User's Manual

8.2.14 RP_NumSlots: Configure the repeater data base
Settings
index
Designation
Permissible
values
Default value Permissions
Number
of
Bytes
17
RP_NumSlots
1 -255
32 read/write
1
An 8 Bit field that 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 frequency channel, when all repeater devices send the received packet at the same
time. To avoid this, the frequency channel is divided in RP_NumSlots time slots, where each
repeater chooses a certain slot by random.
The smallest number of time slots that is needed, depends on the network structure and the
number of the repeaters used. Assume there are NumRP repeater devices in the range of a
sending device, then the probability of two repeated packets collide can be calculated by:
1 −
RP _NumSlots!
RP _NumSlots
NumRP
× (RP _N umSlots − N umRP )!
(2)
Common values are:
NumRP RP_NumSlots 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 6 , 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 RP_NumSlots equal 32 is sufficient to have a collision prob-
ability 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 9 ) times a random number between one and
RP_NumSlots .
Example:
In RADIO_DefaultRfProfile 0 the maximum send time for one packet is about 40ms. If we
use 32 RP_NumSlots , the packet is forwarded latest after 32×40ms=1280ms.
8.2.14.1 Example 1
Set the RP_NumSlots to 64 (0x40) using the CMD_SET_REQ .
Themisto-I reference manual version 1.0 © August 2019
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