Overview

Learn the steps to investigate why a high rate of subscriber busy error may be happening when another operator's home-routed MT-MT SMS traffic is delivered through different RTRs.

The MT message is a long SMS in this scenario, and all of its segments should be routed to the same RTR.

In an MT-MT path, the RTR will replicate the outgoing MT as the incoming MT was received, so you need to check why not all segments of a long SMS were sent to the same RTR that replied to the incoming SRI-SM.

Flowchart

Implement an MTIR rule

Create an MTIR rule to store the incoming MT message on AMS before delivery, so that AMS can deliver all segments in sequence.

Check how incoming MT messages are load-shared to the RTRs

If the traffic is load-shared to all RTRs, that can cause the subscriber busy issue.

The incoming SRI-SM can be load-shared to any RTR using the common GT. Once some specific RTR replies it includes its own GT in the SRI-Resp to receive subsequent MT messages. If STPs are load-sharing incoming MT traffic to any RTR, regardless of the CdPA in the incoming MT, that can be the root cause.

Check for the use of MMTS flag in the MT-fwdSM sent

Check how the other operator is routing long SMSC towards your network. For an MT-MT routing path, it's fine if they send one SRI-SM request for each segment of a long SMS. However, using MMTS flag in one of the MT-fwdSM sent may cause the subscriber busy on the message delivery.

Collect a trace with incoming and outgoing MT traffic of a long SMS

A trace from the foreign network long message containing the incoming and outgoing MT traffic may be useful to understand all the flags and paths involved in the data transmission.

Check if segments can be sent individually, without TP-MMS flag

Check with the foreign network if they can send the long messages in individual segments, without using the TP-MMS flag.