Common VoIP Problems

I am currently undertaking a thesis into the common QoS problems with VoIP, this is based on:

Pinpointing a specific QoS problem i.e. Packet-loss, delay or echo etc…

Analysing two methods for solving this problem and comparing them against each other.

I am currently looking for the specific ‘problem’ to anaylse and its mainstream solutions. If anyone can offer any suggestions based on previous experience it would be greatly appreciated.

Thanks in advance!

Matt