SIP PBXes stake a claim

SIP works. Not only does it work, but it is easier to use, implement, and configure than older VoIP standards.

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The promise of SIP has always been interoperability, and the three SIP-based phone systems we reviewed demonstrate why real standards are important. The interoperability part of the equation works, too. You can mix and match manufacturers to a much greater extent than you could with earlier VoIP solutions.

This is not to suggest that every feature of every SIP product is totally interoperable, because they're not. There are several features on these products for which there is no final standard, and this is where the proprietary nature of PBX products shows up. Nevertheless, it's clear that these companies are embracing SIP, and they're making an effort to follow the standard.

At the University of Hawaii's Advanced Network Computing Laboratory we tested three PBX products: Avaya's Communication Manager 3.0 and Communications Server; Siemens' HiPath 8000 Real-Time IP System; and Zultys' MX250. After putting them through their paces, we found that interoperability generally worked: these PBXes could certainly talk to other SIP devices, but they couldn't network effectively with other PBX products from different makers, for example.

They could also use nearly every SIP phone ever designed, and we were pleasantly surprised to find that these units had similar results when we tested their performance and operation using the Spirent Communications Abacus 5000. They consistently made calls that went through and sounded good -- exactly what you expect from your phone system.

AvayaCommunication Manager 3.0 and Converged Communications Server

Avaya's PBX is really two products: Avaya CM (Communication Manager) 3.0 provides the basic call controlling, and Avaya CCS (Converged Communications Server) runs on the S8500 Media Server and is the SIP proxy. It provides additional extensions and enhancements including the presence engine and IM. CCS also works with a separate and optional Avaya product called Meeting Exchange, which handles large conferences and runs on a separate server. Together they form a full-featured SIP PBX.

For this test CM was installed at the factory on the Avaya S8300 Media Server, a single-board Linux machine installed as a blade into a slot on the G700 Media Gateway. The media gateway, in turn, lets CM work with a wide variety of legacy telephone systems, including other IP phones, digital phones, and even POTS phones.

CM communicates with CCS using SIP. When CCS gets a SIP call request (someone dialing a number from their SIP phone, for example), it accepts the request and passes it along to the CM. CM routes those calls to other SIP phones on the network, to legacy PBXes, or to the PSTN. It also handles all of the traditional telephony features, including the dial plan and voice mail. It sets up the calls, enables communications between end points, controls the gateways and trunking, and manages resources.

One consequence of handling so many different types of legacy phone systems is that the Avaya products must also support calling features currently unavailable in SIP. Avaya uses a second SIP channel for these features, but the company says that when new features are added to the SIP standard, they may be added to the products' standard SIP feature set.

Both of these applications can be installed on platforms other than the ones on which they were tested; running CM on a larger platform would allow you to support more people, for example. However, CM and CCS cannot currently be run on the same machine. (Avaya says that will change eventually.) The result is a pair of Linux machines in your rack -- a 2U G700 Media Gateway with the S8300 installed internally and the 1U S8500 running CCS -- which means you use more rack space and must buy more than one platform.

The Avaya engineers apparently raided their interoperability lab for spare phones, showing up in Hawaii with one of everything they could find: phones and softphones from Avaya, Cisco, Polycom, Snom, and others, including some dubbed "unknown generic phone." All of them worked seamlessly with the PBX.