Figure 4.5 Per-number routing with a) two gateways or b) one gateway P.67
[IP Telephony Cookbook] / Setting Up Basic Services
The first scenario uses two gateways and thus allows the PBX to decide where to route a call to.
IP Telephony servers route calls they cannot resolve locally through a dedicated gateway to the PBX and let it make the routing decision. If both IP Telephony servers support the same signalling protocol, they may contact each other directly (see Section 4.1.1.2).
The second scenario uses only one gateway, so the PBX does not need to know which IP
Telephony server to contact, but merely needs to know the fact that the call has an IP target.
Components in the IP world share a common configuration database that locates a specific number on server A (e.g., a SIP Proxy) or B (e.g., an H.323 gatekeeper).This allows any server or gateway component to make routing decisions.
The described scenario may vary in many ways.The assumption that all IP components use the same routing database is often difficult to achieve, especially if products from different manufacturers are used, because there is no common format for such entries. In this case, it might well be that a separate database is maintained for each component, leading to administrative overhead for their synchronisation with a high risk of inconsistency.
~ 4.1.1.2 Trunking
The bigger an institution is, the more complex its organisational infrastructure.This may be due to the need to support multiple locations (sites) or because certain organisational units need to administer their own communication solutions (and eventually do not adhere the institution's standard procedures). Either way, the IP Telephony system probably consists of more than one server, all with the need to share the same dialling space.
4.1.1.2.1 Prefix-based trunking
One possibility of connecting two or more networks is to give each network a different prefix and make routing decisions based upon those prefixes.This works best if the current PSTN dial plan already provides these prefixes, e.g., in the form of area codes plus subscriber prefixes.