To inventory the nearby tags, the reader needs to receive a message from each
tag that gives the identifier for the tag. This situation is a multiple access problem
for which the number of tags is unknown in the general case. The reader might broadcast a query to ask all tags to send their identifiers. However, tags that replied
right away would then collide in much the same way as stations on a classic
Ethernet.
We have seen many ways of tackling the multiple access problem in this chapter. The closest protocol for the current situation, in which the tags cannot hear each others’ transmissions, is slotted ALOHA, one of the earliest protocols we studied. This protocol is adapted for use in Gen 2 RFID.
The sequence of messages used to identify a tag is shown in figure. In the first slot (slot 0), the reader sends a Query message to start the process. Each QRepeat message advances to the next slot. The reader also tells the tags the range of slots over which to randomize transmissions. Using a range is necessary because the reader synchronizes tags when it starts the process; unlike stations on an Ethernet, tags do not wake up with a message at a time of their choosing.
The reason for this exchange is that EPC identifiers are long, so collisions on these messages would be expensive. Instead, a short exchange is used to test whether the tag can safely use the slot to send its identifier. Once its identifier has been successfully transmitted, the tag temporarily stops responding to new Query messages so that all the remaining tags can be identified.
A key problem is for the reader to adjust the number of slots to avoid collisions, but without using so many slots that performance suffers. This adjustment is analogous to binary exponential backoff in Ethernet. If the reader sees too many slots with no responses or too many slots with collisions, it can send a QAdjust message to decrease or increase the range of slots over which the tags are responding.
The RFID reader can perform other operations on the tags. For example, it can select a subset of tags before running an inventory, allowing it to collect responses from, say, tagged jeans but not tagged shirts. The reader can also write data to tags as they are identified. This feature could be used to record the point of sale or other relevant information.
We have seen many ways of tackling the multiple access problem in this chapter. The closest protocol for the current situation, in which the tags cannot hear each others’ transmissions, is slotted ALOHA, one of the earliest protocols we studied. This protocol is adapted for use in Gen 2 RFID.
The sequence of messages used to identify a tag is shown in figure. In the first slot (slot 0), the reader sends a Query message to start the process. Each QRepeat message advances to the next slot. The reader also tells the tags the range of slots over which to randomize transmissions. Using a range is necessary because the reader synchronizes tags when it starts the process; unlike stations on an Ethernet, tags do not wake up with a message at a time of their choosing.
Example message exchange to identify a tag.
Tags pick a random slot in which to reply. In figure, the tag replies in slot
2. However, tags do not send their identifiers when they first reply. Instead, a tag
sends a short 16-bit random number in an RN16 message. If there is no collision,
the reader receives this message and sends an ACK message of its own. At this
stage, the tag has acquired the slot and sends its EPC identifier.
The reason for this exchange is that EPC identifiers are long, so collisions on these messages would be expensive. Instead, a short exchange is used to test whether the tag can safely use the slot to send its identifier. Once its identifier has been successfully transmitted, the tag temporarily stops responding to new Query messages so that all the remaining tags can be identified.
A key problem is for the reader to adjust the number of slots to avoid collisions, but without using so many slots that performance suffers. This adjustment is analogous to binary exponential backoff in Ethernet. If the reader sees too many slots with no responses or too many slots with collisions, it can send a QAdjust message to decrease or increase the range of slots over which the tags are responding.
The RFID reader can perform other operations on the tags. For example, it can select a subset of tags before running an inventory, allowing it to collect responses from, say, tagged jeans but not tagged shirts. The reader can also write data to tags as they are identified. This feature could be used to record the point of sale or other relevant information.
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