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Initial Configuration Settings on the Cisco Expressway

Chapter Description

In this sample chapter from CCNP Collaboration Cloud and Edge Solutions CLCEI 300-820 Official Cert Guide, you will explore protocol interworking on the Cisco Expressway. It covers both the H.323 and SIP settings and includes interworking on the Expressway. You will also confirm these settings with the verification of registration.

Protocol Interworking on the Cisco Expressway

The Interworking page (Configuration > Protocols > Interworking) lets you configure whether or not the Expressway acts as a gateway between SIP and H.323 calls. The translation of calls from one protocol to the other is known as interworking.

By default, the Expressway acts as a SIP–H.323 and H.323–SIP gateway but only if at least one of the endpoints that are involved in the call is locally registered. You can change this setting so that the Expressway acts as a SIP–H.323 gateway regardless of whether the endpoints involved are locally registered. You also have the option to disable interworking completely.

The options for the H.323 <-> SIP Interworking Mode setting are as follows:

  • Off: The Expressway does not act as a SIP–H.323 gateway.

  • Registered only: The Expressway acts as a SIP–H.323 gateway but only if at least one of the endpoints is locally registered.

  • On: The Expressway acts as a SIP–H.323 gateway regardless of whether the endpoints are locally registered.

Cisco recommends that you leave this setting as Registered Only. Unless your network is correctly configured, setting it to On (where all calls can be interworked) may result in unnecessary interworking, for example, where a call between two H.323 endpoints is made over SIP, or vice versa.

Calls for which the Expressway acts as a SIP to H.323 gateway are Rich Media Session (RMS) calls. The Expressway always takes the media for SIP–H.323 interworked calls so that it can independently negotiate payload types on the SIP and H.323 sides, and Expressway will rewrite these as the media passes. Also, in a SIP SDP negotiation, multiple codec capabilities can be agreed (more than one video codec can be accepted) and the SIP device is at liberty to change the codec it uses at any time within the call. If this happens, because Expressway is in the media path, it will close and open logical channels to the H.323 device as the media changes (as required) so that media is passed correctly.

When searching a zone, the Expressway first performs the search using the protocol of the incoming call. If the search is unsuccessful, the Expressway may then search the zone again using the alternative protocol, depending on where the search came from and the H.323 <-> SIP Interworking Mode setting. Note that the zone must also be configured with the relevant protocols enabled (SIP and H.323 are enabled on a zone by default).

  • If the request has come from a neighboring system and Interworking Mode is set to Registered Only, the Expressway searches the Local Zone using both protocols, and all other zones using the native protocol only (because it will interwork the call only if one of the endpoints is locally registered).

  • If Interworking Mode is set to On, or the request has come from a locally registered endpoint, the Expressway searches the Local Zone and all external zones using both protocols.

SIP endpoints can only make calls in the form of URIs, such as name@domain. If the caller does not specify a domain when placing the call, the SIP endpoint automatically appends its own domain to the number that is dialed. If you dial 123 from a SIP endpoint, the search will be placed for 123@domain. If the H.323 endpoint being dialed is just registered as 123, the Expressway will not be able to locate the alias 123@domain and the call will fail. The solution is to do either of the following:

  • Ensure all your endpoints, both H.323 and SIP, register with an alias in the form name@domain.

  • Create a pre-search transform on the Expressway that strips the @domain portion of the alias for those URIs that are in the form of number@domain.

You will dive into pre-search Transforms in Chapter 7, “Cisco Expressway Call Processing Order,” for more depth on how to accomplish this.

For SIP calls, the Expressway implements RFC 4733 (obsoletes RFC 2833) for dual-tone multifrequency (DTMF) signaling in RTP payloads. For H.323 calls, the Expressway implements H.245 UserInputIndication for DTMF signaling. dtmf is the only supported UserInputCapability. Expressway does not support any other H.245 user input capabilities (e.g., basicString, generalString). When the Expressway is interworking a call between SIP and H.323, it also interworks the DTMF signaling, but only between RFC 4733 DTMF and the H.245 user input indicators dtmf and basicString.

The Expressway can also act as a gateway for calls between IPv4 and IPv6 devices. To enable this feature, select Both for the IP protocol on the IP page (System > Network Interfaces > IP). Calls for which the Expressway is acting as an IPv4 to IPv6 gateway are traversal calls and require a Rich Media Session license.

5. Verifying Registration on the Cisco Expressway | Next Section Previous Section

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