Import T documentation from wiki

common/utils/T/DOC/T/multi.md

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+# Multiple tracers
+
+To connect several tracers to the same process, first launch the `multi`
+tracer.
+
+Run as:
+
+```shell
+cd common/utils/T/tracer
+./multi -d ../T_messages.txt
+```
+
+Then connect other tracers (`enb`, `textlog`, `to_vcd`, etc.) to the
+`multi` tracer.
+
+For example (if you debug the `lte-softmodem`) in one terminal, run:
+
+```shell
+cd common/utils/T/tracer
+./enb -d ../T_messages.txt -p 2022
+```
+
+That will launch the GUI tool to analyze the proper running of the
+softmodem.
+
+And in another terminal, run:
+
+```shell
+cd common/utils/T/tracer
+./to_vcd -d ../T_messages.txt -p 2022 -vcd -o /tmp/openair_dump_eNB.vcd
+```
+
+That will dump the VCD traces generated by the softmodem to the
+file `/tmp/openair_dump_eNB.vcd`.
+
+The `multi` tracer waits for other tracers to connect to the port 2022.
+So you pass `-p 2022` to a tracer so that it connects to the `multi`
+tracer. This port can be changed with the `-lp` option.
+
+For example to listen to port 4321, run `multi` as:
+
+```shell
+./multi -d ../T_messages.txt -lp 4321
+```
+
+Then you would pass `-p 4321` to any tracer for it to connect to `multi`.
+
+As a reminder, to trace `lte-softmodem`, you run it as:
+
+```shell
+sudo ./lte-softmodem -O config_file.conf --T_stdout 0
+```
+
+The important part is `--T_stdout 0`.
+
+The order to launch the various programs (`multi`, the softmodem, other
+tracers) is not important.