CSEM Scientific and Technical Report 2008

A Remote Reconfiguration Mechanism for WiseNET Wireless Sensor Networks
D. Piguet, P. Dallemagne, L. Von Allmen
This paper describes the design and implementation of a remote reconfiguration mechanism for the WiseNET wireless sensor network platform.
With this new feature, the program that runs on the nodes that compose a wireless sensor network can be remotely updated without accessing the
nodes physically. The scheme includes a low-power, reliable dissemination protocol and a patch technique based on the calculation of the binary
difference between the current and the new version of the program. This technique drastically reduces the size of the transmitted data.
Reconfiguration is an update feature by which devices on a
computer network can download and install a new version of
the program they are running. This is a challenge in the world
of wireless sensor networks because such devices have only
limited resources. CPU power, memory and energy are
severely restrained. Only a few nodes (e.g. the sinks, the
device which collect the data coming from the network) are
connected to a more powerful device.
Although the solution is not restricted to this case, a network
with a single sink that is also the distributor of the update is to
be assumed. The two main components of the reconfiguration
scheme are a dissemination protocol and a patching
technique [ 1] . The process, depicted in Figure 1, follows four
steps:
• Build: a new executable image is created (compilation +
link dedicated to the target).
• Patch: a patch is computed from the difference between
the new image and the running one. This step is optional:
it is possible to distribute the entire new image.
• Protocol: a distribution protocol is used to load the object
(patch or entire new image) to the destination nodes. The
object dissemination starts from the sink which received
the data from the host through its serial port. All the
destination nodes must be identical. However, a network
may be made of different types of nodes.
• Re-flashing: on each node, the old image is replaced by
the new image and the node is restarted. If a patch has
been distributed, the new image is reconstructed from the
current one prior to re-flashing.
Figure 1: Principle of the reconfiguration of a WiseNET network.
The protocol allows the distribution of the new program to
every node in the network (or a subset of them), originating
from the sink, over possibly multiple hops. It is fully reliable
and copes with the constraints of WiseMAC [ 2 ] , the
asynchronous, random access MAC protocol used by the
platform of interest in which nodes randomly wake-up and
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sample the air for eventual received data. Its basic principle is
to use multiple unicast communications to propagate the data
to the neighbours since the cost of waking-up all nodes is too
high, in terms of energy spent. The rules that the individual
nodes have to follow to propagate the object are strictly local,
ensuring the scalability of the protocol. It also encourages
spatial multiplexing by fragmenting the object into pages that
progress towards the leaves of the network tree while their
follower start to be downloaded near the root. The protocol is
pull based: the nodes are informed of the existence of the new
program to download and request pages to a node which
represents the next hop towards the sink when the network
operates normally (in this context, it is called the parent node).
The patching technique is used to significantly reduce the size
of the data to be transmitted over the network. A patch, which
represents the difference between the current, running version
of the program and the new version to be installed, is sent
over the network instead of the full, new program image. The
patching technique uses the open-source xdelta 3.0 encoder
which computes the difference between two binary files and
outputs the result in the VCDIFF format defined in the
RFC 3284. Because the VCDIFF format involved look-up and
hash tables that are too big for a decoder embedded on a
resource constrained node, a new patch format more adapted
to low-resource decoding has been introduced. The patch in
the new format is obtained by translating the VCDIFF patch to
the new format. The results show that the new format allows
patches which are almost as small as the original VCDIFF
patches. Typical compression rates are between 90 and 99%
for small changes and between 65 and 75% in presence of a
major upgrade.
The implementation of the entire scheme (protocol + decoder)
takes a little more than 8 kB in program memory. The part of
the patch decoder is about 2 kB. While not negligible, this is a
reasonable price to pay for a reliable dissemination protocol
and a very efficient patch technique. In addition, this technique
is completely independent from the underlying system. It
works equally for an implementation based on a kernel such
as FreeRTOS, a scheduler such as TinyOS or a dedicated
executive.
The project has been partly supported under the FP6 EU
framework (Project IST 034963 WASP).
[1] D. Piguet, et al., “A remote reconfiguration mechanism for
WiseNET wireless sensor networks”, First International
Workshop on Remote Entrusting, Trento (IT), Oct. 15-16, 2008
[2] A. El-Hoiydi, et al.,“WiseMAC: An Ultra Low Power MAC
Protocol for the WiseNET Wireless Sensor Network,” in Proc. 1st
ACM Sen-Sys Conf., Nov. 2003, 302–303