4.8 Peculiarities of the Introduction …
383
Fig. 4.51 Reference frame
these triggers in order to ensure that they are sufficiently reliable (i.e., they cannot
be activated accidentally due to interference in the environment).
In order to assess the reliability of the triggering mechanism No. 4, the researchers
took the frame Query as a reference frame. This frame is shown in Fig. 4.51, and its
content is described in detail in [231].
Since the proposed triggering circuit is based on a CRC calculation, it is necessary
to make sure what is the likelihood of that a transmission error will modify a CRC
calculation in such a way that it can activate the triggering mechanism.
Since for the Query command, the length of the CRC is 5 bits, there are 32 possible
different combinations. Due to the polynomial used by CRC-5, and given that the
probability of modifying 2 bits from the reference frame is less than modifying 1
bit, and so on, simple calculations using simulation show that the polynomial having
the lowest probability at the output of a CRC check in the RFID tag is 10001. This
polynomial is the only one set at the output of a CRC check when a frame is received
with five CRC bits changed relative to the reference frame. The frame that will
activate the triggering mechanism is given in [49] (Fig. 4.52).
In order to assess the reliability of the triggering mechanism No. 3, it is necessary
to check whether only some values of the output of the CRC control can appear
when 1 bit is modified in the frame sent from the reference frame and whether all
values of the output of the CRC control with various modifications of the 2 bits in
the sent frame. Again, the value of 10001 in the output of the CRC control cannot
be obtained by modifying only 1 bit in the frame. It is necessary to modify at least 2
bits in a frame from the reference frame. Therefore, the best value to be used in the
CRC control is 10001.
Fig. 4.52 Frame-triggering mechanism
383
Fig. 4.51 Reference frame
these triggers in order to ensure that they are sufficiently reliable (i.e., they cannot
be activated accidentally due to interference in the environment).
In order to assess the reliability of the triggering mechanism No. 4, the researchers
took the frame Query as a reference frame. This frame is shown in Fig. 4.51, and its
content is described in detail in [231].
Since the proposed triggering circuit is based on a CRC calculation, it is necessary
to make sure what is the likelihood of that a transmission error will modify a CRC
calculation in such a way that it can activate the triggering mechanism.
Since for the Query command, the length of the CRC is 5 bits, there are 32 possible
different combinations. Due to the polynomial used by CRC-5, and given that the
probability of modifying 2 bits from the reference frame is less than modifying 1
bit, and so on, simple calculations using simulation show that the polynomial having
the lowest probability at the output of a CRC check in the RFID tag is 10001. This
polynomial is the only one set at the output of a CRC check when a frame is received
with five CRC bits changed relative to the reference frame. The frame that will
activate the triggering mechanism is given in [49] (Fig. 4.52).
In order to assess the reliability of the triggering mechanism No. 3, it is necessary
to check whether only some values of the output of the CRC control can appear
when 1 bit is modified in the frame sent from the reference frame and whether all
values of the output of the CRC control with various modifications of the 2 bits in
the sent frame. Again, the value of 10001 in the output of the CRC control cannot
be obtained by modifying only 1 bit in the frame. It is necessary to modify at least 2
bits in a frame from the reference frame. Therefore, the best value to be used in the
CRC control is 10001.
Fig. 4.52 Frame-triggering mechanism