Telos Zephyr Xstream - Manual (Page 146)

USER’S MANUAL
Section 7: ISDN BASICS 134
With the digital nature of the modern telephone network is hidden from subscribers, voice and
signaling has been delivered just as they have been since the era of wooden phone sets and
mechanical bells. In the age of digital communication, this “last mile” bottleneck has become
increasingly frustrating for those who have need to send digital information through Ms. Bell’s
wires.
With most of the network now digital, it is clearly odd that we have been using modems to
convert digital information to analog beeps just to accommodate the mile or two of ancient
analog linkage at each end of a thousand‐mile long digital connection.
ISDN is the technology which has evolved to eliminate this analog bottleneck yet still utilize
existing copper infrastructure. It is currently the only technology that allows fully digital access
to the Public Switched Telephone Network.
7.2 The ISDN Basic Rate Interface (BRI)
On one ISDN BRI circuit, there are three simultaneous channels: Two 64kbps “bearer” channels
for the transmission of user information and one 16kbps signaling channel for call set‐up and
status communication. This is Basic Rate Interface (BRI), 2B+D service. In Europe and other
countries it is also referred to as ISDN‐2 or SØ. It can be implemented over most of the millions
of standard copper two‐wire phone circuits already in service.
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CURIOSITY NOTE!
While the D channel is always present for signaling, it can, in theory, be used for connections
to the public packet network. ISDN lines where the D channel is used only for signaling are
sometimes referred to as 2B+0D rather than 2B+D service. D channel packet service is not
widely available and is not used by Zephyr Xstream.
ISDN BRIs are perfectly matched to Zephyr Xstream’s transmission capabilities. One channel
provides high quality mono, while the two channels can carry near‐CD quality stereo.
i
ISDN TIP!
In some cases, your ISDN line, while having 2 B channels, will only have only one phone
number. This is the case of the AT&T Custom PTP protocol, and may be the case with ETS
300 (Euro-ISDN) and INS 64 (Japanese ISDN). This does not generally present a problem.
This works as if the lines are assigned to a short “hunt group” i.e.; the first incoming call
will be assigned to Line 1, and the second incoming call will be assigned to Line 2. Of course
you have the option of which line to use on outgoing calls.