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A05-0473-010
Network Synchronisation
When Agile™ devices transmit data they require a lot of energy.
Therefore, to maintain low battery power consumption, the devices
are not in transmit/receive mode all the time; for much of the time
they will be in a very low power (silent) mode.
To communicate properly, the devices in the network must all
transmit and receive at the same time. To do this, the communicating
periods must be synchronised so that devices wake up together
from their silent state to move data to and fro before going silent
again. This synchronisation of the network is orchestrated by the
gateway which maintains a constant ‘drum-beat’ throughout the
mesh system.
In the Agile™ 200 Series RF Fire System, a complete cycle of
transmit/receive windows takes approximately 5 seconds including
the silent periods.
Figure 5: Synchronised Communication Sequence
5s approx.
~ s
½
~ s
½
~ s
½
Time
REQ 1
(->GW)
REQ 2
(->GW)
RESP
(GW->)
The Back-up Node
A mesh network that is operating normally is kept in sync by the
Gateway. But if a gateway is removed from a system or is powered
off, control of the network will be lost. All the devices will continually
try to re-connect with the missing gateway and this will lead to high
battery power consumption and significantly reduce the battery life,
unless all the batteries are removed from the Agile™ RF devices.
To prevent this situation (for example, during a fire system
maintenance period), a special node has been created in the mesh
that takes over the network synchronisation role should a gateway
go ‘missing’. Hence, the network continues to operate, but in a
low power (idle) state, minimising battery usage across the system
while the gateway is off. Obviously, during this time, the Agile™ RF
system will not be providing fire cover.
It can take up to 12 minutes for a backup node to assume control of
the network, after the gateway has been switched off. It may take
up to 10 minutes for the gateway to reclaim control of the network,
when the gateway is re-powered on.
SITE SURVEY
What is a Site Survey?
Great care needs to be taken when assessing a site and choosing
the right technology and design layout to use; wireless systems
may not be suitable for every situation. Before committing to a
design and physical implementation of a wireless fire system it is
important to understand and ‘visualise’ the field strength of the RF
network to ensure that vital areas of the building have adequate
signal coverage.
A site survey needs to be done to ensure that the RF fire system
will work reliably after installation.
A site survey involves the use of the AgileIQ™ Software Tools
and Site Survey equipment to carry out RF energy scans and RF
link quality checks. The RF energy scan identifies any channel
frequencies that are unsuitable and the link quality check ensures
that RF communications between nodes is acceptable.
Why is it Necessary?
A site RF survey is a critical element in the process of designing
and installing a wireless communications network in an office
or building. The survey will determine the best placement of the
sensors and manual call points to comply with the coverage and
positional requirements of the fire regulations in the designated
location.
In the UK, the Code of Practice for system design, installation,
commissioning and maintenance of fire detection and alarm
systems (BS5839-1: 2002) specifically addresses the need to carry
out an RF site survey. Section 27.2 states that installation of a
radio-linked system should only take place after a comprehensive
radio survey has been undertaken to ascertain the following:
●
There are no other potentially interfering radio sources
● There is adequate signal strength for communication
The Code also requires that only radio survey test equipment
approved by the manufacturer should be used and records of
signal readings should be kept for future reference
.
When doing a site survey, give adequate consideration to how
the site will be used when the Agile™ RF system is working. For
example, make sure that doors and windows are closed when
signal strength measurements are being taken.
And when installing an Agile™ RF system, it is important to ensure
that there have been no changes to the areas within a building,
such as new internal walls or partitions, the introduction of tall
metal enclosures or the introduction of other wireless systems
since the original site survey was carried out. Any changes to the
system design or the building may require an extra site survey to
confirm the wireless fire system will still work reliably.
How to Plan a Site Survey
The RF energy and link quality tests are important as they ensure
the RF fire system will work reliably in the building where it is
installed.
It is preferable to preplan how the tests will be carried out during
the site survey visit. Use a plan-view of the building to identify the
likely positions of devices with respect to customer requests, local
regulations and fire systems requirements. Identify each device
location with a device type and unique code. Consider how the RF
mesh network will provide coverage across the site, being mindful
of the potential attenuation that walls and other objects can cause.
Site layout drawings can be marked up manually to show the
planned positions of devices, or an electronic copy of the site layout
drawings can be loaded into the Agile IQ™ Software Application to
assist with a site survey. Using the Agile IQ™ design feature, it is
possible to draft a layout diagram of the Agile™ RF devices, create
