Telos Zephyr Xstream - Manual (Page 145)

USER’S MANUAL
Section 7: ISDN BASICS 133
7 ISDN BASICS
7.1 Background
It was the introduction of digital transmission services from telephone providers, which has
made the Zephyr family possible. The telephone infrastructure has moved 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.)
t
DEEP TECH NOTE!
Actually, the original Telco technique used only 7 bits for the audio, and the 8 th bit was used
for signaling. Since the digital technology was deployed as isolated links (later over
microwave or coax cable) within an otherwise analog network, channel banks at the
boundaries of each digital interface performed analog-to-digital-to-analog conversion.
It was soon discovered that quantization noise was a problem, given that many calls
underwent 3 or more conversions enroute. Since the signaling did not require 8kHz
resolution, it was decided to use all 8 bits for audio, but to “rob” this low order bit on every
6
th sample for signaling. This improved the noise problem, without the need to revamp the
rest of the system.
This “Robbed bit” signaling is still occasionally encountered with T1 applications. It is
ironic to discover that these bits (2 in each direction) are merely used to emulate the switch
hook leads on much older analog trunk interfaces.
ISDN BRI and PRI finally bring truly modern signaling to the local loop. In fact PRI is very
closely related to the SS7 technology used for interoffice trunks.
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 soon 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 nearly all switching and routing is performed by digital machines.