industrial wireless book special edition - Networking ...

With Diversity Path Mesh (left) all data packets propagate across the mesh, and every
node in the mesh, in a single wave. A strict synchronistic mechanism prevents
transmitted packets from echoing around the nodes as unwanted reflections and loops.
The simple propagation concept requires little processing power for routing algorithms
when compared to that required in isolating a standard single routing path.
[One might imagine that every node carrying every data packet would end up using more
battery power overall, but the authors are adamant that this is not the case – Ed]
routing information. Furthermore, there is no set up time and any number
of nodes can be inserted or removed. As long as the added or remaining
nodes are within the reception range, the network simply continues to
operate – without healing time and network downtime.
Until recently, the use of flooding in standard mesh network architectures
has been avoided for several reasons, commonly known as the
‘broadcast storm problem’. Simply put, nodes within reception range
retransmit the message whenever received. This results in an uncontrollable
series of collisions which degrade network performance and
increase energy consumption to unacceptable levels.
To channel the flooding technique into a practical and useful solution,
Diversity Path Mesh makes use of high level synchronisation. Messages at
each node are relayed to the surrounding nodes with precise timing, thus
forming multiple concomitant transmission paths on the way to the destination.
The retransmission of messages through the network is synchronised
to sub-bit level through using TDMA as the master construction framework.
Instead of interfering which each other, the multiple and identical, ±noise,
transmissions received by the node receivers can then be summed together
in a demodulator function block. This summing action increases the strength
(so decreasing the error rate) of the received signal.
Overall, the scheme returns increased reliability since there is no single
point (node) of failure, and increased propagation robustness due to
the inherent spatial diversity of the propagation through the multiple
relay paths. The same summing mechanism which is at the heart of the
proposition translates to greater range. It also virtually eliminates dead
spots in steady state conditions, multipath occurrence, the effects of
RF interference and the effects of changes in propagation paths.
There is a significant reduction in the probability of a message failing
to reach its destination. With sequential propagation from node to node
in a standard mesh network, the time delay adds sequentially with each
propagation step. With simultaneous propagation across the mesh, the
time delay to get from one side to the other is greatly reduced over a
sequential propagation mode. Relaying the messages also enables DPM
networks to extend as far as needed, with the cell overall range and
robustness rising with an increasing number of nodes. The maximum
number of nodes which could be deployed exceeds any practical requirement
for such applications.
In a nutshell, the behaviour of a DPM network has these characteristics:
● The nodes are the network and there are no routers. Once the nodes
are connected and powered, the network is up and running. Also there
is no need for network management and consequently no need to develop
software to manage the network. Compared to router-based networks,
extended software development and field tests are eliminated.
● The operation of adding or removing nodes is immediate and effortless.
There is no such notion as reprogramming or resetting, hence there are
no corresponding delays and no downtime.
● Messages propagate in simultaneous, parallel paths, thus increasing
the range between nodes and improving the resiliency to external
conditions and RF interference.
● The network traffic comprises practically pure data with no
management overhead, resulting in high data throughput and lower
power consumption as a consequence.
● The number of times the data may be retransmitted by relay (the
number of legs or hops) is practically unlimited, with actual numbers
far exceeding the requirements of real applications. For example, with
the basic range between nodes being some 2km (with a high power
module) a coverage area spanning 60km can be easily achieved within
30 hops within a DPM network cell.
● Increasing the number of nodes either increases the size of the
network cell, or the robustness of the network, or both.
Conclusion
Flooding is arguably the most appropriate technique to be considered
for creating a network mesh as it addresses critical performance factors
of wireless mesh networks, as well as the accompanying system tradeoffs.
However, the flooding technique carries several grave drawbacks which
have precluded its use in the past.
Diversity Path Mesh is probably the first mesh wireless network to use
the flooding technique instead of standard routing; synchronised
flooding produces a surprising performance benefit over and above the
positive benefits of flooding propagation.
Products based on this technology and deployed by its originator,
Virtual Extension, have proven the capability of Diversity Path Mesh,
now successfully deployed in projects where other technologies have
been unable to deliver. These applications include smart metering –
electricity water and gas, smart and emergency lighting, building and
industrial automation, vending machines, agriculture and security.
Marius Gafen is involved with Israeli start-up company Virtual Extension Inc
www.virtual-extension.com
First published in the industrial ethernet book May 2010
Industrial Wireless
sponsored by Advantech
industrial ethernet book
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