Variable Coding and Modulation (VCM) is one of the three coding and modulation schemes for the forward link implemented in OpenSAND.
When using VCM, one forward link carrier can use multiple MODCODs (selected before the emulation), which can be used for different types of traffic, depending on the QoS.
VCM is not the default mode of operation in OpenSAND. However, it can be enabled for each forward link carrier from the OpenSAND Manager.
To do so, in the Carrier configuration window, click on the
Configure button under the MODCOD label. This will open a new window, where the different coding and modulation schemes can be selected. Select VCM in the left box, and the available MODCODs will be shown at the right. Any number of MODCODs can be selected.
For each selected MODCOD, a weight can be specified (named
Ratio), which allows to change the amount of the carrier symbols per frame that use this MODCOD. The graph displayed helps visualize the ratios of each MODCOD.
Then, the MODCOD used for each forward link FIFO must be selected. This can be done from the
Advanced window, in the Configuration tab. This configuration is found under each gateway, in the
dvb_ncc section, and for every spot (e.g.:
For each Layer 2 FIFO (consult the QoS page for more information), the default
access_type selected is ACM. To use VCM for one FIFO, select any of the VCM types.
VCM0 corresponds to the first MODCOD displayed in the VCM configuration window,
VCM1 to the second, etcetera.
VCM is used when scheduling packets in the forward link. This is performed by the
ForwardSchedulingS2 in the DVB Block.
When the scheduling is performed for one superframe, the capacity of each VCM subcarrier (each particular MODCOD) is calculated. Then, for each QoS FIFO using a VCM carrier, encapsulated packets are scheduled using the MODCODs supported by each subcarrier, until there is no capacity left.