"Hugo Cornwall "The Hacker's handbook"" - читать интересную книгу автора

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
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