The following functionality has been implemented to support the APN Backoff Timer functionality:

1. The Signaling Priority Indication IE on the S5 interface in the Create Session Request message identifies the low priority devices.

2. A LAPI APN can be configured. Any call landing on that APN is considered a LAPI call.

3. A back off timer value is configured in the APN. The backoff timer is sent in Create Session Response messages in the P-GW Back-Off Timer IE.

4. If the following conditions are met, the call is rejected with the cause APN Congestion. The configured backoff timer value is inserted in the Back Off Timer IE in the Create Session Response message.

   • Once an incoming call is identified as LAPI due to condition 1 or 2 above.

   • The P-GW is in the overload state.

   • The backoff timer is configured.

5. The M2M license must be present and enabled on the system. The GTP-C Load/Overload feature's configuration and backend reporting of overload parameters works with the M2M license.

6. In some dictionaries (for example, custom35) CDRs have a LAPI field for LAPI calls. This field is populated when the Signaling Priority Indication IE indicates that the call is a LAPI call. However, if the call is of type LAPI due to the LAPI APN configuration, then the CDR should not have the LAPI field.

7. Bulk statistics, counters, and statistics have been implemented at the APN and P-GW service level to show the number of calls rejected due to the APN Backoff Timer feature.

To detect whether the P-GW/SAEGW is in the overload state, the GTP-C Overload feature's framework has been used. It is tied to the M2M license so that only overload configuration and reporting of the overload state is enabled. The remaining overload features such as, sending the OCI towards the peers, throttling, and receiving OCIs from peers do not work. This leads to the following behavioral scenarios:

If a Call is a LAPI call and the Backoff Timer is Configured and the P-GW is in the Overload State:

• Only M2M License: Call is rejected with cause APN Congestion.

• Only GTP-C Overload Enabled: Not applicable as backoff timer cannot be configured.

• Both M2M and Overload Enabled: Call is rejected with cause APN Congestion. That is, LAPI takes priority over the GTP-C Load/Overload feature.

If a Call is Not LAPI or Backoff Timer is Not Configured and the P-GW is in the Overload State:

• Only M2M License Enabled: No impact. Call is accepted normally.

• Only Overload License Enabled: Call accepted. OCI will be sent in the Create Session Response message. Overload feature works normally.

• Both M2M and Overload Features Enabled: Call accepted. OCI is sent in the Create Session Response message. Overload feature works normally. Backoff Timer feature is ignored.

Use the following example to configure LAPI behavior:

configure 
   apn apn_name 
      pdn-behavior lapi 
      end 

To disable LAPI behavior:

configure 
   apn apn_name 
      no pdn-behavior 
      end 

Notes:

• pdn-behavior lapi configures the APN as a LAPI APN.

• no pdn-behavior disables the LAPI APN configuration.

Caution

Do not configure the emergency APN and pdn-behavior lapi settings in the same APN, as these two settings are mutually exclusive. If both settings are configured in the same APN the pdn-behavior lapi configuration takes priority. As a result, if both settings are configured and the system is overloaded, the call will be rejected. To determine if both settings are configured in the same APN, execute the show configuration error verbose command in Exec Mode. The command output contains a warning if both settings are configured in the same APN.

The P-GW requires a fixed value and a jitter to introduce randomness in the Backoff Timer value that is returned to the MME for different sessions; this helps prevent a session storm after the Backoff Timer expiry.

Use the example below to configure the APN Backoff Timer and Jitter Values:

configure 
   apn apn_name 
      backoff timer-value seconds [ jitter seconds ] 
      end 

To disable Backoff Timer functionality:

configure 
   apn apn_name 
      no backoff timer-value 
      end 

Notes:

• backoff timer-value seconds configures the backoff timer value, in seconds. Valid entry is an integer from 0 to 576000 seconds. There is no default setting.

• jitter seconds configures the jitter value, in seconds. Valid entry is from 0 to 1000 seconds. There is no default setting.

To verify the configuration:

In Exec Mode, issue the show apn name apn_name command, apn_name is the name of the APN for which you want to view configuration settings.

In the command output, look for the following fields:

• pdn behavior: <lapi> or <no pdn-behavior>

• Backoff Timer Value: <seconds> Jitter: <seconds>

Verify that the configuration settings are correct. If any of the settings are incorrect, use the configuration procedure in this chapter to reconfigure the incorrect setting(s).

This section describes the Exec Mode show commands and output available to monitor the APN Backoff Timer Support on the P-GW/SAEGW feature.