BSTJ 60: 7. September 1981: Effect of Echo Canceler on Common-Channel Interoffice Signaling Continuity Check. (Fang, G.S.)

7.8 MB·English

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Effect of Echo Canceler on Common-Channel
Interottice Signaling Continuity Check

By @ 5. FANG
(@tanuccrint received December 12, 1980)

[Annoying echoes generated by imredance mismatches can oceur in
Jonge distonce telephone network An echo eanceer contrat cho By
“pnthesizing an soho replica and sublencting Ut rom the actual echo
‘Gmthe return path, For the coho canceler to cork properly, i! mus be
Atle to distinguish between desired signals and echoes. One instance
tuhere this can be difcue is during the cammon-channel interofice
‘ienaling (ects) continuity chech Botween four-wire auitehing offices.
‘On ects trunks the voice and the signaling are routed separately, To
ensure a satisfactory transmission jah, a voice path continuity check
Is conducted before atl setup, The ehech ia performed on lop basis
ty vending 1 check tone wnat lnoplng i Dock at the aistane office
Since the cha canceler addantice iter memorizes information from
the last previous cull, may partially cancel the logped- bark tone. Pe
‘his paper wwe stu the effect of the echo canceleraaptive memory
‘onthe cots continuity check. The analytical and experimental resulls
Idiente that he occurrence of continuity check failure caused by the
presence ofan active coho canceler is s0 infrequent and insignificant
Compared tv the existing ttatistive of ell other pertinent failure
mechanisms Thus, disabling the ron canceler during the es eon

lanuity check ts unmeceesary,

| IwTRoDUCTION

[An echo may be produced in tansmission system whenever there
is an impedance mimatch, This impedance discontinuity cun cause «
signifcane portion of the Uatmuitied signal energy to be reflected
tomard the sgn souree ver an echo path. Noticeable echnes const-
{ute one of theese serious forms ofimpairment in telephnne channels
Tis subjective annoyance increases with both ocho amplitude and
ropution delay. The increasing wo of satellite civeuits for domestic
‘and intemational eal has made che control of echoes even more
important, The taditional approsch of echo contzal using via net: lost
‘or echo suppress it nol aeoepable for fl-hop antlite cireuit, Le,
‘route exrryinghoth directions of transmission va ali A better
tnpronch is Wo uae echo exrelers (hat cntrol wc Iy syne
Feplica of the echo and subtracting it from the returned signal* The
realization of the single-chip invagrated echo ennceler has made its
‘widespread deployment appealing”

‘Figure I modcls the eseential elements ofthe echo path to show how
‘echo caneclors work, When there i farend epeech (bueno near
‘end spoceh o(), the internal registers of the echo cancer adaptively
‘update the estimate of the echo path impulse response Algo form an
echo reple 5 tha is sbteneted trom the real ec 9(0). When the
‘esr-end speech in detected in the prosonce of the far-ond specch
{double talk) the spooch deterior inte further updating, but the
echo canceler al trea to eaeel the ech contained in 3) by using
the moot recent extimate of the echo path impulke reponse. This
property of continued echo canceling during double cal is a nice
feature of the echo canceler. However, its effect on the common
channe!inerotfice signaling (cx) contiouity check between four-wire
‘itching offices has caused some concern

(Common-channal intcrofico signaling is n system for exchanging
information betmeen processor-equipped switching systems over &
network of signaling links, All vigtaing data for cll setup and take-
lover, well network managerens gmat, are exchanged by ene
systema over the signaling links instead of being ent over the voice
path. Thus, a continuity check for voice path ussurance (vPA) is
‘conduetod whenever a cota trunk is selected to switch a eal forward.
‘This cheek not only ensures satisfactory transmission path but alo
prechides billing for an otherwise undetectable faulty, connection
‘Bebween fourwire switching ofies the ¥Pa check ia performed on
Toop basis by connecting a single-frequency transceiver at the righ
ating office und Tooping the (ransmission pire at the distant office
‘The eheck is considered auceeseful hen the vr tone received ut the
originating office ie within accep cransmssion and Ging Bite,

Figure 2showe the vPa choc for the satellite circu, AL the distant
office che same notation x(¢ is used for the received and the tranamit-
ted signals because ofthe looping configuration. ‘Tho echo canceer at
the distant office will detac done tle during the VPa check, Ifthe
cho canosler i not diaabled it may partially caneel the ube tone
tocording to the last estimate of the echo path impulse response
bheenuae ofits continaed canosling property daring double tall There-
fore, the introduction of the ect cancer ino the long-dietance
lelephone network without proper disabling contol wil have some
He

FCHOCANCELER 1518
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1916 THE BELL SYSTEM TECHUCAL JOURNAL, SEPTEMBER 1981
influence on the eis continuity check. Voi path assurance failures
trill lower the performance indices ofthe allt offis and yonerate
ditional mainvenance actives. Unfortunately since call processing
information is oot relly available, no aacafactory solution has been
found auch thot the echo canceler can be digabled al tho desired
‘moment. The teriousness uf the echo eanceler effect on the vPa check
is seed below by evalusling the probabilities of various possible
vents caused by WA failures de to the uze of the echo cancer
‘Section Il describes the circuits nd facilities in the vra path and how
they are modeled wathematcally im the derivations given in the
‘Appendicen, Section Ll presents whultie nnd experimencal results and
discusses the impact of echo eanccler on Une Com continity check.

1, MODELING OF VPA CHECK.

In thie section, the discussion and the derivation will be restricted ta
il satelite circuits beonier the renulee can be

"The ¥ea chock for ects eae No.4 as offices is a two-step tost
in a specified time intarval. The fit step i Wo deeet athe proper
receive level th Iransmitted vea tone which is looped back at the
distant office, The serond rlep isto atop transmitting the VPA tone
land detect its disappearance atthe receiver, AUprowent the specie
‘ime interval for completing the teas sct anywhere between 2(0 9 5
Gepending on the specific fee. Ac the beginning of a VPA check
‘during call atap, the orgmaring procescor ofthe seiiching machine
‘rate cho timer and attaches» 2010Hs eransceiver to the solocted
tlle trunk coneureent with tending un wld message Cheough
terrestrial cets links identfyine tho trnle to he looped back. ‘The
Aistant office, upon recep of tho moswaxe, connects the receive side of
‘he trunk tothe tnnaml nde thergh a 2r0-oss lop. The originating
bce checks the lve f the returning tone to verify that transmission
Joss ie within accaptabo limits, Tf the returning tone has acceptable
level the oriinaling few stone sending the ven tone and verifies that
the receiver mesures below a release level. A vra failure is generated
if for any reaoon the above sen steps nro not completed fore the
timer times out

"The specification of che transceiver levee is given in Table 1
Frequent and periodic tests ae performed in sitching ofiea to make
sure thatthe transceiver levele are within specification. Table Lalo
fives the test requiremens which wre ariter than the specifiation*
Since the test only gives paseo fail ndicaton ic ia aarumed chat the
various levels (eal are uniformly distributed within tho test limits
"Thus, live to each othe, che transmit and the detect levels of the
‘yra tome are aseured to be uniformly dietribuued in (1,1) dB an

ECHO CANCELER 1917
‘Table |—Comman-channel introtice signaling continuity check

(-94, ~66) dim, respectively. The release level in Table T ie not
‘modeled since a properly working echo canceler should not affect i

In addition tothe transeciver level variations, the VPa tone level is
affected by the tmo-eny antolite unk lose the check loop, and the
fcho eanoeler at the distant office. The safollite trunk is a class of
{nvertallerunks with 0B inserted connected los which is specfiod to
bbe normally isteibuted witha standard devieton of eround 0.7 o 08
AR. The chece loop is speci to have a lose of 0 0 dB, ‘This
‘aviation i small and wil bo ignored in the following study. The effect
ofthe echo eancolorroquires detailed consideration.

In discrete-time notation, Uhe nearend wpeech detector inhibits
updating the echo canceler risers at time A if

yb 1) >% max stk—I—n} for 015255, Ut)

‘here the notations are given in Pg. L' At an &le sampling ente,
On = 127 implies using a 16-ms topped delay line adaptive digital
‘ver to model the echo path, while ©2255 mewn thal onc nea

‘end speech is detected, the detector continues declaring its presence
for the hangover time of 52 ms. The factor of one half is based on the
‘esuumption that there willbe atleast a 8 los through the hybrid
‘During the continuity check, che vea tone is leoped back so the near
‘end speech detector finds chat, (1) is satisfied and thus inhibits
Updating the registers. The echo eanceler wl partially cancel the yea

Dulion ofthe whole telephone network ianot known, The only available
BERL data, us shown in Fig. were mono at the Pittsburgh
Feionsl Centar in Uhe second Ball of 1978 during the fil evaluation
of domestic watelive.” These meusurecaenls are wot inconsistent with

revoualy reported echo pach and incecll unk loss Although the
lone distribution was derived from che eatimated echo path frequeney
reqponae aa averages over SN to IK) Ta, it will be aarumed to be
the lose distribution forthe vPa (on, The demily function shown it

1318 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1991
To ETAT a

Fig. 3 is consatent with » normal diseribution, except the small
sucondary peal at about 5B. The following derivation turns out to
be inlependent ofthe los ditsbution below 6

‘The above modeling of the VA path can be used 10 predict eA
failures which may generate various kinds of maintenance activities
fecting trunks and eazrier groups There ar 12 tromks in each carier
froup. I's truck fn a cartier group falls 4 VPA check, the call is
Feattempted on another trunk ina different carrier group, while the
fiat carrer group is vexaporarly tren nut of service, Several records:

ECHO CANCELER 1918
Tater, « secon trunk in the Toekedaut cariar group i rundown
selecied and checked for continuiy. Ifthe second trunk aleo fal a
Sofeware carrier group alarm (suc) is generated for maintenance
accons. Ifthe second trank passes the check, the temporary lockout
fn the frst camer group is lensed und the initially failed trunk is
rechecked Ii fis wg, w single rank Lockout (tt ts generated for
maintenance. The probability of getting a VPA failure is derived in
‘Appendix A. The probability of renerating wn 6 iver ia Appendie
1B. The probability of generating seu cannot be analyzed since the
to failed trunks which share identical facilities fom the awitehing
afficos to tho eatallitecarch seations may be dependent. ‘hus, only the
Upper and the iver bounds of The probally of rea are oboe i
Appentix ©.

li, DISCUSSION OF RESULTS

lures 410 7 show the results obtained in the appendivcs, During
thw UPA chock, the estimated ache 2) in Fig. 2 is aso 1 ing
Trequency tore at 2010 Fh, Tee amplitude aval phase deperd on the
ZEPRE of the last eal. Por the woracnse, 6 my select on the
Teoped back tone (0) varkes from 83 dR he toes add vont
tively) to 6 dB Gi the tones add dexcuccively). With the. Rem.
distsibuton given in Wig. , the effec of the echo canceler on aay VF4
tone level follows the density function shown in Wig, 4, Thi denaty
has a mean value slighty larger thom zu; Ubal is, Ue expected effect
the erho eanceler isto ineveate the tan Tove lightly. "This fc

sis + Poobaiiy Coe ano of eof ie alr nthe Ea tae

1920 THE BELL SYSTEM TECIINICAL JOURNAL, SEPTEMBER 1981
‘ube expected sinoe che resultant za ont contains the powers of wn
Sinaacids2(@) and #() with random relative phases as opposel to the
Single sinusoid 0) without the echo cancclr. However, the reducing
‘fect om the vea tone level hus a gresr impact because it can be at
large aa ~6 dB. This is evident in Fi. 5, which shows that sing echo
‘anceler appreciably increases the probability of va failure. However,

ECHO CANCELER 1321
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the caleulations assume chat all cienits are working property and
‘wiehout outside interferences. Votre path msmurance fallures ext be
‘generated by carrier glitch, fading, and many ather causes. Tiitel
sumpling performed during the spring of 1980 in several No. 4 uss
‘offices not equipped with an echo eanceler yielded ven fallure rates
ranging from 0.5 % 10" to 2.7 % 20", with a sample mean of 5.0 %
10-"! "Thece are of che same order of magnitude af the caleulated
probability of vea failure with echo eanceler, suming the satellite
trunk standard deviation ia between 0.7 to 08 dB, Therefore, if the

1922. THE BELL SYSTEM TEGHNICAL JOURNAL, SEPTEMBER 1081