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

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
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 (view data 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