Motorola VRS750, Service Manual

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Detailed VRS Operation
5
Multi-VRS
Operation
The VRS is designed to accommodate multiple VRS units in the same
area. This is accomplished by each VRS maintaining a priority level or
delay state. During either portable-to-base or base-to-portable
transmissions, only the highest priority VRS will transmit or cause a
mobile PTT. The procedures for determining priority levels in multi-
VRS operation are discussed next.
Determining Priority
Levels
Whenever a VRS is enabled, it transmits a Single Tone burst (except if
portable activity is already present). The Single Tone burst frequency is
fixed at 847.5 Hz. By default, when a VRS (VRS number 1) unit is
enabled, it becomes the priority unit with a delay state of zero. When
a second VRS unit (VRS number 2) is enabled in the area, it will
transmit a Single Tone burst as described. Upon receiving this signal,
VRS number 1 will know that another VRS is active and change its
priority to delay state one from delay state zero. If a third VRS were to
be enabled, VRS number 1 would change to delay state two and VRS
number 2 would change to delay state one. This process could
continue up until delay state seven is reached by VRS number 1,
meaning that eight VRS units have been enabled. After eight VRS units
are activated, they begin to stack up at delay state seven.
Portable-to-Base
Transmissions
If a portable begins to transmit, only the highest priority VRS will PTT
its corresponding mobile for transmission to the base. The other VRS
units, however, will be able to hear the portable transmission and
monitor for mobile channel activity. Channel activity is verified by a
squelch detect of the mobile. For non-priority units, the portable’s
audio will be sent to the mobile’s speaker (unmuted) from the VRS, not
from the mobile. This allows the mobile to monitor the correct
transmit frequency. If mode steering options are enabled then the
non-priority VRS units monitor their mobile’s channel activity on the
channel steered to by the portable’s PL and not on the mobile’s current
user selected channel.
The non-priority VRS units have a short delay in which they expect to
see the squelch detect message. Conventional system units will wait at
least 600 ms and trunking system units will wait at least 1500 ms. This
time period is referred to as one delay state. If in the appropriate time
frame a squelch detect message is not received, the priority VRS unit is
assumed to have been disabled and all other VRS units will increase
one priority level (for example, go from delay state 2 to delay state 1).
The VRS units will continue to count down until a squelch detect
message is received. In an extreme case, in which all but the lowest
priority VRS units are disabled, it is possible that the remaining VRS
unit which was the lowest priority (highest delay state) will have to
wait up to 7 delay states before transmitting. This can lead to a
transmission delay of up to 10.5 seconds for a trunking unit.