Tag identification Message formats

The format of the Query message is shown in the figure given below, as an example of a reader-to-tag message. The downlink rates are limited, from 27 kbps up to 128 kbps so the message is compact.   To identify the message as a Query the Command field carries the code 1000.

• The physical layer parameters for reader transmissions and tag responses are determined by the next flags, DR, M, and TR.  
  
• Three fields select the tags to respond and these three fields are Sel, Session, Target. Whether they have been identified in those sessions and tags keep track of up to for concurrent sessions. Readers are able to select a subset of identifiers. By using different sessions multiple readers can operate in overlapping coverage areas.
  
• Q is the most important parameter for this command. From 0 to 2^Q -1, defines the range of slots over which tags will respond. To protect the message fields CRC is there. It can be seen that at 5 bits, it is shorter than most CRCs. Whereas the Query message is much shorter than most packets too.
  
• Tag-to-reader messages are simpler. It knows what message to expect in response to each of its transmissions as the reader is in control. Such as the EPC identifier, they simply carry data.
  
• Originally, the tags were just for identification purposes.   To resemble very small computers, tags have grown over time. Some research tags are able to run small programs to gather and process data and some have sensors. One version of this technology is the “Internet of Things” that connects objects in the physical world to the Internet.