5.7.2.2 Recording of errors U.K.
The DSRC medium is a dynamic wireless communication in an environment of uncertain atmospheric and interference conditions, particularly in the ‘portable REDCR’ and ‘moving vehicle’ combinations involved in this application. It is therefore necessary to ascertain the difference between a ‘read failure’ and an ‘error’ condition. In a transaction across a wireless interface, read failure is common and the consequence is usually to retry, i.e. rebroadcast the BST and reattempt the sequence, which will in most circumstances lead to a successful communication connection and transfer of data, unless the target vehicle moves out of range during the time required to retransmit. (A ‘successful’ instance of a ‘read’ may have involved several attempts and retries).
Read failure may be because the antennas were not paired properly (failure of ‘aiming’); because one of the antennas is shielded — this may be deliberate, but also can be caused by the physical presence of another vehicle; radio interference, especially from circa 5.8 GHz WIFI or other public access wireless communications, or may be caused by radar interference, or difficult atmospheric conditions (e.g. during a thunderstorm); or simply by moving out of the range of the DSRC communication. Individual instances of read failures, by their nature, cannot be recorded, simply because the communication simply did not occur.
However, if the agent of the competent control authority targets a vehicle and attempts to interrogate its DSRC-VU, but no successful transfer of data ensues, this failure could have occurred because of deliberate tampering, and therefore the agent of the competent control authority needs a means to log the failure, and alert colleagues downstream that there may be a violation. The colleagues can then stop the vehicle and carry out a physical inspection. However, as no successful communication has taken place, the DSRC-VU cannot provide data concerning the failure. Such reporting shall therefore be a function of REDCR equipment design.
‘Failure to read’ is technically different to an ‘error’. In this context an ‘error’ is the acquisition of a wrong value.
Data transferred to the DSRC-VU is supplied already secured, therefore must be verified by the supplier of the data (see 5.4).
Data subsequently transferred across the air interface is checked by cyclic redundancy checks at the communications level. If the CRC validates, then the data is correct. If the CRC does not validate, the data is retransmitted. The probability that data could successfully pass through a CRC incorrectly is statistically so highly improbable that it may be discounted.
If the CRC does not validate and there is no time to retransmit and receive the correct data, then the result will not be an error, but an instantiation of a specific type of read failure.
The only meaningful ‘failure’ data that can be recorded is that of the number of successful initiations of transactions that occur, that do not result in a successful transfer of data to the REDCR.
DSC_82The REDCR shall therefore record, time-stamped, the number of occasions where the ‘initialisation’ phase of a DSRC interrogation is successful, but the transaction terminated before the Data was successfully retrieved by the REDCR. This data shall be available to agent of the competent control authority and shall be stored in the memory of the REDCR equipment. The means by which this is achieved shall be a matter of product design or the specification of a competent control authority.U.K.
The only meaningful ‘error’ data that can be recorded is the number of occasions where the REDCR fails to decrypt the Data received. However, it should be noted that this will only relate to the efficiency of the REDCR software. Data may be technically decrypted, but make no semantic sense.