Added notes for net/mm8

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+= Can
+2020-10-01:s5/net/mm8/noter/1 #2280
+
+Can lever i layer 1 - 3 i OSI modellen.
+
+== Bus
+
+Requires that a single bit is received by all bus members.
+
+1 is recessive, while 0 is dominant.
+Recessive is the default so pull up and dominant is when the line is pulled down.
+
+....
+0 + 0 = 0
+1 + 1 = 1
+0 + 1 = 0 # Because 0 is dominant
+....
+
+CAN defines 0 as the dominant bit.
+
+== Frame
+
+Node id is also used as priority.
+Smaller id gives a higher priority.
+It therefore uses `CSMA/CA`.
+This can be seen below:
+
+1. First everyone starts transits zero.
+2. Then nodes starts transmitting their id.
+3. If a node hears something different than they are transmitting they know they are loosing.
+4. The winner (TAKES IT ALL) can then send the rest of the frame.
+5. Loosing nodes wait for end of frame, so they can try again.
+
+In the ACK field the receiver sends a dominant bit to signal if the frame went fine.
+
+This means that we are constantly listening while receiving and others can also jump in.
+
+=== Addressing
+
+ID is not really a address.
+
+One does not operate on source and destination but more different message ID's.
+Like different sensor values have different id's, and other nodes can then subscribe on these id's.
+
+Nodes only listen for id's they are interested in.
+
+=== Frame types
+
+Data Frame:: a frame containing data.
+Remote frame:: a request for a specific resource.
+Error frame:: transmitted by a node on a error.
+Overload frame:: Whaat
+
+== Bit synchronization
+
+Nodes need to agree on whats on the bus.
+
+Bit time must be twice propagation time.
+This means long lines give slower speeds.
+
+====
+.Problem
+Periodic messages with different priority can give non-deteministic behavior, as their clock go in and out of phase.
+
+This can be solved by finding a worst case and checking if it lives up to a requirement.
+However this can be hard if there are many messages.
+
+One can also run simulations.
+====