DASH7 (Active RFID)


Wireless Standard – DASH7(ACTIVE RFID)



A Brief Introduction To RFID Technology for Asset Tracking

Passive RFID Tags (aka Transponders)

RFIDs are actually electronic identification devices.  Similar to bar codes but able to be read by radio transmission instead of by a light source.  An RFID tag is comprised of a microchip and antenna.   The microchip contains the information and the antenna is used to wirelessly transmit that information to a Reader.  The most basic version contains a serialized identifier that uniquely identifies the item (like an automobile license number or Vehicle Identification Number).  The passive version of these RFID tags can be placed on the item and are about 1” square.  They have no on-board power (thus passive) and the Reader device provides that power to the unit by radio energy at the time the RFID is to be read.  Because it must receive this energy from the Reader, the passive tags are used at short range and in most cases must be less then 3 feet from the Reader.

Since the passive RFID tags have no batteries and broadcast their data only when “energized” by a Reader they must be physically polled by the Reader.  By contrast, active RFID tags are battery powered and have a much longer read-range, often times greater then 100 feet.  The active tags are considerably more expensive then passive tags and require an infrastructure consisting of an Antenna and Reader to complete an installation.  Multiple Antenna/Readers are mounted on poles or at portals in a typical installation (Installations are described in a subsequent section).

The RFID tags themselves can have a variety of form factors.  The least expensive ones look like labels that can be affixed to a product or container with an adhesive coating.

Different Types of RFID Tags

The RFID tags embody different characteristics with regard to performance and how their data content has been stored within them, later to be accessed by a Reader.  They range upward in price depending upon their method of storing the information and are generally supplied in three forms:

1. Those with serialized information that is placed into the tag during manufacture;

2. Those that are "Write-Once" tags that permit a user to write to the tag only once during production or installation;

3. Those that are "Read/Write" tags that can be written multiple times by writing over previous data.  These tags are the most expensive.

Data Storage Capacity

The different RFID tags range in data storage capacity from about 16 bits of information on the low end to several thousand bits on the high end (Meaning from 2 alphanumeric characters to 128 characters or more).  In general, the more the data storage the higher the cost.  The price of Passive tags can be less then $1.00 with some in the 20-cent range.

Standardized Information Content - The Electronic Product Code

The Electronic Product Code, or EPC, is an emerging specification for the content of RFID tags.  It was developed at the Auto-ID Center at the Massachusetts Institute of Technology.  The EPC is a standard for encoding the product data in the tag.  The current structure of an EPC tag looks like this:


The RFID Tag Encoder

Choosing the least expensive approach, the user will purchase preprogrammed RFID tags from the manufacturer with the serial numbers already “burned” in.  The next level is to obtain "Writable" RFID tags along with an Encoder in order to place custom codes into the RFID tags.  This is more expensive but allows the user to tailor his identification procedures.

RFID Tag Readers and Antennas

The RFID Reader energizes a passive RFID tag in order to have it respond by sending its digital information back to the Reader.  There are different types of Readers, from hand-held to stationary, as illustrated below:


Fiigure 1 shows a hand-held Reader that can interrogate both passive and active RFID tags.  The Reader can be tethered to a local logging device or can be integrated into a portable wireless data collection system.  These are often used in warehouses, on shop floors and by transportation personnel.  Resembling a bar-code scanner like those for supermarket checkout, the information is usually sent to a backend data base which then stores or acts upon the content.  Cost ranging from $500 to $3,000.

Active RFIDs can also be read by Antenna/Readers on poles.  The Antennas are about the size of a small pizza plate.  Figure 2 shows an active RFID Antenna (left hand) and an active RFID tag (in right hand).  The active RFID tag is about 9” long, 3” wide and about 1 ½” thick.  It is powered by a 3.2 volt lithium battery identical in size to a commercial AA battery.  Antenna/Reader cost about $4,000.

Figure 2

Fixed Readers are often placed at portals to sense when items pass by.  Figure 3 shows an Antenna/Reader mounted on a temporary tower.  As active RFID tags on the trucks or cargo pass by they are "seen" by the Reader which takes data from the on-board RFID tags.  Readers can also be placed at or near conveyor belts, but in the case of passive tags they must be placed within range of the product as it passes by the fixed Reader.  Usually mounted 25' high, these large Antenna/Readers cost about $4000/unit.


Figure 3

Introduction to DASH7

This document is designed as a very brief introduction to DASH7, an RFID standard using active tags operating at the 433 MHz unlicensed frequency band. It also can be used in standards based sensor networks. The link page points to additional websites where more information is available.


DASH7 is a recently ratified wireless active RFID standard originally intended for asset tracking. It has the following characteristics:


Operation at 433 MHz, globally available, unlicensed spectrum

  • Based on ISO 18000-7 standard

  • Multi-year battery life

  • Range of up to 2 km (potentially farther)

  • Penetration of concrete walls, water, and ability to "bend" around metal objects

  • Low latency protocol that enables reliable tracking of moving objects

  • Small, lightweight protocol stack that minimizes silicon costs

  • Data transfer at up to 27.77kpbs (potentially as high as 250kpbs)

  • Sensor & security support



DASH7 has been designed to operate using the“BLAST” concept: Bursty, Light-data, Asynchronous,Transitive.

Bursty: Data transfer is abrupt and does not include content such as video, audio, or other isochronous (i.e.streaming) forms of data.

Light-data: In conventional applications, packet sizes are limited to 256 bytes. Transmission of multiple, consecutive packets may occur but is generally avoided if possible.

Asynchronous: DASH7’s main method of communication is by command-response, which by design requires no periodic network “hand-shaking” or synchronization between devices.

Transitive: A DASH7 system of devices is inherently mobile. Unlike other wireless technologies DASH7 is upload-centric, not download-centric, so devices do not have to be to be managed extensively by fixed infrastructure (i.e. base stations).

DASH7 provides an ultra low power active RFID for asset tracking with very long range due its operation at 433 MHz.

More detailed data can be found at www.dash7.org.



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