Interrogator Functions

Interrogator functions used to vary by manufacturer. Today, the competition is high and all manufacturers try to provide the best functionality possible within the most competitive price range. Of course, the features these units offer can slightly differ, but the basic functions are pretty much included in all of them.

Read/Write Capability

Interrogators offer read and write capabilities. Some types of interrogators, particularly older handheld units, can only read tags, not read and write to tags. Most of the interrogators support currently used air interface protocols and data standards such as electronic product code (EPC) Class 0, Class 1, Generation 2, and International Organization for Standardization (ISO) 18000-6A and B (possibly C) for ultra-high-frequency (UHF) readers, or ISO 18000-3 and ISO 15693 for high-frequency readers.

Read and write ranges vary based on the interrogator's quality, tuning, the characteristics of the interrogation antenna, tags under interrogation, and present environmental conditions (including interference, RF noise, materials, and humidity). The output power is limited by various regulations and differs around the world.

The read range is the specific distance over which the reader can successfully read data from a tag. The read range can be relatively long, but you cannot forget about the loss in a signal's strength based on a distance (loss follows the inverse square rule, as pointed out in Chapter 1, "Physics of RFID"). The tag usually needs around 30 microwatts to be powered and respond back to the reader.

The write range is the distance over which the reader can successfully write data to the tag. The write range is usually a lot shorter than the read range. The shorter distance ensures higher reliability of the write operation. The writing process is also very sensitive to outside conditions. When writing to a tag, you should try to avoid RF interference from other sources, which could interrupt the operation.


To successfully write tags on a conveyor line, you should consider an RFID tunnel to avoid interference and concentrate the wanted RF signal into one area. The tunnel allows a higher reader output without affecting other tags along the conveyor.

I/O Capability

Many interrogators today can directly operate peripheral devices and provide I/O ports to connect these units to the interrogator. Devices directly operated by an interrogator include the following:

  • Light stacks

  • Audible devices such as horns or beepers

  • Gate triggers for opening the doors

  • Conveyor diverters

  • Box pushers using a hydraulic arm

  • Vehicle tollgates

  • Video cameras

Digital I/O devices are also being integrated as an upgrade to conventional programmable logic controllers (PLCs).


You can find more information on RFID peripherals in Chapter 6, "Peripherals."

User Interface

Interrogators can be configured either through a network by using middleware, or by using a specific reader deployment tool such as ODIN's EasyReader, or directly through the reader's GUI. Every manufacturer provides a different GUI for its reader, and different GUIs can be more or less user friendly. Most of the reader manufacturers, however, have designed the GUI for highly experienced RFID engineers, and they are meant to be used for one reader at a time.

To access the GUI, you have to connect your computer to the reader either directly by using a serial connection through RS-232, through an Ethernet connection through RJ-45 (using crossover cable) or universal serial bus (USB), or through a network. If you are not connecting through a serial cable, in most cases you will need to know the Internet Protocol (IP) address of the device.

If you do not know the IP address of the reader (and this can happen if the IP is assigned dynamically), do not fret; there is a way to find it. You can connect your computer directly to the reader by using a serial connection and establish communication by using HyperTerminal or Telnet. To do this, you will need to set up your communication port (COM) with a correct data rate in bits per second (usually 38,400), data bits (usually 8), parity (usually none), stop bits (usually 1), and flow control (usually hardware or none). After you establish communication, you will be able to retrieve the reader's IP address.

After you have the reader's IP address, you have to set up your network settings in order to communicate with the reader. Then you can input the IP address into your browser (Microsoft Internet Explorer works just fine) and it will take you to the reader's Hypertext Transfer Protocol (HTTP) interface. Sometimes, access to the reader is blocked by a password; sometimes it will let you in without it.

The user interface will give you access to the reader's settings including the active antennas (antennas that are connected to the interrogator and are not disabled by the system or by the user), air interface and data protocol used or enabled, network settings such as Dynamic Host Configuration Protocol (DHCP), IP address, subnet mask and default gateway, read/write power settings, polling frequency, filtering, and other settings as well as a capability to change these settings or upload new firmware. The settings can be reconfigured either by simply clicking on a rollout menu and selecting an option or by typing in commands and parameters by using a command line.

Most of the GUIs that ship with standard readers are meant to be used as a demonstration tool, which is suited for only one reader at a time and is not scalable or automated. The team deploying the largest RFID network in the world-for the U.S. Department of Defense-used ODIN Technologies' EasyReader, which provides a simple drag-and-drop interface to set up many readers and printers at a time. This industry best-practice software allows for very fast design, configuration, and testing of entire RFID networks. This is done by dragging and dropping virtual portals onto the floor plan of the facility to be deployed.

Dumb Reader vs. Smart Reader

Honestly, when I first heard someone referring to an RFID interrogator as a "dumb reader," I was quite offended. This device (although it has already seen several winters) was able to read data in RFID tags, but not much else. However, this capability still seems to me miraculous, and therefore I would never call this device "dumb," even though the interrogators on today's market are bursting with advanced features and functions waiting to be taken advantage of.

What can a "smart" reader do? Here is a list of some of its capabilities:

  • Filtering Instead of reading all available data from tags and transferring the data to the host, a reader acquires only selected data and out of these, only the requested (necessary) part goes to the host/middleware/application.

  • Cycle acquisition A smart reader can be set up for continuous polling for tags, or it can poll in preset time intervals, such as every second, 3 seconds, 5 seconds, up to days. This reduces the amount of data going through the system, and the tags are interrogated only when needed.

    • Real-Time The reader transfers data as soon as it acquires them.

    • Batch Data are collected and sent to the host in batches.

    • Triggered Data are collected and sent to the host only when triggered by a certain event or by the user.

    • Pull from the Host/Push to the Host Data are accessed and copied by the host or sent to the host.

  • Managing Sensors Readers can manage external sensors, such as motion sensors, through their I/O interfaces.

  • Triggering Interrogation Interrogation can be started based on a signal from a sensor (such as a motion sensor), based on time, or manually.

  • Dense Reader Mode Operations Readers are able to operate in an environment with many other readers present and are able to avoid or reduce the risk of interference. This topic will be discussed further later in this chapter.

  • Anticollision capabilities Readers are able to poll for the tags by using certain algorithms to prevent tag collisions (two or more tags responding at the exact same time). These algorithms also will be discussed further in this chapter.


Interrogators differ in the communication methods they use. In active RFID systems, the readers send signals to active tags and receive the signal that the active tags transmit. This signal can be a response to the interrogation signal (referred to as reader-talks-first, or RTF) or a tag can beacon its serial number to the environment (referred to as tag-talks-first, or TTF).

In passive and semi-passive RFID systems, the reader always has to send out a signal first in order to receive data from the tag (RTF). The transmitted signal sent to a passive tag powers the tag's circuitry and provides it with energy to modulate the signal and send the data back to the reader. A semi-passive tag uses a battery to run its circuitry but communicates by reflecting and modulating the reader's signal in the same way as a passive tag. Passive and semi-passive systems can use inductive coupling or passive backscatter to communicate with a tag.

RFID interrogators can operate on various frequencies. Usually, an interrogator can operate on only one frequency band (the band used in its region); however, some manufacturers are coming up with devices that can operate in a wider band to include frequencies used in different regions (such as readers that can work in the United States as well as in Europe). There are also devices that use two frequency bands, such as interrogators that work on low as well as high frequency. (These are usually made up of two readers integrated into one case.)

The operations of the interrogators are regulated by government and standardization organizations around the world. These organizations not only allocate a specific frequency that can be used for RFID communication and publish standards applying to data formats and air interfaces, but also place restrictions on the output power and transmission techniques. Table 2.1 shows the allowed output power, number of channels used for frequency hopping, and other limitations related to operating frequency for interrogators around the world. (Because these restrictions are the most variable in the UHF frequency band, I will concentrate on UHF.)

Table 2.1: Transmission Restrictions by Region
Open table as spreadsheet

Continent or Country

UHF Band in MHz

Allowed Output Power (ERP)


North America (U.S. and Canada)


1 W up to 4 W with gained antenna (EIRP)

Should use frequency hopping-50 channels (1 channel = 500 kHz)

South America

915 (typically accepted)

Same as U.S.







0.1 W

2 W

0.5 W

0.5 W

(1 channel = 200 kHz)

Should use listen-before-talk (LBT)


865–868 (up to 870) (north)

915 (south)

Same as Europe

Same as U.S.



917–922 (temporary)

2 W


China (Hong Kong)

865–868 and/or 920–925

Same as Europe

Same as U.S.




4 W (EIRP)





0.02 W

4 W (EIRP) (need to be registered)

Should use LBT



4 W (EIRP)

Should use LBT and frequency hopping



1 W (EIRP)


New Zealand

864–868 and 921–929

1 W up to 4 W when using frequency hopping



866–869 and 923–925

0.5 W

0.5–2 W


Up to 2 W licensed



Up to 0.5 W

Up to 4 W (EIRP)




868.1 and 919–923

Both freq. up to 0.5 W

Both freq. up to 2 W



Information based on the document by Asia-Pacific Telecommunity: Draft New APT Recommendation on Spectrum for Ultra High Frequency (UHF) Radio Frequency Identification (RFID) Devices.

Tag Population Management

Interrogators manage tags that are present in the interrogation zone via commands defined by the communication and data protocols. The following commands are based on the capabilities of EPC Generation 2 interrogators, and some of them may not be available in devices operating under other protocols. There are three primary commands:

  • Select This command picks out a group of tags based on certain characteristics, such as a specific manufacturer's ID or other parts of the EPC number. Because this command concentrates on only a small part of the data within the tag, it is able to quickly locate and target certain tag groups within the whole tag population.

  • Inventory This command isolates an individual tag from the group allocated by the Select command.

  • Access This command is used to address a specific tag after it is singulated by the Inventory command. After the tag is accessed, other specific commands can be issued, such as the following:

    • Lock This command locks the access to a tag. It is protected by an Access password. The tag can be write-locked to prevent unauthorized changing of the tag's data, as well as read-locked to prevent reading the tag's data. This command also can be used to lock specific memory banks.

    • Kill This command is used to permanently disable the tag. After being killed, the tag is unable to respond to interrogation. The Kill command is also password protected.

Other techniques used by Generation 2 for tag management are sessions and AB symmetry. Each tag has the ability to operate in four sessions. This is useful when tags are inventoried by more than one type of reader. Each reader or group of readers interrogates tags in a separate session, and therefore does not interfere with other readers when interrogating tags. The session number is sent to a tag during the Inventory round.

After the tag is inventoried, it changes its state from A to B or from B to A. This technique is called AB symmetry and replaces the Generation 1 method, which was putting tags to sleep after they were inventoried to avoid interference with interrogation of other tags.

Firmware Upgrades

There are several reasons for uploading a firmware upgrade or patch to your interrogator. However, you should not be upgrading just because a new version is available. With interrogators as well as other electronic devices, one old rule should be applied: "If it ain't broke, don't fix it." Therefore, you should check what features the new version offers and why it was developed, and carefully consider whether it is important to upload this version to your interrogators. Why am I even discussing this? Because a simple power interruption or installation error could leave your interrogator disabled, and you may have to reload the operating system totally before being able to use this device again.

However, there are valid reasons for upgrading the firmware-for instance, if the upgrade is released to fix bugs in the existing firmware that have been causing you problems from time to time, or if you need to add new capabilities such as the ability to support the EPC Class 1 Generation 2 protocol. (RFID printers also can be upgraded to support Generation 2 through firmware.)

New capabilities often can be put into your interrogator through firmware, but sometimes you will have to purchase a new component or a completely new unit to support certain features and perform specific operations.

CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
CompTIA RFID+ Study Guide Exam RF0-101, includes CD-ROM
Year: 2006
Pages: 136 © 2008-2017.
If you may any questions please contact us: