How Do RFID Readers Communicate with Tags? 5 Elements of Air Interface Protocol

Have you ever traveled to a country where you don’t speak the language? It can be pretty hard to communicate. Sometimes you may be unable to get your point across at all. It’s the same with your RFID system, except your tags communicate using protocol instead of language.

If your RFID tags and reader don’t align according to the same protocol, communication becomes impossible, and all the work you put into installing this system becomes null and void.

So how do “conversations” between tag and reader happen? For tags and readers to communicate, they need to use a common language. Within RFID, we refer to the process of selecting a standard means for communicating as the Air Interface Protocol.

The air interface protocol consists of the five elements listed below. The tag and the reader must follow all five. Otherwise, the tag’s data will not communicate with the reader.

5 Elements of the Air Interface Protocol

1. Modulation

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

2. Coding

Through coding, The 1’s and 0’s (bits) of binary code translate 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 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 the kind of data the reader requests from the tag. Command sets communicate between reader and tag through a standard 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 has been 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 how RFID systems ensure that signal only comes from one tag at a time. Although many RFID interactions seem to happen simultaneously, receivers can only take in one tag’s data at a time.

If a reader tries to communicate with multiple tags at once, it will indicate the timing at which a tag should transmit its data through the MAC. The two types of MACs commonly employed are 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 starting with 000 would tag 0001 be able to transmit its data. The reader would then continue down various paths through the tree until all five tags have been able to transmit their data. 

Slotted Aloha

In Slotted Aloha, the reader sends a command for every tag to choose a number within a given range (i.e., 0-20). The reader then sends a message requesting a response from the tag that 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 its 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 to allow for the fewest possible collisions, even if there will be empty spaces. If many collisions occur, the reader will increase the range and begin the process again. EPC Gen 2 employs this method, a common protocol used in UHF systems. 

5. Interpretation

Once the reader has received the tag’s data, it must be 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 critical 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 product until it is utilized or sold to an end user), you need to use one standard protocol within your organization. Suppose your product needs to be traced throughout a supply chain and across various organizations. In that case, you will want to agree with your stakeholders on a protocol that has been standardized for use across industries by EPC Global or ISO/IEC 27002.  

Once you’re confident that all of the components 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. Your RFID system will be secure, reliable, and well-integrated with the right protocol. The right integration partner will help you find the correct protocol for your needs and 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.

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