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GSM Issue: UE Receives DM Instead of UA After Sending SABM

gsm radio link protocol ue network data communication

This article discusses a common issue observed in GSM networks where a User Equipment (UE) sends a SABM (Set Asynchronous Balanced Mode) frame and expects to receive a UA (Unnumbered Acknowledge) response from the network, but instead receives a DM (Disconnected Mode) response.

Understanding the Frame Types

Here’s a table summarizing the frame types relevant to 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

RLP Entity Modes: ABM vs. ADM

An RLP (Radio Link Protocol) entity can operate in one of two modes:

Asynchronous Balanced Mode (ABM)
Asynchronous Disconnected Mode (ADM)

The UE’s RLP entity transmits a SABM frame to initiate the transition from the ADM state to the ABM state. The network is expected to acknowledge this request with a UA response, confirming the successful transition to ABM.

The Problem: Receiving DM Instead of UA

The issue arises when the network is unable to enter ABM for some reason. In this scenario, instead of the expected UA response, the UE receives a DM response. This indicates that the network is either unable or unwilling to establish the asynchronous balanced mode connection.

States of SABM

The following states are associated with SABM:

SABM_State (idle, send, wait)
- if (.._State = send): The SABM PDU (Protocol Data Unit) needs to be sent.
- if (.._State = wait): The RLP entity is waiting for the UA response.

In the described issue, the UE enters the “wait” state after sending the SABM, expecting a UA, but receives a DM instead, signaling a problem.

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