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:

NameCommand FrameResponse FramePossible values in control field of LAPDm
SABMYesNo(7F) because P-bit is always 1
DMNoYes(0F), (1F)
UANoYes(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.


RF and Wireless tutorials WLAN 802.11ac 802.11ad wimax Zigbee z-wave GSM LTE UMTS Bluetooth UWB IoT satellite Antenna RADAR

Data Communication: Interview Questions and Answers

Prepare for your software engineering job interview with these frequently asked questions on data communication, covering OSI model, TCP/IP, security, and emerging trends.

data communication
network
protocol
Bandwidth Delay Product Calculator

Bandwidth Delay Product Calculator

Calculate the bandwidth delay product (BDP) for wired and wireless systems. Understand its formula and significance in data communication.

bandwidth
delay
product
GSM Uplink Packet Switched Data Call Flow

GSM Uplink Packet Switched Data Call Flow

Explore the packet switched (PS) data call flow in GSM uplink, detailing GPRS attach and PDP context activation between mobile devices and network elements.

gsm
packet switching
data communication