Dienstag, 8 März 2011
Using NFC data is exchanged by two inductively coupled coils — one per appliance — generating an magnetic field with a frequency of 13.56 MHz. The field is modulated to facilitate data transfers. For the communication one device acts as the initiator (starting the communication) whereas the other device operates in target mode (waiting for the initiator). Thus not more than two devices can be evolved in the communication.
The rolls of the devices — initiator and target — are assigned automatically during the listen-before-task concept which is part of the mode switching of NFC. In general each NFC device acts in target mode. Periodically the device switches into initiator mode in order to scan the environment for NFC targets (= polling) and then falls back into target mode. If the initiator finds a target an initiation sequence is submitted to establish the communication and then starts exchanging data.
NFC distinguishes two operation modes for communication: passive and active mode.
In passive mode only the device that starts the communication (the initiator) produces the 13.56 MHz carrier field. A target introduced to this field may use it to draw energy but must not generate a carrier field at its own. The initiator transfers data by directly modulating the field, the target by load-modulating it. In both directions the coding complies with ISO14443 or FeLiCa, respectively. This mode enables NFC-devices to communicate with existing contactless smart cards. The term load modulation describes the influence of load changes on the initiator’s carrier field’s amplitude. These changes can be perceived as information by the initiator. Depending on the size of the coils, ranges up to 10 cm and data rates of 106, 212, and 424 kBit/sec are possible.
When in active mode, both appliances generate an RF field. Each side transmits data by modifying its own field, using an Amplitude Shift Keying (ASK) modulation scheme. Advantages compared to passive mode is a larger operating distance (up to 20 cm) and higher transmission speeds (eventually over 1 MBit/sec). To avoid collisions only the sending device emits a electromagnetic field; the receiving entity switches off its field while listening. If necessary these roles can change as often as needed.
Usecases and Applications
An NFC compliant device offers the following modes of communication:
- Reader/Writer Mode: In Reader/Writer mode an NFC system acts as an ordinary reader for contactless smart cards. If two or more cards are present in the reader’s carrier field one is selected using an anti-collision algorithm. NFC also takes care of sensing whether the chosen card is ISO 14443-A/B or FeLiCa compliant. The method used for anti-collision is dependent on the type of card detected. This mode causes the NFC device to act as an active device. From an application’s view there is no difference between a conventional and an emulated terminal, accesses to the contactless token proceed equally.
Operating in this mode, the NFC device can read and alter data stored in NFC compliant passive (without battery) transponders. Such tags can be found on e. g. SmartPoster allowing the user to retrieve additional information by reading the tag with an NFC device. Depending on the data stored on the tag, the NFC device takes an appropriate action without any user interaction. If e. g. an URI was found on the tag the handset would open a web browser.
- Card Emulation Mode: Tag emulation mode is the reverse of reader/writer mode: A contactless token is emulated. Now the device acts soley in passive mode. Due to the fact that the card is only emulated it is possible to use one NFC wdevice to act on behalf of several “real” smart cards. Which card is presented to the reader depends on the situation and can be influenced by software. Additionally an NFC device can contain a secure element to store the information for the emulated card in a secure way.
In this case an external reader cannot distinguish between a smart card and an NFC device in card emulation mode. This mode is useful for contactless payment and ticketing applications for example. Actually, an NFC enabled handset is capa-ble of storing different contactless smartcard applications in one device.
- Peer-to-Peer Mode: This mode is specific to NFC. After having established a link between the two participants (the method is equal to ISO 14443-A) a transparent protocol for data exchange can be started. The data block size can be chosen freely, with an MTU (maximum transmission unit) limited to 256 bytes. Main purpose of this protocol is to enable the user to send his/her own data as soon as possible (i. e. after a few milliseconds). In a peer-to-peer session either both initiator and target can be in active mode or initiator in active and target in passive mode. This helps the target to reduce its energy consumption and is therefore especially useful if the initiator is a stationary terminal (e. g. a ticket counter) and the target a mobile device (e. g. a mobile phone).
The NFC peer-to-peer mode (ISO 18092) allows two NFC enabled devices to establish a bidirectional connection to exchange contacts, bluetooth pairing information or any other kind of data. Cumbersome pairing processes are a thing of the past thanks to NFC technology. To establish a connection a client (NFC peer-to-peer initiator) is searching for a host (NFC peer-to-peer target) to setup a connection. Then the Near Field Communcation Data Exchange Format (NDEF) is used to transmit the data.
The architecture for the integration of NFC into a mobile phone normally looks the following:
- NFC Phones
- NFC Stickers