When it comes to IP PBXs, products from IP-centric vendors and legacy PBX companies are characterized as much by their differences as by their similarities.In general, the two camps can be summarized this way: the legacy PBX vendors include\u00a0Alcatel,\u00a0Avaya,\u00a0Mitel,\u00a0Nortel\u00a0and\u00a0Siemens; the IP newcomers are led by\u00a0Cisco,\u00a03Com,\u00a0Shoreline\u00a0and\u00a0Vertical Networks.The legacy vendors have an edge when it comes to the total number of traditional PBX features. The IP PBX vendors offer a more stripped-down suite of features - call hold, call forwarding, call waiting, conference calling and voice mail. But the newcomers also take advantage of the tighter integration with desktop productivity applications, such as Microsoft Outlook, giving users a unified view of voice mail, e-mail and faxes.When it comes to other characteristics, such as basic architecture, how they provide reliability and how they support standards for features such as call control and inline power, the results are all over the map.IP phone homeYou might assume that all IP PBX vendors support IP phones, but until recently, that was not the case. The legacy vendors supported IP phones, but not upstarts Shoreline and Vertical. They held the view that the value of the IP PBX lay with the distributed architecture and the applications, not an expensive phone. Therefore, they limited their support to analog stations.Another concern often raised about IP phones is the potential need to reengineer the corporate IP network to accommodate - and prioritize - voice traffic. Establishing quality of service entails configuring switches and routers to support\u00a0802.1p\/q, type of service bits, and potentially,\u00a0Differentiated Service. While these are not necessarily difficult tasks, they do require datacom expertise - either in-house or outsourced.IP phone advocates argue that IP phones - with Web-based browsers and desktop\/PDA integration capabilities - add substantially more value than their proprietary digital predecessors. As Shoreline and others have discovered, there is enough corporate interest in IP phones to warrant - perhaps necessitate - support, which is why Shoreline and Vertical have added support for IP phones from Polycom.Cisco has made IP phones an integral part of its architecture for voice and video (AVVID). Cisco provides support for analog devices, such as phones and fax machines, through an optional gateway, the VG-248.For 3Com users interested in keeping their digital handsets while migrating to a NBX IP PBX, Citel Technologies offers a gateway between 3Com's IP PBXs and Nortel Norstar digital handsets.The legacy PBX vendors, including Alcatel, Avaya and Nortel, have incorporated support for IP phones, analog phones and their own digital handsets. IP phone support is consistently accompanied by support for G.711 and G.729a voice coders.Topology tourMany IP PBX vendors, including Avaya, Cisco and Nortel, rely on centralized call-control servers.For example, Cisco's AVVID call manager sits on a Media Convergence Server (MCS) running on Windows 2000; Nortel's Succession CSE 1000 call server runs on VxWorks, as does its Signaling Server; and Avaya's S8700 Media Server runs on a dedicated Linux server.The potential problem with these centralized call-control server architectures is the dreaded single point of failure. However, there are ways to mitigate this problem.For instance, Avaya addresses reliability through a redundant, dual-processor configuration; if one goes down, the other assumes the call-processing load. Similarly, Nortel supports redundant Signaling Servers.Cisco approaches the issue of high availability a bit differently, introducing an element of distribution through server clustering. AVVID supports clusters of up to 10 CallManagers over LAN or WAN links to provide failover and load-sharing support.Clustering approachA cluster can support one or more device pools.Within each cluster, there is a primary CallManager, and designated failover CallManagers. These failover CallManagers can simultaneously serve as primary CallManagers in other clusters, letting large sites set up multiple, overlapping clusters to maximize survivability.Cisco's IP phones can be configured to signal a primary MCS, as well as back-up MCSs, in case the primary MCS becomes inaccessible.Technically, this is a sound approach, but it requires buying additional AVVID MCSs. This might be acceptable to large companies, but might not fit the budgets of smaller organizations. Also, this approach doesn't necessarily address the requirements of small branch offices, where it doesn't make economic sense to deploy an MCS.What happens when one of these small branch offices loses its WAN link and cannot reach a stand-alone MCS or cluster?Cisco's response is survivable remote-site telephony, an IOS-based option for routers and Catalyst switches (which might require an IOS upgrade) that provides limited call processing back-up functionality in the event the Call Manager becomes unreachable.Avaya and Nortel also employ local survivability techniques with their Succession CSE 1000 and S8700 Media Servers, respectively. Call-server and signaling functions, in the form of processor boards, can be distributed to gateway devices - Nortel's Media Gateway and Avaya's branch office S8300 Media Server\/G700 Gateway - for temporary, local call-processing capabilities.By contrast, Shoreline's Shoreline4 is based on a more distributed call-server architecture. Each ShoreGear voice switch has gateway and embedded call-control software, running on VxWorks. If a voice switch goes down, its effect is localized; users connected to other switches still have access to their own call-control server. Shoreline plans to extend this survivability to voice applications such as voice mail through a distributed application server architecture, in the second quarter.From a deployment and management perspective, Shoreline presents a less-complex alternative to the clustering and SRST-like techniques of the other vendors.However, Shoreline's recent support for IP phones has introduced a new issue. Shoreline's IP phones are set up to signal designated gateways (which reside in Shoreware voice switches). If a switch fails, or becomes unreachable, manual intervention is required to switch over all the associated IP phones to a new gateway. A future Shoreline release promises to automate this fail-over function, the company says.For management, most IP PBXs rely on Web servers - either stand-alone or implemented on the call server's main controller, while a browser or Java application is used to access and configure management data. In most cases, Web-based management is sufficient. However, configuring Cisco's AVVID requires some command-line interface expertise for the associated switching and routing infrastructure.Standard stewWhen it comes to call control, signaling, interswitch communications, and even in-line power for IP phones, some vendors are adhering to standards; others are not. For example, Avaya and Siemens support\u00a0H.323. For its recently released IP phones, Shoreline is using media gateway control protocol, while deploying Session Initiation Protocol (SIP) for communication between voice switches. Cisco uses a proprietary standard - Skinny Client Control Protocol - for call control.SIP continues to gain acceptance, but it's unclear if and when it will overtake H.323 as the protocol of choice.With respect to in-line power, most vendors appear to be rallying around the IEEE's\u00a0802.3af\u00a0draft for powering IP phones over Ethernet cabling. But because it has not yet been ratified, some vendors are relying on prestandard solutions. For instance, Cisco's Inline Power is a prestandard implementation that Cisco has indicated it plans to support 8023.af once it is ratified, while maintaining support for Cisco Inline Power.Despite these unresolved issues, the IP PBX market has matured quite a bit over the past year. As a whole, the IP PBX vendors have done a credible job of refining the functionality and resiliency of their products.And because legacy PBX vendors are shifting their development resources away from legacy PBXs to newer IP PBX technologies, customers will have little choice but to explore this new, converged world.