Exam Essentials

Explain the function of an interrogator. An interrogator is a device that reads data from tags, writes data to tags, processes this data, and sends them to the host. Interrogators also perform filtering and aggregation of data, manage I/O devices, help with anticollision, and avoid interference by using a dense reader mode.

Define the types of interrogators. The main types are fixed interrogators and mobile interrogators. Mobile interrogators include vehicle-mounted interrogators, handheld interrogators, and other mobile interrogators (cell phone readers, readers in PCMCIA cards, and reader modules). Fixed interrogators are usually attached to an RFID portal, a tunnel, or a wall. They are connected to the power and network through cables, but can communicate wirelessly. Vehicle-mounted interrogators are usually used with forklifts, where they are either mounted with antennas between the forks or in some cases the reader itself is mounted on the vehicle. In that case, the cables have to be run through specially fitted hydraulic hoses. Handheld readers often are used for exception processing; they are usually powered by a battery and communicate wirelessly with the network. Handhelds have an integrated antenna and often a barcode scanner.

Explain the causes of interference between interrogators and solutions to this problem. Interference can be caused by more than one interrogator operating simultaneously in the area, as well as by RF noise from other RF-emitting devices (such as conveyors, fluorescent lights, microwave ovens, or wireless phones). RF interference can compromise or totally interrupt reading and writing tags. To prevent interference, you should use shielding, frequency hopping, listen-before-talk, and synchronization methods such as time-division or frequency-division synchronization, including dense reader mode.

Explain why the dwell time is important for successful reads and/or writes. Because reading and writing operations take a certain amount of time to be successful, you need to ensure that the tag stays in the interrogation zone for a sufficient amount of time. The dwell time (or time in beam) depends on how fast the tags are moving through the interrogation zone, the data-transfer rate between the tags and the interrogator, and/or whether the tags are being read or written to. Generally, the longer the dwell time, the more successful will be the reading and writing operations.

Define the differences and applications of bi-static and mono-static antennas. A mono-static antenna fulfills both transmitting and receiving functions. These functions are switched by a circulator, which is integrated into the interrogator. An advantage of mono-static antennas is their smaller size as compared to bi-static antennas put into one case; however, they are slightly less efficient because of using the circulator.

A bi-static antenna uses separate antennas for transmitting the RF signal to the tag and receiving the signal from the tag. These two antennas either can be integrated into one case or can be in separate cases. Bi-static antennas are slightly more efficient than mono-static because they are not using a circulator, and each antenna is dedicated solely to transmitting or receiving.

Explain the relationship between interrogator output power and antenna field, including the restrictions posed by regulatory agencies. Increased interrogator power output will provide increased power input into the antenna, which will increase antenna output. This will enlarge the antenna lobes and increase the reading distance. You have to make sure that you do not cross the limits set by any regulatory agencies, however, or you could be fined. In the United States, the allowed transmission power is up to 4 watts EIRP (while hopping minimally across 50 channels); in Europe it is only 2 watts ERP.

Define anticollision protocols. If a collision occurs (two or more tags respond simultaneously to the interrogator's signal), the interrogator cannot successfully communicate with either tag. The main anticollision methods are synchronous (deterministic) and asynchronous (probabilistic). The probabilistic, or asynchronous, method is based on tags responding at randomly generated times. This method includes several specific protocols. The most well-known is the ALOHA protocol. Generation 2 uses a Q algorithm that is based on the slotted ALOHA protocol. The deterministic, or synchronous, method is based on a reader going through the tags according to their unique ID. The most well-known synchronous method is a binary-tree, or tree-walking, scheme.

Understand the challenges of a dense reader environment. In a dense reader environment, the number of simultaneously operating readers is larger than the number of available channels (for the United States it would be more than 50 readers in the area). This creates a lot of RF noise and potential interference; therefore, readers have to use dense reader mode to operate well in such an environment. The dense reader mode usually results in slower data-transfer rates between readers and tags. While in a single reader mode, the data rates can be up to 640 kilobits per second; using a dense reader mode can slow them down about four times (or more). A dense reader environment also affects network traffic (by increasing it). As with any other network device, the reader will use a certain portion of the bandwidth to send the data to the host.