VEMCO – VR4-UWM Manual
6
1.3
Acoustic Modem
The acoustic modem embedded in the VR4-UWM is a Benthos Inc. Band C Compact Modem. It operates in
conjunction with the Benthos UDB-9000 Band C Deck Box and transducer (shown below).
The modem on the VR4-UWM receiver stays in a low power sleep mode most of the time, and must be
woken up remotely by a command from the surface. This command is initiated using VUE and takes less than
30 seconds. If communication is lost for 3 minutes, the modem automatically goes back to sleep to conserve
the battery.
Under good conditions, the modem will work successfully at the full bit rate speed of 1200 bps and at a range
up to 1000 meters. When good conditions are not available, communication can be improved by decreasing
the bit rate (rates are listed in Table 1), decreasing the distance between the surface modem and the VR4-
UWM (300 – 500 meters is a typical distance), or adjusting the variable output power on the Deck Box and/or
the VR4-UWM compact modem.
The table below lists the time required to offload different amounts of data using the modem at various bit
rates. These values assume there are no transmission errors. The VUE software detects transmission errors
and automatically requests a re-transmit of the bad packet, ensuring that data is offloaded correctly. Of
course, the more errors that occur, the longer the offload will take. The error rate depends on the local
conditions, bit rate, range, and power level settings. Errors are rare under good conditions, but as conditions
worsen errors can become more frequent. The lower bit rates are more robust against transmission errors, so
if the error rate is high, it is generally better to reduce the bit rate. VUE is usually set for “Automatic Tuning”
so there should be no need to adjust the settings.
Acoustic Modem Data Approximate Offload Times
Modem Bit Rate
(bps)
Data Offload Time
10 kB With No Errors
(minutes)
Data Offload Time100 kB
With No Errors
(minutes)
300
6.4
63.8
600
4.1
41
1066
2.9
29
1200
2.7
27
Benthos Deck Box
Typical modem transmission range:
300m to 500m
Benthos surface modem.
Benthos Band C Compact Modem on VR4-UWM.