Chapter 3: Making Basic Decisions about Your RFID System
3. Compare the two read rates
You can expect results something like this:
• 1 tag tested for 30 seconds yields 1,000 reads
• 10 tags tested for 30 seconds yields 100 reads
In addition to the metrics, here are some considerations for weighing accuracy against speed and distance, as you decide on the basic design of your RFID system:
If tags move through an interrogation zone very quickly, you need read- ers configured to broadcast and receive data very quickly. Because that setup is focused on very quickly reading every tag that goes by, it is not possible to read more than a few tags at maximum speed.
Similarly, because a tag takes time to respond to an inquiry from a reader, the farther away that tag is from the reader and the weaker that signal is, the longer it takes to get that response back and receive the next one.
So if you want speed and accuracy, you need to keep the number of tags to read within the capabilities of the system, and if you want to emphasize speed and accuracy, you want the shortest distance possible between the tag and reader. The reason for these constraints is because of the frequency hopping— the fact that each channel can broadcast only for a short period of time and then has to switch channels. Therefore, if you’re in the middle of communication and the channels change, there is a higher likelihood of miss- ing a receiving communication from the tags.
Now What about the Tags and Objects?
The tag is in fact a tiny RFID system: It receives an RF wave, processes the signal, and transmits an RF wave. Those waves are subject to the same laws of physics that the readers are subject to. With these laws in mind, you need to consider how the objects and the tags on those objects will work with your system:
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Chapter 3: Making Basic Decisions about Your RFID System
Consider how the object might affect the system:Whether the object is a case, a pallet, or a laptop, you need to know how it will respond to the various factors that make up an RFID system. If the tag is surrounded by liquid, for example, that liquid will strongly attenuate the RF wave. A metal object may change the tuningof a tag (or frequency on which it can receive signals), reflecting the RF waves from a reader, or block communication from a specific antenna.
Remember that the tag and object need to work together while fre- quency hopping: The constraint that you have to face in the United States is the use of frequency hoppingacross 124 channels from 902–928 MHz.
This means that the tag on an object has to be properly tuned to read well in all those channels because, according to FCC regulations, a reader can broadcast on any given channel for only a certain period of time (400 milli- seconds per channel over any 20-second period in most instances). This is considered wideband communication because it takes place over a wide band of communication channels — 902–928 MHz.
See Chapter 5 for more details on understanding the physics of tags and choosing the right one. See Chapter 9 for all the specifics of testing for the proper tag and placement.
Part II
Ride the
Electromagnetic Wave: The Physics
of RFID
In this part . . .
You start to take your first steps down the RFID road in this part. I give you more in-depth knowledge of the technology and its various uses. You explore some of the specific implementations and understand the specifics of how tags, readers, and antennas all work together as a system.
This part gets you up to full speed on all the workings of an RFID system and really goes a long way toward satisfy- ing the geek in you. You find out about air interface proto- cols, details of antennas, and what makes a reader work.
Chapter 4
What Makes Up an RFID Network
In This Chapter
Understanding the basic components of an RFID network Connecting your hardware with middleware
Learning the origins of radio frequency Understanding resonance frequency
If this book is your first bit of reading on the topic of radio frequency identification, this chapter serves to ground you in the basics of radio fre- quency technology and how it came around to tracking small tags through the supply chain. If you’ve been inundated with press about the newest and most disruptive technology in decades, this chapter is your reality check.
This chapter walks you through some of the basics of good old-fashioned physics and the laws that God created for science.
In this chapter, I describe the elements of an RFID network. The description starts with basic RFID components and moves step by step to a full Web- enabled supply chain network. This step-by-step approach helps you under- stand how a single tag the size of a quarter can grow into a global supply chain technology that is capable of saving billions of dollars. The RFID net- work is built up one node, and one tag, at a time. Understanding the individ- ual components and how they fit together helps you frame architecture and deployment strategies.
Elements of a Basic RFID System
Learning the fundamentals of RFID can be overwhelming. You can avoid feel- ing overwhelmed and the sensation of going around and around in circles, and the sensation of going around and around in circles — I’m just messing with you — by understanding the basics of how data travels in waves and then through a network in an RFID system. This understanding gives you a solid foundation for greater knowledge as you explore the global architecture of RFID.
In a basic RFID system, four fundamental components are required for data to make its grand journey:
A transponder(more commonly just called a tag) that is programmed with information that uniquely identifies itself, thus the concept of
“automatic identification”
A transceiver(more commonly called a reader) to handle radio commu- nication through the antennas and pass tag information to the outside world
An antennaattached to the reader to communicate with transponders A reader interface layer,or middleware, which compresses thousands of
tag signals into a single identification and also acts as a conduit between the RFID hardware elements to the client’s application software systems, such as inventory, accounts receivable, shipping, logistics, and so on Figure 4-1 shows the basics of how a simple RFID system works and the four main components of that system.
For now, I focus on a passive RFID system and a passive tag. Here’s an overview of how the system works:
1. The tag is activated when it passes through a radio frequency field, which has been generated by an antenna and reader.
2. The tag sends out a programmed response.
3. The antenna that generated the field originally and is attached to the reader detects that response.
4. The transceiver (or reader) sends the data to the middleware.
5. The middleware sends the information contained in the tags to whatever systems need that information.
Antenna transmits waves to wake up tag
Reader Middleware
Tag on a pallet Figure 4-1:
The basic components of an RFID system.
The following sections discuss the role each piece of the system plays in creating waves and transmitting data through those waves and into your network.
Everything starts with the tag
A tag, in a passive RFID system, is a little transceiver waiting to be turned on (and no, that doesn’t happen by seeing a tag of the opposite sex). The tag has a small computer chip (or memory area) that is programmed with informa- tion that uniquely identifies the tag. This information is sent when the tag is activated (turned on).
A passive RFID transponder does not contain its own power source; rather, it absorbs energy propagated from a reader antenna’s radio frequency (RF) field to supply all the power it needs to wake up its chip and communicate with a reader by sending back (backscattering) the information contained in its memory to a receiving antenna. As tags move into an antenna’s radio field, they are excited, and each one transmits its identification data.
For more on the inner workings and physics of tags, see Chapter 5.
Antennas send and receive radio waves
Both tags and readers have their own antennas because they are both radio devices. A tag antenna, which is only a few centimeters (or less) long, attaches to the integrated circuit (IC, or just chip) to absorb a signal and then transmit out a slightly modified signal. The reader antennas range in size but are gener- ally about the size of a computer flat screen and are specially tuned to transmit and receive RF signals.
Antennas are how readers communicate with the outside world. Reader antennas send radio signals into the air to activate a tag, listen for an echo (or backscatter) from the tag, read the data transmitted by a tag, and, in some cases, write data onto a tag. Antennas act as conduits between the tag and the transceiver and can function continuously or on demand.
Continuously active antenna systems are used when tagged items are present on a regular basis or when multiple tags are passing through the antenna’s detection field.
On the other hand, an antenna’s detection field can be activated only when needed by a sensor of some kind. The on-demand method can be triggered by optical, pressure, or other kinds of proximity sensors.
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