Figure 4.6 Prefix-based trunking
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[IP Telephony Cookbook] / Setting Up Basic Services
This is the classic branch office scenario that is based upon the assumption that both networks have different locations, and that dialling prefixes already exist in the PSTN.This is the same problem as the one already addressed in Section 4.1.1.1.1.
4.1.1.2.2 Static individual routing
Prefix-based routing fails to work if you have more than one IP Telephony server and a PBX, all of them within the same dialling address space, and want to allow users to change their legacy PBX phone for an IP phone without switching to a new phone number.
An example of such a scenario is a university that has no structure in its PBX dial plan and that now introduces IP Telephony, but has a computer science faculty that runs its own IP Telephony server. How does the system know where to route a dialled number?
Obviously, a central database storing routing information for each phone number would be a good idea (see Figure 4.7).This usually works, only if all routing entities support the same kind of database, meaning that the legacy PBX and the new IP Telephony server must share the same database. Such a solution is most probably only possible if the PBX and the IP Telephony server come from the same vendor.
2972 -> A
2973 -> B
2974 -> C
2975 -> D
D
A
2975
2972
2974
IP
2973
C