© Microhard Systems Inc. Confidential 45
3.0 Mesh Configuration
3.2.4 Collision Mitigation
In Mesh topology, as with most wireless systems, one of the primary challenges is the management and
mitigation of data collisions. The following section covers several complex interwoven concepts and
options that are used to provide robust, low latency communication, while minimizing the effects of data
collisions. It is important to plan and design the network layout with these concepts in mind from the
start, rather than try to cope with collisions after the system is deployed.
3.2.4.1 Channel Access Mode
To coordinate transmissions from multiple devices and to manage and minimize data collisions, the Pico
Series uses three different channel access modes. The recommended mode is purely a function of the
type of data being sent. The three modes are Aloha, RTS/CTS and TDMA, which are set using register
S244. In systems that send small bursts of data and do not want account for the overhead of CTS/RTS,
Aloha mode may be the best solution. For systems sending large packets of data with a low tolerance
for data loss or collisions, RTS/CTS may be best.
S244 = 0 - Aloha
S244 = 1 - RTS/CTS
S244 = 2 - TDMA (transmit on slot number)
Aloha
In a Mesh based system frames are used to maintain a time reference, and to keep the system synchro-
nized. The frame is composed of a number of different attributes, one of which is a configurable number
of Aloha slots (S214). Aloha slots are used for all data communication. When the channel access mode
is set to Aloha, the transmitting modem randomly selects a Aloha slot within any frame to begin its data
transmission.
The randomness at which modems attempt to access the channel using Aloha is configurable using
register S115. The importance of this register becomes apparent when a system is such that many de-
vices attempt to either access the channel at the same time, or transmit data often, increasing the
chances of data collisions. Increasing the randomness (S115) will make it less likely that collisions oc-
cur, but at the cost of added latency and reduced throughput. It is difficult to suggest a value for S115,
as it is impacted by the type, and frequency of data transmissions. A good starting point may be to set
the value of S115 to the (number of devices in the system) / 2, so if you have 10 total units in the net-
work, try a value of 5 to start with.
S115 = 1 - Repeat Interval (0-255)
For systems using polling mechanisms (polled protocols, Master/Remote systems) , or if the devices
infrequently send data at staggered times, the default value of 1 is recommended, as these types of
systems have a low risk of data collisions.
If multiple modems in a system need to transmit at the same time, CSMA (Carrier Sense Multiple Ac-
cess) is used to detect if the channel is currently in use. If the channel is determined to be busy, the
modem will select another random Aloha slot to begin its data transmission. Carrier Detect Threshold is
covered in more detail later in this document.
The number of aloha slots have a direct impact on the size of the frame and as a result the hop interval,
which is the amount of time spent on any one channel. Increasing the number of aloha slots will in-
crease the number of slots available for data transmission, but at the risk of being affected by interfer-
ence or noise on a channel. A short hop interval, or reduced number of aloha slots will make the system
more immune to outside interference and noise, but at the cost of reduced throughput.
To Next Page
Loading...
