54 | Section 5
ISDN BRI
ISDN BRI (Basic Rate Interface) lines offer two B channels, supported by one D channel.
These were intended as a residential replacement for POTS lines or for small businesses. One
application envisaged by its inventors was to allow a simultaneous voice call and data connec-
tion. With DSL providing much higher data rates, ISDN BRIs are moving ever closer to obsoles-
cence. Nevertheless, when they are available, they could be useful for small installations that
need only a few lines. Most VoIP gateways have cards to interface with these lines. As with PRI
lines, take care to set the gateway’s configuration to match the type of signaling that your line
uses.
SIP Compatibility
VX implements the basic set of SIP functionality described in RFC 3261, together with the
related standards:
♦ RFC 4566 and RFC 3264 for describing and negotiating supported audio codecs
♦ RFC 3550 and RFC 3551 for audio transmission
♦ RFC 2833 codec for sending DTMF tones separately from audio
♦ RFC 2617 digest authentication
We have tested VX with different SIP providers, gateways, PBX’s, soft- and hardphones, and
other gadgets, to make sure it works out of the box. You will find a list of some of these in this
very manual, and on our web site. We strongly recommend using VX together with a tested
device, but sometimes it might not be an option.
Telco-provided SIP trunks are one such case. Chances are that these will work, “just like that.”
However, it is not possible to test every SIP-talking-thing out there, and different vendors tend
to interpret some parts of the standard differently. SIP started as a simple text protocol to set
up phone calls, but over it has grown quite complex. While we strive to do the right thing,
no one is perfect, and it’s possible that we have missed a line or two. If you are experiencing
problems, let us know and we’ll figure it out.
That said, there are a couple of things that VX isn’t likely to support in near future. One of them
using multicast for both SIP signalling and RTP. This is an optional feature in the SIP standard,
not widely used, and disabled in VX because it can cause conflicts with Livewire, which is mul-
ticast, too. The second is telephone URI handling, which might sound surprising for a phone
system. Don’t worry, it’s fine. Simply put, it means that VX identifies each line with a single SIP
address (extension@server entered in the configuration), and expects that any mangling of
numbers (like adding or removing prefixes, or changing between internal and external num-
bers) is done by a gateway or PBX. For VX to do that would require it to know a lot more about
the phone network, and believe us, you don’t want to configure that.
Axia Element Console as VX Controller
The Element console makes a perfect partner to the VX. (With apologies to our colleagues in
the Axia division, who would prefer to put that the other way ‘round – “the VX makes a nice
partner to the Element…” They would also point out that other Axia consoles, such as the IQ,
are capable and compatible VX partners. Control and audio are tightly integrated for a talent-
leasing all-in-one place control of telephone operations. Both selectable and fixed lines are
supported. For the former. The Element’s Call Control Module includes line select buttons and
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