Selection Criteria for RFID Systems

There has been an enormous upsurge in the popularity of RFID systems in recent years. The best example of this phenomenon is the contactless smart cards used as electronic tickets for public transport. Five years ago it was inconceivable that tens of millions of contactless tickets would now be in use. The possible fields of application for contactless identification systems have also multiplied recently.

Developers of RFID systems have taken this development into account, with the result that countless systems are now available on the market. The technical parameters of these systems are optimised for various fields of application –ticketing, animal identification, industrial automation oraccess control. The technical requirements of these fields of application often overlap, which means that the clear classification of suitable systems is no simple matter. To make matters more

ISO 14443 868/915 MHz, 2.45 GHz

ISO 15693, ISO 18000 ISO 14223

Figure 2.18 RFID systems can be classified into low-end and high-end systems according to their functionality

binding standards are as yet in place for RFID systems.

It is difficult even for a specialist to retain an overview of the range of RFID systems currently on offer. Therefore, it is not always easy for users to select the system best suited to their needs.

In what follows there are some points for consideration when selecting RFID systems.

2.6.1 Operating Frequency

RFID systems that use frequencies between approximately 100 kHz and 30 MHz operate using inductive coupling. By contrast, microwave systems in the frequency range 2.45 – 5.8 GHz are coupled using electromagnetic fields.

The specific absorption rate (damping) for water or nonconductive substances is lower by a factor of 100 000 at 100 kHz than it is at 1 GHz. Therefore, virtually no absorption or damping takes place. Lower-frequency RF systems are primarily used due to the better penetration of objects (Sch¨urmann, 1994). An example of this is the bolus, a transponder placed in the omasum (rumen) of cattle, which can be read from outside at an interrogation frequency of<135 kHz.

Microwave systems have a significantly higherrangethan inductive systems, typically 2 –15 m.

However, in contrast to inductive systems, microwave systems require an additional backup battery.

The transmission power of the reader is generally insufficient to supply enough power for the operation of the transponder.

Another important factor is sensitivity toelectromagnetic interference fields, such as those gen-erated by welding robots or strong electric motors. Inductive transponders are at a significant disadvantage here. Microwave systems have therefore particularly established themselves in the production lines and painting systems of the automotive industry. Other factors are the high mem-ory capacity (up to 32 Kbyte) and the high temperature resistance (up to 250^◦C) of microwave systems (Bachthaler, 1997).

2.6.2 Range

The required range of an application is dependent upon several factors:

• the positional accuracy of the transponder;

• the minimum distance between several transponders in practical operation;

• the speed of the transponder in the interrogation zone of the reader.

For example, in contactless payment applications – e.g. public transport tickets – the positioning speed is very low, since the transponder is guided to the reader by hand. The minimum distance between several transponders in this case corresponds to the distance between two passengers entering a vehicle. For such systems there is an optimal range of 5 –10 cm. A greater range would only give rise to problems in this case, since several passengers’ tickets might be detected by the reader simultaneously. This would make it impossible to allocate the ticket reliably to the correct passenger.

Different vehicle models of varying dimensions are often constructed simultaneously on the production lines of the automotive industry. Thus great variations in the distance between the transponder on the vehicle and the reader are pre-programmed (Bachthaler, 1997). The write/read distance of the RFID system used must therefore be designed for the maximum required range.

The distance between the transponders must be such that only one transponder is ever within the interrogation zone of the reader at a time. In this situation, microwave systems in which the field has adirectional beam offer clear advantages over the broad, nondirectional fields of inductively coupled systems.

Inductive coupling Electromagnetic coupling (backscatter) nondirectional

Electromagnetic coupling (backscatter) directional

1 2 3 m

Figure 2.19 Comparison of the relative interrogation zones of different systems

The speed of transponders, relative to readers, together with the maximum write/read distance, determines the length of time spent in the reader’s interrogation zone. For the identification of vehicles, the required range of the RFID system is designed such that, at the maximum vehicle speed, the length of time spent in the interrogation zone is sufficient for the transmission of the required data.

2.6.3 Security Requirements

Security requirements to be imposed on a planned RFID application, i.e.encryption and authen-tication, should be assessed very precisely to rule out any nasty surprises in the implementation phase. For this purpose, the incentive that the system represents to a potential attacker as a means of procuring money or material goods by manipulation should be evaluated. In order to be able to assess this attraction, we divide applications into two groups:

• industrial or closed applications;

• public applications connected with money and material goods.

This can be illustrated on the basis of two contrasting application examples. Let us once again consider an assembly line in the automotive industry as a typical example of an industrial or closed application. Only authorised persons have access to this RFID system, so the circle of potential attackers remains reasonably small. A maliciousattackon the system by the alteration or falsification of the data on a transponder could bring about a critical malfunction in the operating sequence, but the attacker would not gain any personal benefit. The probability of an attack can thus be set equal to zero, meaning that even a cheap low-end system without security logic can be used.

Our second example is a ticketing system for use in public transport. Such a system, primarily data carriers in the form of contactless smart cards, is accessible to anyone. The circle of potential attackers is thus enormous. A successful attack on such a system could represent large-scale financial

sale of falsified travel passes, to say nothing of the damage to the company’s image. For such applications a high-end transponder with authentication and encryption procedures is indispensable.

For applications with maximum security requirements, for example banking applications with an electronic purse, only transponders with microprocessors should be used.

2.6.4 Memory Capacity

The chip size of the data carrier – and thus the price class – is primarily determined by itsmemory capacity. Therefore, permanently encoded read-only data carriers are used in price-sensitive mass applications with a low local information requirement. However, only the identity of an object can be defined using such a data carrier. Further data is stored in the central database of the controlling computer. If data is to be written back to the transponder, a transponder with EEPROM or RAM memory technology is required.

EEPROM memories are primarily found in inductively coupled systems. Memory capacities of 16 bytes to 8 Kbytes are available. SRAM memory devices with a battery backup, on the other hand, are predominantly used in microwave systems. The memory capacities on offer range from 256 bytes to 64 Kbytes.

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