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Design Articles

Near Field Communications

With a range measured in centimeters, NFC is the ultimate in short-range wireless for portable devices, with a wide range of uses. This article explains how it works.

By Steve Rackley

nfc phone

Near field communication (NFC) is an ultra short range wireless communication technology that uses magnetic field induction to enable connectivity between devices when they are in physical contact or within a range of a few centimeters. NFC has emerged as a technology for interconnecting consumer electronic devices from the convergence of contactless identification (e.g. RFID) and networking technologies, and aims at simple peer-to-peer networking through automatic connection and configuration.

The key difference between NFC and standard RF wireless communication is the way in which the RF signal is propagated between the transmitter and receiver. Standard RF communications, such as a Wi-Fi, is described as "far-field" since the communication range is large compared to the size of the antenna. Near field communication relies on direct magnetic or electrostatic coupling between components within the communicating devices rather than free space propagation of radio waves.

Because of the very short range, NFC devices can communicate using extremely low electric or magnetic field strengths, well below regulatory noise emission thresholds, so that there are no limitations on frequency band usage due to licensing restrictions.

NFC technology is a joint development of Philips and Sony, and is based on the ECMA 340 standard. The technology is being promoted by the NFC Forum, whose sponsor members also include MasterCard, Motorola, Nokia and Visa International.

The ECMA 340 standard was adopted by the ECMA General Assembly in December 2004, and defines NFC communication modes using inductive coupled devices operating at a centre frequency of 13.56 MHz. The definition is also known as the near field communication interface and protocol (NFCIP-1). Similar to the more familiar IEEE standards, ECMA 340 specifies the modulation and data coding schemes, data rates and frame format for NFC device interfaces. A simple link layer protocol addresses link initialization and collision avoidance, and a transport protocol, covers protocol activation, data exchange and deactivation.

NFC PHY Layer

ECMA 340 specifies a magnetic induction interface operating at 13.56 MHz and with data rates of 106, 212 and 424 kbps, which is compatible with Philips' MIFARE® and Sony's FeliCaTm contactless smart card interfaces.

Rather than measuring transmitter power and receiver detection levels in dBm as is the case for far-field RF communication, the strength (H) of the magnetic field used in NFC is measured in amps/meter (A/m). ECMA 340 specifies the field values as shown in Table 1.

table 1
Table 1: ECMA 340 NFC Magnetic Field Strength Specification

The ECMA 340 standard defines two communication modes — active and passive. In the active mode, communication is started by an RF field generated by the initiating device (the Initiator) and the target device (the Target) also generates a modulated RF field to respond to the Initiator's command. Modulation and bit coding methods used in active mode are shown in Table 2.

table 2
Table 2: ECMA 340 Active Mode Modulation and Bit Coding Methods

In the passive mode (Table 3), the Initiator starts the communication using an RF field but the Target responds by load modulation rather than by generating an RF field in response. Load modulation entails modulating the load in the target device that the initiating RF field is applied to. This generates sidebands on the original carrier frequency (13.56 MHz) that are detected by the Initiator.

table 3
Table 3: ECMA 340 Passive Mode Modulation and Bit Coding Methods

Protocol Stack

Since NFC is not attempting to provide the full range of network features captured in the OSI model, the protocol stack is very limited and consists of a single simple transport protocol, which defines activation, data exchange and deactivation on an NFC link.

The vestiges of a Data Link layer are also evident in the form of media access control based on CSMA/CA. An Initiator checks for an existing RF field before commencing communication and similarly a Target device in active mode checks for an existing RF field before responding.

figure 1
Figure 1: NFC Collision Detection with Multiple Responding Targets

A single initiating device can interact with multiple targets, each of which generates a random 40 bit ID at the start of the device selection process. The discovery of target device IDs involves an elegant process to resolve collisions which will occur when several targets respond at the same time, particularly when targets are responding in passive mode (Figure 1).

Collision detection at the bit level is made possible by the use of Manchester coding, since a collision is detected when a full bit period occurs without a transition being sensed. This can only occur when a 1-bit transmitted by one target collides with a 0-bit transmitted by another target. Bits received before the collision can be recovered and the targets are requested to re-send data starting with the unrecovered bit. A random delay used by responding targets ensures that this process does not get stuck in a repeating loop.

The data link between devices is transaction based, with initiation and termination occurring around a single data transfer. Initiator and Target negotiate a communication speed, starting with the lowest (106 kbps), in a parameter selection step during transport protocol initiation.

NFC in Practice

Four basic NFC usage models are currently envisaged, as shown in Table 4.

table 4
Table 4: NFC Usage Models

Apart from these usage models in which the NFC connection is used to transfer end-user data, NFC can also be used to securely initiate another connection between two NFC enabled devices. For example, NFC-enabled Bluetooth or Wi-Fi devices may use NFC to initiate and configure the longer range link. Security is assured by the close proximity requirement for NFC operation. Once the Bluetooth or Wi-Fi link is configured, the devices can be separated for longer range communication.

figure 2
Figure 2: PAN Technologies; Range vs. Data Rate

Current and Future Developments of NFC

The first examples of NFC in use have been trials followed by commercial deployment for transport ticketing and payment on a local bus network in Hanau, Germany and on Taipei's Mass Transit Rail system in Taiwan. These trials have been based on the Nokia NFC shell, which clips on to a Nokia 3220 mobile phone.

Future data rates up to 1.7 Mbps are currently planned, approaching the 3 Mbps of Bluetooth 2.0, and market research points to 50% of mobile phones being NFC enabled by 2010.

Summary

The simple PAN landscape, dominated by IrDA and Bluetooth, is becoming increasingly diverse, as shown in Figure 2, with many new technologies being developed that will offer the user a wide range of choices in terms of data throughput, range, power consumption and battery life.


Rerinted with permission from Newnes, a division of Elsevier. Copyright 2007. “Wireless Networking Technology, From Principles to Successful Implementation” by Steve Rackley. For more information about this title and other similar books, please visit www.elsevierdirect.com

This article first appeared in the September, 2008 issue of Portable Design. Reprinted with permission.

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