Please tell the factors due to which ASR and ACD gets low?
There are so many factor due to which ASR (please see SIP cause codes for this), however for ACD there are few factors which make it low.
1) Jitter ( variance in delay)
2) Delay ( more than 300 ms delay decreases the ACD)
3) Packet Loss
4) Codecs have low bandwidth usage has low sampling rate, for example if we increase G729 sampling rate (ptime) to 30 ms instead of 20 ms, it will consume almost same BW as G723 but decreases the ACD
5) PPS (packet per second) capacity of ethernet can be the bottle neck as well.
6) Number of hops ( SBC passing media through them) will also decrease ACD
7) Last but not least, traffic profile also contributes toward the low ACD.