SABM has it starting point with X25 and ISDN.
GSM is built on ISDN therefore you will see LAP-D (LAP-B points
to X25).SABM(E) stands for Set asynchronous balanced mode extended.
It is used to establish a Link Layer. For example with ISDN
several INFO messages are exchanged before a SABME is sent by the
TE. ISDN Network will send a with UA.