Proximity Readers Tutorial

By Brian Rhodes, Published May 20, 2013, 12:00am EDT

One of the most common credentials is also the most confusing. How exactly do proximity cards 'read' into an electronic access system? Despite being widely used by more than a half-dozen 'contactless credential' types, that these credentials use nothing more than a simple wave in front of a reader can discount just how complex the process is. In this note, we look at 'proximity credentials', how they work, and the technology behind them.

What is 'Prox'

'Prox' is both a brand specific offering from credential giant HID Global, as well as a term to describe a range of contactless RFID credentials that are read close to, but do not touch the reader. The individual data formats of the credentials differ, but these 'contactless' type use the same general technology to authenticate and are composed of the same basic components.

Proximity Types

A few of the best known 'proximity' types are:

  • 125 kHz Prox: Older generation of HID Prox. Proliferation of 'other' 125 kHz RFID chips, costs of manufacturing, and limited storage bits have yielded to 13.56 MHz.
  • 13.56 MHz Prox: Despite a shorter wavelength, the quicker reading speed, increase in storage bits, and decreased amount of copper in antenna offers advantages over older 125 kHz.
  • Desfire MIFARE: Outside the US (HID's strongest market), MIFARE uses a 13.56 MHz based system build to conform to international format standards.
  • Hybrid/Multiclass: These credentials include more than one 'contactless' technology, that may include 125k, 13.56M, or other proprietary RFID chips that may include write/rewrite abilities.

Even though these products operate using the same methods, they are not interchangeable. Like mechanical keys, proximity credentials are read/used in the same basic way, but the format of the information they contain differs. An automobile ignition key is bitted and has a different physical profile than an office door key, but they both must be inserted into a lock to work. Likewise, a Desfire card will not register on a 125 kHz reader, but both credentials use the same mechanics.

The components in the respective formats are tuned for different frequencies; this allows 'muticlass' prox credentials to exist on the same card without interference issues.

Resonant Energy Transfer

How can a plastic card or fob, with no internal battery, powerlessly communicate over an air gap of 3 inches or more? Proximity cards are "passive RFID" elements that rely on some other device to supply power to the credential. At it's core, resonant energy transfer uses minute voltages, wirelessly transferred via a small radio field, and collected by an antenna embedded into a card or fob.

While minute, the field emits sufficient power over a few milliseconds to be stored within the credential. For a familiar demonstration of 'resonant energy transfer', see the short clip below of putting a lightbulb into a microwave:

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In the case of microwaves, the field excites the inert gas inside the bulb and causes it to alight. For proximity credentials, the total energy transfer is much smaller and tightly contained than the dramatic explosions of YouTube clips, but the energizing method works the same, with the credential taking place of the bulb and the card reader supplying power.

Inside Prox Cards

All 'Contactlesss' RFID credential technology uses variations of three basic components, identified in the image below:

  • Antenna Coil: A lengthy fine copper winding, measuring 20' or more, is coiled within the credential and acts as a bidirectional antenna, both receiving (and energizing) the capacitor, but also broadcasting the ICC Chip's information to a reader.
  • Capacitor: This component stores, for a brief period, the energy emitted by the reader. When the capacity of the capacitor is reached, it simultaneously discharges into the ICC Chip and the antenna.
  • ICC Chip: The chip stores the information contained by the credential, including discrete details like facility codes, unique ID numbers, and other static details stored on the card. This information is broadcast to the listening reader via the Antenna Coil.

A key characteristic of this technology is the low power requirements. While the needed electricity falls well beneath human perception, it means the credential must be positioned close to the energy source - the reader. This describes why 'proximity' card must be mere inches away from the reader in order to be activated:

In the case of 'long range' readers, the size of the unit is larger. Because the power of the signal cannot be increased beyond FCC defined limits, the physical size and sensitivity of the reader's antenna must be larger. A typical proximity reader will measure under 2" x 3" x 0.5", a unit built to read credentials from distances up to 6' can measure 10" x 12" x 2".

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