Modems

Every account of what a modem is and does begins with the classic
explanation of the derivation of the term: let this be no exception.
Modem is a contraction of modulator-demodulator.
A modem taking instructions from a computer (pin 2 on RS232C)
converts the binary 0's and 1's into specific single tones, according
to which 'standard' is being used. In RS232C/V24, binary 0 (ON)
appears as positive volts and binary 1 (OFF) appears as negative
volts.
The tones are then fed, either acoustically via the telephone
mouth-piece into the telephone line, or electrically, by generating
the electrical equivalent direct onto the line. This is the
modulating process.
In the demodulating stage, the equipment sits on the phone line
listening for occurrences of pre-selected tones (again according to
whichever 'standard' is in operation) and, when it hears one,
delivers a binary 0 or binary 1 in the form of positive or negative
voltage pulses into pin 3 of the computer's serial port.
This explanation holds true for modems operating at up to 1200
baud; above this speed, the modem must be able to originate tones,
and detect them according to phase as well, but since higher-speed
Hacker's Handbook
working is unusual in dial-up ports--the hacker's special interest,
we can leave this matter to one side.
The modem is a relatively simple bit of kit: on the transmit side
it consists of a series of oscillators acting as tone generators, and
on receive has a series of narrow band-pass filters. Designers of
modems must ensure that unwanted tones do not leak into the telephone
line (exchanges and amplifiers used by telephone companies are
sometimes remotely controlled by the injection of specific tones) and
also that, on the receive side, only the distinct tones used for
communications are 'interpreted' into binary 0s or 1s. The other
engineering requirements are that unwanted electrical currents do not
wander down the telephone cable (to the possible risk of phone
company employees) or back into the user's computer.
Until relatively recently, the only UK source of low-speed modems
was British Telecom. The situation is much easier now, but
de-regulation of 'telephone line attachments', which include modems,
is still so recent that the ordinary customer can easily become
confused. Moreover, modems offering exactly the same service can vary
in price by over 300%. Strictly speaking, all modems connected to
the phone line should be officially approved by BT or other
appropriate regulatory authority.
At 300 baud, you have the option of using direct-connect modems
which are hard-wired into the telephone line, an easy enough
exercise, or using an acoustic coupler in which you place the
telephone hand-set. Acoustic couplers are inherently prone to
interference from room-noise, but are useful for quick lash-ups and
portable operation. Many acoustic couplers operate only in
'originate' mode, not in' answer'. Newer commercial direct- connect
modems are cheaper than acoustic couplers.
At higher speeds acoustic coupling is not recommended, though a
75/1200 acoustic coupler produced in association with the Prestel
Micronet service is not too bad, and is now exchanged on the
second-hand market very cheaply indeed.
I prefer modems that have proper status lights--power on, line
seized, transmit and receive indicators. Hackers need to know what is
going on more than most users.
The table below shows all but two of the types of service you are
likely to come across; V-designators are the world-wide 'official'
names given by the CCITT; Bell-designators are the US names:
Service Speed Duplex Transmit Receive Answer
Designator 0 1 0 1
V21 orig 300(*) full 1180 980 1850 1650 -
V21 ans 300(*) full 1850 1650 1180 980 2100
V23 (1) 600 half 1700 1300 1700 1300 2100
V23 (2) 1200 f/h(**) 2100 1300 2100 1300 2100
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V23 back 75 f/h(**) 450 390 450 390 -
Bell 103 orig 300(*) full 1070 1270 2025 2225 -
Bell 103 ans 300(*) full 2025 2225 1070 1270 2225
Bell 202 1200 half 2200 1200 2200 1200 2025
(*)any speed up to 300 baud, can also include 75 and 110 baud
services
(**)service can either be half-duplex at 1200 baud or asymmetrical
full duplex, with 75 baud originate and 1200 baud receive (commonly
used as viewdata user) or 1200 transmit and 75 receive (viewdata
host)
The two exceptions are:
V22 1200 baud full duplex, two wire
Bell 212A The US equivalent
These services use phase modulation as well as tone.
British Telecom markets the UK services under the name of
Datel--details are given in Appendix V.
BT's methods of connecting modems to the line are either to
hard-wire the junction box (the two outer-wires are the ones you
usually need)--a 4-ring plug and associated socket (type 95A) for
most modems, a 5-ring plug and associated socket (type 96A) for
Prestel applications (note that the fifth ring isn't used)--and, for
all new equipment, a modular jack called type 600. The US also has a
modular jack, but of course it is not compatible.
Modern modem design is greatly aided by a wonder chip called the
AMD 7910. This contains nearly all the facilities to modulate and
demodulate the tones associated with the popular speed services, both
in the CCITT and Bell standards. The only omission--not always made
clear in the advertisements--are services using 1200/1200
full-duplex, ie V22 and Bell 212A.
Building a modem is now largely a question of adding a few
peripheral components, some switches and indicator lights, and a box.
In deciding which 'world standard' modem to purchase, hackers should
consider the following features:
Status lights you need to be able to see what is happening on the
line.
Hardware/software switching - cheaper versions merely give you a
switch on the front enabling you to change speeds, originate or
answer mode and CClTT or Bell tones. More expensive ones feature
firmware which allows your computer to send specially formatted
instructions to change speed under program control. However, to make
full use of this facility, you may need to write (or modify) your
terminal emulator.
Auto-dial - a pulse dialler and associated firmware are included in
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some more expensive models. You should ascertain whether the
auto-dialer operates on the telephone system you intend to hook the
modem up to--some of the US 'smart' modems present difficulties
outside the States. You will of course need software in your micro to
address the firmware in the modem --and the software has to be part
of your terminal emulator, otherwise you gain nothing in convenience.
However, with appropriate software, you can get your computer to try
a whole bank of numbers one after the other.
D25 connector - this is the official 'approved' RS232CN24 physical
connection--useful from the point-of-view of easy hook-up. A number
of lower-cost models substitute alternative DIN connectors. You must
be prepared to solder up your own cables to be sure of connecting up
properly.
Documentation I always prefer items to be accompanied by proper
instructions. Since hackers tend to want to use equipment in
unorthodox ways, they should look for good documentation too.
Finally, a word on build-your-own modems. A number of popular
electronics magazines and mail-order houses have offered modem
designs. Such modems are not likely to be approved for direct
connection to the public telephone network. However, most of them
work. If you are uncertain of your kit-constructing skills, though.
remember badly-built modems can be dangerous both to your computer
and to the telephone network.