[Serusers] SER/* design schemes

[maka]

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?