Six burning VoIP questions

5. Will SIP ever be ready for the desktop?

The VoIP industry has touted SIP for most of this decade as the future of IP telephony. Proponents say the open-standard nature of SIP, its flexibility and elegance are among its virtues (besides being a great acronym for marketing PowerPoints and trade magazine headlines).

The problem is, most companies must still rely on proprietary VoIP protocols, or vendor-tweaked (and thus, vendor-exclusive) versions of SIP in large IP telephony deployments.

"SIP really describes a limited number of features in terms of it being an industry open standard," says Anne Coulombe, senior product manager at Avaya. "So invariably, a proprietary protocol will have more features."

Most major vendors such as 3Com, Avaya, Cisco, Nortel, Mitel and Siemens who ship phones that run proprietary VoIP protocols also offer standard SIP software stacks that can be loaded onto the devices. This allows the phones to work with so-called "pure" SIP backend IP PBXs or media servers. Even the open source Asterisk IP PBX system - touted by users for its openness and flexibility - has its own non-SIP protocol for communicating between servers and end-point devices. (Although Asterisk fully supports SIP-based endpoints and peering servers.)

With desktop phone features, the most important ones vary widely depending on users. People who live on conference calls want a button that can hold all parties without dropping anyone. Those who pop in and out of the office need a message-waiting light. This is why protocols such as Cisco's SCCP, Siemens' CoreNet, and others still come as standard on respective IP and phones and PBXs.

But the demand for SIP is increasing, as users look to integrate presence and multimedia features into a VoIP network. To accommodate, vendors are also creating proprietary extensions to SIP to give the protocols a few extra features - enough to make or break an enterprise VoIP system sale, in some cases.

"It's commercially unreasonable to say to customers that they must be purists about a certain protocol," Microsoft's Duffy says. " If we need to make changes to a protocol, or other scenarios, we'll do that" in order to meet customer's needs, he says.

Avaya calls its SIP extension Avaya SIP Telephony, which extends the number of features a SIP phone supports to around 62 - twice as many as are available on basic IETF-based SIP phones.

Vendors such as Avaya and others are also extending basic SIP phone functionality with feature access codes. This involves passing dual-tone multi-frequency (DTMF, or tone-based signaling based on dial pad buttons) signals through standard SIP packets to a PBX or IP PBX backend, which allows users of SIP-based phones to access features normally available only to proprietary systems.

"So features you could normally turn on by dialing 1234# on your phone, it will turn on the backend," Coulombe says. "That's 100% SIP-compliant, but you've actually extended the capabilities of all SIP phones attaching to the [non-standard] backend."

Some users rolling out large deployments of SIP endpoints say a lack of features is not an issue. (Albeit, these users say they chose to use SIP phones in basic office settings where advanced PBX features are not commonly used.) The University of Pennsylvania is one such organization in the process of deploying thousands of IETF-standard SIP telephones to faculty and staff offices at its Philadelphia campus locations.

"The truth is that the vast majority of services people want, we can provide," says Deke Kassabian, senior technology director at the university. "And the ones we can't yet provide, we're working on those."

Bridged-line appearance and busy-indicator lights are among some features that are hard to do well in an open, standard environment right now, he says.

At toolmaker Stanley Works, plans are in the works to widely use Polycom IP phones with a SIP-based VoIP system from Interactive Intelligence. IT executives at the company have said they expect to see cost savings of $200 to $300 per seat in using the SIP-based phones vs. proprietary VoIP handsets and systems offered by Cisco or Avaya.

"I have not heard of any problems or issues about shortcomings in terms of SIP's features," says David Cote, global telecommunications manager for the company.

As SIP becomes more mainstream, increased interoperability and the expansion of features should be expected, industry insiders says.

Microsoft's Duffy says users "won't be having conversations about SIP interoperability in five years." Over time, VoIP systems and SIP will operate similarly to Web applications over TCP/IP. "No one would for a minute realistically wonder if those systems would work."