I found a very easy explanation in this site, explained in which cases we could obtain  SNMP requests timed out.

I am analysing alarms within monitoring system and I would like to know:

1. In which cases we get ping failed

• How does a ping work?
• If we get ping failure, this means that we will get SNMP time out?
• As per explanation on SNMP, could you please confirm that if we get SNMP requests time out it does not mean that we will get ping failure?

1. In which cases we can get packet error?

Have a pleasant day