ASR (Answer Seizure ratio) is a statistical parameter which determines the quality of connection in a given direction through a specific telephony node, such as PBX or switch (or SoftSwitch in IP telephony).

ASR is defined by the International Telecommunication Union (ITU) in SG2 recommendations: E. 411: "International network management — Operational guidance".

ACD (Average Call Duration) is the statistical parameter of the PBX, telephony switch (or SoftSwitch in IP telephony), which shows the average duration of calls in a particular direction.

The ACD value is usually calculated based on the detailed call data records (CDR). Often, ACD is used by providers to assess the demand for directions, as well as determine indirectly the connection quality of a specific direction. For example, if the average statistical ACD value decreases in a certain direction, this means that subscribers do not like the connection quality.

ASR is calculated as a percentage of the answered calls number to the total number of call attempts in a specified direction. While situations such as subscriber unavailability and other rejected calls are considered as unsuccessful call attempts, the calculated value of the ASR parameter may vary depending on the behavior of the dialed subscriber.

Wikipedia