manuals: Add description from README.md to manual
Change-Id: I38db6f5723b8f02920abd70bf10c2a134060d183
diff --git a/doc/manuals/chapters/remsim-bankd.adoc b/doc/manuals/chapters/remsim-bankd.adoc
index 11a8811..27abe69 100644
--- a/doc/manuals/chapters/remsim-bankd.adoc
+++ b/doc/manuals/chapters/remsim-bankd.adoc
@@ -1,8 +1,62 @@
== remsim-bankd
+The `remsim-bankd` (SIM Bank Daemon) manages one given SIM bank. The
+initial implementation supports a PC/SC driver to expose any PC/SC
+compatible card readers as SIM bank.
+
+`remsim-bankd` initially connects via a RSPRO control connection to
+`remsim-server` at startup, and will in turn receive a set of initial
+[client,slot]:[bankd,slot] mappings. These mappings determine which
+slot on the client (corresponding to a modem) is mapped to which slot on
+the SIM bank. Mappings can be updated by `remsim-server` at any given
+point in time.
+
+`remsim-bankd` implements a RSPRO server, where it listens to connections
+from `remsim-clients`.
+
+As PC/SC only offers a blocking API, there is one thread per PC/SC slot.
+This thread will perform blocking I/O on the socket towards the client,
+and blocking API calls on PC/SC.
+
+In terms of thread handling, we do:
+
+* accept() handling in [spare] worker threads
+** this means blocking I/O can be used, as each worker thread only has
+ one TCP connection
+** client identifies itself with client:slot
+** lookup mapping based on client:slot (using mutex for protection)
+** open the reader based on the lookup result
+
+The worker threads initially don't have any mapping to a specific
+reader, and that mapping is only established at a later point after the
+client has identified itself. The advantage is that the entire bankd
+can live without any non-blocking I/O.
+
+The main thread handles the connection to `remsim-server`, where it can
+also use non-blocking I/O. However, re-connection would be required, to
+avoid stalling all banks/cards in the event of a connection loss to the
+server.
+
+worker threads have the following states:
+* INIT (just started)
+* ACCEPTING (they're blocking in the accept() call on the server socket fd)
+* CONNECTED_WAIT_ID (TCP established, but peer not yet identified itself)
+* CONNECTED_CLIENT (TCP established, client has identified itself, no mapping)
+* CONNECTED_CLIENT_MAPPED (TCP established, client has identified itself, mapping exists)
+* CONNECTED_CLIENT_MAPPED_CARD (TCP established, client identified, mapping exists, card opened)
+* CONNECTED_SERVER (TCP established, server has identified itself)
+
+Once the client disconnects, or any other error occurs (such as card I/O
+errors), the worker thread either returns to INIT state (closing client
+socket and reader), or it terminates. Termination would mean that the
+main thread would have to do non-blocking join to detect client
+termination and then re-spawn clients, so the "return to INIT state"
+approach seems to make more sense.
+
+
=== Running
-remsim-bankd currently has the following command-line options:
+`remsim-bankd` currently has the following command-line options:
==== SYNOPSIS