industrial wireless book special edition - Networking ...

RFID technology
Active RFID tags have an onboard battery to power the microchip’s circuitry and transmit signals to
readers. Typically, active tags can be read from distances of around 30m. They also allow more
applications by offering two-way communications, sensor integration, independent system
intelligence, and constant viability. Active RFID tags work using Received Signal Strength Indication
(RSSI) or Time Difference of Arrival (TDOA) technologies.
RSSI: Some tags associate with APs to provide a regular beacon or ‘chirp’ (a 416-bit 802.11 frame).
This reduces tag/AP interaction to a simple unidirectional packet – there is no state for the AP to maintain
and no IP address required. Also, less radio time means a longer battery life. This is common amongst
IEEE 802.11 wireless protocol families, whether a, b, g or the newer n flavour. Other tags send out a
more constant signal, become associated with the AP, providing a more constant awareness with
stronger integration. However, battery life is shorter.
In an 802.11 Wi-Fi network, the APs are arranged so that they receive signals from the RFID tag and
triangulate its position based on the received signal strength at each AP. The stronger the signal, the
closer the tag is to the AP. Each AP sends it’s strength metric to the wireless control system, which
interprets the signal strength via an appliance and middleware. This provides coordinates via APIs to
user interface software, which – in turn – specifies location. The results can be sent as coordinates
or shown on a visual map.
For Lightweight Access Point Protocol (LWAPP) environments, location information is sent to the Location
Appliance and Wireless Control System (WCS) rather than an alternative separate non-LWAPP engine,
which can also calculate tag positions. Accuracy can be 2 – 3m, depending on environment, location
and AP density.
TDOA: This is similar to RSSI in that the network APs triangulate tag locations based upon the signal,
but here it is the time it takes for the signal packet(s) to arrive at the AP that is important. The shorter
the time taken for the signals to be detected, the closer the tag is to the AP, and a position is estimated
using one or more locating algorithms in the location appliance and WCS. This method is more resilient
for high ceilings and outdoor applications, and is less disturbed by building structures.
In another application there are chokepoints (‘exciters’). This works using strategically placed readers
or triggering devices – the exciters. When a tag passes within range of a 125 kHz exciter, the tag emits
a short burst of Wi-Fi data that includes that exciter’s unique identification. With an exciter, immediate
tag recognition occurs as the tag is ‘excited’ and emits a response. Conventionally, it can take several
minutes before tags chirp and the location is updated. Exciters, therefore, provide a more real-time
experience for tracking moving items. Additionally, tags can be set to turn themselves on or off as they
go through chokepoints.
In summary, RSSI is good for most indoor tracking applications and uses existing standards-based Wi-
Fi networks. TDOA uses a more proprietary approach, but can be backhauled via Wi-Fi networks and
is good for outdoor and difficult environments. Choke-points are good for tracking movement of assets
through doorways and designated areas, so helping to prevent asset loss or tracking mislaid items.
Organisations that define standards and regulate the use of RFID include ISO, IEC, ASTM International,
DASH7 Alliance and EPCglobal.
Open standards platforms
With different protocols and standards abounding,
a key technology development has been the introduction
of open standards based platforms.
The pace of adoption is starting to pick up.
The customer stories below tell the business
benefits side, but there are also IT and process
benefits. With an open-standards WCS4, the
application can integrate with warehouse, ERP
and other business systems. Tracking WIP can
become part of the overall methods of manufacture,
not only for the supply chain, but also
plant-floor activities, such as MES and WIP.
Challenges remain. Firstly, manufacturers want
reliable and available systems. A system is now
available that uses a wireless interference
monitoring technology. This comprises special
hardware built into certain APs, plus advanced
interference identification algorithms that
automatically detect radio interference and can
map its source, rather like an integral spectrum
analyser. Microwave and blue-tooth devices,
for example, can cause interference. The system
identifies the interference and detects where
it’s coming from and can mitigate it, providing
a more reliable network.
Secondly, 802.11n adoption continues to
grow. These are higher performance than the
older networks, but manufacturers’ IT departments
may still have legacy deployments, so
technologies are still needed to cope with both
old and new. There are ways of improving the
network performance of legacy 802.11a and g
devices (older Wi-Fi networks), whilst still
enjoying the benefits of a higher performance
802.11n Wi-Fi network.
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