The tag transmits its own ID to the reader, and then the
reader consults an external database with the ID to recognize the object. RFID is fast
replacing bar code-based identi?¬?cation mechanisms because (1) communication between
a reader and a tag is not limited by the requirement of ??????line-of-sight??™??™ reading and (2) each
tag is allowed to have a unique ID.
139
Reader transmissions or tag transmissions lead to collision because readers and tags
operate within the same frequency band due to cost considerations. Collisions are divided
into reader collisions and tag collisions [2,3]. When neighboring readers interrogate a tag
simultaneously [4,5], reader signals collide and the tag cannot decode any reader signal.
On the contrary, when multiple tags transmit IDs to a reader at the same time, tag signals
collide and tag collision prevents the reader from recognizing any tag [6]. Collisions make
both communication overhead and transmission delay often lose their usefulness. As a
result, either the reader may not recognize all objects or retransmissions are required for
successful recognition. Especially, since low-functional passive tags can neither detect
collisions nor ?¬?gure out neighboring tags, tag collision gives rise to the need for a tag
anticollision protocol that enables the recognition of tags with few collisions, and also
executes in real time.
Pages:
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283