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

computer conversations simultaneously or can send several bits of one
computer conversation in parallel, just as though there were a ribbon
cable between the two participating computers. Either way, what
happens is that each binary 0 or binary 1 is given, not an audio
tone, but a radio frequency tone.



Synchronous Protocols

In the asynchronous protocols so far described, transmitting and
receiving computers are kept in step with each other every time a
character is sent, via the 'start' and 'stop' bits. In synchronous
comms, the locking together is done merely at the start of each block
of transmission by the sending of a special code (often SYN). The SYN
code starts a clock (a timed train of pulses) in the receiver and it
is this that ensures that binary 0s and 1s originating at the
transmitter are correctly interpreted by the receiver; clearly, the
displacement of even one binary digit can cause havoc.
A variety of synchronous protocols exist, such as the length of
block sent each time, the form of checking that takes place, the form
of acknowledgement, and so on. A synchronous protocol is not only a
function of the modem, which has to have a suitable clock, but also
of the software and firmware in the computers. Because asynchronous
protocols transmit so many 'extra' bits in order to avoid error,
savings in transmission time under synchronous systems often exceed
20-30%. The disadvantage of synchronous protocols lie in increased
hardware costs.


One other complication exists: most asynchronous protocols use the
ASCII code to define characters. IBM ('Big Blue'), the biggest
enthusiast of synchronous comms, has its own binary code to define
characters. In Appendix IV, you will find an explanation and a
comparison with ASCII.
The hacker, wishing to come to terms with synchronous comms, has
two choices: the more expensive is to purchase a protocol convertor
board. These are principally available for the IBM PC, which has been
increasingly marketed for the 'executive workstation' audience, where
the ability to interface to a company's existing (IBM) mainframe is a
key feature. The alternative is to see whether the target mainframe
has a port on to a packet- switched service; in that event, the
hacker can use ordinary asynchronous equipment and protocols--the
local PAD (Packet Assembler/Disassembler) will carry out the
necessary transformations.

Networks


Which brings us neatly to the world of high-speed digital networks