GSM Issue-UE sends SABM and gets a DM response instead of UA response

This issue is most commonly observed in GSM networks where in UE sends SABM and expects network to send UA response but it ends up receiving DM response from the network.SABM stands for Set asynchronous balanced mode, UA stands for Unnumbered Acknowledge and DA stands for Disconnected Mode.

Frame Types of the Abis-Interface

Name Command Frame Response Frame Possible values in control field of LAPDm
SABM Yes No (7F) because P-bit is always 1
DM No Yes (0F), (1F)
UA No Yes (73) because P-bit is always 1

An RLP entity can be in one of two modes:
• Asynchronous Balanced Mode (ABM)
• Asynchronous Disconnected Mode (ADM)

SABM is transmitted by RLP entitity of UE(Mobile) to terminate ADM state. Network respons UE with UA response. If network is not able to enter into ABM state due to any reason, it sends DM response back.

There are three states as mentioned below:
SABM_State (idle, send, wait)
if (.._State = send) the SABM PDU has to be sent
if (.._State = wait) the RLP entity waits for the UA response

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