© Microhard Systems Inc. Confidential 55
3.0 Mesh Configuration
As seen in the first table of the illustration, using a cost of hop of 100, results in Path 1, which is the
shortest path, being the lowest cost and the preferred route. It has the least amount of hops and even
though the signal strength between hops may not be ideal, it is considered the preferred route and
added to the routing table.
The second table of the illustration shows the effect of changing S245 to a small value, S245 = 10, low-
ering the cost of each hop. The results for this scenario have much different result. Since the cost of
each hop is very low, and the strength of the links between hops is very strong, Path 3 results in being
the lowest.
As seen, changing the value of S245 can drastically change the results. It should also be noted that the
S245 can be modified on each unit, again changing the “best” path as seen by the route discovery proc-
ess.
3.2.8 Hop Pattern
Frequency Hopping modems hop from frequency to frequency to allow for multiple networks to share
the same frequency spectrum. The pattern at which the modems hop is known as the hopping pattern.
In the Pico Series modems the hop pattern is pseudo-randomly generated using a complex combination
of the Network ID (S104), Register S106, and S180, which define which channels are to be included in
the calculation. This ensures that no two networks have the same hopping pattern, which would cause
interference and collisions.
Register S106: Hop Pattern selects the hop pattern to use for the current system. The value set in this
register must be the same for each unit in the system
S106 = 0 - Selects Hopping Pattern
FCC requires that FHSS systems hop on 50 different channels within the 900 MHz ISM Band. The
maximum time spent on any one channel must not exceed 400ms.
To calculate the center frequency represented by each channel only the starting frequency and the size
of each channel (Channel Bandwidth) need to be known. For the 900 MHz ISM Band, the starting fre-
quency is 902.4 MHz, and the Channel Bandwidth is dependent on the current link rate.
Freq channel n = 902.4+ ((n-1) x BW)MHz.
Example: Channel 75 @ 172 kbps = 902.4 +((75-1) x 0.250) MHz
902.4 + (74 x 0.250) MHz
902.4 + 18.5 MHz
920.9 MHz
Link Rate
(bps)
Channel
Bandwidth
(kHz)
57600 100
115200 200
172800 250
230400 280
276480 350
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