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By default, OpenSAND uses a star topology. The picture below shows an example of such topology (in case of Multi-Gateway configuration - see note (*) below).
This type of architecture uses a transparent satellite that serves as a signal relay between the gateway and the terminals: on return channels, messages from the STs are sent to the satellite to be relayed to the proper gateway; on forward channels, messages from the GWs are sent to the satellite to be relayed to the proper terminals.
Since the gateway is the entity that does the routing and switching, all traffic between terminals pass through the gateway, hence the star shape. This means that a packet going from, for example, ST2 to ST1, must be first sent to the associated gateway, GW0, (step 1 in the picture). The gateway will route the packet, and send it to the destination ST1 (step 2 in the picture). This whole exchange amounts to two RTTs, which is around 1s in GEO links (with taking into account possible additional MAC delays).
The use of a transparent satellite also affects the choice of forward and return standards and encapsulation protocols.
(*) Multi Gateway topology is an extension of a star topology model/unique GW. In multi-Gateways systems, all traffic are still agregated by central GW points, but with the main difference that all satellite terminals are not associated with the same GW. Also Multi-GW systems are needed for the support of feeder link diversity.
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Star topology (transparent satellite) is the default mode of operation of OpenSAND. However, it can be changed from the Configuration tab, when the Advanced mode is active (via Simulation in the menu bar).
When using transparent mode, DVB-S2 is used on the forward link, and DVB-RCS2 in the return link. Several encapsulations protocols are supported, depending on the standard used on each link.
The choice of a star topology impacts OpenSAND functioning at many levels, since it is a parameter that affects all three components (STs, GWs and SAT), and their interactions.
One main consequence of the use of this topology (and of a transparent satellite), is the architecture of the satellite. In transparent mode, satellites are composed of only three of the blocks that form OpenSAND: the Satellite Carrier block, the Physical Layer block, and the DVB block.
Since actual DVB frames packets are not processed on the satellite instance, no encapsulation/desencapsulation is needed (unlike the case of mesh topologies).
Other features affected by the type of topology include the routing (which is performed at the gateway), the transmission of attenuation measurements, and the type of standard used on forward and return links. The impact (design-wise) of the two different topologies is detailed on each of these features page.
⚠️ Even though the satellite does not host a DVB layer, a DVB-RCS2 block is needed for sharing messages between channels; it does not provide any DVB services.