Answer to Seizure Ratio-Based Routing

New SIP session agent constraints set a threshold for Answer to Seizure Ratio (ASR) has been implemented. ASR is considered when determining whether session agents are within their constraints to route calls (in addition to session and rate constraints).

The new session agent constraints indicate the minimum acceptable ASR value and computes the ASR while making routing decisions. ASR is calculated by taking the number of successfully answered calls and dividing by the total number of calls attempted (which are known as seizures).

If the ASR constraints are exceeded, the session agent goes out of service for a configurable period of time and all traffic is routed to a secondary route defined in the local policy (next hop with higher cost).

The two session agent constraints are:

• minimum seizure: determines if the session agent is within its constraints. When the first call is made to the session agent or the if calls to the session agent are not answered, the minimum seizure value is checked.

  For example, if 5 seizures have been made to the session agent and none of them have been answered, the sixth time, the session agent is marked as having exceeded its constraints and the calls will not be routed to it until the time-to-resume has elapsed.

• minimum ASR: considered when make routing decisions. If some or all of the calls to the session agent have been answered, the minimum ASR value is considered to make the routing decisions.

  For example, if the you set the minimum ASR at 50% and the session agent’s ASR for the current window falls below 50%, the session agent is marked as having exceeded its constraints and calls will not be routed to it until the time-to-resume has elapsed.