Easiest solution: buy phones which have QoS tagging build in. Second easiest solution: but your phones into a dedicated sub-network. Then it is easy to identify phone traffic according to the IP.
Well ... first solution is nice ... but I have many phones allready. Second one has many problems - 1: If it is a softphone, I'll be giving high priority to the PC holding the soft phone and I just want to prioritize RTP. service, not all the services of that PC. - 2: Lets say I use fix ports for softphone's rtp ... but kind of messy if too many softphones. - 3: If doing NAT, according to the KTPD ( Kernel Travelling Packet Diagram ), I could not prioritize by source IP, because, at QoS STAGE, the packet was allready nated, and so, all packets would match.
Regards,
Lucas