RFID Tags, a closer look to them and to their configuration


Sun, 06/15/2014 - 11:31
Nov 10th, 2010
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An RFID tag is a WIFI 802.11  device equipped with a transmitter and an antenna. It does not associate  to access points so it is not acting like other wireless clients. An  RFID tag transmits information on a regular basis. This information can  be called beacons. The Nature of the beacons is multicast traffic sent  at low rates. Every X amount of seconds the RFID tag sends Y amount of  beacons on its configured channels. When completing a cycle across all  configured channels, the RFID tag sits on standby and waits for the next  round to do the same actions over and over again.

When deploying RFID for asset tracking in WIFI, you may want to consider the following:

1.How many beacons per channel will the RFID Send?

Due  to the nature of Multicast traffic, shoot and forget, no ACK,  increasing the amount of beacons per channel is a good practice.

Under  clean RF environment Access points will receive tag updates and report  it to their WLC even if the tag is configured to send one beacon per  channel.

However,  under challenging RF environment, there is a possibility to miss a tag  update on a certain Access Point. Missing a tag update on an important  nearby AP can lead into inacurate location. By increasing the beacon  amount per channel to two or three instead of one, we will be lowering  down the possibility of the tag update not to be heard by the Access  Points around it.

You have to take into consideration the Battery life affected by the extra transmission for your RFID tags from the vendor.

2. Which Channels?

In  2.4 GHz deployment, channels 1, 6 and 11 are the non overlapping  channels in the spectrum. The recommended channels to be configured on  an RFID tag will be 1,6 and 11. Note that in some scenarios, an Access  Point will be able to hear RFID tag updates on a channel that is  different from the one it is operating on. By design, the Access Point  will drop these updates and will not forward them to the WLC.

3. How much time between Iterations?

Configuring  the Beaconing interval plays an important role for Location Tracking  since it is defining how much time separates every location calculation.  For stationary tags you may want to define a beaconing interval of 60  seconds.

Keep  in mind that when a tag moves you need more real time information to  base the calculation upon. For this enable motion beaconing on the RFID  tag for it to beacon every 10 seconds.

4. How much time does an RFID wait between each packet transmission?

Time  between each packet transmission can be configured on an RFID tag.  Example configuring: 1 packet per channel with 512 msec spacing results  in having 1.5 seconds required for an RFID to finish a complete  iteration on all three channels.

Further  more, there is a factor that could add up additional time for an RFID  tag to complete all 3 channels updates. It is called CCA (Clear Channel  Assessment). Most of the RFID tags do carrier sensing before  transmitting. If the medium is busy they back off for additional time  and refrain from transmission. After some time, if the medium is clear  they will transmit and move to the next channel. If the medium is still  busy then they will not transmit on that channel during that iteration  and they will move to the next channel. The maximum amount of time for  the back off is not a fixed and may vary from vendor to vendor.



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