RR SABM + MM CM SERVICE REQUEST
TCH, SAPI = 0
This is the first message that is sent after tuning to the
channel. The Mobile initiates a LAPm connection with the
BSC by sending a Set Asynchronous Balanced Mode
(SABM) message. The service request message meant
for the MSC is also sent in this message.
RR UA
TCH, SAPI = 0
The BSS replies with Unnumbered Acknowledge (UA) to
complete the LAPm setup handshake
BSS Expert