Previous pages: Can I trust Microsoft with VoIP?
Reliability: Microsoft and the fifth 9
What really happens when I dial 911?
How secure is VoIP?
Do I need a $1,000 IP phone?
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.
The problem is, most companies must still rely on proprietary VoIP protocols, or vendor-tweaked (and thus, vendor-exclusive) versions of SIP.
"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 IP 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 to allow users of SIP-based phones to access features normally available only to proprietary systems.
As SIP becomes more mainstream, we can expect increased interoperability and an expansion of features.
Microsoft's Duffy says users won't be having conversations about SIP interoperability in five years'. Over time, he says, VoIP systems and SIP will operate similarly to Web applications over TCP-IP.
Continued: How do I run my business on Skype?
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