How Does Your Reader Communicate With The Right Tags?

Have you ever traveled to a country where you don’t speak the language? It can be pretty hard to communicate, at times you may not be able to get your point across at all. It’s the same way with your RFID system, except your tags communicate using protocol instead of language. If your tags and your reader aren’t aligned according to the same protocol, communication becomes impossible and all of the work you put into installing this system becomes null and void.  

We have already shared our knowledge about the various types of RFID systems and their capabilities and limitations, but how are these “conversations” between tag and reader actually happening? In order for tags and readers to communicate, they need to use a common language, so to speak. Within RFID, we refer to the process of selecting a common means for communicating as the Air Interface Protocol.

The air interface protocol is made up of the five elements listed below. If all five of these are not practiced in the same way by both the tag and the reader, none of the tag’s data can be conveyed to the reader. 

1. Modulation

RFID tags and readers communicate through the modulation of radio waves. This occurs through changing the amplitude of radio waves through Amplitude Shift Keying (ASK). 

2. Coding

Coding refers to the way in which the 1’s and 0’s (bits) of binary code are translated into radio transmissions through Amplitude Shift Keying. In the case of RFID, Pulse Interval Encoding (PIE) is used to modulate 1’s and 0’s. This method works slightly more slowly than a code in which the signal is simply on to represent a 1 and off to represent a 0 (NRZ coding), but it ensures that a passive tag won’t lose power and turn off in the case of a long string of zeroes. 

3. Commands

A command indicates what kind of data the reader is requesting from the tag. Command sets are communicated between reader and tag through a common format called a packet. A packet can be 1-150 bits of data separated into a preamble, a command, a parameter, and an error check. 

The preamble is always the same, it helps the tag know that a command sequence is being initiated.

The command indicates which information the reader wants to receive from the tag.

The parameter indicates some values describing which tags should respond.

The error check serves to ensure that the packet was received correctly. 

4. Medium Access Control (MAC)

Medium Access Control is the method by which RFID systems ensure that signal is only coming from one tag at any given time. Although many RFID interactions seem to happen simultaneously, receivers can actually only take in one tag’s data at a time.  If a reader is trying to communicate with many tags at once, it will indicate the timing at which a tag should transmit its data through the MAC. The two types of MACs which are commonly employed are called Binary Tree Navigation and Slotted Aloha.

Binary Tree Navigation

In Binary Tree Navigation, the reader requests a response from all tags whose code begins with 0. If there is a collision (i.e. multiple tags respond at once), communication between one tag and the reader is prevented (note: tags 0001 and 0011 in this example would collide).

The reader will then continue down the tree, requesting a response from tags beginning with 00 (again, there would be a collision). Not until the reader requests a response from tags beginning with 000 would tag 0001 be able to transmit its data clearly. The reader would then continue down various paths through the tree, until all 5 tags have been able to transmit their data. 

Slotted Aloha

In Slotted Aloha, the reader sends a command that every tag choose a number within a given range (i.e. 0-20). The reader then sends a message requesting a response from the tag which has selected 0, if no collision occurs, the reader will receive the unobstructed data from one tag. Then, the reader will request that every tag reduce their chosen number by 1.

The reader again requests a response from the tag which has selected 0.If no tag responds within an allotted response time, the reader will command that the tags again decrease their number by 1, and again request a response from the 0 place.  The given range should always exceed the number of tags present, in order to allow for the fewest possible collisions, even if it means that there will be empty spaces.  If many collisions occur, the reader will increase the range and begin the process again.  This method is employed by EPC Gen 2, a common protocol used in UHF systems. 

5. Interpretation

Once the reader has received the tag’s data, it is important that it is interpreted correctly. The reader interprets the data by combining the object identifier obtained from the tag with context provided by other tag readings, sensor information, business rules, and other associated data.   

All of the above factors come together to constitute a protocol. A protocol aligns your RFID system, so that it can function as a cohesive unit. Without a consistent protocol, for example, your tags may be configured according to Slotted Aloha while your reader uses Binary Tree Navigation. In such a case, none of the important data on the tags will be received by the reader. 

If your RFID system serves an intra-organizational purpose (i.e. to stock and store your own product until it is utilized or sold to an end user), you simply need to use one standard protocol within your organization. If your product will need to be traced throughout a supply chain and across various organizations, you will want to agree with your stakeholders on a protocol which has been standardized for use across industries by EPC Global or ISO/IEC 27002.  

Common protocols used in various RFID systems:

Once you’re confident that all of the component parts of your, and your operational partners’, RFID systems are speaking the same language through a common protocol, you’re one step closer to having a successfully implemented RFID process.

While protocols can feel quite technical, they are the key to success for your automated data identification and inventory solutions. With the right protocol, your RFID system will be secure, reliable, and well-integrated. The right integration partner will help you find the right protocol for your needs and will also keep in mind any other factors that might make your RFID implementation unique. Computype is a dedicated RFID integration partner and we are always here to help you along your RFID journey. For more guidance continue reading our blogs or contact us. 

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