Section 7.7. Heterogeneous Signaling

7.7. Heterogeneous Signaling

Heterogeneous or mixed-standard signaling occurs when a single call path must be established using two or more devices of different signaling standards. A SIP endpoint, for example, could call an H.323 endpoint via a softPBX that understands both SIP and H.323. An H.323 endpoint could call an analog phone on the PSTN or connected to a media gateway. A SIP phone could call another office via the PSTN, only to end up ringing a second SIP phone at that office. All would be hetero-signaled calls because they involved endpoints that support opposing standards: SIP, H.323, analog FXS/FXO, and so on.

The call path, that is, the logical path across the network that carries the sound stream for a call, must be softPBX-routed in hetero-signaled calls, because there is no capabilities negotiation between endpoints of opposing signaling standards. Figure 7-12 shows how a SIP server equipped with an FXO interface handles both a SIP signaling leg and a POTS signaling leg in order to set a call up from an IP phone to the PSTN.

Figure 7-12. A SIP proxy server with a POTS interface (like an Asterisk server with an X100P) is able to handle a setup on both the IP leg and POTS leg of the call

Figure 7-13 shows how a single softPBX might handle a call from a SIP phone to an H.323 phone, if it supported both standards. Asterisk supports both standards, and so does a Cisco media gateway.

As in Figure 7-13, the call setup signaling for a call from 3001 to 3002 through a softPBX could work like this:

1. Endpoint 3001 sends a SIP INVITE method to the softPBX, which is its SIP proxy.

2. The softPBX responds with a 100 Trying SIP response while...

3. ...sending an H.225 Setup message to endpoint 3002, which has been resolved through the softPBX, which is also an H.323 gatekeeper.

4. 3002 sends a provisional H.225 Call Proceeding message to the softPBX.

5. 3002 sends an ARQ Verify request to its gatekeeper, which is the softPBX.

6. The softPBX responds with an ACF, indicating the call is legitimate .

7. 3002 sends an H.225 Alerting message to the softPBX; meanwhile, it rings.

8. The softPBX sends a 180 Ringing SIP response to endpoint 3001.

9. When the called party picks up the phone, 3002 sends an H.225 Connect message to the softPBX.

10. The softPBX sends a 200 OK SIP response to 3001.

11. 3001 sends a SIP ACK method to the softPBX.

Figure 7-13. A single softPBX can facilitate calls between legs of opposing signaling standards

7.7.1. Heterogeneous Signaling and the SoftPBX

Different vendors implement heterogeneous signaling differently, and if you're really interested in tracing the signaling steps taken during a particular multiprotocol call, your best bet is to capture them using a packet sniffer or study the standards recommendations themselves . In the meantime, Figure 7-13 should give you a working idea of heterogeneous signaling.

7.7.1.1 Cisco AVVID/CallManager

Cisco's approach to signaling is distributed, meaning that specialized, separate devices are the preferred mechanism for signaling non-SCCP endpoint connections. A media gateway such as the Cisco 2650 is a modular chassis that contains a variety of interface hardware and IOS firmware that is specific to the type of signaling desiredH.323, SIP, and so on.

These media gateways refer to endpoints or remote gateways as dial-peers regardless of their individual signaling standards. A POTS link, an H.323 softphone, and a SIP hardphone are all called dial-peers.

Independently, Cisco media gateways can be used as very basic PBX systems, since they support a programmatic dial-plan, which is quite similar in concept to an IOS routing configuration and can use a variety of different phones. But the preferred, and more enterprise-savvy way to build a phone system is using CallManager, Cisco's full-fledged softPBX.

CallManager supports one endpoint-signaling protocol: SCCP. On the server, capabilities negotiation and other setup functions are handled by H.323, but it's SCCP that signals call progress events to the phones themselves. So, it's fair to say that Cisco's endpoint signaling is mainframe-like, because some big pieces of the signaling process occur on the server rather than "over the wire" between the server and the phone.

In order to use analog, SIP, or H.323 phones with a CallManager PBX, those phones must be connected to a media gateway, where their signals are trunked back to the CallManager, preferably using SCCP. In Cisco's environment, it's the media gateway that is multiprotocol awarenot the PBX.

7.7.1.2 Asterisk

A softPBX built with Asterisk is probably more like an Avaya or Nortel softPBX than like a Cisco one. Asterisk has all the signaling capabilities it needs on one PC server. Whereas a Cisco setup requires trunks to media gateways in order to support non-SCCP endpoints, Avaya's MultiVantage and Nortel's Meridian IP-enabled softPBX systems, like Asterisk, support all the necessary signaling standards in one machine.