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We didn't just take a sip. We took a good long drink of SIP technology in this round of iLabs testing.
We gathered more than 50 devices from five SIP server vendors and 13 SIP endpoint vendors to prove that multi-vendor SIP telephony deployments are possible.Our results show that while basic SIP interoperability is outstanding, advanced features such as call forwarding and conferencing might not work so smoothly between all SIP devices.
Protocol simplicity is an argument in favor of SIP over the more complicated H.323 VoIP standard.This simplicity minimized interoperability problems and made device configuration easy Compared with previous iLabs tests involving H.323, SIP let us connect more devices to more servers more quickly.With the exception of an older device an engineer brought from his own network, all SIP endpoints passed our basic call tests.
Starting from scratch
We defined our telephony environment in the context of a midsize company wanting to build a SIP-based VoIP system from the ground up. Designing a VoIP network is much like designing a LAN:You have to plan all aspects, from numbering of phones and IP addresses, to setting up services such as voice mail and call conferencing, to enabling voice encoders, to naming devices.
One of the first VoIP planning steps is setting up the dial plan (see "Dialing for VoIP dollars," page 60) that defines how long phone numbers are, how gateways to the public switched telephone network (PSTN) are addressed and how the internal network is divided between SIP servers.
In a traditional telephony environment, the dial plan is built into the PBX. In a SIP network, the dial plan has to be configured on all phones individually. If you don't program the dial plan into the phones, end users will either wait for the phone to "time out" when dialing or hit a terminator character (the pound sign is common) when they're done to get the phone to dial (like hitting "send" on a cell phone).
Because of the expanse of our SIP test bed, we had a fairly complex dialing plan and found that not every phone could support that. On our network, users could dial phone-to-phone with a four-digit extension. But to dial through to the PSTN, Interop's eNet or Free World...