P-GW did not distinguish signaling traffic from Delay Tolerant or Low Priority
devices such as low priority machine-to-machine traffic.
The UE was able to
indicate its device profile to the MME via NAS and Attach Request messages. The
MME was able to pass this information to the P-GW via the Signaling Priority
Indication Information Element (IE) on the S5 interface. Some UEs may not have
supported the sending of the Signaling Priority Indication IE on the S5
interface to the P-GW. As a result, the P-GW could not distinguish between the
signaling types. With the current release, the P-GW can distinguish between
these signaling types.
during overload situations, the P-GW previously allowed new sessions from Low
Access Priority Indicator (LAPI) devices and treated the traffic from LAPI
devices with the same priority as the normal UEs. With the current StarOS
release, during overload conditions, the P-GW can be configured to back off the
traffic that is identified as LAPI. The identification is based on either the
APN configuration or the Signaling Priority Indicator IE.
The backoff timer
algorithm and the R12 GTP-C Load/Overload Control algorithm work together. This
feature provides the benefit of rejecting low priority calls in turn allowing
more bandwidth for high priority calls.
functionality has been implemented to support the APN Backoff Timer
Priority Indication IE on the S5 interface in the Create Session Request
message identifies the low priority devices.
A LAPI APN can
be configured. Any call landing on that APN is considered a LAPI call.
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.
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.
incoming call is identified as LAPI due to condition 1 or 2 above.
The P-GW is
in the overload state.
timer is configured.
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
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.
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 and the M2M License
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
If a Call is a LAPI call and
the Backoff Timer is Configured and the P-GW is in the Overload State:
License: Call is rejected with cause APN Congestion.
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.
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
The APN Backoff Timer feature requires that the M2M license be
enabled on the P-GW/SAEGW. Contact your Cisco account or support representative
for licensing details.
Backoff Timer Support on the P-GW/SAEGW
describes how to configure APN Backoff Timer Support on the P-GW/SAEGW. The
procedure consists of the following tasks:
Use the following
example to configure LAPI behavior:
configureapn apn_namepdn-behavior lapiend
To disable LAPI
configureapn apn_nameno pdn-behaviorend
pdn-behavior lapi configures the APN as a LAPI
no pdn-behavior disables the LAPI APN
Do not configure
the emergency APN and
lapi settings in the same APN, as these two settings are mutually
exclusive. If both settings are configured in the same APN the
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,
show configuration error
verbose command in
Mode. The command output contains a warning if both settings are
configured in the same APN.
Configuring the APN
Backoff Timer and Jitter Values
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: