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inserted between calls before feedback becomes a problem.
6.
Digital call setup and supervision.
Analog lines use a strange mix of signaling to convey
call status. Loop-current drop signals that a caller has disconnected, and blasts of 90
Volts at 20 Hz mean someone wants you to answer. ISDN uses a modern digital approach
to controlling calls and conveying status information about them. ISDN call set-up times
are often only a few tens of milliseconds, enhancing production of fast-paced shows.
Perhaps more importantly, when a caller disconnects while waiting on hold, the ISDN
channel communicates this status change instantly. This contrasts with the usual 11-sec-
ond delay on most analog lines. One of the most common complaints of talk hosts is that
they go to a line where they expect a caller to be waiting, only to be met with a blaring,
annoying dial tone. The chance of this happening with an ISDN line is nearly zero.
ISDN lines come in two varieties: Basic Rate Interface (called BRI, SØ, or ISDN 2 in various
parts of the world) and Primary Rate Interface (PRI, S2 or ISDN 30). BRI lines are the kind we
normally see in broadcast stations, as these are what are used with MPEG codecs such as the
Telos Zephyr and Zephyr Xstream. BRIs have a capability of one or two active 64 kbps channels.
Since the Telos iQ6 is used with BRI lines, we will only consider that type here.
An ISDN line from the central office is a single copper pair identical to a normal (unloaded)
analog line. When it arrives at the subscriber, it is called the U-interface. It is a two-wire connec-
tion, usually via an RJ-11 style modular jack.
The S-interface is at the user side of the Network Termination Type 1 (NT1) device. The NT1
is sometimes generically called a NCTE (Network Channel Terminating Equipment) or, in
some countries an NTBA. It is a four-wire connection, via an 8-pin RJ-45 style modular jack.
(Sometimes the S-interface is called S/T. There is a subtle distinction between the two, but it is
not relevant for our purposes here, and the two may be considered to be the same.) In the USA
& Canada NT1 functionality is usually included in the terminal equipment, and indeed the iQ6
in these countries supports the U-interface. In other parts of the world, the telephone company
provides the NT1. Only one NT1 may be connected to a U-interface. However, as many as eight
terminals may be paralleled onto the S bus.
In the USA & Canada a direct connection to the “bare copper” 2-wire U interface is required.
Therefore, the USA & Canada interface module includes an integral NT1 and has RJ-11 style
jacks. In the rest of the world, the telephone company provides the NT1, and the 4-wire ISDN
S-interface will be used with an 8 pin RJ-45 style jack. Normally we ship to North America with
the U-interface module and to other areas with the S-interface module.
In either case each ISDN interface has three connectors. Since each ISDN connection has two
channels, this means each interface module handles 6 ‘dialtones’ or ‘lines’.
Data and Voice
ISDN lines may be used for voice signals encoded in standard fashion to allow inter-working
with analog telephones, or may be used to transmit digital data streams. The latter mode is used
for such applications as high-speed Internet access. It is also the mode used with MPEG codecs,
in which case the ISDN line may be carrying voice signals, but is doing so in a format that is
not compatible with the analog part of the telephone network. The distinction is made in the
automatically conveyed Setup message that begins each call.
Normally, the Telos iQ6 uses only the voice mode, so data capability is not necessary. Often
voice costs extra, whereas this is rarely true for data. Of course, you may use a line with both
capabilities. Just be sure the BRI circuit supports the Circuit Switched Voice (CSV) capability
as well as data.
