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TheRadioSystem
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TheradiosystemusedbytheSontaySonNetdevicesisdividedinto3sectionsor‘layers’.
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1. The radio layer is where physical control of the radio signal is done. This conforms to
international standard 802.15.4, and determines the frequency of the
radio signals, the
number of ‘channels’ available for use, the bandwidth and power level of the signal etc.
Thereare 16channelsavailable,andthebestone isautomaticallyselectedbythereceiver.
ThefrequenciesusedareintheISM (Industrial,ScientificandMedical)2.4GHzband,witha
maximumdatarate
of250kb/s.
2. The network management layer is where the self‐healing tree functionality is run, which
controlsnetworktopology.‘ZigBee’isanexampleof anetworkmanagementMESHprotocol.
SonNet does not use ZigBee, but instead uses a ‘self‐healing tree’ protocol to control
networktopology.
3. Theapplication
layeriswhatdetermineswhatthedevicedoes–i.e.makesitatemperature
sensingdevice,a router or areceiver. SonNet devices usespecific applications, andinclude
featuressuchasconfigurationproperties.
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Security
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AllSonNetsystemdeviceshavethesame,uniquenetworkidentifier.OnlydeviceswiththecorrectID
willbe
allowed to join the network. The ID used by system devices is different from the ID used for site survey kit
(SSK) devices. Hence, SSK devices cannot join a system network and vice versa. When a SonNet system
network has been formed, it can be ‘locked’ to prevent
any unauthorised devices joining, even if they are
SonNetdevices.TheCMScanbeusedtoauthoriseextraSonNetsystemdevicesifrequired.
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AlldatatransmittedbySonNetdevicesisencrypted.
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HowtheSelf‐HealingTreeNetworkIsFormed
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Thenetworkisformedbasedon3rules,andin
aspecificorderofpriority:
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1. Howmany‘tiers’adeviceisawayfromthereceiver.
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If a device can communicate directly with the receiver, it will, even if the link quality is
poorerthanif itwent througharouter.Ifa devicehasa choiceofmorethan
onerouter,it
willalwayschoosetherouterclosesttothereceiver(theleastnumberoftiersaway),evenif
thelinkqualityismarginal.
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2. Thenumberof‘child’devicesarouteralreadyhas.
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A router can have a maximum of  16 ‘children’. If a device has a
choice of more than one
router of the same tier level, it will always choose the router with the least number of
children,evenifthelinkqualityismarginal.
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3. SignalStrength(linkquality).
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Finally,ifadevicehasachoiceofmorethanonerouterofthesame
tierlevelandthesame
numberofchildren,itwillchoosetherouterwiththebestlinkquality.
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If,foranyreason,adevice(nodeorrouter),losesitspreferredpathbacktothereceiver,itwillautomatically
searchforanalternative–still obeyingthe 3rulesabovein
sequence.If, despiteemploying DirectSequence
SpreadSpectrum(DSSS)techniques,interferenceonthecurrentlyoccupiedchannelpreventscommunications,
thereceiverwillautomaticallylookforanotherchannelwhichisclear.Allotherdevices,havinglosttheirlinks
to the receiver, will then also automatically scan the 16 channels until they find
the receiver again, and  the
networkwillre‐formwithoutuserintervention.
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