Support User Manuals

Telos Zephyr Xstream Two-Way Radio User Manual

Open as PDF

of 381

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v

oice‐

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th

ese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Nyq

uist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.(8

kbyte/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

flexibili

ty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perform

edbydigitalmachines.

previous next