© Microhard Systems Inc. Confidential 46
3.0 Mesh Configuration
In a clean system or in areas where interference and noise are not an issue, it is ideal to allow as many
aloha slots as possible. Although the maximum configurable number of aloha slots is 255, it may not be
possible to use this number and the system will generate an error. The maximum number is determined
by considering the link rate, the number of sync slots and the number of aloha slots. FCC limits the hop
interval at a maximum of 400ms. The ATI255 command shows the current frame duration
S214 = 60 - Number of Aloha Slots (1-255)
S214 must be set to the same value on all units in the system. Failure to do so will prevent the unit from
synchronizing and being able to participate is data communication.
RTS/CTS
When register S244 = 1, the Channel Access Mode is set to RTS/CTS. In this mode, before sending
data, the modem will request a channel from the Coordinator. Once the Coordinator fulfills this request,
the modem is free to transmit its data. This type of channel access mode is best suited for systems that
require large packets of data with a low tolerance for collisions or retransmissions.
As with Aloha mode, RTS/CTS also uses S115 to provide randomness to minimize the number of units
attempting to send a RTS (Request to Send) packet to the coordinator at the same time. The larger the
value of S115 the more random the channel access becomes, reducing collisions. The cost of making
the value of S115 too large, thus increasing the randomness at which devices attempt to access the
channel, is increased latency and reduced throughput. Therefore it is important to select a value that
minimizes collisions, yet not too large to impact system performance.
TDMA
When register S244 = 2, the Channel Access Mode is set to TDMA. In this mode each modem in the
system starts its transmission on an assigned slot number within the frame. The slot number is defined
by S221.
3.2.4.2 Carrier Sense
Carrier Sense Multiple Access (CSMA) is enabled by default and is used to detect if other units are
transmitting data by measuring the signal level and determining if the level detected indicates that an-
other unit is transmitting data. The CS Threshold can be modified using S81.
S81 = 60 - Carrier Detect Threshold in -dBm. (0-255)
The default value for S81 is –60dBm, meaning that if a modem has data to send, it will first check if
there is RF energy present and then measure the level of that energy to determine if it is noise or a valid
signal. If the detected signal is stronger than –60 dBm, if will assume that the channel is busy. If the
signal detected is weaker than the threshold, the unit will determine that the channel is free.
The Carrier Detect Threshold can be set individually on each unit in the system, as the noise floor, inter-
ference and received signal strength will vary by location. It is not recommended to adjust the value of
S80, unless the environment is well understood.
Caution: Setting a
value of S81 too low
will prevent all
communication.
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