FROM THE BLOG

Using MAC extensions for OpenTag and DASH7

Recently, I have been asked about the potential for DASH7 (and OpenTag) to accept extensions in the MAC layer.  These requests have come from multiple sources, who are interested in having special “in-network” behavior combined with generic, standards-based DASH7 behavior.  The short answer is that DASH7 was designed from the beginning to compete with proprietary embedded RF, so it is enormously extensible, and any MAC that can reasonably be implemented on low-power devices can be synthesized in DASH7/OpenTag.

Asynchronous MACs:

DASH7 has a lot of RFID heritage, so any kind of asynchronous data-collection MAC will run great on DASH7/OpenTag.  There isn’t much else to say — there’s no more-capable asynchronous MAC in the world than DASH7.

Synchronous MACs:

By this I am referring to TDMA-type synchronization.  DASH7 can do this too, using an RTC as a source of synchronization.  So, the resolution is 30.5 µs (1/32768 sec), but that’s usually sufficient for the kinds of applications DASH7 addresses.  DASH7 is a strange animal, the way it implements scanning and transmitting via a list-processing engine, but once you figure it out, you will realize that it can implement very intricate TDMA-type MACs.

Semi-Synchronous MACs:

Semi-synchronicity  is DASH7′s playground.  This was the major design requirement, because a semi-synchronous MAC design is the most low-power MAC that is possible using current or next-ten-years technologies.  DASH7 includes a hyper-optimized type of packet called an “advertising packet” that is designed for one purpose: group synchronization of asynchronous devices.  Once asynchronous devices get connected to a master, for a period of time (e.g. one minute) they can be entered into a synchronous mode.  This allows low-duty cycle devices to ramp-up their duty cycles in an event-driven manner.

Defining the MAC:

All DASH7 devices require a system registry that controls many of the operational features.  This registry is standardized.  Anything you can enable in the registry, you can enable on basically any DASH7 device, and certainly any OpenTag device.  The registry can define any kind of MAC between the most simple RFID to something approaching 2G GSM.

Doing Even More:

Unlike IEEE 802 specs, DASH7 is not limited to being merely a PHY/MAC.  There are defined methods of interaction between the upper layers and the MAC, and these include collision avoidance.  Advanced, data-driven CA routines can be synthesized into the DASH7/OpenTag MAC as well.

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