[Serusers] SER/* design schemes

Hi all, I am trying to design a VoIP solution covering multiple prefixes (cities) and providing SIP-SIP, SIP-PSTN and PSTN-SIP calls. The alternatives I am considering are: - a single SER installation to register SIP clients (that would be PC softphones mostly) and multiple asterisk servers in each city, and then have SER route calls directly to the appropriate asterisk for PSTN termination; - central SER should forward all PSTN-destined calls to the central asterisk(residing on the same PC), which would then rforward it to the appropriate remote asterisk via IAX. I think this would be more suitable for billing, since all billling should be done by the asterisks; - The other option I'm considering is installing multiple SER+asterisk machines in each remote location, have local users register to the appropriate SER and the forward calls between asterisks via IAX; Which of these would you find most suitable? I think this would interest other people as well. Another thing I find interesting is bandwidth planning - I am in doubt whether to deploy RTP Proxy in my initial design, since that would increase bandwidth requirements dramatically. AFAIK SER only handles signalling messages while most of the bandiwdth (RTP stream) would go directly between the two endpoints without traversing my ISP link. This way, only calls to and from the PSTN should burden my ISP link. Am I right on that?