Telos Zephyr, User Manual

Page: 86 / 258

Section 5 ISDN
86
ISDN BASICS
Background
It is the introduction of digital transmission services from telephone providers which
has made the Zephyr possible.
The telephone infrastructure is moving from analog to digital. Telephony made the first
significant use of digital audio techniques: In the mid 60’s a digital transmission method
called “T- carrier” began to be widely deployed to expand the voice- channel carrying
capacity of existing copper wires. While they were intended originally for simple single-
channel analog, engineers discovered that the common copper wire pairs were capable
of much higher bandwidth than the 3.4 kHz required for speech. Indeed, it was
determined that two of these pairs could be made to relay 24 voice conversations – if
they were digitized and appropriately multiplexed. Thus was born the basic technology
used for digital telephony today.
The standards developed then continue to define the digital telephone network: an 8
kHz sampling rate (resulting from the desired 4 kHz Nyquist frequency to accommodate
a 3.4 kHz audio bandwidth, with guard band) with 8 bits of amplitude resolution
(instantaneously companded to provide performance roughly the same as a 13 bit linear
system producing 78 dB dynamic range for speech signals). Thus the basic voice
channel bit rate was established to be 64kbps. (8kbyte/sec x 8bits = 64kbps.)
These early applications of digital technology were invented by the telephone industry
for its own benefit. The fact that they were digital was neither obvious nor important to
customers. However, telephone engineers learned to appreciate digital audio for the
same reason we in the pro audio community have: immunity to noise and other quality
impairments, ease and flexibility of routing and multiplexing, and lower cost due to
compatibility with the electronics and media invented for the rapidly advancing
computer industry.
Nearly all long- distance calls are now connected from city- to- city using digital paths
on fiber cables and most switching and routing is performed by digital machines.
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 had 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 connection.
ISDN is the technology which has evolved to eliminate this analog bottleneck yet still
utilize existing copper infrastructure.