rea_470_i1 switched special services and private branch exchange services

7/30/2019 REA_470_I1 Switched Special Services and Private Branch Exchange Services

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Rural Elec trific atio n Administ ra tionTelecorrrnunications Engineerin g an d Constructio n Manual

1.2 3.4.

General

SWitched Special Services an d Priv ateBranch Ex change ServicesCCNI'ENTS

Descr iption of N on- PBX Related Switched Special Services Description of PBX Related ServicesTransmis sion Considera ti ons

Section 47 0I ssue No. 1June 1980

5. FX an d WA.TS Lines Design Objectives fo r Switched Non-PBX Relate dSpecial Serv ices5 .1 5.25 .3 6.

Gene ralTransmiss ion Obje ct ivesS ignaling Considerat ionsOff -Premises Extensio n Lines fo r Sw itched Non -PBX Related SpecialServices6 .1 Gen er al6.2 Tr ansmiss ion and Sig naling Design7. PBX Related Services Des ign Objectives7 .1 Gene ra l7.2 PBX-CO Trunks7. 3 s ta t ion Loops7. 4 Tie Trunks Genera l Infonnatio n7.5 N o n-T a nde m Tie Tru nks Tra nsmissio n Objecti ves7. 6 Foreign Ex ch ange and Re roo te WA.TS Trunk De si gn Object ives7. 7 T a nde m an d Intert andem Tie Tr un k Design Objectives8. Glossary9. Referen cesTablesTab le 1Ta bl e 2Table 3Figures

Cable Resistance an d Att enuation lo ss Per KF (RM:)T an dem PBX Through BalanceT a nde m PBX Ten ninal Bala nc e

Figure 1 FX/WATS Lin es2 Off-Pre mises Ext ens io n (OP) Sta tio n3 Secretaria l Service4 lV.fa.in PBX s Interconnectedvia No n-T n.ndem Tie Tru nk5 T a nde m Tie Tr unk Network6 Lang Distance Tr unk Circu it7 Inserted Connected Loss (I CL) and Insert ion loss Calcula tions8 1 KHz Loss of No n-Loaded Cable wit h 900 Ohm Tennin al andSource Ir rq;leda nc es9 2-Wire/4-W ire Voi ce Freque nc y Rep eater Circui t Design

10 4- Wir e Repeater Levels Relative to CO Transmit Level


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1) PBX- Trunks2) TieTr unk s3 ) FX Tr unks4) WATS Tr unk s5) Of f-Premises Extension Lines6) long Dis ta nce Tr un ks (I D )7) Centr ex Lin es & Tr un ks8) Au tan a.tic Call D istribution (ACD) Tr tmk s

2. DE SCR IPTION OF NCN-PBX REIATED SWI'ICHID SP ECIAL SE RVICES2.1 No n-PBX re la ted sw itched spec ial service c ircu i ts ca n generall y be toought of as extensi cn s. of POI'S lo ops. They usually pro vid e thesubsc rib er wit h a circ u i t beyond h is nonnal exchang e area. These c ircui t s usuall y requir e both a lo op f ac il i ty and a dedicated in teroff ice fac i l i tywhich, w h e n combined, provide th e sa ne fu nction as an ordinary subscri berloop .2. 2 For eign exchange (FX) c ircu i ts , as i l lus t rated in Fig ure 1 , provide

a d irec t co nnection between a su bscriber in one exchange area,through th e local central off ice (s er vin g CO) , to a central off ice (FX CO)located in san e oth er exchange area. The centra l office located in theforeign ex change area provid es the only connectio n to the rressage ne tw or k.Th e FX c ircu i t , th erefore, requir es two li nks (1) subscrib er to CO (loo pfaci l i ty ) , and (2 ) CO to CO ( in te ro ffice fac i l i ty ) The in teroffice fac i l i ty w ill no rmally be routed ov erex is ti ng to l l fac i l i t ie s an a dedicate d basis.Dia l tone, r ingin g sig nals an d lo op current o rig in a te a t th e FX CO.

14 , Intero ffi ce Fac i l i ty - L oo p - -1 Facility----1 ,--, ,- --i J I I I f -+ - I I I I- -_ I '-- .J I __FX/WATS Intermed ia te Nom al Sta t ionco co Serving CO Set

FIG URE 1 FX/WATS LINES

2.3 W id e area te lecamrunicatians serv ic e (WATS), se e Figure 1, providesthe custaner d irec t ac cess to se lectedwi de in ters ta te and in t r a -s tate geo gr ap hic al areas (bands) fo r a fi xed rr on thly charge. As of th isw rit in g, separ ate ac ces s l ines fo r int ras tate and in te r s ta te servic e a rerequired to the rr os t co nvenien tly lo cat edcen tr al off ice eq uip pe d fo r WATS.Where the local central office i s eq ui ppe d for WATS service , the one l inkWATS l ines are iden tic al in desi gn to PBX- tr unks. W here the loca l centra lo ffice do es not prov id e WATS, ra rote WATS l ines , sim ilar to FX circu its ,

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REA TE&CM 470

connect th e subscriber through the serving m to th e WATS co. WATS l inesare exclusively one -way outgoi ng or one-way incaning c i rcu its connecting to the direct d is tan t dialing ne twork.2.4 Off-prEmises extension (OP) lines, as i l lus t ra ted in Figure 2,provide a bridged connection between a rarotely located extension

sta t ion an d th e main s ta t ion . The ex tension l ine i s usually bridged a t them butna y be bridged a t the main sta tion. Bridge d ta p iso la tors (b ridgel i f t e r s ) nay be re quired to rr ee t th e loss re quirements.

Serving co CO(I)

Bridged Th.p IL- - 'Bridge d Tap Isolato rs/. _ _ _ _ _ . - - - - - - ~

M ain Stat ion

FIGURE 2 Off-Premises Extension (OP) Stat ion

OP Extensionsta t ion

2. 5 Secretarial service lines (SL) , as i l lus t ra ted in Fi gur e 3, are simila r to off-premis es extension l ines . The subscriber nainsta t ion loop i s bridged to the secretar ia l serv ic e l ine a t th e serving m.The secretar ia l l ines are usually one-way inward (receive only). As withoff-premises l ines , bridged ta p iso la tors (bridge l i f t e r s ) may be required.3 . DESCRIPTICN OF PBX RELATED SE RV ICES3.1 A PBX i s a m anual or d ia l (autanatic) switching machine lCY '.-ated onth e subscriber pr emises. Most ne w PBXs are autanatic switchingmachines (PABX) controlled by microprocessors . However, a t th is time, sOJ:TEPBXs are s t i l l available which require manual co nnection of a l l incoming

and a l l outgoing ca l ls . The primary function of th e PBX i s to inte rconnect PBXstat ions and to connect PBX sta t icns to the message network or other PB Xs.Several PBXs ma y be interconnected in a t i e trunk ne twork. PBXs in a t i etr un k network are classif ied ac cording to the i r function in th e network.3.1.1 Main PBXs have a directory nmnber an d an at tendant . They co nnectPBX stat ions via sta t icn conductor loops to the local cen tral of f ice trunk fo r both incoming and outgoing cal ls . The PBX may also us e

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ServingCOI

BridgedTap

REA TE&Q1 4 70

Secretarial Service\Line- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ C i r c

' ridged Tap Is olatorsJl'r-1ain Station

(a ) DirectLoop Secretarial Service

Serving CO Secretaria l SefficeI Concentra tor\ /Ident i f ie r \

! /Briged Tap Bridged Tap Isolator/ - - - - - J'B in Stat io n(b) Concentrator - Identifier Secretarial Service

FIGURE 3 Secretarial Se rvice

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FX tr unks, WATS tr unks, long dis tance tr unks, t i e trunks, an d off- premisesl ines fo r services other than PBX to lo cal cen tral office. Figur e 4il lustrate s the_ inte rconnecti on of two ma in PBXs.

3 .1 . 2 A tandem PBX pe rf onn s the sane fu nctions as a main PBX an d is alsous ed as an intennediate switchingpoint for main PBXs and othertandan PBXs as sh ow n in Figure 5.

OO(A)

Off-Prern ises Li ne

I PBX-CD TrunkII .II

-1 PBX Tie Trunk

tJ ---..+-!-----100 (B ) r ~ _ - : . . . : . ; . . . : ; ; ; = : . : ~--- ___ _I

PBX(A)

Off-PremisesExtension

FX COr - - - -'-- - - -" - - - - - ' - - - - - ------+-tJI

FIGURE 4 M ain PBXs Interconnecte d via Non-tandem Tie Trunk3.1.3 sa te l l i te PBXs hav e no directory num b er an d are un attended. Theyare connected to th e ir main PBX, usu ally located in th e sa m e exchange area, via t w o- way sa t e l l i te t i e t runks. Since th ey have no direct incanin g connect ion fran the message network , a l l inc aning ca lls are throughthe main PBX. They r r ay, however , ha ve one-way out going trunk connect ions tothe cen tral office . Figur e 5 il lu s t ra te s a s a te l l i te PBX.3.2 PBXs are connected to th e local centraloffice or to other PBXsvia trunk faci l i t ie s . Co nn ection to the central offic e i s viath e PBX CO tr unk and conn ection to other PBXs i s via a t ie tr unk . Tietrunks are classified accordi ng to th e ir functio n in the t ie trunk ne twork.

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- - - - ~ Of.f-Premises LineMain PBX(A)

PBX-CO Trun.lc

- Tandan PBX (B)I .1 ill'a.ndan Tie Trunk

CO{B}

Intertandan Tie Trunk

tJ"----r-CO(C)

....I

Main PBX(D)

u - - ~ ~ - - - ~ rCO(D)

Satel l i te Tie Trunk

FIGURE. 5 Tandan Ti e Trunk Network

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sa te l l i t e PBX(E}


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REA TE&CM 470

3. 2. 1 PBX-CD T r U nks co nnect the PBX to the lo cal cen tral office. Th eset runks nornally appear as sta t ion lines a t the central office andma y be ar ranged fo r one-way in caning, one-way outgoing, or two-way {incaningand outgoing) sig naling. Carrie r fac i l i t ies ma y be used for PBX-m trLmkswhi ch require a la rge nurriber of c i rcui ts to th e serving central office.3.2.2 Non-tandem t i e t runks connec t main PBXs, as i l lust rated in Figure 4,

for the purpose of interconnecting stat ions a t the two PBX locations.The non-tandem t ie trunk may be connected to a PBX-m trunk, FX trLmk, orWATStrunk a t ei ther end of the ci rcui t . Non-tandem t i e t runks are not connectedtogether or to oth er t ie tr unks .3. 2. 3 Ta nde m t ie trunks interconnect main PBXs an d tan dem PBXs. Tandem

t ie t runks have ac cess to other t ie trunks a t the tandem PBX only .Tandem t ie trunks are analogou s to t o l l connecting trLmks in th e messagenetwork. Tandem t ie trunks are i l lust rated in Figure 5.3. 2. 4 Intertandem t i e tr unks interconnect tandem PBXs. In tertandem t i etrunks ha ve access to oth er t i e trunks a t both ends. Th ey are,

therefore , analogous to in ter to l l trunks in the message network. In tertandemt i e t runks are i l lust rated in Figure 5.3.2.5 Sa tel l i te t ie tr unks interconnect th e main PBX with a sa te l l i t e PBX.T hey are designe d as no n-tandem, tandem, or intertandem t i e tr unksdepending an t he i r functi on. Figure 5 i llustrates a sa te l l i te t ie tr unk.3.2.6 L ona distance tr unks connect a PBX direct ly to a class 4 or hig her

to l l switching point. These trunks are no:rnal ly used in rrotel/hoteltelecommun ications networks. Long distance trunkc i rcu its are i l lustratedin Figure 6.

Toll SWitchboardI \

serving m~ - - - ,. PBX

, . F . ' - - - - - - - - - - - - - - - - ~I '

FIGURE 6 Long Distance Trunk Circui t

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3.3 An auto rrati c ca l l distributo r (ACD) i s a swit chi ng rrachine whichd is tr ibu tes larg e numbers of in can ing cal ls equitably betwee ns ta t ions and in order of arr iva l. Unli ke most other PBX ty pe sw itc hin gsyst ans, the ACD sy stemusually ha s more incani ng tru nks th an s ta tion lin es. 3. 3. 1 NIJ tr unks are trea ted as PBX-CO tr unks.3. 3. 2 In addit ion to ACD-CO tr unks, there ma.y a lso be ACD-FX trunks,ACD-WATS tr unks, and ACD-overf low tru nks. ACD-overf lo w tr unksare one-way tru nks connec te d between two ACD' s to reroute t ra ff ic whenth e ca pac it y of on e NIJ i s exce ed ed.3.4 Cen tr ex i s a PBX type servicewit hdirec t inw ard dialing to acen tr ex s ta t ion fra n th e message net wor k. Cen tr ex sw it chi ngequipnent.is nor mall y loca te da t the cen tral office . With centr ex service ,the ce ntr ex s ta tion se ts are conne cte d to th e ce ntr ex sw itc h v ia subscriberlines .3.4 .1 Bas ic centrex featu re s a re as fo llows:3.4.1 .1 Direct ou tward d ialin g (OOD) offers direc t acces s to the message ne twork wit hou t atte ndant assis tance.3 .4 .1 .2 Station to s ta tion calling permits dia l ac cess between PBX s ta t ionswithout attendant assistance.3. 4. 1. 3 Sta ti onhu nti ng directs cal ls to alternate s ta tions when the called sta tion is busy .3.4 .1 .4 Sta tio n restric tion de nie s specific sta tions the ab i li ty to pla ceoutg oin g ca lls . 3. 4 .1 . 5 Call transfer all ow s the calle d s ta tion , whil e conn ect ed to thein can ing l ine , to signal the attendant and ha ve the ca l l transferred

to another s ta t ion with in the PBX sy ste m.3.4 .1 .6 Direct inward d ialing (DID) pe rmits calls fran th emessage net wor kto ha ve d irec t acc es s to a PBX s ta tion wit hou t attendant assistance.3 .4 .1 .7 Automatic identified out war d diali ng (AIOD) iden t if ies the cal lingsta tion on outg oin g to l l ca lls fo r b il l in g pur pos es.3. 4. 2 Any sto red program control PBX available to day can pro vid e an y of

th e above features.4. 'IRANSMISSICN CONSIDERATICNS

4.1 In sert io n Loss (I L) i s the loss which re su lts when a net wor k i s , inserted into a tra nsmis sion sy ste m. The networkmay be anyc ircu it it em (such as th e l(X )p trea tm ent equip:rent or the load ed cab leinsert ed between CO (A) and CO(Z) in Fig ur e 7) The inse rtio n loss i s ameasure of the power loss resulting fra n the network being inserted in to the sy ste m. In Fig ure 7 the in serti on loss between CO(A) an d CO{Z) i s th e sum of the lo sses of theDLL and the loaded in teroff ice fac i li ty .

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RFA TE&CM 470

For loaded cable, insertion loss an d th e cable at tenuati on loss as calculatedfrom Table 1 are equivalent. However , for nonloaded cable the inser tion lossi s sarewha t less than th e cable at tenuatio n lo Ss calculated fran Table 1.The cable attenuation constant i s calc ulated with th e cable tenninated in i t scharacteristic irrq;:ledance. Since non-l09,ded cable te.rminated in either 600 or90 0 ohm s does not look l ike non-loaded cable terminated in i t s character is t ici r rpe.dance th e attenuation constant does not giv e a good indicat ion of th ecables insertion loss. The 1004 Hz insertion loss for non-loa de d ca bletenninated with 90 0 ohms should, therefore, be dete.rmined fran Figure 8.

CO(A) CO(Z) STA..:._. .....__VF Repeater I \ I \DLL XXX Qj

~ IL l "IE IL2CLIL = DLL Loss + CD{A) to CO.(Z) 1004 Hz Cable Attenuation Loss from Table 11IL2 = co (Z) to STA 1004 Hz Cable Insert ion Loss fo r 900SG Terminations fr anFigure 8.ICL = IL l + IL 2 - GA

GA = CD (A) p e a t e r Gain

FIGURE 7 Inserte d Connected Loss (ICL) an d Insertion Loss Calculations

4. 2 Inserted co nn ected loss (ICL) i s the 1004 Hz loss to which the c ircui tis designed. I t i s , therefore, the circu it net loss wi th a l l th eloop/trunk treatment equipnent in place. IC L includes the circui t net lossbetween th e originating an d te nninating outgoi11g sv-litc.l} appearances fo r a tr un kand be tween th e l ine-side switch ap pe arance and custaner sta t ion for a l ine .

4. 3 As show n in Fi gur e 7, ICL i s calcula te d from the circui t insert ionloss (IL) an d any circui t gain. Ca ble attenuation values use d tocalculate the cable attenuation loss are l is ted in Table 1. Values for nonlo ade d cable insertion loss are in Figure 8. Office wiring an d a l l eq uipnentlosses should be included in the calculat ions where s ignif icant . Tabl e I I Iin REA. TE&CM 426, "SUbscriber lD op Corrputation Design-by-Loss Method" containsna nin al 1000 Hz loss values which may be found in a typical subscriber loopfac i li ty . In designing special services circui ts th e ICL should be chosen togiv e minimum contrast be tween th e special services c i rcuit an d th e POI'S typec i rcui t to the sane custaner . No:rnally up to a 2 dB contrast between circui tswil l not be detected by th e custaner. At a l l ti mes transmission object ives(return loss, frequency response) must be maintained.

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10 l II tII II t l t . l t l l t l l l l t t l , , , , , ,,,,,,!, 11 11111 , , , , , I L\'

8.

6/=

l1l /'1ls.-i-eQ) 41lI::H -

2

4 6 B 10 12 14 16 18 20

cable Length kF

FIGURE 8 1kH z Los s o f Non -loa ded Cable with 90Qg Te rmina l & Sourc e Imp eda nce

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4. 4 Attenuation loss i s te rrperature depend en t for aer ia l cable butv irtual ly independent of te mperatu re for buried cable . Atte nuationlo ss fo r aer ia l ca ble varies by abo ut 1% fo r each 2. 6 C change in tempera ture. T he reference te mperatu re i s tha t te mperature a t which the in i t i a lloss measurerrents -were made. For exar rple6 i f th e in i t ia l loss measurement-were made a t an am bi en t ~ r a t u r e of 20 C an d the temper ature a t san efuture date wer e to go to 40 C th en the loss would in crease by approxi mately 8% 4.5 Many of the ex isting fac i l i t ie s which may be use d fo r special services circu its may have br idge d taps. Preference should be given to fac i l i t ie s without bridged taps. Although, bridged ta ps general lywill not be a problem an d may be le f t in tac t . I f problems ar ise the bridgedta ps should be suspect. The 1004 Hz loss introduced by the bridged taps, about0.25 dB per kilofoot (0 .3km) sh ould be accounte d fo r in th e c ircui t design. However ,the bridged tap los s varies with fr eq uency an d may irrq:lair the frequen cy re -sp onse of the special service circu it . I t sh oul d be noted tha t where th especial services c ircui t ma y carry data th e bridged taps co ul d cau se problems.Ref er to TE&CM 428, "Application an d Use of Bridged Tap Isolators {BT I) for SUbscriber Loops" for rrore in fornat ion on bridged taps. .4. 6 Fo r interof f ice l inks a minimum of tw o load points i s perm issiblepr ovided tha t transmissio n objective s fo r frequency re sponse, andreturn loss are me t {see TE&CM 431) .4.7 Tb insurecircu it s tability, nornally nomore th an tw o 2-wire re peaters should be us ed in an y special service c i rcuit .4. 8 Rep eater gain re qui rements are de te rminedby calcula ting theoverall loss of the systemminus the des ired IC L {see Fig ur e 7).Re peatered circu its sh oul d be designed by the s tabil ity des ign method ofTE&CM 444 .4. 9 Fo ur wire voic e frequency circuits ma y us e 2-w ire/4-wire voic efrequency repeaterso r ter .m se ts , l ine amplifiers , an d equali zers.4.9 .1 As i l lus tra ted in Figure 9, th e transmit direc ti on of 2-w ire/4-wir evoice frequency re peaters i s towar ds th e 4-w ir e side . Co nverselythe rec eiv e direction i s tow ards the 2-w ir e side.

CO{A) CO(Z)I TX \ 1 RX \ I

ST A \_ _ ; ; O d a n ~ _ _ ~ X Xx-4dan. -3.5 da n

I 0OdBn.

TX

r--_ IL_1_=_l_O_dB_ - - - - - ' ) ~ ~ < . - - 50Bj.r--- ICL =3.5 dB-------4--l..;FIGURE 9 2-Wire/4-Wire Voice Frequency Rep eater Circuit Design

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The gain in the tr ansmit directio n shoul d not exceed an outputpower level of +6 dBnpennitt ed for good cro ss ta lk perfon nance.4.10 When carr ier or single fr equen cy (SF} sig nali ngequipmen t i s used ,a tr ansmit and receiv e le vel se paratio n of 23 dB should beThat i s the SF equip nent normal ly tra nsmit s a t -16 dB and re ceives

a t +7 dB as sho wn in Fig ure 10.

CO(A)I

-1 6 dB

CO(Z)

+7 dB

4-Wire Repea te rs(a)CO (I) CO(Z ) STA

1 - - ~ - :+7 ~ - X - - - ~ - X - ;:- ;.]dB==" ;--\.-16 dB - l ~ X : ~ - < 1 -

4-Wire Repeat ers &Carri er

FIGURE10 4-WireRepe ate r Lev el s Relative to CO Transmit Lev el

4.11 Si nce the canputed losses are estimates, minor gain adjustm entswill be requir ed duri ng l ine up to obtain the desired ICL . How ever, manufacturer 's su gges tedoperatingpa ramete rs must be obs erved .4.1 2 The loss youexpect to measu re with the actu al te s t se t up i s theexpec ted measured loss (EML) andmay be about 0. 5 dB grea ter thanthe ICL. The dif ference can arise beca use thepointwhere ICL loss ca lcu lations beg in in th e CO may notbe accessib le fo r measurement purp ose s. I t is , therefore, necessary to be aware of any addit ional losswhich may be a t tr i -butable to the te s t se t up.

5 . FX AND WATS LINES DESIGN OBJEX ::I'IVES FOR SWITCHED NON-PBX RELATED SPECIALSER VICES

5.1 Genera l-13-


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CX> (A) CX> (Z)r I \,- ...., , - 7Iu'>' XXXXX \(-1\ I \'-...) L-

IL1 - - - - - - - ----- ICL

Cable In sertion Loss

CO(A} to CX>(Z) CX> (Z) to STA CX> (A) to STAMax IL1

4d B6dB9dB

15 dB

M ax IL24 dB (NL)*5dB (L)4dB (NL) *?dB

l ldB15 dB

Notes : * See Paragraph 5 .1 . 6NL_= Non-Loade d Cable

L = Loaded CO (Z ) to STA Loop\ -7

Max IL

5 dB12 dB15 dB23 dB

)


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RI'A TE &CM 4 70 CD(A)

CD(A) to CD(Z)MAX IL . 1__ 15 dR

CO(Z)I \

cahle Insertion lo ss. CD(Z ) to STA

.MAX IL215 dB

CD(A) to STAMAX I ~ - -23 dB

Figure 12Guideli nes fo r 4-W ir e Des ig n of FXor HATS LinesCO(A)

Carri er Sy st em

CO(Z )

-l \{! \ ;

L

IL --+-ILICL

Cab le Inser t lon Lo ssCO(Z ) to STA

NL/L

3 .5 dB Ob j5dB M il l{

STAI \

CD(A) to CD(Z ) Max IL ?- \A rir e Repeater Requi rem en tsCarrie r 5 dB No repea te r requ ir edCarr ie r 11 dB Pepeater requ ir ed a t CO(Z)

Note s: NL = Non -lo ade d Cab le CD (Z) to STA LoopL =Loaced Cable CO (Z ) to STA Loop

\ -1

' , 1 = 2-Wire Repeater '\'Then Requir edf\L_\Figure 13 Guid elin es fo r Tr unkCarri e r Desig nof FX o r WATS L ines

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des ign pro blem . This ex ample i s typical , and a s im ila r app ro ac h wo uld beused fo r other design applic ations.5 .2.2 .1 Consider the fo llo wing f a c i l i t ie s ava i lable fo r th is FX desig n

1) CO(A) to CO(Z): 50 Kf (1 5.2 of 24D662) OO(Z) to STA: 15 Kf (4 .6 Km ) o f 24NL

5.2.2 .2 Calcul ate th e CO(A) to OO(Z) attenuation loss using Ta ble 1 values: 50 Kf X 0.2 3 dB/K f = 11.5 dB

5.2.2 .3 De termine the CO(Z) to STA attenuation loss using Fi gu re 8 values:15 Kf of 24 NL = 5.2 dB

5 .2 .2.4 Calculate the OO(A) to STA a ttenuation loss usin g ab ove re su l ts11.5 dB + 5.2 dB= 16. 7 dB

5.2.2 .5 Fra n Fig ure 11 we see tha t the OO(A} to CO(Z) attenuation lo ss may ra nge up to 15 dB for 2-w ir e voic e frequen cy c ircu i ts . Als o

we see tha t for a to ta l insertion loss grea ter th an 15 dB we sho uld use ate rmin al repeate r a t CO (A) and inter m ediate repeater a t co(Z) The repeaters sh oul d be adju ste d to y ie ld an IC L of 3.5 dB. The fo ll ow in g ru le may be use das a guide to de term ine repea te r gain.G(A} = (L1 + L2 + LO - ICL}/3G(Z) =2 (L1 + L2 + LO - ICL )/3

Wher e G (A) = repeater gain a t CO (A)G(Z) =repeater.ga in a t CO(Z)L 1 = CO(A) - CO (Z) insert ion loss L2 =CO(Z) - STA in serti on lossL 0 =CO in sertion loss

Ther efo re the CO(A) repea ter i s adjuste d for a 4.5 dB gain an d the repea te ra t CO(Z) i s adju sted for a 9 dB gain. Th e gain val ues determin ed are onlydesig n gu ides . The ac tual gain i s determin edby the s ta bi l ity design Ir ethodof TE&CM 44 4.5.3 Signa ling Consi dera ti ons 5.3 .1 Sign alin g fu nctions (dial to ne, r in g ing , an d loop curre nt) orig inate from the FX central off ice . Signaling requ irements a re generallythe sa ne as no rma l subscriber loop opera tio n . Methods of pro vidin g signalingincorporate both tr unk and subscriber lo op signaling te chniq ues. Basic to a l lsignaling schemes i s the min inrum loop current require d fo r th e superv isory,

alert in g , addre ssi ng, and r in g - tr ip ft mcti ons . Genera ll y, a mininrum lo op current of 23 rna i s req uire d a t the sta t ion se t .

-1 7-


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RFA TE&01 4 70

wit hout r inger iso la to rs , serious nois e pr obl ems can be ex pec te d. Fo r rro reinfonnat ion on bridged tap isola to rs re fe r to RFA TE&01 428, "Applic ati onand Use of Bridged Tap Iso la tors for Subscrib er IDo ps ".6.2 6.2 .1

6.2 .2

6 .2 .3

Tr ansm issi on and Signali ng Des ig nOP l ines should be des igned fo r an obje ctiv e IC L of 3.5 dB witha maximum loss of 8 dB.OP lines in the san e exc hange as the mai n s ta t ion are trea ted asnormal lo op p lant exce pt fo r the us e of bridged tap iso la to rs .OP l ines ex te nding in to a different exc han ge th an themai n s ta tion are treated as FX l ines except for th e brid ged tap iso la to rs .

7. PBX RELATED SERVICFS DESIGN OBJECI'IVES7.1 General 7 .1 .1 The desig n of PBX systems can be divid ed in to two categories :(1) PBX sys tem s which may include sa te l l i te and no n- tandem t i e trunks, and (2 ) PBX syste ms whic hmay in clu de tandem and in te rta ndem t i e trunks . The prim ar y dif ference be tween the two cate gori es i s tha t thosesys tem s which in clude tandeman::l in ter tan demt ie tnm ks require tenn inalbal anc e and thro ugh bala nce a t th e tande mPBX. Termi nal bala nce and th rou gh bal ance are measurements of ech::> return loss (ERL) a t the vario us switch ingpoin ts in a sw itche d networ k. These measu rer ren ts are neces sary to insuresys tem s ta b i l i ty . 7.1 . 2 The PBX subscriber loop i s ca nposed of the PBX trunk and s ta tion lo ops. The tru nk lo op consis ts of th e conn ec ti on bet wee n the

ori gin atin goutg oi ng swit ch appe aran ce of th e calling CO or PBX an d th e tennin ating out goi ng switc h appe ara nce of the call ed PBX or CO (refer toFigure 14) . And th e PBX s ta tion lo op consis ts of the conn ec ti on bet wee nth e s ta tion se t and the line s ideof the PBX sw it ch ap peara nce .7 .1 . 3 A PBX may be of the cut-t hro ughor th e no n- cut- th rough type. The cut- through type PBX uses CO ta lk battery for s ta tion se t ft mc-tions. The non -c ut- thr oug h type PBX uses lo ca l ba tt ery.for s ta t ion se tfu nc tio ns. Unl ik e the cut-t hrough PBX, non-cu t-t hro ugh PBX s ta tion lo opsignaling i s inde pende ntof tr unk signalin g ex ce pt fo r eme rgency transferc ircu i ts . Gene ra lly , PBXs availa ble to day are of th e non-cu t- thro ugh type.7 .1 .4 PBXs may redu ce th e norm al su pe: rvi sor y ran ge of the CO. The PBXmanuf ac tur er shoul d be cons uled for th e PBXs tr unk and sta tion lo op ranges.7 .1.5 The PBX trunk c ircu it may be e i th e r 900 ohns or 600 ohns ir rpedance .7. 2 PBX-CO Trunks

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REA TE&CM 4

7. 2.1 Transmis sio n De sign fo r rrost PBX-CO tr unks i s sim ilar to norrr ello op desi gn. The main dif ference i s the trunk lo ss-obje ctive .The objecti ve IC L, as i l lu s tra ted in Figure 14, i s 3.5 dB. The ICL i s allowed to go to 4 dB before add in ggain. The fa c i l i t i e s may bee itherm etall ic or carr ier. Except as noted here in, circuits should co nfon n torequirare nts of TE&CM Secti on 424, "Design of 'IWo-Wire SUbsc riber Loop and PABX TrunkPlant" , and TE&CM Section 911, "Station C arr ie rAppli catio n".

m(z) PBXI \ r \S ta t io n

PBX-CO Tru nk LoO p ~ - - - - - - - '6 ' L

St at: wnr14---- 3.5 dBObj I 4.0 dBI Calle dICL Station Calle dStatio n Sta tion

FIGURE 14 PBX-CO Trunk an d Sta tio n Loop IC L' s

7. 2. 2 Signali ngDes ig n depen ds to a great exte nt on the ty peof PBX(m anual or au tarat ic) . The vastmajo ri ty of PBX eq ui pnent avail -able today i s of the autanatic variety (direct- outward-diali ng) wit h a d ialatt endant or un at te nde d co ns ol e. However , san emanual PBXs and sare cordswit chb oa rd s are s t i l l available .7.2. 3 M anual PBXs requir e lo op s ta r t signaling. Loop s t a r t signalin g is the ty pe used on a nonnal loo p between the centr al office an dsu bscriber sta t ion set . 7. 2. 4 Aut anatic PBXs usin g two-way tr unkswill generally errploy grounds ta r t sig nal ing. Ground s ta r t signalingwas dev eloped to prevent

simult aneous seizures {gl are) fran both ends of the trunk . Ground s ta r t signaling requir es tha t the l ine circu i t be m:r lif ied by raro vi ng gro un d fran th e t ip con du ct or when the PBX-CO trunk i s id le .7. 2. 4.1 When the CO switc h seizes th e tru nk for a ca l l towards th e PBX,

it sim ul ta neo us ly places a grou nd on the t ip conducto r an d negativ ebatt ery on th e ring co nd uc to r. The tr unk c ircu it a t the PBX re cognizes thet ip ground as a seizure an dmakes i t s e lf buS y to ou tg oin g ca lls on tha t trunk .7.2.4 .2 When the PBX seizes a tru nk, itplaces a gro und on th e r ing con-du ctor toward the CO and su ppli es batte ry and loca l gro und to th e

PBX s ta tion se t. The ro rec og ni zes the r ing ground as a request for servic efran the PBX and prepare s i ts e l f to receive th e adare ss signal. When rea dy to re ceive the addres s sig nal, the CO places gro un d on th e t ip con du ct or towardthe PBX. The PBX th en con ne ct s the sta t ion to the CO.-21-

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7. 2. 4. 3 Care sh ould be taken to ensure t ha t the grounds a t th e PBX an dCO are a t the sane potential fo r ground s ta rt systems.

7.2.5 PBXs using DTMF signal ing require precision d ia l tone from th ecentral off ice .7. 2. 6 M::>st signaling equiprent available today ca nwork wit h ei the r

loop s tar t or grotmd s tar t ope.ration.7.3 Station Loop s7.3 .1 The maxim um ICL for on-premises s ta t ion loops, as shown inFigure 14 , i s 4.0 dB . However, it should be kep t in mind tha tthe s ta tion se t may be connected to the message network via a t ie trunk anddis tant PBX-ro tnmk. Therefore, station loops should be designed for

minimum loss .7.3.2 The objectiv e ICL for off-premises stat ion l ines , as shown in Figure 15 , is 4.0 dB an d the maximum ICL i s 4. 5 dB . Off -p remises

stat ion l ines functio n th e sane as on-premises station l ines . 'Ihey haveac cess to other PBX stations, to the message network , to t ie t runks, to FXtrunks, or to WATS t runks . Loop treatment equiprent may be located a t thePBX orco.

PBX CO(I ) srA1 \ I \ I \

t J XXX z XXX @14( ICL 4. 0 8B Ob j ,I4.5 dB !YiaxX= 2-Wire Repeater

FIGURE 15 Of f-Premises Station Loop ICL

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7.4.4.3 Loop signaling applies loop current to a relay or other sensingdevice to indicate a change in loop s ta tus . There are three typesof loop signaling - high lo w signaling reverse battery signaling and batteryand ground signaling.7.4.4.3.1 High-low signaling circui ts apply a voltage to th e t i e t runk whenth e local PBX originates a cal l . The trunk c i rcui t resistance a t

th e dis tant PBX is decreased by about two orders of magnitude when th e calledsta t ion i s answered.7. 4. 4. 3. 2 Reverse - bat tery signaling circui ts are open c i rcui t a t the localPBX during th e idle s ta te . Upon seizure the loop i s closed. Th et i e trunk circui t a t th e dis tant PBX reverses t ip an d r ing to indicate answer.7.4.4.3.3 Battery an d ground signaling circui ts are open c i rcui t a t the localen d during idle s ta te . Upon seizure th e local t ie trunk ci rcui tapplies a ser ies aiding voltage to the loop. Th e t ie trunk c i rcui t a t th e

dis tant en d reverses t ip an d r ing to indicate answer. The local PBX holdsth e circui t by rerroving the ser ies aiding battery source.7.5 Non-tandem Tie Trunk Transmission Objectives7.5.1 Non-tandem t ie tr tmks should be designed for an objective ICL of4 dB, as shown in Figure 16 . The ICL i s allowed to go to 4. 5 dBbefore a repeater or addit ional repeater i s required.

PBX(A) CO(A) CO(B) PBX(B)I \ I \I \ I \

Calling M----tStation '6 ' u ~ Calledu Station

7.5.2

7.5.3

7.5.4

7.5.5

7.6

ICL 4.0 dB Obj4.5 dB ~ . a x

FIGURE 16 Typical Non-Tandem Tie Trunk

Idle circui t tenninations may be required to prevent repeatersinging when used with sane t i e trunk circui ts .I.Dading must confonn to requirements of TE&01 Section 431,"Voice Frequency I.oa.ding for Trunk Cables".' Irlmk treatrrent equipnent may be ins ta l led a t the PBX and/or a tt h e m .Carr ier equipnent may be used in an y or a l l of the possible threelinks or between PBX's as the s i tuat ion warrants .Foreign Exchange an d Rerrote WATS Trunks Design Objectives

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7.6 .1 'l be PBX FX trunks a re sim il ar to PBX ro tru nks in des ig n. PBXtr unks are sim ilar to remot eWATS lines in de sig n.7.6 .2 'l be FX CO and remote class 5 WATS ro are definedas CO (A ). 'l beserv ingCO i s def in ed as CO(Z). !my in ten red iate ro i s defined asCO(I ). CO(A) to OO(Z) consti tu tes th e in te ro ffice l ink an d OO{Z) to

PB X i s the PBX -CO l ink .7.6.3 'l beOO(Z) to PBX l ink consis ts of a 2-wir e, no n-l oa ded or lo adedc ircu i t ; 4-wir e, no nlo ade d or lo ade d c ircu i t ; o r 4-w ire ca rr ie rderi ved c ircu it . 'l be OO(A) to OO(Z) l ink m ay be a 2- or 4-w ire loa dedvo ic e fr eq uen cy fac i l i ty , ca rr ie r fac i l i ty , or sare can bi nation of voicefr eque ncy or ca rr ie r fac i l i ty . Nonna ll y the CO to ro l ink i s a ca rrie r

faci l i ty . 7.6 .4 FX and WATS tru nks sh oul dbe de sig ned fo r an objective ICLof3. 5 dB as sho w n in Fig ure 17 . 'lb e rraxim um IC L sh oul dnot ex ce ed

4 dB. Fig ure s 18 an d 19 contain desig n guideli nes fo r 2-wire and 4-wir evoic e freq uen cy c ircu i ts for FX and WATS t r tmks.

7.6 .5

FX orWATSCO{J._) CO(I) CO(Z} PBX STAI \ I \ I \ I \ r - - \LJ u 0

Called IC L - - - 1 C a l l i n g Stat ion s ta tion 1 ICL

')o t Call ed Call ing


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CO (A )

XX>


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ICO(A) CO (Z)\ I \

u NL/LOb.

4.0 dB Max.

CO {A )

ff i(A) to CO{Z)Max IL.1_15 dRGab)e Inser tio n Wss

CO(Z) to PB XMAX IL2

15

m(z)I \

ffi(A) to PBXMAX IL22 dB

PBX

---lSTA\ ~

L NL/L -Wk_t---WIVFRl!_j---ICL

Cable Inser ti on Lo ssm(A) to co (Z )Max IL . CO(Z) to PBX MAX IL2

Notes :15 dB 15 dBNL =Non-l oad ed Cable L =Loa ded Cable

CO(A) to PBXMAX IL30 dB

Fig ure19 Gui deline s fo rDesi gn of 4-Wire FX o r WATS Tru nk s

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7.7.2 VNL i s discussed in the AT&T publicat ion, "Notes on DistanceDialing" . The main points of VNL are as follows:7. 7. 2.1 VNL i s "the loss value in dB assigned to a t runk to carrpensatefo r1i t s added propagaticn delay, tenninal delay, and lossvariabi l i ty ." In equation fonn

VNL = { L: (VNLF X Length} + 4} dBWhere VNLF i s the via net loss factor . VNLF fo r a one-way distance inmiles has the following values:

(a) 2-wire exchange cable (loaded or non-loaded) : VNLF = 0. 4dB/mile(b) 4-wire v f circuits : VNLF = .017 dB/mile(c) carr ier circuits : VNLF = .0015 dB/mile

The 0. 4 dB loss factor accounts fo r t runk connection loss variations.7. 7. 2. 3 The VNL value i s th e minimum net loss allowable fo r a givent runk.7. 7. 3 Tandem an d intertandem t ie trunks should be designed fo r anobject ive ICL of (VNL + 2) dB for through connections an d (VNL+4) dB for tenninal connections.7.7.4 Tie t runk circui ts fo r tandem PBX's should be equipped with aswitchable 2 dB pad. Th e pad should be switched out on throughconnections an d switched in on tenninal connections.7.7.5 Minimum values fo r echo return loss {ERL) an d singing returnloss (SRL) are given in Tables 2 and 3.7. 7. 5.1 Throu;rh balance i s ra:ruired when 4-wire ~ are switched a ta 2-wire PBX. I t i s measured with hybrids a t both por ts an dwith the 2 dB pa d switched out.7. 7.5.2 Tenninal balance i s required when a tandem t i e t runk i s switched

to a non-t ie trunk or to PBX s ta t ion l ines .7. 7.5.3 Tenninal balance i s measured with 900 ohms an d 2.16 ll f a t thecentral off ice for non-tie t runk cor.nections. When tenninat ing

a t the PBX th e tenninat icn should be 600 ohms and 2.16 llf.7.7.6

7.7.7

7.7.8

End sections should be half (0 . 5) a fu l l section length. Endsect ioos for trunk circui ts are discussed in TE&CM 431.Idle circui t tenninat ions may be required with repeaters locateda t th e PBX.The t ie t runk a t on e PBX ITR.lst be coopatible with th e t i e t runka t th e dis tant PBX.

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8. GI.DSSARYAddress Signals - convey cal l destination infonnation suc h as stationnumbers.Alert ing Signal - in dicates an incoming cal l .Attenuati on Loss - th e loss re s'u l ing when a two-port network i s te nninatedin i t s charac ter istic or image impedance.Centrex - a PBX type servic e with direct inward dialing where the switchin ge:;[Uipnen t i s generally located a t th e cen tral office .Channel - transmission path l:::etween two points .Circui t - a ca rrnunication pa th l:::etween two or rrore points .Expected Measured Loss (Er-1L) - th e calculated loss quantity which specif iesth e 1004 Hz loss th a t on e would expect to measure on a te nninate d t e s tconnection l:::etween read ily accessible t e s t points .Foreign Exchange (FX) - connect s a subscril:::er in one exchange area to acentral office located in sare other exchange area.Inserte dConnected loss (IC L) - the 1004 H z loss be tween the origin at ingand te nninating outgoin g switch appeara nce s fo r a t ru nk an d J:::etween thel ine-side switch appearance an d custarer sta t ion fo r a l ine .Ground Star t - a supervisory signal us ed for PBX 1 s given by connectingone side of the l ine to ground.Insertion loss (I L) - the loss which occurs when a ne twork i s inser tedin to a tr ansmission rr edium. That i s , the rat io of power delivered to th a tpar t of the systan fo llowing the network, l:::efore th e network i s in ser ted,to tha t same par t of the system af ter the network i s inserted.I.oop - a channel betwe en a custare r s tenninal an d a central office .Message N etwo r k - th e public switched telecarrnunications network.Off-Pranises Extenstion {OP) Li nes - connec t a rerotely lo cated extensionsta t ion to the rrain sta t ion.POTS - Plain old telephone se rvice.Private Branch Exchange (PBX) - a switching rr achine located on th e custarer ' spranise s for th e pu rpos e of interconnecting cus tarersta t ion se ts andconnecting cus tarer sta tion se ts to other PBXs or the message network.PBX, Main - has a directory number and access to the message network.

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PBX. sa te l l i te - has no directory m:unber an d no attendant. Th e main PBXprovides atte ndant services fo r the sa te l l i t e PBX.PBX, Tandem - pr ovides san e ftmction as main PBX plu s acts as an intennediateswit ch point fo r tw o o r I ro re PBXsPBX-00 Trunk - co nnects PBX to the loca l centra l off iceand no rmal ly appearsas a sta tion line a t th eCO.R in atripSignal - raroves the rin ging signal a t the cen tral off ice when thecalled te leph:>ne i s answered.Secreta ri al Servir e Li ne - Connects the custan er s ta t ion se t to a telephonean sw eri ng service.Tie Trunk, In ter ta ndem - interconnects tandemPBXsTie Trunk, Non -tandem - interconnects main PBXsTie Trunk, sa t e l l i te - inte rconnects main PBX an d sa t e l l i te PBXTie Trunk, Tandem - interconnects main PBX and Tandem PBXWid e Ar ea Telecorn nu nic atirns Servic e (WATS) - pr ovides d irec t access to selectedwide in ters ta te an d in t ras ta te geo grap hic al areas (bands) fo r afixed rro nth ly ch arg e.WATS Line, Inward - used exclu si vel y for incomin g cal ls fran a designatedgeogra phic al area to a cust aner.WATS Lin e, Outward - used ex cl usiv el y fo r outg oin g bul k ra te ca l ls fran thecustaner to a designated geogra phic al area .

9. REFERENCES1. AT&T Engineeringan d Network Serv ires Dep ar trrent Sy stems Pla nningSection, "Notes on Distance Dia ling" American Tel ephone an d Tele graphCarpany, 1975 .2. Technica l Pe rsonnel AT&T, Bell Telephone Cc np an ies , an d Bell Laboratories ,"Telecarmunications Transmission Engineering"; Volume I , Secon d Editi on 1977;Volume I I , F irs t Edition, 1977; Vol um e I I I , Second Edition, 1977; WesternE lectric CaT pany, Inc ., Techni.cal Public ati ons; Winston - salem, North

Carolina.3. "Inte roffic e Tru nk ing an d Sig nal in g" REA TE&CM, Section 319, Issue 4,Oc tobe r 1978.4. "Guide for U tiliza tion of Centr a l Office Loo p Extenders", REA TE&CM,Section 332, Issue l, August 1969.

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5 . "Transnission Object ives", REA TE&CM, Section 415, Issue 4, May 1973.6. "Design of Two-Wire Subscriber Loop an d PABX Trunk Plant" , REA TE&CM,Section 424, Issue 3, May 1973.7. "Subscriber Loop Canputations - Design by loss Method", REA TE&QJI,Section .426, Issue 2, December 1977.8. "Application an d Use of Bridged Tap Isolators for Subscriber L::>ops",

REA TE&CM, Section 428, Issue 1, October 1965.9. "Voice Frequency I..oading for Trunk Cables", REA TE&QJI, Section 431,I s sue 2, October 1962.

10. "Negative Resistance an d Negative Impedance Voice Frequency Repeatersand Voice Frequency Repeatered Trunks", TE&QJI, Section 444, I s sue 2,November 1961.11. "Station Carr ie r Applicat ion", REA TE&QJI, Section 911, I s sue 2,September 1971.

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TA BLE 1CABLE RESISTANCE AND ATI 'ENUAT ION IDSS PER KF (KM)

Res istanc eCable Type Loading Ohns/kf (km ) dB/kf (km)

22 NL 32.8 (108) .34 (1 .1 2)24 NL 51.9 (170) .43 (1 .41)26 NL 83.3 (273) .55 (1 .8 0)22 H88/D66 Note (1} .16 (.52)24 H88/D66 Not e (1) .2 3 (. 75)26 H88/D66 Note (1) .35 (1 .15 )

Offi ce & PBX NL 40 (13 1) No te (2) .4 (1. 31)

NarES : (1} 66 MH Coi ls = 6 ohms/co il88 MH Coil s =8.5 ohns/c oil

(2} Carpr anis e resis ta nceva lue fo r several typ es of cabling.

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TABLE 2

TandemPBX ThroughBalance (PadOUt)

VNL Tie Tnmk to VNL Tie Trunk

TABLE 3

ERL 23 dBmin SRL 16 dB min

Tandem PBX Term inal Balan ce

VNL Tie Trunk to Non-VNLTie Trunk

Tie Trunk toNon-Tie Trun k

Tie Trunk to Test Termin ation(GOOn & 2.16 pf)

*U .S . PRINT ING OFFI Ci$ : 1980 Q-}l.0 -94 4/REA-75

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ERL 16 dBmin SRL 11 dBmin

ERL 13 dB minSRL 10 dB min

ERL 20 dB minSRL 14 dB min

rea_470_i1 switched special services and private branch exchange services

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