tree 5f08460d2f27fbf8fc761c1ed1aeca38e53d6788
parent 7895e0456a4e3fb48344932e4ca83d216038eadf
author Philipp <pmaier@sysmocom.de> 1476895138 +0200
committer Harald Welte <laforge@gnumonks.org> 1478078581 +0000

lapd: adding support for ericsson's lapd dialect

Ericsson's RBS2111-BTS (and similar) implements a lapdm dialect which uses
a link establishment method that differs from the standard. While the BSC
is transmitting sabm frames on one specific timeslot, the BTS will
periodically scan through all timeslots to check for incoming sabm frames.
When the BTS detcts the sabm fames on one of the timeslots it will stay
there and continue to commence the link establishment.

The described procedure requires a slightly modified lapd profile, the t200
retransmission timeout has to be configured to exactly 300 msek. Otherwise
the link establishment will fail. Since the BTS will switch from timeslot
to timeslot most of the sabm frames will not be seen by the BTS, so the
maximum retransmission has to be increased. This patch suggests a maximum
retry count of 300, which is an educated guess and has worked fine during
our tests.

Change-Id: I892af57013d7ab4216e9e2d0873a69129aaeb8e5