WIRELESS CONNECTIVITY & SENSORS
User manual Thyone-I
9.12. RF_RP_NUM_SLOTS: Number of retries
Settings
index
Designation
Permissible
values
Default value
Permissions
Number
of bytes
12
1-255
32
read/write
1
This value contains the number of time slots to be used for the packet repetition.
When using several repeater devices in a single network, repeated data packets may collide
on the radio channel, when all repeater devices send the received packet at the same time.
To avoid this, the frequency channel is divided in
time slots, where each re-
peater chooses a certain slot by random.
This calculation assumes exactly one radio frame over the whole duration of
the repeating (i.e. a low throughput scenario). With higher throughput require-
ments, the Poisson distribution of frames together with the selected channel
access scheme needs to be included in the calculation.
The smallest number of time slots that is needed, depends on the network structure and the
number of repeaters used. Assuming there are
NumRP
repeater devices in the range of a send-
ing device and there is only one radio packet present over the entire duration of all repeating,
then the probability of at least two of the same repeated packet collides can be calculated by:
1
−
RP
_
N umSlots
!
RP
_
N umSlots
N umRP
×
(
RP
_
N umSlots
−
N umRP
)!
Common values are:
NumRP RF_RP_NUM_SLOTS
Collision probability
2
32
3.1%
3
32
9.2%
4
32
17.7%
5
64
14.8%
6
64
21.5%
7
128
15.4%
In the example network shown in figure
, there are only two repeaters that can conflict each
other. Repeater 2 and 3 are forwarding the packet received from Sender 1 "at the same time".
Thus,
NumRP
equals 2 and
equal 32 is sufficient to have a collision proba-
bility of less than 5 %. The time delay used by the repeater device can be determined as
the time needed to send one packet (see table
) times a random number between one and
Example:
In
0 the maximum send time for one packet is about 16 ms. If we use 32
, the packet is forwarded latest after 512 ms (32 × 40 ms).
Order code 2611011021000
98
Version 2.3 , July 2022
