28. New readers, RF intruders in wireless alarm systems

[-pull@shoot]

Radio Frequency (RF) intruders.

The whole RF spectrum is divided and regulated by international
committees who define the allowances on each frequency.
 The Wireless Alarm Systems have free multi user "shared" frequencies
assigned with no waranty that it is free for communication at any
requested time.
 Keeping in mind that only one operator can transmit on the same
frequency at the same time, a protocol is laid-out to circumvent the
shared frequency restrictions.
 Each user has to respect some transmission rules like:
- Limit the transmit power;
- Each transmission has to have an identifier;
- transmit in a BURST transmission mode,
 to mention the most essentials.

Limited transmission power.
 By limiting the transmission power the signal reception range is
reduced and by consequence the possible mutual interference on that
frequency.

Personal identifier (ID).
 As multiple users uses the same frequency and in order to retrieve
only the information belonging to hiss personal system, each user
identifies themselves during hiss transmission with a unique ID.
Some transmitters uses ID rolling codes, the code changes each time in
order to make sure that at repetition he has a unique ID.
 Realize that sometimes this rolling code may not be unique, the
individual receivers have to keep trace of the temporary intruder ID
in order to retrieve only hiss data.
 This requires multiple transmission/reception attempts before true
validation of the corresponding ID and by consequence causes a
reception DELAY.

Burst transmission.
 Burst transmission is one way to circumvent mutual interference.
 Each transmitter sends hiss data information during a small period of
time hoping that the frequency is unoccupied while he transmits.
 When two or more transmissions occur at the same time, interference
is generated and the receivers are disturbed, they can't decode the
data information.
 This is not a real problem, the transmission is repeated several time
with the hope that at some time during the repetitions the frequency
is free.
 This require multiple transmission/reception attempts before data is
passing true, by consequence this causes a reception DELAY of the data
involved.


 The whole burst transmission protocol has restrictions, the burst
length and repetition rate, the number of possible systems and
different types using the same frequency and more..

Now, back to the header, intruders.
 As described above, the validity of data transmission of all
transmissions using the shared frequency spectrum are depending on the
respect of the burst transmission requirement/principle.
If one, lets call them INTRUDER, uses a different protocol and
transmits without the respect of the burst principles, the whole
frequency utilization goes bingo in the distance range of hiss
transmitter.
 That's the case during Radio Frequency Interference (RFI).

Hope this clarifies a little the wireless alarm system shared
frequency operation fragility.

NOTE: I don't mind if "so called professionals" explain it in a
      better and more detailed way but it never happened...
      Criticism is easy, explaining instead...

Paul