home
This Wiki describes the OpenSAND emulated satcom features.
[[TOC]]
⚠️ TODO: This page needs to be updated.
This section presents an overview of the Satcom System.
A satellite communication system is composed of a satellite network on the operator side and satellite terminals (STs) at the client side.
The satellite operator part is composed of a backbone network between satellite gateways (GWs) to interconnect them to external networks. It is also composed of several control entities (e.g. NCC, NMC), in charge of resource management, satellite control, and access control.
Satellite communications have been dominated (and still are) by broadcast TV, and this has influenced the actors. DVB-S2, a protocol originally conceived to broadcast TV over satellite, has become a de facto standard for forward links in SATCOM data links. In order to provide a return link data channel, DVB-RCS2 was defined.
In satellite communication systems, the physical characteristics of the medium cannot be disregarded. In particular, geostationary satellite links have two defining characteristics: the propagation delay, and the link attenuation.
The former, an inevitable product of the distance between terminals on Earth and geostationary orbits, makes satellite communications distinct from wired communications, for example, due to the higher latency. The latter is important, not only because of the link budget requirements it entails, but also due to its changing nature: satellite radio signals are affected, for example, by atmospheric phenomena varying in time, and which may affect the normal functioning of the system temporally.
Thus, satellite communications standards have been designed to cope with these effects. For example, DVB-S2/RCS2 implements adaptive coding and modulation techniques to counteract changes on the link attenuation.
In order to access the radio resources, a terminal has to gather the necessary information to communicate with the gateway in a forward link procedure. The information that the terminal can obtain is related and not limited to: identification of satellite and its gateway, identification of superframe sequence number, satellite ephemeris, and identification of logon timeslots.
Once the terminal has this information, its next step is to perform the logon procedure, in order to signal its presence to the NCC, and perform capacity requests if needed. These requests are usually carried in a shared medium, and multiple access techniques are implemented in order to allow multiple terminals to communicate.
At the end, and in a common data plane transfer, the user plane packets coming from the user client computer/device go through the return link allocated physical slots and they are received by the gateway. These packets go through the different layers of satellite gateways in order to be sent to the remote application servers.
The response from servers goes back to the satellite gateway within the forward link process, and then they are scheduled/encapsulated and multiplexed with all the forward traffic, which allows to be sent back to the terminal with the correct addressing.