Subject: RE: [Rfid] Re: Rfid Digest, Vol 6, Issue 9

From: "David Husak" <[email-masked]>
Date: Fri, 25 Mar 2005 08:46:16 -0500

Hi Margaret, RFID List,

> BTW, the theoretical EPC C1 limit is much closer to 1000 reads per
> second.  Matrics claims...

There are two different performance metrics being discussed here that
need to be kept separate: singulation rate (what Krishna was quoting)
and read rate (Margaret's example). Singulation rate is the pace at
which a reader can dis-ambiguate [sic] a randomly-distributed population
of tags, and takes into account the protocol overhead necessary to do
that and read a nominal amount of data from each tag in the population.
Maximum read rate in the degenerate example you gave, does not account
for the singulation protocol/statistics necessary for the
dis-ambiguation [sic++] and I agree, it is in comparison not all that
interesting a metric.

> The FCC requires random frequency hopping for readers at 400ms
> intervals.  In addition to significantly lowering the overall read
> rate ...

There is no reason why frequency-hopping will significantly lower the
overall read or singulation rate for a well-designed reader -- a
pedestrian reader implementation can hop in a millisecond or two, and
there are readers that can hop in as little as 50us. (This is the time
it takes to shut off the transmit power amp, re-tune the local
oscillator, let it settle, and re-enable the power amp.) 

Tag state persistence (on which the Gen2 UHF protocol relies much more
that the Class 1 protocol) is specified to be at least one second, so
the tags will suffer no loss of state during the hop 'blip.' The tags
are not frequency-selective, so they don't even notice that the reader
hopped. BTW, the sequence is not actually random, it is pseudo-random.
The other requirement the FCC imposes is that the hopper must dwell on
each frequency in its hop sequence for an equal amount of time.

So for a reader that follows the letter of the law and hops every 400ms,
the reduction in efficiency is on the order of 1%, usually a lot less.

It is also noteworthy that frequency-hopping is not used at all in
Europe and Japan. (The band- and channel-width, as well as the allowable
transmit power is different in these jurisdictions as well.) These
markets should be supported as well.

>         ...The FCC also forbids active coordination between the 
> readers to share the RF band.  So, ... 

In Europe, coordination, in the form of Listen-Before-Talk, is
mandatory. 

> In general, RFID readers do not need to have a real-time connection
> to the back office databases.  The readers timestamp their tag
reads...

For retail and CPG inventory tracking applications, you are correct.
There are however, applications of RFID that use the data produced to
servo real-time controls and processes. These applications should be
supported as well.

Dave

_______________________________________________
Rfid mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/rfid

---

From: Margaret Wasserman <[email-masked]>
Date: Fri, 25 Mar 2005 11:13:12 -0500

Hi David,

>servo real-time controls and processes. These applications should be

Do you know of any cases where tag data is used for real-time control 
of a device (such as a diverter) where the tag data goes back to a 
central (multi-reader) controller that that central controller issues 
a command to the device (i.e. the diverter) under real-time control? 
What is your definition of real-time in that application (accuracy in 
milliseconds?).

In most cases, I have seen real time control accomplished locally, 
via a GPIO line on the reader that is triggered based on tag data 
and/or via serial line communication with a host device (in the 
embedded case).

Margaret