I have Inter rat success rate issue in LTE, where I put IU trace in RNC level and found RANAP_RELOCATION_CANCEL To-CN. With cause Invalid-RAB-ID(30). What could be the reason of this and solution for this.
I decided to request some help from one of the AI Assistants I use often. Perhaps this info can help you resolve the problem.
This is a specific issue often encountered during LTE to 3G (UMTS) Inter-RAT (I-RAT) Handover or PS Handover. Finding a RANAP_RELOCATION_CANCEL message with the cause Invalid-RAB-ID (30) at the RNC level indicates a fundamental mismatch in how the Radio Access Bearer is being identified or mapped between the two technologies.
In I-RAT handovers, the source (eNodeB/MME) must provide the target (RNC/SGSN) with a consistent set of parameters to “hand off” the user’s data session. If the RNC receives a request for a RAB ID it doesn’t recognize or that violates protocol rules, it will trigger a cancellation.
Likely Root Causes
1. RAB ID Mismatch/Mapping Error
The most common cause is a discrepancy in how the RAB ID is mapped from the E-RAB ID (LTE) to the RAB ID (UMTS).
- In UMTS, the RAB ID is typically a 4-bit or 8-bit value (range 0–15 or 0–255 depending on the release).
- If the MME or the eNodeB sends an E-RAB ID that the RNC interprets as out-of-range or already reserved for a different service type (e.g., trying to map a non-GBR bearer incorrectly), the RNC returns “Invalid RAB ID.”
2. Missing Context in the Handover Request
During the Relocation Request from the Core Network (SGSN/MME) to the Target RNC:
- The Source-to-Target Transparent Container might contain corrupted or incomplete information regarding the bearer’s QoS or Identity.
- If the RNC cannot find a valid RAB configuration that matches the ID provided in the container, it sends the RELOCATION_CANCEL.
3. Core Network Version Incompatibility
There may be a protocol version mismatch between the MME (S1 interface) and the SGSN (Iu interface) during the context transfer. If the MME uses a newer IE (Information Element) for the Bearer ID that the older RNC/SGSN cannot parse, it defaults to “Invalid.”
4. Multi-RAB Configuration Issues
If the UE has multiple active bearers in LTE (e.g., Default + Dedicated Bearer for Volte/Video) and the target 3G cell does not support the same number of RABs or fails to map one of them correctly, the entire relocation may be canceled with this error.
Recommended Solutions & Troubleshooting Steps
1. Check the “Relocation Request” Message
Perform a trace on the Iu-PS interface and inspect the RANAP: Relocation Request.
- Look at the RABs To Be Setup List.
- Cross-reference the RAB ID value here with the E-RAB ID in the LTE Handover Required message. They should match or follow the specific mapping rules defined in your vendor’s IRAT parameters.
2. Verify QoS Profile Mapping
Ensure that the LTE QCI (Quality of Class Identifier) is correctly mapped to the UMTS Traffic Class (Conversational, Streaming, Interactive, Background). If the RNC receives a RAB ID for a service it cannot support in its current admission control state, it may reject it as invalid.
3. Review Vendor-Specific Bearer Management
- Huawei/Ericsson/Nokia Specifics: Check if there are parameters governing “RAB ID Offset” or “Max Number of RABs.”
- In some cases, the RNC requires the RAB ID to start from a specific integer (like 5). If LTE starts assigning from 1, a conflict occurs.
4. UE Capability Audit
Verify if the UE involved in the trace supports the specific UMTS frequency or “Inter-RAT PS Handover” capability. Sometimes the network attempts a handover that the device cannot technically execute, leading to a signaling collapse at the RNC.
5. Check SGSN/MME Logs
The RELOCATION_CANCEL is sent To-CN. Check the SGSN logs to see if it received a Forward Relocation Response from the RNC before the cancel. If the RNC never even tried to reserve resources, it points to a configuration/software bug in the RNC’s RANAP handling.
Summary Table for Quick Reference:
| Element | Checkpoint |
|---|---|
| Interface | Iu-PS (RNC to SGSN) |
| Message | Relocation Request / Relocation Cancel |
| Key IE | RAB ID (should be 0–15 usually) |
| Mapping | E-RAB ID (LTE) \rightarrow RAB ID (UMTS) |
| How frequently is this occurring? If it’s isolated to a specific RNC or Site, it’s likely a configuration mismatch in the Neighbor Cell Relation or the RNC’s local bearer setup parameters. |