Add T2-MI Networks page

SUMMARY.md

@@ -28,6 +28,7 @@
 * [Concepts](configuration/concepts.md)
 * [Electronic Program Guide](configuration/electronic-program-guide.md)
 * [Digital Video Recorder](configuration/digital-video-recorder.md)
+* [T2-MI Networks](configuration/t2mi-networks.md)
 * [Web User Interface Customisation](configuration/web-user-interface-customisation/README.md)
   * [Theme](configuration/web-user-interface-customisation/theme.md)
   * [Date/Time Format](configuration/web-user-interface-customisation/date-time-format.md)

configuration/t2mi-networks.md

@@ -0,0 +1,45 @@
+# T2-MI Networks
+
+Some providers deliver a complete DVB-T multiplex inside a single PID of a
+satellite transponder, rather than broadcasting it terrestrially. The inner
+multiplex is either wrapped in T2-MI baseband frames (ETSI TS 102 773) or
+carried as a plain transport stream (data piping, ETSI EN 301 192), and it is
+often scrambled. The Abertis/Cellnex regional DTT feeds on Hispasat 30°W work
+this way.
+
+A T2-MI network lets Tvheadend receive those inner services directly, without
+an external decapsulation tool. It is available when Tvheadend is built with
+T2-MI support (the default; disable with `--disable-t2mi` at configure time).
+
+## How it works
+
+A T2-MI network does not tune a transponder itself. Instead you point it at one
+or more source muxes that already exist in another network (typically your
+DVB-S/S2 transponders). Tvheadend looks at the services on those muxes, finds
+the ones that carry a T2-MI or piped stream, and creates one inner multiplex for
+each carrier.
+
+Each inner multiplex subscribes to the carrier service on the source mux. The
+stream therefore passes through the normal service and descrambler path first,
+so a scrambled carrier is descrambled exactly like any other service. Only then
+is the inner transport stream extracted and handed back to Tvheadend, where it
+is scanned and its services are mapped to channels like those of any other mux.
+
+## Setting one up
+
+1. Create a network of type **T2-MI Network**.
+2. In **Source muxes**, select the transponders that carry the feed.
+3. Scan the network. Tvheadend discovers the carriers, creates an inner
+   multiplex for each one, and scans those for services.
+
+For feeds that do not use a T2-MI descriptor, leave **Detect private-stream
+carriers** enabled so private data streams are treated as carriers.
+
+An inner multiplex whose carrier temporarily disappears is kept, so its services
+and channel mappings survive a change in the feed. Inner multiplexes are removed
+only when their source mux is removed or the T2-MI network is deleted. Set
+**Network discovery** to Disable to stop new carriers being added automatically.
+
+## Related
+
+Feature pull request: [tvheadend/tvheadend#2199](https://github.com/tvheadend/tvheadend/pull/2199)