9.68 Auto-RollOff

The Auto-RollOff logical device adds an automatic roll-off selection to IRDs which lack from this feature or which do not behave properly with the roll-off set to AUTO.

The Auto-RollOff device is linked to an IRD's rollOff and symbolRate parameters, it provides an rollOff parameter which the GUI uses instead of the IRD's one. The device's rollOff parameter offers the same selections as the the parameter at the IRD, with an AUTO setting added.

If the rollOff parameter is set to one of the 20%, 25% or 35% settings, this value is passed to the IRD. If set to AUTO, the device computes and sets the rollOff setting from the symbol rate using these formulas:

1. Compute the occupied bandwidth BWocc = symbolrate * 1.2
2. From the occupied bandwidth evaluate the allocated bandwidth BWalloc by extending it to the next defined slot bandwidth in the list (see below).
3. Evaluate the roll off factor by comparing the allocated bandwidth to the symbolrate: if symbolrate * 1.35 <= BWalloc then rolloff = 35% else if symbolrate * 1.25 <= BWalloc then rolloff = 25% else if symbolrate * 1.20 <= BWalloc then rolloff = 20% else raise fault The evaluated roll of factor is commanded to the IRD, the same applies if the symbol rate changes.

Device window pages

The following table shows which device window pages are available with this individual device type. Tool-bar functions not mentioned here are described at the general description of device windows .

• --- Sets the roll of factor
• --- The faults page.
• --- The device info page.
• --- The maintenance page. The configuration parameters from this page are described below.

Configuration parameters

At the maintenance page of the device window there are a couple of configuration parameter which must be set to interconnect the Auto-RollOff device with the IRD device.

List of allocated bandwidths

The device provides a setup parameter to specify the list of allocated bandwidth value to use. When specifying this list, the following rules apply:

• The bandwidth values are floating point values, representing the bandwidth in Hz with the same magnification the IRD's symbol rate parameter uses. The IRD driver coming with the sat-nms software all specify the symbol rate in terms of Msym/sec, hence the bandwidth values are in MHz.
• The values must appear in ordered sequence, starting with the smallest bandwidth, ending with highest one. The last value also works as fallback for symbol rates which produce a higher occupied bandwidth.
• The list element must be separated by comma characters, spaces or other delimiters are not allowed.

Fault flags

The Auto-RollOff device provides one fault flag which is raised if in AUTO mode a symbol rate is recognized which occupies a bandwidth wider than the widest allocated slot, even with 20% roll off set. The flag remains active until a valid symbol rate is set at the IRD.

Variables defined by this device driver