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Cisco Bug: CSCvi00931 - SRVCC HO failing: DL NAS COUNT mismatch

Last Modified

Jul 09, 2019

Products (2)

  • Cisco ASR 5000 Series
  • Cisco ASR 5500

Known Affected Releases

21.2.8 21.2.9 21.5.5 21.8.0 21.8.ca1

Description (partial)

Symptom:
Nearly 30% of SRVCC handovers are unsuccessful (that is, handover to 3G does not happen) and silent voice call drop occurs.

UE vendor says that this happens in cases when NASSecurityParametersfromE-UTRAN value is wrong from the UE perspective.

According to 24.301, NASSecurityParametersfromE-UTRAN contains 4 least significant bits of the current DL NAS COUNT:

 

Table 9.9.2.6.1: NAS security parameters from E-UTRA information element

DL NAS COUNT value (short) (octet 2, bit 1 to 4)

 

This field contains the 4 least significant bits of the binary representation of the downlink NAS COUNT value applicable when this information element is sent.

UE vendor says that UE cannot handle the "HANDOVER PREPARATION" request from the MME in case if MME sends an unexpected value of NASSecurityParametersfromE-UTRAN.

 

Find attached 2 scenarios - "good" and "bad";

 

In the "good" scenario, DL NAS COUNT is increasing for 1 in each DL NAS message;

when ACTIVATE_DEDCT_BEARER_REQUEST is sent, the DL NAS COUNT is set = 5; in "HANDOVER PREPARATION" command, NASSecurityParametersfromE-UTRAN  = 0x06 as expected and SRVCC HO is completed fine.

 

In the "bad" scenario, DL NAS COUNT jumps.

when ACTIVATE_DEDCT_BEARER_REQUEST is sent, the DL NAS COUNT is set = 5; in "HANDOVER PREPARATION" command, NASSecurityParametersfromE-UTRAN  = 0x0C instead of 0x06 and SRVCC HO fails.

Conditions:
N/A
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