4 ms·

How do they deal with many chips crossing the line at the same time? How do they define the line in the first place? The transmitter has to be extremely direct

How do they deal with many chips crossing the line at the same time? How do they define the line in the first place? The transmitter has to be extremely directional and relatively high power.

bradfa 10y ago
There's an anti-collision system which is used to inventory all tags in the field of view of a reader. On the order of 500+ tags per second can be picked up by a single reader. For more details, see the GS1 spec, page 49 or so http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_1_2_0-standard-20080511.pdf http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_1_2_...
kens 10y ago
> How do they deal with many chips crossing the line at the same time? The EPC Gen 2 spec has a few ways of avoiding collisions. One is Random-slotted collision arbitration, where a tag waits a random amount of time before responding. There's also a somewhat complex algorithm; basically the interrogator says "I heard from tag X, so X be quiet, anyone else?" and repeats until everyone has been inventoried. The interrogator can also ask for subgroups of tags at a time, "Anyone whose number starts with 1? 2? ..." See [1] for details. The tags are designed to work for inventorying when many, many tags are visible at once. I'm not sure which methods are in use during races; taking inventory doesn't have as much time pressure, of course. The tags are fairly omnidirectional, but the detection mats are directional. I read somewhere that the timing system has about 300ms accuracy, but I can't find that now. I assume that for the race winners they use highly accurate finish line technology, rather than RFID. (It would also be very embarrassing if the winner's tag didn't scan: "Sorry, we didn't catch your time.") [1] http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_2_0_0_standard_20131101.pdf http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_2_0_...

How do they deal with many chips crossing the line at the same time? How do they define the line in the first place? The transmitter has to be extremely directional and relatively high power.

bradfa 10y ago

There's an anti-collision system which is used to inventory all tags in the field of view of a reader. On the order of 500+ tags per second can be picked up by a single reader. For more details, see the GS1 spec, page 49 or so http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_1_2_0-standard-20080511.pdf http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_1_2_...

kens 10y ago

> How do they deal with many chips crossing the line at the same time? The EPC Gen 2 spec has a few ways of avoiding collisions. One is Random-slotted collision arbitration, where a tag waits a random amount of time before responding. There's also a somewhat complex algorithm; basically the interrogator says "I heard from tag X, so X be quiet, anyone else?" and repeats until everyone has been inventoried. The interrogator can also ask for subgroups of tags at a time, "Anyone whose number starts with 1? 2? ..." See [1] for details. The tags are designed to work for inventorying when many, many tags are visible at once. I'm not sure which methods are in use during races; taking inventory doesn't have as much time pressure, of course. The tags are fairly omnidirectional, but the detection mats are directional. I read somewhere that the timing system has about 300ms accuracy, but I can't find that now. I assume that for the race winners they use highly accurate finish line technology, rather than RFID. (It would also be very embarrassing if the winner's tag didn't scan: "Sorry, we didn't catch your time.") [1] http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_2_0_0_standard_20131101.pdf http://www.gs1.org/sites/default/files/docs/epc/uhfc1g2_2_0_...