Hacking_and_Penetration_Testing_with_Low_Power_Devices

Saving power
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on the modem. The PL parameter can be set between 0 and 4, with 4 (the default)
being the highest transmit power possible.
Modems may be put into one of the available sleep modes. When powered down,
XBee modems consume less than 50 mA at 3.3 V. Recall that traffic for Series 2
modems will be stored by the parent coordinator or router until an end device comes
back online. Packets destined for sleeping Series 1 modems will be lost unless resent
by the other party.
There are four sleep modes, not counting the default of not permitting sleep. Half
of these are pin sleep modes and the other half are cyclic sleep modes. Sleep modes
are selected by setting the SM (sleep mode) parameter on a modem to a value
between 0 and 5. The default value of 0 disables sleeping.
Sleep mode 1, pin hibernate, is the most efficient sleep mode. In this mode, applying
3.3 V to the sleep pin (pin 9) will cause the modem to sleep after finishing any
current receive, transmit, or association operations. Dropping the pin to ground will
cause the modem to wake up in about 10 milliseconds. Modems sleeping in this mode
consume less than 10 mA.
Sleep mode 2, pin doze, is similar to pin hibernate but with faster wake-up times.
Typical wake-up time for modems operated in this mode is 2.6 ms. The price for this
faster wake-up time is a higher sleeping current. According to Digi, modems sleeping
in this mode consume less than 50 mA. Considering that a BeagleBone Black has an
average current draw that is more than 4000 times this amount, there is no practical
difference between this mode and pin hibernate mode for our purposes. The cyclic
sleep modes also use less than 50 mA.
In order to use pin sleep modes, the sleep pin jumper must be installed on the
XBee cape or mini-cape described in the previous chapter. Recall that the sleep
pin is gpio69. The modem can be put to sleep using echo 1 >/sys/class/gpio/
gpio69/value. Echoing the value 0 (low) to the same pseudo file with the command
echo 0 >/sys/class/gpio/gpio69/value reawakens the modem.
Sleep mode 4 (there is no sleep mode 3), cyclic sleep remote, puts a modem to
sleep. The modem will wake up once every sleep cycle and poll its coordinator for
any stored data. The sleep cycle period is determined by the value of the SP (cyclic
sleep period) variable. If there are any queued data to be exchanged, the modem will
remain awake after the transfer is complete until the SP (time before sleep) timer
expires. The ST parameter sets the awake time before the modem returns to sleep.
If there are no queued data, the modem immediately sleeps for another sleep
cycle period.
Sleep mode 5, cyclic sleep remote with pin wake-up, is similar to sleep mode
4. The only difference is that in addition to the periodic waking, the modem may
also be awakened via the sleep pin. Unlike the pin sleep modes, a falling edge
transition (high to low) and not the voltage level will awaken the modem. In other
words, the sleep pin is edge-triggered, not level-triggered as in the pin sleep
modes. The advantage of this mode over mode 4 is that the modem is more easily
awakened prematurely to send urgent data before the next scheduled wakeup
time.