dvb s2

DVB-S2

Table of content

• DVB-S2
  • OpenSAND Exploitation
    • Encapsulation
    • BBFrame Scheduling
      • Probes
    • Fade Mitigation Techniques and MODCOD
  • OpenSAND Software Design

OpenSAND implements a protocol based on a simplified emulation DVB-S2 standard for the forward link.

This protocol fragments and encapsulates network packets into BBFrames (whose structure and implementation complies with the DVB-S2 specs) which are sent through dedicated carriers and then rebuilt into network packets at their reception.

The BBFrame scheduling is based on a Strict Priority algorithm. Priorities are assigned to different QoS FIFOs.

To optimize the performance in various conditions, DVB-S2 allows the use of fade mitigation techniques (FMT) among ACM, VCM or CCM. All these modes are supported in OpenSAND by means of appropriate MODCOD / Access type configuration.

Note that the forwarded signaling plane of DVB (namely DVB and MPEG -SI and PSI tables) are currently NOT implemented in OpenSAND. However some signaling channel and mechanisms to send data "out of band" are implemented by the Opensand sotfware to allow signaling exchanges in real time.

OpenSAND Exploitation

The DVB-S2 protocol provides the following features.

Encapsulation

In case DVB-RCS2 is the protocol used on the return link on a star topology (or downlink on a mesh topology), the encapsulation protocol used on DVB-S2 is GSE. Otherwise, the possible encapsulation protocols are either GSE or ULE with MPEG2-TS.

For more information about encapsulation, please consult the encapsulation page.

BBFrame Scheduling

Every spot of each gateway has a specific DVB-S2 scheduler. Its Strict Priority algorithm associates a priority on each QoS FIFOs. Thus, the packets with the highest priority are sent on the first place, continuing with the next lower priority, etc.

For more information, please consult the QoS page.

Probes

The probes related to the DVB-S2 scheduling available on OpenSAND are detailed below (other probes are detailed on their specific feature pages):

• Frames interval: Available on gateways under the Perf section, it shows the time (in milliseconds) passed between two BBFrames.
• BBFrame number: Available on the gateways, for each Spot and category. It displays the number of complete BBFrames created during one scheduling loop.
• Queue size probes: Available on the gateways, for each Spot and category. They display the size of the FIFO queues at each scheduling loop. One probe exists for each queue (AF, BE, EF, NM, SIG). The size is displayed in kbits and in number of packets (packets used in the Encapsulation layer, e.g. GSE) inside the packets section.
• Queue loss probes: Available on the gateways, for each Spot and category. They display the data lost in the FIFO queues at each scheduling loop when the FIFO surpasses the maximum length. One probe exists for each queue (AF, BE, EF, NM, SIG). The loss is displayed in kbits and in number of packets (packets used in the Encapsulation layer, e.g. GSE) inside the packets section.
• Throughput probes: Available on the gateways, on the terminals and on the satellite.
  • On the terminal, the probe L2_from_SAT (in the Throughputs section) displays the throughput of the layer 2 on the forward link (BBFrames).
  • On the gateway, the throughput probes are located in the Throughputs section of each category of each spot. They are divided in two sections:
    • L2_to_SAT_after_sched: contains probes displaying the throughput at the output of each FIFO queue (and one for the total) during one scheduling loop.
    • L2_to_SAT_before_sched: contains probes displaying the throughput at the input of each FIFO queue (and one for the total) during one scheduling loop.
  • On the satellite, the throughput probes are located in the Throughputs section of each spot and each gateway. The probes corresponding to DVB-S2 are those on the forward link (L2_to_ST and L2_from_GW) for transparent mode, or in the donwlink (L2_to_ST and L2_to_GW) for regenerative. The throughput considered is that of the BBFrames.

Fade Mitigation Techniques and MODCOD

Depending on the applied fade mitigation technique (ACM, VCM or CCM), different MODCODs can be enabled.

This list of enabled MODCODs is defined for each forward link carrier on a star topology (or uplink carrier on a mesh topology).

The list of all DVB-S2 MODCODs is defined on a unique file.

For more information about MODCOD, please consult the MODCOD page.

OpenSAND Software Design

The DVB-S2 scheduling is implemented on the block DVB in each SpotDownwardTransparent object of the Gateway in a star topology, or in each SatGw object of the Satellite in a mesh topology.

The encapsulation protocol, fade mitigation techniques, and MODCOD restrictions for the DVB-S2 protocol are implemented by the OpenSAND Manager and the configuration.