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On Sun, 12 Mar 1995 4700gbera@um*.cc*.um*.ed* wrote:
> OK, I'll ask another possibly silly question; if two (or more) divers in
> close proximity are using hoseless air-integrated computers, what prevents
> diver A from displaying info from diver B's sending unit, etc? PArticularly
> if both happen to have the same computer? I can easily get my wrist closer
> to a buddy's 1st stage than is his own wrist.
The question is not at all silly, and I wonder how much attention has been
given, or needs to be given, to this problem by the manufacturers of
air-integrated computers. I can think of three solutions to the problem,
the first two probabilistic:
1. Adjust different units so as to broadcast on different
frequencies. [Does anyone know what frequencies, incidentally,
computers use? Do they come under FCC regulations?]
This solution is probabilistic, because at some point one runs out
of frequencies, and has to reassign frequencies.
2a. Assign to each unit a unique, n-bit identifier, where n is a sufficiently
large number. [Global network addresses for schemes like Internet
addressing currently depend on centrally-registered, unique IDs.]
The software in the wrist unit for a particular transmitter
must check the n-bit identifier before accepting incoming data--
and flash an "intruder alert" ("...humanoid countdown :)") warning
if it sees packets destined for another address.
2b. Use an Aloha-based transmission protocol to handle "collisions". A
transmitter should first "listen" for transmissions from other units
before attempting to transmit. If the transmitter detects a collision
while transmitting--and this would require the use of a receiver in
the transmitter--then the transmitter should wait a random period of
time before attempting to retransmit.
Incidentally, the receiver might decode the transmitted message even
in the event of transmission; relative signal strengths would be the
key consderation here, as they are in ham radio.
3. Design the units so that collisions don't matter. Sense all crucial
data in the wrist unit, and transmit only non-crucial data from the
transmitter. Also, transmit a total--rather than an incrementally
updated--representation of the data gathered at the transmitter, to
prevent any problems that would result from data that was either lost,
or captured spuriously. The worst that would then happen is that the
wrist unit might become a little confused about the status of the
non-essential, transmitted data for as long as the buddy is close to
the "wired" diver.
====
Phil Pfeiffer, Computer Sci. Dept. | Kindness in thought leads to wisdom.
East Stroudsburg University, | Kindness in speech leads to eloquence.
East Stroudsburg, Pa. 18301-2999 | Kindness in action leads to love.
phil@es*.ed* (717) 424-3820 | -- Lao-Tsu
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