6 Group 3 fax protocol (half-duplex)
V.32 9.6
V.32bis 14.4
V.17 14.4 Group 3 fax protocol (half-duplex)
V.34 28.8
V.34bis 33.6
With some modems you simply select the highest speed and they’ll choose their own speed, compression, and error-control (see below) at the start of a session. Some devices can’t determine these parameters until you press certain keys, often repeatedly — such as Return for a carriage return (CR), or Control-C (or †-C) for an end of text (ETX) message.
- Always use the highest speed that both ends can accommodate — if one modem runs
at 14.4 kbit/s and the other 9.6 kbit/s then both must use 9.6 kbit/s.
Fax-modems
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  A fax-modem also acts as a Group 3 fax machine. Although operation is usually at
9.6 kbit/s such machines automatically slow down to 7.2, 4.8 or 2.4 kbit/s to suit
another device or when there’s a poor circuit. They can’t work at over 9.6 kbit/s,
although 14.4 kbit/s can be used with other fax-modems.
Typical speed combinations include:-
Fax Send Fax Receive Data (kbit/s)
9.6 9.6 2.4
9.6 4.8 2.4
14.4 14.4 14.4
14.4 14.4 28.8
Special Systems
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The UK’s Prestel system uses split rates — enquiries are at 75 bit/s and replies at 1.2 kbit/s. Not all modems can be programmed to work this way but some do provide bit rate conversion — essential if a computer or its software can’t use certain data rates.
A Telecommunications Device for the Deaf (TDD) allows anyone with hearing disabilities to communicate via a keyboard. It operates at the rate used by a teletypes (TTY) and Telex machines — 45.5 bit/s in the USA and 50, 110 or 300 bit/s in other countries. To simulate a Baudot terminal you require a special TDD modem and a communications application that can handle 5-bit Baudot code. It only uses upper-case characters for communication.
Modem Settings
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Unless otherwise indicated, you should set your communications application to:-
Parameter Setting
Data bits 8
Parity None
Stop bits Auto or 1
If this doesn’t work try Data Bits = 7 and Parity = Even or Odd.
Data Bits
This setting depends on the type of communication. Communication channels that accommodate binary files (containing any value from 0 to 255) require 8-bits. Other services, such as e-mail, only use pure ASCII files and must use the 7-bit setting.
Parity
Used to correct minor transmission errors — the parity flag indicates whether the value in the data byte contains an odd or even number. If this doesn’t correspond with what appears at the receiver a correction is made. This only works if the byte has a single error — if there are two errors the corrected result is still wrong!
See below for more advanced error correction systems.
Stop Bits
Although 1, 1.5 or 2 stops bits can be used this rarely needs to be adjusted.
Hayes Protocol
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Intelligent modems can be controlled by sending them commands from the computer — they invariably use Hayes protocol. Fortunately you don’t need to understand any of this — your software and modem should speak to each other without you knowing about it!
Each command string sent by computer begins with AT to bring the modem to attention.
This example tells the modem to dial a number:-
ATDT * 70, 777-2424
The instructions are:-
AT Attention
D Dial a number…
T … using tone dialling
* 70 Dial *, 7 and 0 to disable call waiting
, Pause for two seconds
777-2424 Dial the number 777-2424
in which the letter T could be replaced by a P for pulse dialling.
Here’s another example:-
ATDT 9,1(323)444-7487
The instructions are:-
AT Attention
D Dial a number…
T … using tone dialling
9 Dial 9 for outside line
, Pause for two seconds
1(323)6… Dial the number 13234447487, ignoring other characters
If you send a modem this message:
ATA
followed by Return it will switch into readiness for automatic answering.
Hayes modems often need a specific pre-dial init instruction to set them up.
Here’s an example for use in Italy:-
AT&F1S0=0E1M1V1X3W2&
The init itself is contained inside the ampersands (&). The code X3 tells the modem to disregard dialling tone — it’s not used on Italy’s phone system!
Error Correction
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The blocks or packets of data making up a file must be sent correctly. Following an initial connection both modems must agree on the type of error detection. if any, to be used. The following systems are commonly employed:-
MNP4 and V.42
Microcom Networking Protocol Level 4 (MNP4) provides error correction, either in the modem’s hardware or in Mac software — a modem with V.42 error correction also supports MNP4. When using these protocols you must set the modem to 8 data bits, no parity and one stop bit.
Hardware MNP is confirmed when a modem receives a string such as /REL.
Not all devices or software support all versions of MNP — for example, MNP10 is only used for unreliable links over a cellular telephone link. Some devices use LAP-B or LAP-M error correction instead.
Kermit and Others
Common Mac software protocols such as Kermit, XMODEM, YMODEM and ZMODEM include error correction but may be slow to operate in some situations. Kermit and XMODEM are available as communications tools that can be used with any suitable communications application.
√π See the Communications chapter for details about XMODEM and Kermit
Data Compression
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Data compression speeds up data transmission. When using compression you must increase the baud rate (the speed between modem and Mac) by up to to four times the actual modem speed. For example, for a modem operating at 14.4 kbit/s just select a baud rate of 57.6 kbit/s in your application — for 28.8 kbit/s you should choose 115.2 kbit/s.
MNP5 and V.42bis
Microcom Networking Protocol Level 5 (MNP5) compression is invariably incorporated into MNP modems (see above). Modems with V.42bis compression also support the MNP5 version but are up to 40% more efficient in V.42bis mode.
