Contactless Smart Card - ISO 14443bAuthor: Chris Vanderbles Date: 04/14/2008
Objective: This white paper will cover communication methods for 13.56 MHz contact-less smart cards, more specifically it will cover the communication standard set forth by ISO 14443b. Other ISO specifications for contact-less smart card communications will be briefly reviewed, but not covered in detail. Literature Review: The contact-less smart card is a relatively new technology belonging to the family of radio frequency identification (RFID) technology. This piece of technology differs in nature from previous RFID technologies by having a micro-processor on the card itself to provide intelligent communications to the end device, rather than just a static ID number. There are many different card technologies that operate in the 13.56 MHz spectrum, but this paper will focus on the implementation of the ISO 14443b specification for contact-less smart card communications. The other specifications that will be briefly covered in this paper are ISO 14443a, and ISO 15693. Before delving into the technical specifications of contactless smart-cards, it is important to understand the history of the technology and how it has developed over time. The first known implementation of RFID was used by the British during World War II; it was used to identify planes returning from mainland Europe. This system was know as the “Identify: Friend or Foe” system, or IFF. By 1977, the US government released their developed RFID technology, developed at the Los Alamos National Laboratory, to the public. In the early to mid 1980’s, companies started utilizing RFID cards for controlling physical access to their facilities. By 1986, a “de-facto” standard for proximity based credentials had emerged; this was initially developed by the Atmel Corporation. This standard utilized a 125 KHz carrier wave for transmitting and receiving RF data and has a capacity of up to 256 bytes of data. The 125 KHz standard is still common-place technology today. The first smart cards were also developed in 1986, but operated at much lower frequencies than today’s technology, and required multiple processors for implementation. On July 1, 2001, the International Standards Organization (ISO) published the operational standards for the 14443 specification. As of 2002, there are more than 50 million ISO 14443 compliant, and 200 million pre-standards smart cards in use around the world. Table 1-1 below contains a summary of the different specifications for contactless communications:
Power supplies for cards are bulky and require maintenance and replacement, so this technology is designed to utilize a passive card technology, and use inductive coupling as a means of deriving power from the RF field being produced by the reader. The ISO 14443a method of sending information to the card (downlink) is 100% amplitude shifting key (ASK) modulation. This method of downlink communications adds complexity to the cards, as they must be able to store enough power to continue to operate when the reader station is sending 0 (no RF signal). The ISO 14443b specification addresses this issue by using a 10% ASK modulated signal for downlink communications, providing continuous operating power for the card while in range of the reader. The encoding method for downlink communications in ISO 14443b is standard non return to zero (NRZ) code. The uplink communications portion of this specification operates on a 847 KHz sub-carrier frequency, which is one sixteenth of the primary carrier frequency. The uplink modulation scheme is binary phase shifting key (BPSK) and also uses NRZ code for its encoding scheme. Figure 1-1 below depicts the modulation and encoding method for downlink and uplink communications respectively. Since the card does not produce its own RF field, it uses load modulation to modify the carrier signal of the reader to transmit its uplink data. Figure 1-2 below depicts the process of load modulation. The load on coil Lc is varied by switching R2 in and out of parallel in the circuit. This causes the load on Lt to vary, and modifies the current U0 flowing through Ri. U0 is monitored by the reader and is used to decode the uplink data transmission.
Figure 1-1: Modulation and encoding graphical representation Figure 1-2: Load modulation diagram (source RFID Handbook, second ed.)
Uplink: Maximum theoretical bandwidth (Nyquist) calculation, using number of states (n) = 2, and sub-carrier frequency = 847 KHz. BW = log2 (n) * fb Max theoretical bandwidth = log2 (2) * 847 KHz = 847 Kbps Standards defined bit rate = 106 Kbps Calculated downlink baud = bit rate = 106 Kbaud/sec Bit rate utilization ratio: defined/maximum = ~ 1/8 Data Communications for Contact-less Smart Cards: The data communication process for the 14443b specification has been designed to allow for communications with multiple cards at concurrently using time division multiplexing. The process involves using an anti-collision technique called “slotted aloha”. The process of card to reader communications is as follows. While idle, the reader continuously generates REQB messages. This message is used to pass information to any card in its field range so that it knows how to respond to its signal. The REQB frame format is shown below in Figure 1-3. The data fields encoded within the frame are the anti-collision prefix (APf), the application family identification (AFI), the PARAM parameter, and a cyclic redundancy check. The anti-collision prefix is a one byte value that is fixed and is used to mark the beginning of each REQB frame. The value assigned to this field is 0000 0101 (binary) or 05 (hex). The application family identification is further used to prevent unnecessary transmissions by smart cards that do not contain the type of information the reader is looking for. Table 1-2 below covers the pre-determined values of this one byte frame and their assigned designations.
Figure 1-3: ISO 14443b Command and Frame Structures Figure 1-4: ISO 14443b card read process block diagram. Courtesy ISO/IEC.
Finkenzeller, Klaus, RFID Handbook Second Edition: Fundamentals and Applications in Contactless Smart Cards and Identification, John Wiley & Sons. International Standards Organization, ISO/IEC JTC1/SC17 N 1531 HID Corporation, ISO Standards, iCLASS Compatibility and Market Position, http://www.hidcorp.com Smart Card Alliance, Contactless Technology for Secure Physical Access, http://www.smartcardalliance.org |
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13.56 MHz Contact-less Smart Card Communications |