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

tend to be abbreviated and formal. Typically they are used, not by
the eventual customer for the information, but by professional
intermediaries--librarians and the like-- who have undertaken special
courses. Originally on-line hosts were accessed by dumb terminals,
usually teletypewriters like the Texas Whisperwriter portable with
built-in acoustic modem, rather than by VDUs. Today the trend is to
use 'front-end' intelligent software on an IBM PC which allows the
naive user to pose his/her questions informally while offline; the
software then redefines the information request into the formal
language of the on-line host (the user does not witness this process)
and then goes on-line via an auto-dial modem to extract the
information as swiftly and efficiently as possible.
On-line services require the use of a whole series of passwords:
the usual NUI and NUA for PSS (see chapter 7), another to reach the
host, yet another for the specific information service required.
Charges are either for connect-time or per record retrieved, or
sometimes a combination.
The categories of on-line service include bibliographic, which
merely indexes the existence of an article or book--you must then
find a physical copy to read; and source, which contains the article
or extract thereof. Full-text services not only contain the complete
article or book but will, if required, search the entire text (as
opposed to mere keywords) to locate the desired information. An
example of this is LEXIS, a vast legal database which contains nearly
all important US and English law judgements, as well as statutes.

News Services

The vast majority of news services, even today, are not, in the
strictest sense, computer-based, although computers play an important
role in assembling the information and, depending on the nature of
the newspaper or radio or tv station receiving it, its subsequent
handling.
The world's big press agencies--United Press, Associated Press,
Reuters, Agence France Presse, TASS, Xinhua, PAP, VoA -- use telex
techniques to broadcast their stories. Permanent leased telegraphy
lines exist between agencies and customers, and the technology is
pure telex: the 5-bit Baudot code (rather than ASCII) is adopted,
giving capital letters only, and 'mark' and space' are sent by
changing voltage conditions on the line rather than audio tones.
Speeds are 50 or 75 baud.
The user cannot interrogate the agency in any way. The stories
come in a single stream which is collected on rolls of paper and then
used as per the contract between agency and subscriber. To hack a
news agency line you will need to get physically near the appropriate
leased line, tap in by means of an inductive loop, and convert the
changing voltage levels (+80 volts on the line) into something your
RS232C port can handle. You will then need software to translate the
Baudot code into the ASCII which your computer can handle internally,
and display on screen or print to a file. The Baudot code is given in