Re: Purchasing a Home Alarm

[-pull@shoot]

On Sat, 3 Sep 2005 16:41:29 +1000, "Jamie" <askme.com> wrote:

>    I am currently looking to purchase an alarm for my house, I was
>wondering if anyone could tell me of any good brands that i could look into.
>I live in australia and i am not sure what the good brands are down here, i
>will be installiong this alarm myself and i think i want a wired system over
>a wireless.
>Jamie

What is the basic difference between wired and wireless alarm systems?
 It boils down to the "connections link" between sensors and control
panel of both systems, besides that sensor data connection link both
systems are alike.

So the comparison is easy, the WIRED WIRES versus a WIRELESS data link
between sensors and the control panel.

WIRED:
- Each sensor has "hiss own" cabling, hiss own data transmission link.
 All attempts to tamper sensor boxes, short or cut sensor wires are
detected without any ambiguity an with no delay.

- At a first glance the wired links are of the non-frequency selective
type and as such are more prone to capture a greater Radio Frequency
(RF) spectrum range of signals.
 The data transmitted on sensor wired lines are of the low frequency
type and as such can easily be filtered at all inputs entering the
control panel.
 This makes the system data link low frequency selective by
attenuating the eventual High Frequencies reaching the equipment by
huge power transmitters.

- The level of the transmitted signals on the wired lines are around
"2Volt" to switch from on to off (to be more precise, 400mV
interference free immunity for TTL circuits).

WIRELESS:
- All the sensor data is vehicle via ONE wireless data link composed
of a low power transmitter in the sensors and a sensitive receiver at
the input who has around 4 microvolt input sensitivity (wired 400
millivolt (mV) / wireless 4 microvolt (uV) = ratio 100 000 times less
power required to disturb wireless systems)

- The wireless data link contains all the information required to have
a reliable connection as long as there are no other transmissions
present who block the data communication.
 The wireless RF receiver collect in normal circumstances the data and
decode it. This data contains an ID (rolling code), alarm, tamper
attempt, battery low and more information. When the signal is
disturbed, EVERYTHING is, ID can't be recognized...

The receiver:
  The receiver is made as much as possible frequency selective and
sensitive at signals on the frequency in order to capture the week
signals emanating from the sensors (see below).
 They "attenuate" more or less, depending on the quality of the
receiver, the frequencies beside that privileged frequency, its called
the band-pass attenuation range (essential quality comparison data not
provided and published by the manufacturers in order to mask how bad
the bandpass is).

The sensor transmitters:
  In wireless alarm systems the transmitting power is limited by law
and by reasonable battery live time.
  The RF transmitter power of the sensors is of the order of 10
milliWatt (mW), low, very low.

COMPARISON:
- The receive end is 100,000 times more sensitive to signals in
wireless versus wired (4uV / 400mV);
 The ratio is even higher because the high frequencies, where
disturbing transmissions occurs, are attenuated by low pass filters at
the input of the wired lines.

- The link in wired systems are wires who can be filtered/shielded
against RF interferences (and by location of the wires); in wireless
it is the open air reachable by everybody, no shielding possible.

CONCLUSION:
- An external RF transmitter can disturb both systems but the power
required to do this is much higher in wired systems (>100,000 times).
- In wireless systems, the power required to interfere and disturb the
system is similar to the sensor power (10mW) when generated at the
same distance, RFI power should be increased if the distance is
increased.
- Wireless alarm systems are not reliable, they can be interfered and
disturbed/muzzled due too and by an outside transmission.

FINAL NOTE:
- Don't forget that in order to interfere wireless alarm systems that
the RF disturbing signal source should satisfy some frequency
requirements.
 For example; cell phones with theyre 2 watt power don't satisfy that
frequency dependent requirement and as such don't disturb normally.