add more chapters

satrs-book/src/SUMMARY.md

@@ -2,5 +2,6 @@
 
 - [Introduction](./introduction.md)
 - [Design](./design.md)
-
+- [Communication with Space Systems](./communication.md)
+- [Working with Constrained Systems](./constrained-systems.md)
 

satrs-book/src/communication.md

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+# Communication with sat-rs based software
+
+Communication is a huge topic for satellites. These systems are usually not (directly) connected
+to the internet and only have 1-2 communication links during nominal operation. However, most
+satellites have internet access during development cycle. There are various standards provided by
+CCSDS and ECSS which can be useful to determine how to communicate with the satellite and the
+primary On-Board Software.
+
+# Application layer
+
+Current communication with satellite systems is usually packet based. For example, the CCSDS space
+packet standard only specifies a 6 byte header with at least 1 byte payload. The PUS packet
+standard is a subset of the space packet standard which adds some fields and a 16 bit CRC, but
+it is still centered around small packets. `sat-rs` provides support for these ECSS and CCSDS
+standards to also attempts to fill the gap to the internet protocol by providing the following
+components.
+
+1. UDP TMTC Server. UDP is already packet based which makes it an excellent fit for exchanging
+   space packets.
+2. TCP TMTC Server. This is a stream based protocol, so the server uses the COBS framing protocol
+   to always deliver complete packets.
+
+# Working with telemetry and telecommands (TMTC)
+
+The commands sent to a space system are commonly called telecommands (TC) while the data received
+from it are called telemetry (TM). Keeping in mind the previous section, the concept of a TC source
+and a TM sink can be applied to most satellites. The TM sink is the one entity where all generated
+telemetry arrives in real-time. The most important task of the TM sink usually is to send all
+arriving telemetry to the ground segment of a satellite mission immediately. Another important
+task might be to store all arriving telemetry persistently. This is especially important for
+space systems which do not have permanent contact like low-earth-orbit (LEO) satellites.
+
+The most important task of a TC source is to deliver the telecommands to the correct recipients.
+For modern component oriented software using message passing, this usually includes staged
+demultiplexing components to determine where a command needs to be sent.
+
+# Low-level protocols and the bridge to the communcation subsystem
+
+Many satellite systems usually use the lower levels of the OSI layer in addition to the application
+layer covered by the PUS standard or the CCSDS space packets standard. This oftentimes requires
+special hardware like dedicated FPGAs to handle forward error correction fast enough. `sat-rs`
+might provide components to handle standard like the Unified Space Data Link Standard (USLP) in
+software but most of the time the handling of communication is performed through custom
+software and hardware. Still, connecting this custom software and hardware can mostly be done
+by using the concept of TC sources and TM sinks mentioned previously.
+