Figure 4.2 SIP/H.323 zone using a signalling gateway
The gateway is used to translate signalling between the two worlds.The media stream may still be exchanged directly between the endpoints, but eventually (see Section 5.1.4.4) the gateway needs to transcode different codecs, even if both endpoints support the same codec.This problem might occur if either the gateway or the entity calling the gateway (endpoint or gatekeeper) does not support FastConnect (see Section 2.2.1.5.1).
An architectural problem similar to the last one is the use of servers that feature a proprietary IP
Telephony protocol and provide a SIP or an H.323 interface that is limited to basic call functionality without any supplementary services or security features. An example of this is the popular Cisco CallManager.
~ 4.1.1.1 PSTN gateways / PBX migration
The most common scenario for introducing IP Telephony systems is to integrate them with an existing PBX. On the technical side, this usually involves a gateway that translates between H.323
or SIP and QSIG or other protocols over an S2M interface.The services that can be provided between the legacy PBX and the IP world will depend on the QSIG implementation of the PBX
and the gateway vendor.There is no general advice here but to test before buying.
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[IP Telephony Cookbook] / Setting Up Basic Services
Besides technical aspects, organisational aspects of PBX-VoIP integration call for careful planning and analysis.The question is how to integrate legacy and IP Telephony equipment into the dial plan of an organisation. Is it necessary that the phone number reflects whether the participant is a VoIP user or a PBX user.
Generally, there are three possibilities to explore, as explained below.There are more details on setting up an IP Telephony gateway in Section 5.1.
4.1.1.1.1. Routing based on a number prefix
This option requires that, in the dial plan, there is a numberblock available for IP Telephony use.
This is easy to implement on legacy PBXs and IP Telephony servers but the result is new phone numbers for every user that switches from legacy to IP Telephony.
Figure 4.3 Routing based on number prefix
In this example, the PBX is configured to forward every call starting with the prefix 8 or 9 to the gateway.The gateway passes the call to one of the IP Telephony servers depending on the prefix 8
or 9. Unknown target prefixes are routed to the legacy PBX. An IP Telephony server only needs to route all internal calls with unknown prefixes to the gateway.
4.1.1.1.2. Per-number routing, one server: per-number routing on the PBX.
When migrating from a legacy PBX towards IP Telephony, provision of seamless migration is often required for users switching over from the PBX to the IP world, e.g., when a user decides to switch to IP Telephony but wants to keep his telephone number.
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