User's Manual

MPI 6000 Multi-Protocol Reader System Guide
4-6
3 reads @ 4 milliseconds each = 12 milliseconds
3 writes @ 4 milliseconds each = 12 milliseconds
5 GENACKs @ 0.8 milliseconds each= 4 milliseconds
= 28 milliseconds total, each full hand-
shake
To complete 4 full handshakes (simply a rule of thumb), the vehicle would need to be
in the footprint for 112 milliseconds. If the agency requires 100 mph operation, the
vehicles travel one foot in 6.8 milliseconds. At this speed, the footprint would need to
be 16.47 feet long to satisfy this requirement.
This footprint value can change depending on the use of time division multiplexing
(TDM), which will increase the footprint requirements, or by using more sophisticated
polling methods, which may reduce the footprint requirements. Furthermore, the times
presented in this example for the individual components of the transaction can vary.
For example, a password-protected read or write operation can take longer to com
-
plete than an ordinary read or write and can impact the overall statistical reliability of
the transaction.
Note: Please consult with TransCore to assess the impact of the more sophisticated
types of transactions.
Given the uncertainties of any RF link due to reasons already discussed in this section,
a short transaction of only a few milliseconds has a statistically better chance of suc
-
ceeding than will a complex, longer 30-millisecond transaction. The tag is assumed to
remain in the footprint for a minimum period of time relative to the maximum vehicle
speed and the size of the footprint.
Once the length of the footprint has been determined, the presence of light curtains or
detection loops may dictate the point at which the first tag read should occur. Also,
manned lanes or mixed-use lanes typically require that the tag read occur at least a few
feet in front of the toll collection point. The speed requirements may be reduced for
these lanes and, hence, the footprint size. The point of the first read may be controlled
by antenna placement, uptilt angle, and RF power, which are discussed later in this
chapter. Likewise, if the lane is exceptionally wide or if there is a need for better cov
-
erage toward the lane sides, the antenna may be mounted higher or in line with other
antennas. A lower gain antenna may be used to increase the side coverage.
RF Factors
The RF factors involved in tuning an AVI system may include the following parame-
ters:
• The downlink and uplink transmitted RF power
• Range control adjustments that can be made to the receiver
• Antenna type
• Antenna mounting, that is, lane position (relative to payment point, angle, and
height)