RFID - a basic introduction

Transcription

1 RFID - a basic introduction Sophie Bruce Supervisor: Jerzy Dabrowski May 10, 2016

2 Contents 1 Introduction 1 2 What is RFID? Transponders Physical principles of RFID systems Passive RFID Tags Active RFID Tags Read-Only vs. Read/Write or Smart Tags Readers Anticollision and data integrity Parity Checking XOR checksum CRC Privacy 7 5 Conclusion 8 Sources

3 1 Introduction As many are aware of, RFID (radio frequency identification) is a technique used to read information from a distance. It is a technique that contactless transfers data between the data-carrying device and its reader which makes it very flexible. (Finkenzeller, 2007) The basic principle is that a signal is sent to a transponder, which then either reflect the signal back (passive system) or broadcasts it (active system) (RFID Journal, 2016). Barcodes are widely used, almost everything has some sort of barcode printed on it to help manufacturers and retailers keep track on and identify the product. They can be seen as the products fingerprint, since its creation in the 1970s (How Stuff Works, Tech 2016). It has some disadvantages that could explain why the RFID-technology is becoming increasingly popular. For starters, barcodes can only be read and can therefore not send out any information. Also it needs to be read in a direct line of sight by an optical scanner and it can only hold a limited amount of data. Whereas RFID can efficiently store data and can carry information by radio waves. THe RFID technique is also becoming less and less expensive to implement. The history of RFID dates back to the Second World War when radar technology first was used to identify enemy and friendly aircrafts, IFF (identify friend or foe) by putting a transmitter on each British plane. (RFID Journal, 2016) Nowadays RFID are used in many applications and industries such as pet identification, road tolls, transportation and assembly lines. 1

4 2 What is RFID? As mentioned in the introduction, RFID is using radio waves to transfer data information. Data is stored on the transponder (tag), which is located on the object that is being read. The device that is identifying the transponder, the reader (interrogator), can come in different versions depending on if it can only read or also read/write information. These two, the transponder and the reader, are the main components of every RFID system. (Finkenzeller, 2007) In addition there is also the controller, which usually takes the form as a computer with a software that runs a database. When the transponder enters the zone of a reader, the reader signals the transponder to transmit its stored data. When the reader is in possession of the data, it is sent to the controller through a standard network interface such as a LAN-cable or through the internet. (Hunt, Pugalia and Pugalia, 2007) The transponder only operates when it is supplied with power. There are two types of transponders, active and passive. An active transponder contain an on-board power source, i.e. a battery whereas a passive transponder have no power source if its own, instead it uses the signal sent by the reader to derive power, usually through load modulation (Paret 2005). When talking about the reader-transponder relationship Paret (2005) defines the uplink as the data transfer from the reader to the transponder and the downlink as the data transfer from the transponder to the reader. Frequencies used in the RFID vary from 30 KHz to 3 GHz and applications are usually divided into three bands. LF RFID (low frequency) is usually in the band ranging from 30 KHz to 300 KHz. HF RFID (high frequency) is the name for applications using the band ranging from 3 to 30 MHZ. The third group is called UHF RFID (ultra-high frequency) and covers the range from 300 MHz to 3 GHz. There are ISO-standards that specify in which frequency range certain RFID applications operates.(impinj, 2016) 2.1 Transponders Shortly explained a transponder is something that emits an identifying signal once it has received an interrogating signal. In RFID technique one differs between active and passive transponders. But they both have similar physical functionality Physical principles of RFID systems This energy transfer system is caused when the readers antenna is producing an alternating electric field, H(t), due to an alternating current I(t), see equation (1), where H is expressed in amperes per meter (A/m). Where N is the number of turns the readers coil (antenna) have per meter while carrying the current I. (Paret, 2005) In the medium where the magnetic field travels there is a corresponding induction B(t) created, see equation (2). The induction is expressed in teslas (T ) and µ is the permeability of the medium the waves travels through. Since RFID tags are used at different distances, depending what kind of tag it is, it is useful to examine the variations of magnetic induction as a function of the distance between the reader and the transponder. (Paret, 2005) From equation (3), and by holding r, the radie of the readers antenna, constant one can derive the variation of B(d). Similarly one can solve for the optimal value of r, which is useful for a system that is 2

5 operating at a fixed distance. (Paret, 2005) H = NI (1) B(t) = µh(t) (2) r 2 B(d, r) = µ NI (3) 2(r 2 + d 2 ) 3/2 ) If the d is set to be d = ar then the magnetic field strength or magnetic induction takes the general form in equation (4). (Hunt, Pugalia and Pugalia, 2007) 1 B(a, r) = µ (r 2 + a 2 ) 3/2 ) Depending on if the distance, d, between the base station antenna and the transponder compared with the radius, r, of the antenna, the transponder is either in the very near field (d << r, or a << 1), near field (d > r or a > 1) or far field (d >>> r, or a >>> 1) of the transmitting antenna.(hunt, Pugalia and Pugalia, 2007) Passive RFID Tags As mentioned in the beginning of this section, the transponder is only activated when it is within the interrogation zone of a reader. The power that is needed to activate the tag is derived through the transponder antenna from the magnetic or electromagnetic field the reader provides. (Finkenzeller, 2007) In passive transponders, the variation of the magnetic flux causes an induced potential difference, u(t), to appear. It is from this voltage that the transponders derive its power supply through a data carrier. In order for this to happen the tag needs to be tuned to the carrier frequency. The amount of energy transfer depends on how well the frequencies of the two antennas (reader and transponder) are tuned.(hunt, Pugalia and Pugalia, 2007) When out of range from the reader, the transponder does not have any power supply and can therefore not send out or receive any signals. A passive transponder normally consists of a transponder coil, chip and some sort of housing. The housing can consists of glass (for situations when it will be injected under the skin, i.e pet identification), plastic or the most common one, the disk (coin).(finkenzeller, 2007) Figure 1 shows the components of a glass transponder. The coil functions as the antenna, the chip capacitor is the circuit used for storing a charge temporarily. The cip is a microprocessor used to incorporate functions from a central processing unit (CPU) on a single semiconducting integrated circuit (IC), everything is put onto the printed circuit board (PCB). (RFID Journal, 2016) Active RFID Tags Active RFID tags has the same components as passive ones but with an additional power source, usually some sort of battery but it can also be captured from photovoltaic cells or other sources. (RFID Journal, 2016) NI 2r (4) 3

6 Figure 1: Layout of a glass transponder (Finkenzeller, 2007) Since an active transponder has this source of power it has a longer read range than most passive tags. This is because they do not need to modulate the received signal in order to receive power, that means that the transponder is capable to detect a much weaker reader signal. It does however need the reader signal in order to transmit data between the reader and transponder. It can not generate a high-frequency signal of it own, therfore these kind of transponders are also sometimes called semi-active transponders. (Finkenzeller, 2007) Some active transponders has the same functionality as a classic radio with an active transmitter (TX) and sometimes a high-quality receiver (RX).They are sometimes considered to be short-range radio devices (SRD), since they can emit high-frequency electromagnetic fields, see figure 2. Figure 2: Comparison between passive and active transponders (Finkenzeller, 2007) Read-Only vs. Read/Write or Smart Tags Read-only tags have permanently encoded data set, usually in the form of a serial number. When it is in a reader field it starts to continuously broadcast this number. The flow is only one way, wich means that the reador cannot access the transponder, it can only receive the information. It is important to make sure that there is only one transponder in 4

7 the readers interrogation zone. Otherwise continuously transmit from the transponders creates a data collision and the reader cannot distinct one transponder from another. (Finkenzeller, 2007) Read-only tags are much used as a substitute for barcodes in identification of pets, products, containers or in other measures where very small amounts of data is needed. Read/Write-tags are of wide diversity of different memory sizes. These usually have some sort of anticollision features that ensures that the reader can differentiate between transponders. Many of them also have cyptological procedures between the transponder and reader and data stream encryption. (Finkenzeller, 2007) 2.2 Readers The reader (also called base station) usually consists of a microcontrolled part, an analouge part and an antenna. The analouge part have some sort of bit coding, carrier modulation and a power stage to ensure transmission. To ensure reception it needs reception of the transponders load modulation of the transmitted signal. It also needs to be able to process the signal through amplification, filtering, demodulation and validation of the bits. (Paret, 2005) The microcontrolled part ensures anticollision, manages the encryption algorithm, display information on a LCD screen or ensures connection to the host if needed. (Paret, 2005) As maybe understood, readers can look very different depending on the application. It is therefore hard to state a standard circuit or description. In short words explained by Paret (2005), in order to use the information sent by the transponder it is neccessary to: amplify and filter the signal demodulate it detect it validate the value of the bit (parity checking, see section 3). 3 Anticollision and data integrity Every communication channel is limited by its capacity determined by the maximum data rate and the time span of its availability. In order for several transponders to transmit to a single reader without collision the channel have to be divided between the number of senders. A technical procedure that handles multi-access without any interferences is called an anticollision system. Competition between manufactures has lead to publications of what kind of anticollision procedures that are being used are rare. (Finkenzeller, 2007) In order to clarify how a multi-acess procedure can work, the ALOHA procedure will be shortly explained. It is a simple procudere developed in the 1970s and is exclusively used with read-only transponders that generally send out small data packages with long pauses between transmissions. The variable G in equation (5), is the averaged offered load i.e. the number of transponders transmitting simultaneously. T is the observation period and τ is the transmission duration of a data packet and r is the number of packets that are transmitted by transponder n during the observation period.the throughout S of a transmission is calculated with equation (6) and the probability that a packet can be transmitted without collision is calculated with equation (7). If equation (6) is derived 5

8 and set to zero, the maximum will occur at G = 0, 5 which results in S being equal to 18, 4 %. Hence, only 20 % of the channel is used, but more load would quickly lead to collision between transponders. (Finkenzeller, 2007) G = n i=1 τ n T r n (5) S = Ge 2G (6) q = S G = e 2G (7) In order to be able to identify transmission errors or other interferences, checksum is used to detect these errors. The most common checksum procedures are parity checks, XOR sum and CRC. 3.1 Parity Checking A parity bit is a bit that is added at the end of a string, so that nine bits are sent for every byte. There are two parity types, even parity bit and odd parity bit, which makes the each byte have either an odd number or an even number. Both the sender and the reader needs to be configurated with the same parity sense, i.e. need to know whether party is to be odd or even. If the parity bit is even, the bits with value one is counted for the given set of bits making the whole set an even number. If the parity bit is odd, it is set to one if the set has an even amount of one s. It is set to zero if the set already have an odd number odd one s. (Finkenzeller, 2007) The transmitter sends the data and at the same time count the data. If two devices communicate with even parity, the transmitter set the parity bit in each byte so that the set is even. The reader then checks so that it has an even number of set bits. If it finds an odd set it knows where there has been an error in the transmission. (Webopedia, 2016) The method is a simple form of error detecting code and therefore popular. However, it has some drawbacks. If for example an even number of bits is changed due to electrical noise it goes undetected (Webopedia, 2016). Similarly, an odd number of inverted bits will always be detected, but for an even number (2, 4, 6,..) the errors cancel each other out and the parity bit will appear to be correct (Finkenzeller, 2007). 3.2 XOR checksum This method is also known as the longitudinal redudancy check, LCR. It is primarily used for fast checking of small data blocks. It functions similar to parity checking in the sense that if you XOR the bits together it is possible to tell if a bit string has an even or odd parity. The reader then perform the same checksum procedure to see if it matches the sent checksum. Problems occur if multiple errors cancel each other out, the check cannot detect if bytes have been transposed within a data block. (Finkenzeller, 2007) 3.3 CRC The CRC procedure is reliable even for large data quantities. The name, cyclic redundancy check, suggest the procedure is a cyclical one. It uses polynomial division, where 6

9 the remainder of this division is set to the CRC value. The receiver then calculates the CRC value of the received data including the received CRC byte. The result is always zero unless there is an transmission error involved. It is well suited for error recognition in data transfer via wireless interfaces such as RFID. If even one bit is incorrect the CRC value will not match up. (Finkenzeller, 2007) 4 Privacy Attacks on RFID systems can be done by attacks on the transponder or the RF interface. Common measures of attacking the transponder is through exposure to a strong field which thermally destroys the transponder since the waste heat created at the shunt regulator cannot be transported away enough. More direct measures of destroying the transponder is to simply smash the antenna or any other part of the device. A slightly more sophisticated method is transponder shielding/tuning which blocks out the transponder from the reader s magnetic or electromagnetic radiation. A simple method is to wrap aluminum foil around the transponder. The third and most sophisticated method mentioned here is to use spoofing or cloning of transponders. For this a clone of a read-only transponder is built but the PROM is replaced with an EPROM (multi-programmable memory). A reader is then needed in order to get hold of the transponders serial number before putting it in the transponder clone. After that the cloned transponder can send out teh serial number and the reader cannot determine if the signal is sent by the original transponder or the cloned one. Attacks on the RF interface is more common due to fact it does not require access to a reader or a transponder and can therefore be done from a distance. Common measures is eavesdropping via interception or jamming through interruption of the communication between the reader and the transponder by a interference signal. (Finkenzeller, 2007) For security, RFID signals can be encrypted and RFID applications that are easy to physically access should avoid using read-only transponders. 7

10 5 Conclusion The field of RFID-technique is far more complicated than this basic rapport has presented. The technique is evolving together with new components, algorithms and transmission methods. The future for the RFID technology looks bright and it is increasingly used in a variety of different industries and products due to its more and more cost effective implementation. Because of this increasing use it is also facing more privacy issues. The tracking functions RFID offers could be misused. But as long as personal and data integrity can be protected the usage of RFID applications will probably increase even more in future. 8

11 References Printed sources: Finkenzeller, K. (2010). RFID Handbook Fundamentals and Applications in Contactless Smart Cards, Radio Frequency Identification and Near-Field Communication. West Sussex: John Wiley and Sons Ltd. Paret, D. (2005). RFID and Contactless Smart Card Applications. West Sussex: John Wiley and Sons Ltd. Hunt, V.D., Pugalia, A and Pugalia, M. (2007). RFID: A Guide to Radio Frequency Identification. West Sussex: John Wiley and Sons Ltd. Electronic sources: RFID Journal (2016). The History of RFID Technology. [ ] Impinj (2016). The different types of RFID systems. [ ] How Stuff Works, Tech (2016). How RFID Works. tech-gadgets/rfid1.htm [ ] RFID Journal (2016). Glossary of RFID terms. [ ] Webopedia (2016). Parity checking. http : // ERM/P/parity checking.html [ ]