ET STANDARDS:Fed Hwy Admin Admits Stalemate over Data Link

FHWA originally hoped to be able to help engineer a new single standard for the next generation of DSRC equipment. But it was too late. Having to accommodate the burgeoning first generation systems stalemated over the divide between asynchronous backscatter and synchronous active transponders was too much. In essence it is Amtech versus Mark IV.

The feds are now addressing the more modest objective of:

(1) nationwide (and hopefully Canadian/Mexican) interoperability for heavy truck operations

(2) regional interoperability for passenger vehicles

This involves quite small departures from the status quo.

The major action involved in the proposed rulemaking is that the FHWA will adopt an active synchronous mode in a new standard called “Dedicated Short Range Communications for Commercial Vehicles” (DSRC/CV). The feds have leverage here since they provide funds for many weigh station bypass, electronic logging and border crossing projects for trucks. They are under a legal mandate to set standards.

Sandwich standard

Says the FHWA in its 12/30/99 Federal Register filing: “It is the Data Link Layer where the current standards process is stalemated. The current version of this standard allows the two fundamentally incompatible protocols, synchronous and asynchronous, to exist. Since there is no clear industry agreement on this protocol, interoperability can best be achieved by continuing to use the Data Link Layer functions found in the legacy systems that conform to ASTM Version 6.”

Data link standards can be written that accommodate both backscatter and active transponders, but there was agreement there is no current demand for such a system. Moreover it involved development expenditures, testing and almost certainly increased cost and reduced performance. No manufacturer therefore would commit to building to it, so the Feds gave up on the exercise.

The standards development thrust in the US is for the more modest goal of developing a system of interoperability for tolling heavy trucks in a limited number of toll lanes at each toll plaza around the country. In most cases the far right toll lane would have both the present local ET reader plus the new DSRC/CV reader, though in heavy truck corridors there might be a dedicated DSRC/CV reader.

Under contract to the Orlando (OOCEA) toll authority Transcore has demonstrated at its San Diego facility that Amtech’s asynchronous backscatter tags can be made to operate adequately in the same toll lane as Mark IV active synchronous tags. It should be no more difficult to operate the antennas for backscatter tags, whether Amtech or CA Title 21 alongside active tags such as the ASTMv6 or the new DSRC/CV standard antennas.

Sandwich or Donut?

DSRC/CV is sometimes called a ‘sandwich’ standard since it wraps slightly different Physical Layer 1 (called ASTM PS 111-98) and a completely different Applications Layer 7 specification (called IEE P1455) around the existing Data Link Layer 2 of the ASTMv6 standard used in current heavy truck equipment for weigh station bypass. The physical layer (One) provides for both active and passive operations. It is agreed upon and approved through the standards setting process and the applications layer (Seven) is very close to formal adoption too.

ASTMv6 was developed by Hughes but is substantially an open standard using time division multiplexing, allocating tiny blocks of time so that communications can be conducted almost simultaneously with several vehicles in the read zone. The new DSRC/CV will be compatible with the existing ASTMv6 equipment in use across the US and much of Canada.

Like the existing vehicle-to-roadside communications in north America, the new DSRC/CV uses the existing 902-928MHz frequency band, though the FCC not long ago awarded the transport sector a large chunk of higher frequency in the 5.9GHz band, but there are no planned uses of this spectrum.

The FHWA filing says that the DSRC/CV will require only small costs in developing and modifying existing ASTMv6 equipment. The new standard will not require scrappage of existing ASTMv6 installations, only that after Jan 1, 2001 procurements must be DSRC/CV compliant to be part of any federally funded program.

“On the benefits side, this provisional standard eliminates the need to purchase equipment with proprietary interfaces, thus freeing agencies of long-term commitments to specific vendors and their systems with proprietary interfaces. This standard also enables operation with reduced mutual interference, so that co-site and inter-site frequency coordination is greatly simplified. The application layer portion of the provisional standard also makes possible the use of the device for applications other than CVO.”

The feds say they want all CVO applications to “eventually” comply with DSRC/CV but it will be left to the discretion of implementing agencies and companies how they proceed with legacy equipment.

The new applications layer IEEE P1455 is a major advance on previous arrangements for handling applications in that it allows the single transponder to be used for a variety of different functions. There is a chance that ICCC P1455 may be adopted internationally, which would mean that other transponders could use the same software as north America.

The FHWA in its filing says that it adopted the active mode for the data link simply because most CVO operations already have that with their ASTMv6 equipment. The new DSRC/CV provides for operations both in a lane-constrained environment (as at a conventional toll plaza) and in the open road environment. The standard has message identifier codes for toll system entry, enrolment, vehicle classification and variable price tolls. It will work in abbreviated or short data mode handling most existing applciations or in a longer data transfer mode for applications yet to be designed. (Contact William S. Jones, ITS Joint Program Office FHWA 202 366- 2128 william.s.jones@fhwa.dot.gov http://www.access.gpo.gov/nara The DSRC/CV regulation is a substantial book. AOL had great difficulty handling the file.)