VoIP in the corporate network - Voice Networks

Communications News, July, 2003 by Olivier Hersent

When evaluating a voice over Internet protocol (VoIP) solution, the option of using an "intelligent" phone (usually associated with the SIP or H.323 protocol) instead of the usual stimulus-controlled business phone should be considered. This is a fundamental design choice that will significantly influence an enterprise's future IP-PBX experience.

Before deciding on an IP phone. an organization should look closely at the protocols supported by the phone. Most traditional PBXs use proprietary "stimulus protocols" to control business phones, since standardized protocols, like ISDN, do not offer enough flexibility to provide many of the features required from business phones. The stimulus protocol allows phones to be completely controlled, at a low level, from the central PBX, which facilitates management and provides more features than standard analog or ISDN phones.

Although many IP-PBX systems still use proprietary protocols to control the IP phones, this is only because most of them are ports of traditional PBXs, where the proprietary stimulus protocol is simply tunneled-over-IP packets. With media gateway control protocol (MGCP) and its business phone extensions, however, a standard for stimulus protocols exists, providing the necessary low-level control required for even the most complex business telephony system features.

Providing telephony service with a smart phone may introduce problems, which led the PBX industry to use stimulus protocols in the past. For this reason, protocols such as SIP or H.323 have a key role to play in network infrastructure or on the network side of IP-PBXs replacing CAS or ISDN.

An IP-PBX should use a proprietary (such as Cisco SCCP or Nortel UniStim), or standard (such as MGCP or H.248) stimulus protocol for all business phones and analog line adaptors, and allow SIP or H.323 for videoconferencing stations and PC softphones. Such an IP-PBX would use H.323 and/or SIP on the network side in order to provide an easy connection to VoIP service providers, without requiring a local VoIP gateway.

Some IP phones can run either in proprietary stimulus mode or with MGCP. These phones can seamlessly migrate from an IP-PBX model to an IP Centrex model.

Once you have selected the proper IP phone, migrating your PBX installation to VoIP can be done without major risks. If you do not trust or do not need to use an IP-based Centrex service, you can use a new generation IP-based PBX. In both cases, you will enjoy the same services as before, with a greatly reduced management cost due to the unified network and the ease of move/add/change operations associated with VoIP technology.

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