7.7 IP PBXs

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A relatively new development is the IP-based PBX, which transports intraoffice voice calls over an Ethernet LAN and, via an IP-PSTN gateway, transports off-net calls over the PSTN. In a nutshell, voice is converted to data, the data is compressed, and the compressed data is arranged into packets using the IP. At the other end, this process is reversed, allowing calls to be made even to conventional phones through the PSTN.

Full-featured digital phone sets link directly to the Ethernet LAN via a 10BaseT interface, without requiring direct connection to a desktop computer. Phone features can be configured using a Web browser. Existing analog devices, such as phones and fax machines, can be linked to the LAN via a gateway. In addition to IP nets, calls can be placed or received using T1, ISDN Primary Rate Interface (PRI), or traditional analog telephone lines.

All the desktop devices have access to the calling features offered through the IP PBX management software running on a LAN server. The call management software allows hundreds of client devices on the network, like phones and computers, to perform functions such as call hold, call transfer, call forward, call-park, and calling party ID. Other advanced capabilities—such as multiple lines per phone or multiple phones per line—are also implemented in the call management software.

All of the major PBX vendors offer IP interfaces to their conventional PBX systems. Lucent, for example, offers an IP trunk interface for certain models of its DEFINITY product line. The IP interface supports 24 ports and allows businesses to integrate least- cost routing and class-of-service features, giving network managers the ability to add the Internet and corporate intranet as alternative routes for voice and fax services. The ability to add the IP trunk interface directly into DEFINITY products also reduces the cost of obtaining Internet telephony capabilities by eliminating the need to buy a separate IP-PSTN gateway. Nortel also offers a 24-port IP interface for its Meridian communication system, enabling the routing of real-time voice and fax calls over IP data networks, rather than the PSTN.

7.7.1 Administration

Among the vendors offering IP-based phone systems is Cisco. Administration of the phone system is performed through the company’s CallManager, a software application that provides the intelligence necessary for implementing PBX-like features. The application is installed on a Windows server and provides basic call processing, signaling, and connection services to IP and virtual phones, VoIP gateways, and other local and remote devices. This includes the management and control of various signaling protocols such as Q.931 for ISDN WAN control and H.225/H.245 for IP packet control. With total switch and network independence, administrators can create a virtual campus environment using the familiar interface of a Web browser.

An example of an administration system for an IP PBX is Cisco’s CallManager Administration, a Web browser-based application used to configure devices such as telephones and gateways. It allows the administrator to add a new IP phone by configuring the telephone number (called a directory number), protocol information, and other parameters. CallManager Administration stores all the information about the IP PBX system in a database.

Supplementary and enhanced services such as hold, transfer, forward, conference, multiple-line appearances, automatic route selection, speed dial, last-number redial, and other features are extended by CallManager to IP phones and gateways via parameters stored in a configuration database. Microsoft’s Internet Information Server (IIS) is installed at the CallManager server to provide a browser interface to this database. With administrator approval, users can be granted access to configure their own phones. Any user with appropriate access privileges on the IIS can administer the CallManager from any location through a Web browser.

CallManager manages the resources of the IP PBX by signaling and coordinating-call control activities. It sets up a call by instructing the calling party to set up an RTP audio stream to the other device, either telephone or gateway. Once the audio stream is set up between the two devices, the CallManager is out of the picture until a new request is made, such as transfer or disconnect. Should the CallManager fail during a call, the two parties stay connected and can complete their call. The application also supports the RSVP—which, combined with routers that support the standard—reserves network resources and maintains a stateful connection over the IP network. RSVP also prioritizes voice traffic to guarantee QoS over the IP network.

An optional suite of integrated voice applications can be added to CallManager to provide basic voice messaging, voice conferencing, manual attendant console, click-to-call, and other functions. Because CallManager is a software application, these and other options and enhancements can be loaded for quick implementation, eliminating expensive hardware upgrade costs and installation delays.

CallManager supports the industry standard Station Message Desk Interface (SMDI) to provide connectivity to various voice-mail and IVR systems, along with call detail record reporting for tracking call activity and billing. Call records are kept in a standard CDR database, from which a variety of summary and detail reports can be generated.

CallManager is implemented on a single server, but the architecture allows for a scalable network of multiple, redundant CallManager servers. This enables any IP phone to receive CallManager services from a primary, secondary, or tertiary server. In the event that communications between the active CallManager and the IP phones is disrupted, the phones will register with the backup CallManagers for their call processing service.

7.7.2 Managed Services

PBXs can also be interconnected over IP nets. PSINet (now owned by Cogent Communications) offers a service called Voice iPEnterprise, a fully managed service that delivers internal voice traffic over the same connection as corporate Internet data. This service leverages existing investments in PBXs and uses a T1 dedicated connection to the PSINet network, allowing businesses to carry voice services between their offices.

The quality of this type of managed service is virtually indistinguishable from that of traditional tie line or dedicated voice services between PBXs. Such services are aimed at companies with branch offices, especially those that must scale rapidly due to internal growth or operate in widely dispersed geographic locations. Service providers claim corporate customers can save up to 50% on call costs over traditional carrier services. IP desktop fax, conference calling, and unified messaging capabilities are options that companies can consider to further reduce their communications costs.

AT&T has married aspects of VPNs and corporate PBXs to extend PBX functionality to remote users via VPNs. Called Virtual Connection Services (VCS), the service includes an Internet-based IP VPN, which is used to send PBX signals from a remote user’s PC to the corporate PBX located at a node on the VPN. Those signals instruct the PBX to forward inbound calls to the site where the intended recipient of the call happens to be located. The signals also instruct the PBX to place outbound calls over the public phone network and then ring up the remote user and link the two calls together when the call has been established. That effectively lets a remote user make calls directly from the PBX and take advantage of the lower rates corporations get for their phone trunks.

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