ASA - Average Speed of Answer is a unit of time. It is used to derive service level efficiency for the inbound calls.
for e.g a business has set a target of 70/30 that would mean 70% of calls should be answered within 30 secs. These 30 seconds are counted moment the call is dialed, passes through the IVR or a VDN and is queued up to be picked.
Even if you answer the call at 29th second, the SL for that particular call would be 100%, however at 31st seconds the SL for the call will be 0%.
There are lot of processes/business that use ASA as their service effeciency criteria rather than SL.
Call1 ASA - 29
Call2 ASA - 31
Call3 ASA - 21
Call4 ASA - 35
Call5 ASA - 10
For the business with these 5 calls, the consolidated SL would be 60%