GB2342014A

GB2342014A - Add-drop multiplexers

Info

Publication number: GB2342014A
Application number: GB9930548A
Authority: GB
Inventors: Masahiro Shinbashi; Akihiko Oka; Kazuo Takatsu; Hideaki Mochizuki; Junichi Ishiwatari; Koji Iwamoto
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1995-02-23
Filing date: 1996-02-23
Publication date: 2000-03-29
Anticipated expiration: 2016-02-23
Also published as: GB2342257B; GB9929817D0; GB9930548D0; GB2342257A; GB2342014B

Abstract

An add-drop multiplexer comprises: a first demultiplexing unit (51) for receiving at least one of a plurality of first signals, and demultiplexing each of the at least one of the plurality of first signals into a plurality of second signals which are contained in each of the at least one of the plurality of first signals; a second demultiplexing unit (52) for receiving at least one of a plurality of third signals corresponding to the at least one of the plurality of first signals, and demultiplexing each of the at least one of the plurality of third signals into a plurality of fourth signals which are contained in each of the at least one of the plurality of third signals; a lower multiplicity level signal selecting unit (53) for selecting one of the outputs of the first and second demultiplexing units (51, 52); a multiplexing unit (55) for receiving the selected one of the outputs of the first and second demultiplexing units (51, 52), and multiplexing the plurality of second signals in the selected one, to generate at least one fifth signal corresponding to the at least one of the plurality of first signals; and a selecting unit (56) for selecting one of the of the at least one of the plurality of first signals, the at least one of the plurality of third signals, and the at least one fifth signal.

Description

2342014 ADD-DROP MULTIPLEXERS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an add-drop multiplexer, which is used in a synchronized communication network. The add-drop multiplexer according to the present invention can be used in an optical 'synchronous communication system in which a signal which is composed by multiplexing a plurality o-f signals each being in a transimission frame of a first -form, and transmission frames of the first form each can contain a plurality of transmission frannes in a second form. The add-drop multiplexer according to the present invention can be used, for example, in the SONET W4 - (Synchronous optical Network) system in accordance _Lth ANSI T1-105, or in the SDE system in acc-ordance with the ITU-T recommendat ions, G707, G708, and G709, or in the New Synchronous Communication Network in accordance wit-hL the standard JT-G707, JT-G708, and JTT-G709 by the Telecommunication Technique Committee Li Japan. When constructing a large scale network using these communication technologies, accessibil-LL-v to signals in different, hierarchical levels and flexibi-Iii-ty in various services and circuit setting operations are re(n_,ired.

Hereinafter, explanations are provided based on the SONET system only since the correspondence bez.-Ween the SONET system and the SDH system or the New Synchronous Communication Network system is well knc-,,n.

2. DeSC-?i]D-t-icn of the Re-', ated A-r-- ANSI Tl-105 defines the add-drop multiplexer (ADM) as "network elements that i)rov-':-de access to all N, or some subsets M (M is a standard hierarchical level ≤'-N-), of the STS line signals contained within an OC-N optical channel. The STS signals are added to (inserted), and/ol- dropped from (extracted), the OC-N signal as it passes 1-hrough the ADM".

As explained below, the conventional add-drop multiplexers lack sufficient fe Laccessibility to signals in d_Jf rent hierarchical levels and flexibility in vari-ous services and circUlt setting operations.

2.1 Conventional Techni-cue And The Problem Thereof Which Is To Be Solved BY The Present Tnvention In conventional add-drop multiQ-,Lexers, which are located on a bidirectional -k--a-n-s-.nission line th-rouch which signals each containing a multiplexed plurality of first transnrission frames (of a plurality of first channels) in a first forni are transmitted in two crame furt directions, where each first --ransr-.Lss.on her contains a plurality of second transmission frames (of a plurality of second channels) in a second fo-r-m, and the -ibut add-drop multiplexers can add t -ary signals to the above signals in both directions and drop tributary signals from one of the above slunals transm-Itted in a selected one of the two directions, i is not DossIble ierarchy leve! at operations are k-o choose a h-ions of signals :)er-io--med for select.--ng the transmitted through the bidirectional trans-,-,i-ission line between the level of the plurality of first cnannels and 1 41 _he plurality of second channels. in convent-onal adddrop multiplexers, which are located on a bidirectional transmission line through which OC-3 signals, each containing three multiplexed STS-1 sicMials, are transmitted in two directions, where each STS-1 signal further contains a plurality of VT signals, there is provided only a function for switching the directions of the STS-1 signals. Therefore, when trouble occurs in one of the plurality of second channels for the 'vTT signals, the switching of the direction of the signal cannot be perfo=ed at the level of the VT siLgnals, and the Ecrmed at the --ion must be per-L switching of the direc-IL level of the STS-1 signals.

SUM24-ARY OF TEE INVENTION An- object. of L-he present invention is to provide an add-drop multiplexer in which accessibility to signals in di.-Eferent hierarchical levels and flexibility in various services and circuit setti-nq operations are improved.

An object of the present invention is to provide an add-drop multiplexer, which is located on a bidirectional transmission line through which signals - f irs t each containing a multiplexed plurality OL first channels) transm.-ission frames (of a plurality of i in a f irs t f orin are transmitted in two directions, where each first transmission frame further contains a plurality of second transmission frames (of a Dlurality of second channels) in a second form, and the add-d-!cp multiplexer can add tributary signals to the above signals in both directions and drop tributary signals from one of the above signals transmitted in a selected one of the two directions, and are enabled to choose a level of operations of selecting the directions of signals transmitted through the bidirectional f the pluralitV cf trans-m-ission line between the level oL first channels and the plurality of, second channels.

-4 According to the present invention, there is provided an add-drop multiplexer which conta-ins: a first - demultiplexIng unit for receiving at least one of a plurality of first signals, and demultiplexing each of the at least one of the plurality of first signals into a plurality of second signals which are contained in each of the at least one of the plurality of first signals; a second demultiplexing unit for receiving at least one of a plurality of third signals cor-responding to the at least one of the plurality of fl-1-st signals, f the at least one of - and demultiplexing each o. (-he plurality off third signals into a plurality o:f fourth signals which are contained in each of the at least one of the plurality of third signals; a lower -multiplicity ff the level sianal selecting unit- for selecting one o-ff outputs of the first and second demultiplex-ing units; a multiplexing unit for receiving the selected one of the outputs of the first and second derriultiplex-ing units, and multiplexing the plurality of second signals in the selected one, to generate at least one fifft-a signal cc-rresconding to the at least one of the pluralizy of first signals; and a selecting unit for selecting one of the at least one ollf the plurality of first sianals, the at least one of the plurality of --hrd signals, and the at least one fifth signal.

(1-1) In the add-d-rop multiplexer according to invention the selecting ianit may t-he present MU', - 4 --ng unit contain a higher/1--ower _LpliCi.y level select_ for selecting one of the at least one off the piuz:-al-it--y of first signals and the at least one fifth sLgnal, and a higher multiplicity level signal selecting unit for selecting one of the at least one of the D!u-7-alJi--"v of third signals and the signal selected by the higher/lower multiplicity level se.lecting unit.

(1-2) The add-drop multiplexer acco-rding to the above ( 1-1) may further contain a lower multiplicity level signal selection control unit for monitoring the outputs of the first and second demultiplexing units and controlling the lower multiplicity level signal selecting unit so that the lower multiplicity level signal selecting unit selects the output of a specific one of the first and second demultiDlex41.ng units according to L-he monitored outputs, and a higher - multiplicity level signal selection control unit for monitoring the at least one third signal and the cutput of the higher/lower multiplicity level selecting unit, and controlling the higher multiplicity level signal selecting unit so that the higher multiplicity level signal selecting unit selects a specific one of the at least. one third signal and the output of the higher/lower multiplicity level selecting unit, according to the monitored outputs.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Fig. 1 is a diagram illustrating a first eaTple of an acd-d= rniItp rzt eTb3tyL,-,g the pres-at Fig. 2 is a d -ac:razi illustrati -L1( e- W a, 1 add-drop Multiplexer not errbodying the present Fia. 3 is a a third -xei.-le cf:

an add-drolD multi-clexer not erbcclvinc_ the present in-,7ent:icn; Fig. 4 is a diagram illus--rat--,:.,,q a -Fourth example of an add-drop multiplexer not embodying the -present inventiont; Fig. 5 is a diagram illust-ratL-ina a fifth- exam-ple of an add-d=op mulltiplexer Pot embodying the present invention; Fig. 6 is a diagram illustrating a sixth exwmle of an add-drop multiplexer rot embodying the present -;r.-.7ent-ion; Fig, 7 is a diagram illus trating a seventh example cf an add-drop multiplexer 1-0-L e-rrbodyi-,ig the present invention-, Fig. 8 is a diagram illus-tra---Lna an eia,.ith examle ofE ar. add-drop multiplexer not errbodvi:-..g t_he present invention; Fig. 9 is a diagram illust-rating a nirth example of an add-drop m-7"Ltip- ,exer not ernbodying the present invention; Fig. 10 is a diagram i_l 1US trat r.g a tenth exaTle cf an, addd--oo mu- ltiD-l--xer not embcdv-l:Ang -Lie present Fig. 11 is a diagram --liustra-t.T-n( an eleventh example of an add-d=-o multiplexer not embedying the nresent inverticn; Fig. 12 is a d--,agra.-,n -illus-t-ratL-in( a twelfth example of an add- drop multiplexer not embodying the present inventinn; Fig. 13 is a diagram il-lustt---atin( a thirteenth exarmle - embodying the present invention; of an add-drop multiplexer no.

Fig. 14 J1 s a d i a am il, 1 u s t r a t i n g a fourteenth example bodying the present-- of an add-d-rop MUlLiplexer not ern J nventicn; Fig. 15 is a diagram illustrating a f-ifteen-dl- example of an add-drop multiclexer not en-bodying the o-resent invent-lon; Fig. 16) is a diagrari illustk--.ra-L--Jnc. a sixteenth examle of an add-drop multiplexer not embodying the present irvention; Fig. 17 is a diacram --1-1ust-r-a---ing a seventeenth examsle 0 f- an add-d=r) mul. -i::)lexer not eribodving the present icrl; Fig. 18 is a diagram illustratinc: an e-Jc-,hteenth exa-m"e of an add-drcp multiplexer roc aTbcdying the;)----sent FLc. 19 is a diacram i us-,a -::ia a nineteenth exa.=,,,!e -- - - - L_ - - - - o' an adddror) multiplexer Pot e-bcdv-i-9 the p--esert Figs. 20A and 20B are diagrams tw-a-tie-th eaTrLle of an c-dd-d=p mjlti rnt eTbo:tviM Cr-=3a-,t ir%ert=-; Figs. 21A and 21B are diagrams illustrating a -7,a-itvfexaTnl-- of ar c- rb-d=p rrulti rnt eTb=b7irig tne PresEnt irlvctim; Figs. 22A and 22B are diagrams illustrating a r,a-t:v- se::=-d exaTnl-- of an caJ:I-Ctrcp nultj mt eTbx Ce PcEr--L. =; Fig. 23 is a diagram illustrating a twenty-third examole of an add-drop multiplexer not embodying the presert Fig. 24 is a diagram illustrating the basic operation steps of an exaTple ci=uit s4it=hire M.thid tz use in the Fig. 23 add-drop multiplexer; Fig. 25 is a diagram illustrating -he construction of an embodiment of the present invention; Fig. 26 is a diagram illustrating the construct_Jon of another embodiment of the present invention; Fig. 27 is a diagram indicating the con-l-i(guratio.-,i of Figs. 27A to 27D; Figs. 27A, 27B, 27C and 27D are diag-rars illustrating the construction of another embod.,,r.ent of the present invention; and Fig - 28A and 28B are diagrams illus-.rat-i-nQ an example construction of the VT path protection switch i-n Fig. 27D DESCRIPTION OF THE PREFERRED EMEODIMENTS EXPLAN.,I.TIONS 0-7 EXAMPLES7NOT EMSODYING T-L7E PRESENT i'N7ENTTON (Fics... I to 24):e-ence to

First -.'-,-ie--e will be described, with re:

Fias. 1 to 24, various examples o..LE add-drop multiplexers which, although not embodying the present invention, may nevertheless aid in an understanding of L the description of the e=,od-imen--s o the -Lnventon i -olexers that follows. The examoles of add-dros multl described with reference to Figs. 9 to 16 and 19 to 22 are errbodiments of an invention described and claimed in the parent application (no. 9603922.7) of this divisional application. The examp.-s described with reference to Figs. 23 and 24 are embodiments of an is invention described and claimed in another divisonal ion [Agent's ref --!-Lj54315/00-11 of t ' ne Oarent a,or)'- cat application (no. 19603922.7, o_l -h-s divisional acclication.

Fig. 1 is a diagram I'llust-ating the construc:ion 23 o a first example o--:: an add-drop multiplexer not e-.nbod-v-'i.ng the presen-, 4 in tne cons t I-uct ion of Fig. 1, the -,)ar-ial dro-2 unit i selects n first signal(s) of n channels(s) among N sianals of N -rom a irst channels, which are received 25 "I- of- n channel(s) on at least cne second 1_1.ne, NI and n are intecrers sat J S E---- - c: 1.2ke 1 -- a= t, -ia, add uni.. 2 _rece ives n second. s i =-a, 1 z) o- n channel(s) from at least Ore and first sic--a](s) of (N-r.) C4-Larr-e-", (S) a=cr-c th.e frs signalls of the N channels, -w-tuc- are:,,: _.

L - - i!c) selec-ted by tIne par.---L-al d=or) un-it I., ar-d tra-nsmits on zi-rs line the n second (s) of n L-he f channel(s) a-rid the first signal(s) C channel(s) as N third signals cl' N channels.

z - -s ke-ol-ding to t_he cons truction c -10- the I_.

exa mrle, it is cassible to access only an or channels Am ng a pluralli. ty of c--,.anne-ls _= ; -ansmit sig-nals t= through. the IL L-ra.:,-s:n-,ss-- i:Lpe, k_ the tran mission framies in the othe- a-nd a make - - -- - -- -- c-a.-el c::- criar-.z.e-,s of12 the C-7 Cn;n-le--4s::)aSS t_=CU= th anc the add-drop Without be--Lng Cut-put tin-put from the side.

Fig. 2 is a diagram, illustrating the c:cnst-,--uc:t---cn of a o- ar ":L-I-Liz)lexer nott- ertodvL.-Iig the presw add-d=p Generally, the N first sia-nals of N Fig. 1, may re transmitted through the transm-ission line in a form o-.iL-- a multiplexed signal in which the N first signals are and =,,Z the 11,1 third signals of N cha=els may be --o= o a -lircuah the f -irstL. trans-art S-1on line _L.I a second multiplexed signall in which t_ne N tk-hJ4--1-d si-rls are- multiPlexed- in.1khe cons -k--ruc--.;:-on or m7g. 2, t_e 4 m t o add-drop multimlexer contains, in addi t-c constructlio.n of Fig. 1, a 3 and a . 1 =ltiplexing unit J=. The j iplexed th- N 1: p I X - ignals into the second the N thi i d s Fig. 3 is a diagrw., illustrating the cons trUC- exa='-- of an add-d=,r) IRI-21 Lnlexe-- noL and 1dicates a 2 J-.

U=., t ---se-L.S -;Lj-- tne n secc:.-.d of n. ch-ai=iel(s) azd the si(Tna', (s) -ames to be c (Nn) c.-.a=ei ( s), Int-o;t.-.l--e ion _f t.ransm.itte,,'., in with -4t-im4nc: of add- drow and -.he add-drom ofF F-j-(. 3 con-ta.:---s,:L-ri add-4, tien to the cons t= ct ion orl F-icj. 1, a plhase and frequency contzol -init S. The 2hase and _z cOntroll 55 or, L-In.ases of P- chlannel(s) a-nd the f-Lrst- sic-nal 1 - -) (s) of on the phases Zhe -frames .he above -hase a-nd ccn-t-7-ol 5 may con -a for ob a u. i t "aj--iizig the phases in th.e trans-rission.

--ails ss c -n fi-ames based on nhases of the tZ frames to be -L---ar-smL;t-4-ed or. the -2--1Lstt transm-ission line, at least one phase off the in the n second si-.,al(s) c. n a-.--,d at least one -Ohase cc=?t-1 on con- of- t_he i=l L-a-f----d in the received Slc-ial,(s) ofF (IN-n) char el(s).

the above phase and f-requency contzol and mav contai-i a LL the si=ais, based on o, -::S b.v,-- ah-eve 1 C ct a 1 = C Cn l= 1 =t- 1r)= 7 - i-, Cf, -w- = -:7; 3 -'- - Ln add.t--n. o -1-e coi--s.1. T.'ne t_2. Ple=lg Unif t_ 3 and the =ultk de-multt--4p-ie=g unit 3 and a. L.":.- a s e c - 4 functiorri in the same manne- ---esccncl-r-z 0 F.: f eipmei--L.-s in t"c=strucition c. 2, is a diagra-ti illustrating the ccns=,.:c-cicn cf- a f_--of an, add-drco - ,L,,ILiple-xer not e-Lmcdvir.g the -,)reSentL the construction of Fig. 1, the abc-u--9 n f-st of the ri chaznel(s) may _be contained in at t one transmission frame of a f c= as _ - L.

when the n JEi-rsIC (s) of the r_ c.,.,aT-nel (s) is tra. ar =T tted an the at least: one second tr s,, ss--,.-r.

i5 and each tra.-,-ism-ission frame c--ntta--.i to be and a phase of the be as wel -2 as STY- signals -Ln the SDE s-rst-am and.

svs- s t an, ar, -em in jaman, conain such In this case, a transmissilon frame =-st be "ia---sm--.ting the trans-mission fra=e, accerd-:--g to the of the add-drop mul.1L.-iL.Dlexe-r. the add d.--cc =1-1--iplexer fourther contains a phase and f Fig cor,.-tz-o-- un-it- 6, in addition to the ccns.r-actlon o_.

chase and frec-aL=ncv ccntL..ro-', unit 6 contained in the n first s------a-l(s) cff n as the- n first signal(s) C-E n char.:-.el(s) iS -r-aceJvc-d, into the at least one -which on the at least one second tt-rans-,iss:,on in with ti-mL.ig c--, t_ne =dd-d=op cb-,-a-Tns second on at le-ast ci-.e of the inserted Li. the at least --ne f=ne of the n s-c:-;z"(cz, t_he at i east one 21- the Second on _ase the at- least. one f::ar.e as t_e ce-- e- _= 7_ In the f 5, the chase and the at. least one phase ---I the a-- jeas c, tra.r.sm.:.ss-4c,-i frame based, on at. least one refe-rence at least one transmission to be i eas-:.on 02-mse at ed, and at - one the first is recelVed.

ir- add-t--,'-on, the an-c-7e phase and 'irecrier-cy c-r.- 1 may corita-in a for perfermIng stuffing and des---u-J2f-L-ig control in the transml-ss'l-o-ri 12-rames ccn-taig t --- signal(s) of n cnaT-nel(s), based on at. leasst he n fi-rst one frequency of the n s-,-cjna--, (s) to be t-ans-,nit-ed,, =,d at one -I _ L- y of the n f=SZ S-L=. al(S) -whe.-,i the n fL-Jrst s-:w-.-.al(s) of _rl --ece -,--7ecl Fig. 6 is a diagram illus-La-(ng the ccr.st=,ict-icr. cf a s-h ex--rn.", - - '-lexe- nct e-rcd-;ir.g m t Of an add-d--.01D nLIL-= - _e ras en L f: g. 6 Ot C0I.Lai:1S - the phase w-d he add--L-L-CI:) IMI-_Li2IeXe 0' -ac--.,ency c-p-tro", 5 in 7.1g. 3 and the phase and -0 6:Ln F -4. g. 5, as the f -i-,s t an. d, frecuency cont-r - second phase and control 5 and 6, respectively.

7 is a dia=a-n ti-- GE a m add-drop MUltiplexer rct e=.cd.vi-ng. me iDr-2sen-:: invention. The add drop multiplexer of 'Fig. 7 contains, in adlldi, -t--Jor. to the Construction of Fig. 6, the 3 and the =u-1-k-iplexizg u-nLt- 1-, whic-11 iT. the Sam. e as in the co-ristruction of Fg. 2.

:-z. 8:Ls a dia==-,, af: a C:f m addd---zo ret, embcdv-ng tlie -01-esent con-s -L-=c:±-J.-c)rL of Fic. 8, it iine each. conta- - a p-, ty of subC17an -7els (C'P- S a l owe-- mu-1 t i:D-1 -, c:: -v o- ower hierarchy level). The add-drop multi Dlexer of 8 contains a demultiplexing unit 11 and a unit 12 on the tribut-ary side. The denult,--,p-ex2-ng un-,.L-. c '-'n a- -least one n.

ll demi-,lt-iclexes at---Leastone of first signal(s) of the n channel(s) to m fourth s-.gr-als of m channels (in a lower multiplicity level or lowel hierarchy level) to be output to at least one foul-th transu-iission line, where m is an integer and the multiplexing unit 12 receives:n signals of m channels (in the lower multiplicity level or lower hierarchy level) from at least one fifth --ransn,-J,..ss-ion line to generate at least one- of the n second signal(s) of n channel(s) and supply the at least one of the n second signal(s) of n channel(s) to the partial add unit i5 2. The provision of the demultiplexing unit 1211 and the multiplexing unit 12 an the tributary side, enables access to the signall transmitted an the main L.ransm.ission line from the lower multiplicity level (lower hierarchy level) on the tributary side.

Fig. 9 is a diagram illustrat-ing the constructic-n cf a n=th enm--ole C:E cm add-d=P mll:"jPlEEr rr-t e7rbo:i tj-e -m:ESEl l 25 me -dd-cl=p rnlitiplew,=ry-rTdincr tD ttn ninth exaTole contains, in addition to the construction of Fig. first, and second crossconnecting un-k-s 7 and 8 on the it tributary side, as indicated in Fig. 9. The rs t crossconnecting unit 7 establishes at least one c-ireu--,'= through which each of the n first signal(s) of n channel (s) can be transnitted to one of the at least one t a second -L-ransnLss-ion I.ine, -Lin accordance w-4L.,correspondence relationship between the n f-:.-st - n channel(s) and channel (s) in a signal (s) signal(s) c-1 on the at least one second transmission 1-Lne. The second --uJ crossconnecting unit 8 establishes at least one ci which each of the n second signal(s) c-' ed as cne c -E tne N third, c-annel(s) can be su-cr)l signals, in accordance with a ccr--.-esiendenczrelatmionship bet-ween the n second s--:-cnal(s) of n channel(s) and the N third signals.

ion c-c 9, it is Acco-rding to the construct J_ - - nossible to access an arbitrarv one o-- the olurality of first k_ channels in each of the N from each second channel on the tributanr side and letting :on frames in the other channel o- channels t r a n s mi s s _;_ - io pass through the add-drop multicLexers without being outcut to and input from the --r:Lbut-ar-v side.

-g. 10 is a diagram illustrating the construction of a ba:d ple of W, ad:!-dm:P m-lltiPlE>2er =. e-Tboci-lrU ttn prese.t 1'ne P-dd-d=p MUlt:plexer of Fig. 10 contailnS, in additiCr. tO th- construction ctFig. 9, a phase and control unit 9, which functions;-,i the sane manner as the chase and freauency control unit 6;-n Fig. S.

Fig. 11 is a diagram illustrating -he constructle- cf w. ewrJ-- of an aJJ-dizp -LTiiltjnle>p-- ret eTbody:izg the prEEi--,, _. - J - - In the cons trUCt iCr c:E Fig. 11, 'L is assumed that at least cr-en f- c:E tlIce signals trans=tted tl,.rou-- the main ( _. 1-rs-transmission line each contain a plural Lty of sub- 2 J - -1 eve"i c Tcnannels (channels in a lower multimilc.-v lower hierarchy level). The add-drcp rzul-lttir)lexer of 1! contains, in addition to the cor-stt-rjc-.-Jon of Fig. 9, a der,-,ult- Jplexi.ng unit. 17 and a multiplexing unit 18 on t lexinc he tributary side. The provision of the unit 1,7 and the unit 18 on the y de, S in addition to -ne first and second crossconnecting units 7 and 8, enables access to t_he s4c:nals in the various channels trans7.,i-,-ted on the ma:,-----ans:-,-ission 1 ine from a lower Tw-,lt-ipl--c--4ttv level ( 1 ow,= level) on the tL---ibu-.av side.

Jllustrating the const-----or 12 is a diacram -I - tiolaxer rnt aTbi:il-i9 the Presa-C c= a tAell-th axamle of an aJ5-d= ffult i'llne add-dro. nuitiplexer of Fig. 12 contains, ir addit-icn i:-, the c c n s --::- -- c -- i c n c f F _; rj. -1 -, a u",. a s e a- d f, r a c.- u e n c v c c n --:7 c 1 unit 9, which tfunctions in the same rnanner as the ph p-ase and---ea ency control unit 6 in Fic. 5 and the nhase P-j A- - - U L- 1 ------- --- ---- -r_ treauency control unit 9 in Fig. 10.

Fig. 13 is a diagram illustrating the cf a tr=temth exaT-ule of an addd=P MIltiPL-Er rnt E!TcstP'g tm -uLaIn tin c=tn== & rig. LI, it is assmEd t= the sig-i-zls trans=itted throuah the main (first) transmission each contain a plurality of sub-channels (channels in a lower multiplicity level or lower hierarchy level), and that the integers N and n satisfy Nkn>O. The add-dro-o on th I.IIUJ-L--'r)leXer of Fig. 13 contains, in add4t -0 is construction of Fig. 1, the demultiplexing unit 9 and L-he multiplexing unit 8 on -he tributary s ide, and further contains first and second crossconnee-t--Jna un-,z-12:

tinlexinc 13 and 14 on the tributary side of the unit 9 and the -Liultimlex-ina_ unit 8, respectively. 1-he de--,-nul---lulexing unit 9 and the multiplexing unit 8 in Fig. 13 are the same as the correspondincj elements irst crossconnect ig. 8. The f ing un-it 13 establishes at least one circuit through which each C-LE: the M si(nal(s) of m channel(s) (in the lower 1 or lower hierarchy level) can be t-rans=ittled to level one of the at least one fourth transmission line, in accordance with a correspondence relationship between Lourth signal(s) of m channel(s) and the at least -he n 1 L 1.. l_ one fourth transmission line, and the second crossconnecting unit 14 establishes att least one s c t h at e ach o -E t he m f if th s ignal ( s) c if m ch an n e s (in the lower muitiplicitv level or -sowe::

level) can be supplied to one input por-- o-E r-,,ultii:)lexi-ng unit.112, in accordance with a corresuondence relationshiP between the m fifth si(na-2(s) and the input ports 'exJi Ot the multt.--p- -ng un-12.

Acco--(--ing to t-he construction of the th-i--teent-i examle, i t i s p c s s ib 1 e t o a c -- e s s a n a r b r-v c ne 0 the c)'iu7-a-l-: tv c ft sub-e-anriels (cliannels in level or lower in at least one of the signals ---1-r-ouch the ma.in transmission. line, from each channel --,-n the lower 4- - r-,ul--IiD!.---'---v level on the s Fig. 1-41 is a d-:agram t construct on c-T a = ea-th earple of cm aJJ--cbr-p ff ulL' rr-t eTb=briM t-le prese Ir te rTulti-aL--.r=r of Fig. 14, it is asmurad tf-c -,-.e integers IN and n satisfy Nkn>O, and the m fourth signal (s) and the m f th signal (s) (in the lower i-,,ult-ipl-ic-itv level or lower hierarch.y level) are each.

0-ame c f a ccnta--j:-ned in at least one -L-rans,-7.iiss-lon:- is predetermined form when they are -t---ans--,,n-ittted, and each rame can contain _Lniformation t transinis s c_n L' o be transrrL-lted and poi.nter -which -ind-ica--es a phase of the informatilon to be transnitted, in the ion For example, t -rans-,-,LLss-:

rap..sm-iss- Lne t -Ion trames of VT, siana-'s in the SONET system, as well as VC (vi=tual signals in L-he SDH sys--em and t ' n ITI standard sysze,-n in JaDan, con-ta= su--n oo.---izei7 information. In this situation, the pointer -i---fc-rmat.-on in a received transi-ussion frarme must be rew---i--ten ore the trans--.n-ssion framme, a--co--d-i-. o bel -ranszL, tting ng t the -L-in-,Lr.g of the add-d-roD mulltiplexer. Therefore, the add-droQ multiclexer of Fig. 1 further contains cirst and second ffirst phase and '1recuencv control units 15 and 16. The first Dhase and frec-i-,ency con-.rol unil t 15 inserts first -in'lori,-,a-.'-on contained in the -,i -z -Ecu-t -:(al(s) is s-cj-.ial(s) of m channel(s) as the m h s_ rece-ved, into the a-:: least one -,--a-,s,-,-ss-;lon ---ame to be i n s-,;nch-. Lzai on w c - - ad,4 - r-cn.; o:

ob- ormation on at leas-- dro,, -,Lu-l-- -a--,ns second --,n-f, c -he fi-s,-- inforination inserted in the one chase o- L i least one -.-ansnLiss-:on fra-ine, and second rra--,ie as the pointer one transmission 41 nse,-s second phase and frequency control unit _I66 _t_ 4formation conta4 'th-, signal(s) of r..'I _-LIZ ned in the m fif fi-I"th signal(s) channel (s) as the m is recelived, Jnto the at least one transmission f-rame to e t -- a.-. s =, t t e d in synchronization with tiaLing olo t"-e add-droo multiplexer, obtains fourth inf-ormat-icn on a-leas. One phase of the th-ird information in the at least one --ansmission frame, and inserts the ormaz-, on least one on the at least one phase into the at transm-issicn frame as the pointer in-for-mattion.

Fig. 15 is a diagram illustrating thie ccnstr-:ct_-- c-- cf a f if teer:th exarple of am aJd-d=:P MjI:n-1=->P_r rf-t -- ltcd_ViTg t')e Pr-----Ljr.;,tjm. TE ald-d2= mltiplExer of Fig. 15 ou-tairs, Ji, ajditti- tz) =X construction of Fia. 8, a through connection unit 19. least one he through connection unit 19 supplies at L.

it 15, the m fourth signal(s) to the mult-JiLplex-Lng un' instead of -he az least one of &the m f-J, f-th signal(s), where the integers INT and n are assumed to sat-'s-fy _NR:n.>O.

According to the construction of t--;.e fif-Leenth examle, it is possible to make signals in a sub I 4c4 channel or sub-channels in the lower mu 1 tiol- 1-tv level -:pl -.y level other than a signal(.s) in the lower mult 4 c i t which is recuIred to be accessed fron, the trLbutarv side, pass through the add-drop multiplexer without being output to and input from the tributary sde when O both signals in a sub-channel or sub-c'-_Iannels in t-_e lower multiplicity level are contained in the same ch-annel iri the higher multiplicity level Fig. 16 is a diagra-ra illustrating -,'-e cc--st__-uc-__cn of a sixtea- ztn eK3Tple of a! aJd-d=P mlltiPlEmEr Mt eM09-Varxg tE -=es= ar7,-. T-ie afd-dcp mj1tjnjexL-x of Fig. 16 oat=, J:, ad==cr to construction of Fiq. 15, the first and saccnd zlqase a7r requency control units 15 and 16, waicm _1-7c-zlon I 18 - same manner as the cori-espcnding el ements in Fic.

J ustrat -L 1-he cc--str Fi a - 17 is a d_acram -L - k_ - n --, ct Lo n cf a sae-merit-1 exaTple of c-r. aid-d= rrultiiilt----- mt sTbo aM presat -c-g t -:-2 Sever-LeE.--th,t2 cf the to- e=rLh e-cr-,ules, 1:-,, krr-h the L-ine is a un.Jd-ir-ectional -t---a7Lsmiss-ion line, -is extended to cases wherein the main trans-mssion line is a bidirectional transm-ission line or a mair cf transmission lines in oncosite directions.

For the above nu--.-oese, the add-drop multiDlexer of Fig. 17 contains: apartial drop unit 211, a second oartial drop unit 23, a drop signal selection unit 25, a 1 ial add unit 24, L.Lrst pa-rt--i-al add unit 22, a second nartI ina unit 25. m-he first partial and an add signal supp-ly- I drop un-,-.!,--. 21 selects n signal(s) of n channel(s) among '_,-4 first signals of N chan-nels,---whichare received from a first transmission line, and sunplies the n firstI ion signal(s) of r. channel(s) to a drop signal se-lect - unit 25, where N and n are integers sa.-islov:Lng N>n>O.

The second partial drop unit 23 selects n second signal(s) of n channel(s) a-mong N second sig-nals of N 2-5 channels, which are received from a second transmission line and correspond to the n first signal(s), and supplies the n second s-ianal(s) oil In channel(s) to the drom silanal selection unit 25. The d--oc signal selection unit 25 -receives the n first. signall(s) and the n second signal(s) corresponding to the n f-J-rst. signal(s) as n pai_r(s) of corresponding s-Lgnals, sejects one signal gna f-om each pair of the n pair(s) of cc--esoc)nc--ng s o obtain n selected signal(s) and the n selected signal(s) on at least one transmission 33 line. The first part-iial add unit- 22 receives n th-4rd signal(s) of n c,-.yann-e-.(s from at 17east one feurth t icna,(s) of (N.-n) t-a---sm.iss-ion line, and (,T-n) first. s_ channel (S) among t.neN first signals cf, the -N cha-n-ne-', s, which are not selected by the first partial drop 21), and for tkransmi-t--.4Lng on the first transmission line Eirst signal(s) as N the n third signal(s) and the (N-n) fi. L- fourth signals of N channels. The second martial add unit 2lreceives the n third signal(s) of n channell(s) from at least one -fourth transmission line, and (X--n) second signal(s) of (N-n) channel(s) among the -.N' second signals of the N channels, which are not selected by the second martial drop unit 23), and transmits on the second transmission line the n third signal(s) and the (N-n) Second signal(s) as N fifth signals of N channels. The add signal supplying unit 26 receives the n tnird signal(s) of n channel(s) from the at least one fourth transmission line, and supplies the n third sanall (s) to the first and second partial add units 22 and 24.

According to the seventeenth exarrple, it iS POSSil-C-1-- -LC access only to an arbitrary channel or channels of the pluralLty of channels in the signal transmitted in a selected direction, while letting the -.rans-,n,-ssicn -her channel or channels of frames in the ot plural -1-v of channels pass through the add-drop multiplexers -without being output to and input from the -L--7-ibu-ia=,,r side.

Fig. 18 is a diagram illustrating the cons truct _Jo n cf w. eicfttiaaith e>oTple cf cT, aJd-drco rrultjplfi!mer rct t-e 1r-ivrzr,t=. Ir the c=di=L-= cf Fig. 18, a and frag-ia---,-v ccr=-l unit 27'is provided an the tributary side of the drop signal selection unit 25. The function of the phase and frequency control unit 27 is basically the same as the phase and frequency control unit 6 in Fig. S. Namle-,v, the phase and frecraency control unit 27 inserts f-J--st information contained in the n selected cfF n : n c'--nnel's) channel(s) as the n selected signal(s) ol- s received, into the at least one transmission f-rame -Lo be -t,-7-ans.-, nilk-.-ked on the at, least one third line, in synchronization with timing cz: the add-drc-- I ob-a-4-s second -Jn--lo=a---ion on at _Jeast- one nhase c:O the firs t i n formation inserted in- the at 7 east, i inserts the second one -!L-rans-,=ss--,cn frame, and in-joc=.nat-ion on th-e a-least one phase intc the at Ileast one z-,-a.-is=.ss-ion firame as the pointer zig. 19 is a d-iag--am illustratking the construction of a rirútea-th. werrple of a, wb-d= m-,ltj mt eTbodviM -t - Pc=tiM t.,, th-e the t r 1 Cf irld t- twelfth examples, in whi-ch the main transmission lire is a unidirectional!-,ne, is extended to -L - cases wherein the main transmission lire is a is bidirectional transmission I ine or a pa-i- of t --ar-s-,rLY ss ion lines in oiDDos i te direct ions.

For the above purpose, the add-drop multiplexer of Fig. 19 contains, in addition tc the construction of fou---h crossconnect.inq units 28 to 3-1.

cr. 1 -17, f- -4Lrs t t o -l - L 1 20!--n-e first c--ossc--n--iect-fng unit 28 establishes a-- least o f - first signal(s) one c-4-cui-Lt.h-roucjh wii.;-ch eac-l. L-he n o- n channel(s) can be transferred to one of the at , a least one th-i--d rans-,-,Li-ss-ien line, 41-n accordance wi-.

w-resccndence --e!a4--- - --.onshiD between the n first E n channei(s) and the at least one second s -Jcjnal (s) of t The second cros s connect ing un-Jit 31 transm-ission line. establishes at least one e.Lrcuit. throuah which each c:E 7(s) of n channel(s) can be k_he n second sicinal ransf-ei-red to one the at east one third transmission line, in accordance with a correspendence bet-ween the n second s--,anal(s) of n channel (s) and tine at -,east one second ±--ans,-.niss ion 1 ine., he third crossconnecting 30 est--abl-is----s a- leas;-- one circuit through which eac.n. c-10- the n. third sIgnal(s) c.;'- n channel(s) can be transi -o one o_l the at least one first transmission line, in accOrdance witIn a correspondence relationship between the n signal(s) c-LE n c-',na:,-nel(s) and the at least one tirst rans-,-,i- ission 1---e. The fourth crossconnee-ti---C 1-n-z 29 establishes at least one circuit throuch which, a respective one of the n -L.-n-:Lrd signal(s) of n channell(s) 5 can be transfLerred to one of the at least one second t-ransm_ission line, in accordance with a correspondence relat-lonship bet-Ween the n third signal(s) of _n channel(s) and the at least one second transmission line.

According to the nireteenth examle, it is p=sible t- of channels -Jaccess an arbitrary one of the plu L the ma-in s-ignal transmitted on the main (first) transm-ission line in a selected direction, -f--cm each channel on the tributary side while letting transr-,iss-cn is frames in the other channel or channels in the main signal pass through the add-drop multiplexer -without being output to and input from the t---buta---y sicie.

Figs. 20A and 20B are illustrating t_he u-jstrL=t= of a tw---T=-th E%mr)le & w. c-dd-d= iultLm:d--er nt =-Tb= presat P=--- t:) the twmtieth e>wnle, t- uastzL==' of Fig. 9, in tAl ttn main tra--rdssim lire is a unidirectional transmission line, is extended to cases wherein the main transmission line is a bidirectional transmission line or a pair of transmission lines in opposite directions.

For this purpose, the add-droD multiplexer cJ Figs.

20A and 20B contains: a first partial drop unit 211, a second nartial drop unit 23, a first drop signal J selection unit 62, a first partial add unit -92, a secon.

martial add unit 24, a first add signal supplying unit 63, a sat of at least one first 64,,, a set of at least one second demultiplexing un-65,, 65., a set of at. least one second d-7-on selectt--ir-n unit 60,,... 60,, a set of at least one first one r.ult i p7 ng unit 6.,., 67) a se-_ o z at leasz.

second mult_4I:)1exi'_nc unJLt 66., 66 a set of at ar(:i eas t one second add s Lgnal supply-J, na unit 65 L., 61_.

TIne f-Jrst partial drop unit 21 selects n f-4-st signal(s) of channel ( s) amcnq N firs t s _J gnal s of -N,- c.--annels _S TI -whicn are received from a Z-1 t transmission I ne, and o-nu- re u+ L-s the n first signal(s), where N and n a J_ntegers satis-f-ving N-L-n>O. Tne second partial dr-op unit 23 selects n second signall(s) of n cnannel(s) among N rom a second signals of N channels, which a Ic- second transmission line and corresocnd to the n first s-i-anal (s), and supplies the n. second s ignal (s) of n the drop signal select-Lon unit 25. The channel(s) to 41 first droo signal selection unit 62 receives (n-p) first signal(s) among the n first signal(s) and (n-p) second s 4 ignal(s) among the n second signal (s) corrascond=g to f - I(s) as (n-p) pa.__ I_h.e (n-p) fi--St signal r(s) o: 7- corres pond Ing signals, selects one signal from each pa_.L_ off t.re (n- p) pair(s) of corresponding signals to obtain -: t - ected (n-p) selected signall(s) and transi-LI he (-.---p) sel - s-'gna7(s) on at least one third transmiss-: I i Tie, where - L_ --on 4- T J --st pcart. a! add isfying n2:p>O.

o is an- nteger s a t -he f- L J_ uni t 22 receives (n-p) third siana-(s) of (n-p) channel (s) from at least one fourth transaiss-ion line, Irs- p multiolexed S4 -L - _ g n al ( s) o f p c'.Ii a n n e I ss s u p p 1 ii e d from a first multiplexing unit 67,,... 67_, and (N-r.) r first L, ong N k- signal(s) of (N-n) channel(s) a.7 -,-,--e '-. fl-s signals of tne N channels, which are not se-leected by the ransmit L-he f -st first. partial drop unit 211), and t Lls on - J_ i- first P trans-in-iss ion line the (n-n) third s -ignal (s 7 '-Dlexed s (s), and the (N-n) f irst s --cnal (s), as nuit I ,N-, -fourth signals of N channels. The second pa-rzial add f unitt 24 receives the (n-p) th.-4-rd signal(s) least one fou=h trans,-_,Lssion cin-annel(s) from the at t- 7 -' p7exed signal(s) of p c-;---r.-els 3-9 second p nut-l - supplied from a seco7d multiplexing unit 56.,... 66_/ and second sig-al(s) of IN-n) chan:,=--(S) aMLCI"a t-"Ie % I N second signals of the N channels, which are not selected by the second partial drop u--i-i- 23), and transmits on the second transmission the (n-2) tnird signal(s), the second m multiplexed. signall (s) and the (Nn) second signal(s), as N --4.L s--,crnals c-i' N channels. The fi- rst add signal supplying unit 63 receives the (n-o) third signal(s) of (n- c) channel(s) from the at least one fourth transmission llne, and supplies the (n-iD) third signal(s) to the 'L-i-st and 10 second partial add units 22, 24. The set of at least one f Lirst demultiplexing unit 641, 64.. receives p firs-.

signal(s) other than the (n-p) first signals among the r.

first signal(s), and demultiplexes each of- the p first signal(s) to m sixth signals of m channels to output p set(s) of m sIxth signals. The set oil at least one second demultiplexing unit 651,... 65. receives p second signal(s) corresponding to the p first signal(s), c-ther than the (n-p) second signals among the n second s gn-al(s), and demultiplexes each c-LE the p second signal(s) to m seventh signals of -rLi channels for each of -'gnal(s), corresponding to t--e m s_ -h L-he p second s- L-b -:x- s 4 _Ignals of m channels for each of the p f=st- sic----al(s) t o output p set(s) of m seventh signals- --he set of at least one second drop signal selection unit 60,, --- 60 receives the m sixith signals for each of the c fk rst signal(s) and the m seventh signals for each of th.e c second signal(s) corresponding to the m sixth signals for each of the p first signal(s), as pxm pairs of corresponding signals, selects one signal -from each pa4- of the -oYin pairs of corresponding signals to cb-L-a-7--l M selected signals for each of the c first o- second signal(s), and transmits the selected signals for eacin of the p first or second signal(s), on at c--e t k-hi--d transmission line. The sett off at least one mul---:1-plex-l:-ng unitt 67,,. 67. receives and exes c (S) -L. ' se of m eighth signals of m channels -f1roI7 at least one s 'xth transmis s Lon line to genera te the p f -4, r of c channels and -he i) first 7multiplexed sJL=nal(s) cf p channels to f_st martial add unit 22- The set of at least one second -- - -0", - ex JI... 66 _ng unit 65,, the p set(s) of m eighth signals of -L, channels from at least one si=h transmission line to generate thle m second multiplexed signal(s) of p channels and, supply the D second mul tiplexed signal (s) of p channels to the 'al add un-L- 21. The set o' a- least one second car- second add signal supplying unit CL, 61_ receives L and k_he j set(s) Cf, m e-i--.-i-L-h signals of Tn cha, suoi:)lies the p set(s) of m eighth signals to the first.

and second multiplexing units 67,, - - - 67 - -- 66.

_, 66., According to L-he twentieth exam-le, it is wssible to access the plurality of lower leval i -1 y channels, contained in L-he main signal in -7espectivearbitrary ones ol L- ne two dIrections from lower 'rierarch-r level channels an the tr-b.uza--; side. Namely, i--- i f it ILS 1DOSS-LIble to SW-1tCh (Cr select) tne d_ction o a s-Lgnal to be accessed, for each of the o' lower hierarchy level channels.

Figs- 21A and 21B are d-iaw-7-a= -i-1-1ustra-t--inc t.--e camtrL=I=-i. of a toa,tv-first e>OTpl-- cf w. aiJd-cb= eTbcd_v-,i-g the =ega,-:-L =':E ce=dire t3 the t.,-.tyfir-,-L t-e P=V-1siw_ in ti. ecrsdizticr. of the t17-irbea-.th and fcurteErth e:a" l,, in wnich the main z-7-ansm-yss--.--n is a un-ici--, recticna' transmission line, is extended, -o cases wherein the -.T.-,-in transmission line is a b4.di7-=c--:onal ine or a par of t 2 4 nes transmission 1- -rans----.n-iss-ion - cpoos-:1--e directions.

For thils purpcse, the add-drop multiplexer of F: ic-s 21A and 21B contains, in addition to it-he cons tructi on. of Fics. 20A a _nd 202, a set of at least one first.

c-ossconnecl_ing u--.ii second crossconnecting unit 71., 71-, a set of at least one third crossconnecting unit 72,, 72,, and a set of at least one fourth crossconnecting unit 73,, 73,, a set of at least one first crosscorLnect-Jng unit 70,, 70.,, a set of at least one second crossconnectIng unit 71,, 71., a set of at least one third crossconnecti-ng unit 72, f - - - 72., and a set of at least one fourth crossconnecting unit 73,, - - - 73,. The set of at least one first crossconnect-Lig unit 70,, 70P each for establishing at least one circuit through which a respective one of the p set(s) of m sixth signals'can be transferred to one of the at least one fifth transmission line, in accordance with a correspondence relationship between the p set(s) of m sixth signals and the at least one fifth transmission line. The set of at least one second crossconnecting unit 71,,... 71, each for establishing at least one circuit through"which a respective one of the p set(s) of m seventh signals can be inserted into one of m time slots of the p multiplexed the at least one sixth transmission line, in accordance with a correspondence relationship between the p set(s) of m seventh signals and the at least one sixth transmission line. The set of at least one third crossconnecting unit 72,, - - - 72, each for establishing at least one circuit through which a respective one of the m seventh signals in each of the p set(s) can be transferred to one input port of one of at least one second multiplexing unit 66, f - - - 66 corresponding to the above each of the p set(s). The set of at least one fourth crossconnecting unit 73,, - - 73 each for establishing at least one circuit through which a respective one of the m seventh signals in each of the p set(s) can be transferred to one input port of one of at least one first multiplexing unit 67,, - -.67.

corresponding to the above each of the p set(s).

According to the twentY7first example, it is possible to access an arbitrary one of the plurality of lower hierarchy level channels contained i n the -main signal in an arbitrary one of the two directions from each channel, in the lower hierarchy level on the tributary side.

Namely, it is possible to access an arbitrary one of the plurality of lower hierarchy level channels in the main signal from each channel in the lower hierarchy level an the tributary side, and at the same time to switch (or select) the direction of a signal in one of the plurality of lower hierarchy level channels in the main signal for each of the plurality of lower hierarchy level channels.

Figs. 22A and 22B are diagrams illustrating the construction of a twa-t:v---e=-d exmpl-- of w- add-d=- m.Lltn-laer Mt eTb:d-viM t:he In the add-drop multiplexer of Figs. 22-A and 22B, it is assumed that the p set(s) of m sixth signals, the p set(s) of m seventh s-l-gnals, and the p set(s) of m eighth signals are each contained J1-n a plurality of transmission frames of a predetermined form when being transmitted, and each transmission frame can contain information to be transmitted and pointer information which indicates a phase of the information to be transmitted, in the transmission frame. Therefore, the add-drop multiplexer of Figs. 22A and 22B contains additional provision for phase and frequency control as in the construction of Fig. 14. Namely, the add-drop multiplexer of Figs. 22A and 22B contains, in addition to the construction of Figs. 21A and 21B, a set of at leas-;L-. one first phase and frequency control unit 74,, 74,, a set of at least one second phase and frequency control unit 751, 75P, and a set of at. least one third phase and frequency cont=l unit 76,,... 76_. in Figs. 2215. and 22B, the first drop signal selection unit 62 and the first add signal supply unit63 are not shown, since these portions do not directly relate to the above additional provision for phase and frequency control.

The se--- of at- least one first phase and frequency control unit 74,,... 74. inserts first information contained in the p set(s) of m sixth s-;'cnals when the n set(s) of m sixth signals are received, into the ransmitted, - n plural-iL.y of transmission frames to be t- sync hroniz ation with timing of the adddrop multiplexer, obtains second information an a plurality of phases of the first information inserted in the plurality of transinission frames, and inserts the second information on the plurality of phases into the plurality of transmission frames as the pointer information. The set of at least one second phase and frequency control unit 751, 75P inserts third information contained in the p 1: m seventh signals when the p set(s) of m set(s) o-L seventh signals are received, into the plurality of transmission frames to be transmitted, in sync hron iz ation with timing of the add-drop multiplexer, obtains fourth information on a plurality of phases of the third information inserted in the plurality of transmission frames, and inserts the fourth information on the plurality of phases into the plurality of transmission frames as the pointer information. The set of at least one third phase and frequency control unit 761,... 76P inserts fifth information contained in the p set(s) of m eighth signals when the p set(s) of in eighth signals are received, into the plurality of transmission frames to be transmitted, in synchronization with timIng of the add-drop multiplexer, obtains sixth information on a plurality of phases of the fifth infoir- mation inserted in the plurality of transmission frames, and inserts.the sixth information on the plurality of phases into the plurality of transmission frames as the pointer inf ormation.

Fig. 23 is a diagram illustrating the construction of a twenty-third example of an adddrop multiplexer not embodying the present invention. The twenty-third exaTple is _'S_ applicable to add-drop multiplexers locat ted on a transmission line through which a signal containing a olurality of higher hierarchy level- signals is transmitted, and each of the highe-hierarchy level signals further contains a multiplexed signal containing a plurality of lower hierarchy level signals.

The add-drop multiplexer according to the twenty-turd eoTple contains: a higher multiplicity level circuit switching unit 41, a demultiplexing unit. 42, a lower multiplicity level circuit switching unit 43, a multiplexing unit 44, and a circuit switching level -y level circuit selecting unit 45. The higher multiplicit -st signals switching unit 41 receives a plurality oil fL at the higher hierarchy level, and exchanges channels of the first signals, where each of the 3Dlural-ity of first signals contains a plurality of second signals at the lower hierarchy level in a multiplexed form. The demultiplexing unit 42 receives at leas-I one of the plurality of first signals, and demultiiDlexes each of the at least one of the plurality of firs-I signals to obtain the plurality of second signals contained in each first signal. The lower multiplicity level circuit switching unit 43 receives the plurality of second signals which are obtained by the demultiplexing unit 42 for each of the at least one of the plurality of f irst signals, and exchanges channels of the plurality of second signals, at the lower hierarchy level. The multiplexing unit 44 multiplexes the plurality of second signals for each of the at least one o-L the plurality of first signals, after the channels of the plurality of second signals are exchanged by the lower multiplicity level circ,_,it switching unit 43, to obtain at least- one which contains the multiplexed "hird signal each o- L -te at least one plurality of second signals, where t third signal- corresponds to the at least one of the plurality oil first signals, respectively. The circuit ts one of tile at switching level selecting unit 45 select one of the plurality of first signals after the channels thereof are changed by the higher multiplicity level circuit switching unit 41, and the at least one third signal obtained by the multiplexing unit 44.

According to the t--ty-thild exaTPle, it is =.sslbl-- to choose either crossconnecting operations at the higher hierarchy level or at the lower hierarchy level.

Fig. 24 is a diagram illustrating the basic operation steps of m: eenple cimuit sdt: aet-nd f--r changj-ng a level at which ci-rcua-t switching is performed, from a higher multiplicity level (higher hierarchy level) to a lower multiplicity level (lower hierarchy level), in an add-drop multiplexer having the construction of Fig. 23. According to t-Lis method, in step Tl, at least one circuit is established in the above higher multiplicity level circuit switching unit 41. Next, in step T2, channels are exchanged at the level of the plurality of second signals (lower hierarchy level) for the at least. one of the plurality of first signals, by the lower multiplicity level circuit switching unit 43, so that an f second signals output from the order of the plurality o.L multiplexing unit 44 is the same as an order of the plurality of second signals contained in the at least one of the plurality of first signals when the plurality of second signals contained in the at least one of the plurality of first signals are output from the higher 4 multiplicity level circuit switching unit 41. Then, I_n step T3, the output of the multiplexing unit 4-4- in the circuit switching level selecting unit 45 is selected.

According to this method, connections (circuit setting) at the level of the plurality of second channels are not changed before and after switching the level of the crossconnecting operations from the level of the plurality o_-1- second channels to the, level off the plurality of first channels, so that switching of the level of the crossconnecting operations from the level of the plurality of second channels to the level of the plurality of first channels can be performed continuously.

EXPLANATIONS OF BASIC CONSTRUCTIONS OF THE PRESENT INVENTION (Figs. 25 and 26) Fig. 25 is a diagram illustrating the construction -4 Lor of an embodiment of the present inven. The present invention is applicable to add-drop multiplexers located on a bidirectional transmission line through which signals each containing a plurality of higher hierarchy level S 4Lgnals are transmitted in two directions, and each of the higher hierarchy level signals further contains a multiplexed signal containing a plurality of lower hierarchy level signals.

The add-drop multiplexer of Fig. 25 contains: a first demultiplex-ing unit 51, a second demultiplexing unit 52, a lower multiplicity level signal selecting Un4, t unit 53, a multiplexing unit 55, and a selecting 56. The first demultiplexing unit 51 receives at least one of a plurality of first signals at the higher hierarchy level, and demultiplexes each of the at least one of a plurality of first signals into a plurality of second signals which are contained in each of the at least one olf the plurality of f-irst signals and are at the lower hierarchy level. The second demultiplexing unit. 52 receives at least one at' a plurality o;f third sianals at the hiaher hierarchy level, corresponding to t irst signals, and the at least one of the plurality of f__ L_ demultiplexes each of the at least one of the plurality of third signals into a plurality of fourth signals which are contained in each of the at 'Least one off the plurality of third signals and are at the lower L_ hierarchy level. The lower multiplicity level signal selecting unit 53 selects one of the outputs of the first and second demultiplexIng units 51 and 52. The multiplexing unit 55 receives the selected one of the outputs of the first and second demultiplexing units 51 and 52, and multiplexes the plurality of second signals in the selected one, to generate at least one fifth signal at the higher hierarchy level, corresponding to the at least one of the plurality of first signals. The selecting unit 56 selects one of the at least one of the plurality of first signals, the at least one of -he plurality of third signals,. and the at least one fifth signal.

in the add-drop multiplexer according to the present invention,. it is possible to choose a level a-, which operations are performed for selecting the directions of signals transmitted through the bidirectional transmission line (or a pair of transmassion lines transmitting signals in oppos I e directions) between the level of the plurality of first channels and the plurality of second channels. Fig. 26 is a diagram illustrating the construction of another ernbodiment of the present invention. 25 In the add-drop multiplexer of Fig. 26, the selecting unit 56 contains a higher/lower multiplicity level selecting unit 57 and a higher multiplicity level signal selecting unit 59. In addition, the construction of Fig. 26 may further contain a lower multiplicizy level signal selection control unit 54 and a higher multiplicity level signal selection control unit 58.

The higher/lower multiplicity level selecting unit 57 selects one of the at least one of the plurality of first signals and the at least one fifth signal, and the higher multiplicity level signal selecting unit 59 selects one of the at least one of the nlu-rality o' third signals and the signal selected by the higher/lower multiplicity level selecting unit 57.

Further, the lower multiplicity level signal selection control unit 51 monitors the outputs of the first and second demultiplex_ing units 51 and 52, and can control the lower multiplicity level signal selecting unit 53 so that the lower multiplicity level signal selecting unit 53 selects the output of a specific one c-LO the first and second demultiplexing units 51 and 52 according to the monitored outputs, and the higher multiplicity level signal selection control unit 58 monitors the at least one third signal and the output of "he higher/lower multiplicity level selecting unit 57, and can control the higher multiplicity level signal selecting unit 59 so that the higher rnultiplicity level signal selecting unit 59 selects a specific one of the at least one third signal and the output of the higher/lower multiplicity level selecting unit 57), according to the monitored outputs.

EMBODIMENT OF THE PRESENIT INVENTION (Fias. 27 to 28B) Fig. 27 is a diagram indicating the conficruration of Figs. 27A to 27D, and Figs. 27A, 27B, 27C and 27D are diagrams illustrating the construction of another embodi.ment of the present invention.

In Fig. 27A, reference numeral 191 denotes an east side line (OC3)-side interface unit, 192, to 192, each denote an east-side lineside path overhead detection unit, 193. to 193 ide line-side ', each denote an east-s pointer control unit, 194 denotes an east-side line-side outout selector, and 195, -L-0 1953 each denote an east side line-side oath overhead insertion unit. in Fic.

27B, reference numeral 196 denotes a STS add signal switch unit, 197 denotes an eastside STs drop signal switch unit, 198 denotes a west-side vr-r add signal switch unit, 199 denotes an eas-kside VT drop signal switch unit, 200 denotes an east-side drop signal selector, 201 denotes an east-side STS add signal switch unit, 202 denotes a west-side STS drop signal switch unit, 203 denotes an east-side VT add signal switch unit, 204 denotes a west-side VT drop signal switch unit, and 205 denotes awest-side drop signal selector.

In Fig. 27C. reference numerals 2061 to 206, each denote a west-side line-side path overhead insertion unit, 207 - denotes a west-side line-side output selector, 208 denotes a west-side line-side interface unit, 209, to 209 3 each denote a west-side line-side path overhead detection unit, 210, to 210. each denote a westside line-side pointer control unit, 211 denotes a timing control unit, 212 denotes a PLL unit, 213 denotes a pulse generation unit, 214 denotes a timing pulse generation unit, 215 denotes an external ter-m-inal, and 216 denotes a control unit. In Fig. 27D, reference numeral 220 denotes a VT path protection switch unit, 2211 to 221 3 each denote a tributary-side path overhead detection unit, and 222, to 222 3 each denote a tributary side painter control unit.

The add-drop multiplexer indicated in Fig. 27A to Fig. 27D, is located on a bidirectional transm-ission line through which OC-3 signals are transmitted in two directions, where the OC-3 signals are each formed by byte multiplexing three OC-1 signals, and each OC-1 signal contains in its payload a plurality of VT signals. The adddrop multiplexer drops three STS-1 signals to the tributary side, and adds three STS-1 signals from the tributary side.

The timing control unit 211 in Fig. 27D generates master timing for the system, the PLI, unit 212 generates a master clock which is synchronized with the master tim-ing. The pulse generation unit 213 generates a read clock by using the master clock from the PLL unit 212. The read clock is used to synchronize a plurality of 'v7 signals contained in a plurality of STS-1 siqnais with the tim-ing of the add-drop multiplexer by a clock change. The timing pulse generation unit 214 generates a VT timing pulse for aligning phases of a p.urality ol signals contained in the plurality of STS-! signals.

The east-side line-side interface unit 191 in Fig. 27JA, in this embodiment, receives an OC3 signal which is transmitted through a bidirectional optical transmission line in one direction, converts it into an - electric signal, and demultiplexes the electric signal into three STS-1 signals- The eas-t"--side line-side interface unit 191 further inputs three STS-1 signals, multiolexes them to an STS-3 signal, and converts the STS-3 signal into an OC-3 signal which is to be transmitted onto the bidirectional optical transmission line in the other direction. The east-side line-side oath overhead detection units 192, to 192, innuts the above three STS-1 signals which are output from the eastside line-side inter-face unit 1191, detects path overhead in the STS1 signals, and performs overhead processing. The east-side line-side pointer control units 193, to 193. corresoond to the aforementioned phase and frequency control unit, and perform, -rewriting of 'v-T pointer and stuffing and destuffing contl-ol by using the.Li-ning pulse from the timing pulse generation unit 214 and the read clock from the pulse generation unit 213. The insertion and deletion of a byte for the stuffing and destuffing control may be performed by the above east-side line-side path overhead detection units 1921 to 1923.

Similarly, the west-side 1, -ineside interface unit 208 in Fig. 27C, in this embodiment, receives an OC-3 signal which is transmitted through a bidirectional optical transmission line in one converts it into an electric signal, and demultiplexes the electric signal into three STS-1 signals. The west-side 12Lne-side ace unit 208 further inputs three STS-1 signals, interf multiplexes them to an STS-3 signal, and converts the STS-3 signal into an OC-3 signal which is to be transmitted onto the bidirectional optical transmission line in the other direction. The west-side line-side path overhead detection units 209,, to 209, inputs the above three STS-1 signals which are output from the west-side line-side interface unit 208, detects path overhead in the STS-1 signals, and performs overhead processing. The west-side line-side pointer control units 210, to 210 3 correspond to the aforementioned phase and frequency control unit,. and perform rewriting of VT pointer and stuffing and destuffIng control by using the timing pulse frain the timing pulse generation unit 214 and the read clock from the pulse generation unit 213.

The insertion and deletion of a byte for the stuffing and destuffing control may be performed by the above west-side line-side path overhead detection units 209. to 2093.

The tributa-r--y-side path overhead detection units 221, to 221 3 in Fig. 27D inputs three STS-11 signals which are output from the tributary-side interface unit, detects path overhead, and performs overhead processing.

The tributary-side pointer control units 222. to 222, correspond to the aforementioned phase and frequency control unit, and perform rewriting of VT pointer and stuffing and destuffing control by using the timing pulse from the timing pulse generation unit 214 and the read clock from the pulse generation unit 213. The insertion and deletion of a byte for the stuffing and L L.L.

destuffing control may be performed by the above tributary-side path overhead detection units 221. to 221_,.

The west-side STS add signal switch unit 196 in Fig. 27B, is a switch (crossconnection unit) in which one off the three STS-1 signals input frona the east-side line-side interface unit 191, or one of the above three STS-1 signals input from the above tributary-side interface unit is selected as each o- F the above three 1 ignals to be output from west-side line-side STS-1 s- interface unit 208. The eastside STS drop signal switch unit 197 is a switch in which crossconnection between the above three STS-1 signals input from the east-side line-side interface unit 191 and the three STS-1 signals tput to the tributary side, is perf to be out -orinred. The westside VT add signal switch unit 198 is a switch in which one of VT signals contained in one of the above - three ST-1 signals which are output from -he east-side lineside path overhead detection unit 192. to 192, and subjected to phase frequency control by the east-side 1:Lnes de pointer control unit 193, to 193,, or a corresponding one of W signals contained in one ol- the above three STS-1 signals which are output from the tributary-side path overhead detection unit 221, to 2213f and subjected to phase frequency control by the tribut -side point -ol unit 222 to 22231 is Lary L-er cont selected as each of VT signals which are to be contained in the three STS-1 signals to be output from the west side line-side interface unit 208. The east-side 'li-T drop signal switch unit 199 is a switch in which crossconnection between VT signals contained in the above three STS-1 signals output from the east-side line-side path overhead detection unit 192. to 192, and subjected to the phase and frequency control by the east- side line-side pointer control unit 193. to 193,, and VTE signals contained in the th-ree STS-1 signals to be output to the tributary side, is performed.

The east-side STS add signal switch unit- 201 in Fig. 27B, is a switch (crossconnection unit) in which one of the three STS-1 signals input from the west-side -ace unit 208. or one of the above three line-side interf STS-1 signals input from the above tributaryside interface u--7--t is selected as each of the above three STS-1 signals to be output from westside line-side interface unit 208. The west-side STS drop signal switch unit 202 is a switch in which crossconnect-Lon between (-he above three STS-1 signals input from the west-side line-side interface unit 208 and the three STS-1 signals to be output to the tributary side, is performed. The east-side VT add signal switch unit 203 is a switch in which one of VT signals contained in one of the above three STS-1 signals which are output from the west-side io line-side path overhead detection unit 209, to 209 3 and subjected to phase frequency control by the west-side line-side pointer control unit 210, to 210,, or a corresponding one of VT signals contained in one of the above three STS-1 signals which are output from the is tributary-side path overhead detection unit 221. to 221 3 f and subjected to phase frequency control by the is l_ributary-side pointer control unit 222, to 222.P selected as each of VT signals which are to be contained in the three STS-1 signals to be output from the east side line-side interface unit 191. The westside VT drop signal switch unit 204 is a switch in which crossconnection between VT signals contained i_i the above three STS1 signals output front the west-side line-side path overhead detection unit 209, to 209 3 and subjected to the phase and frequency control by the west-side line-side pointer control unit 2101 to 2103f and VT signals contained in the three STS-1 signals to be output to the tributary side, is performed.

The east-side line-side path overhead insertion units 1951 to 195. in Fig. 27A insert path overhead for an STS SPE, into the output signals from the eastside VT add signal switch unit 203 in Fig. 27B. The eastside line-side output selector 194 selects one of each of the k_hree STS-1 signals output from the east-side STS add signal switch unit 201, and a corresponding one of the three outputs of the east7side line-side path overhead insertion units 195, to 1955 to supply the selected one J3 I to the east-side line-side interface unit 191. The eastside line-side output selector 194 selects one of the three STS-1 signals output from the east-side STS add signal switch unit 201 when circuit setting is performed at the level of the STS signals, or selects one of the three outputs of the east-side line-side path overhead insertion units 195. to 195, when circuit setting is performed at the level of the VT signals.

The west-side line-side path overhead insertion units 206, to 206 in Fig. 27C insert path overhead for Erom the westside an STS SPE, into the output signals J - W add signal switch unit 198 in Fig. 27B. The westside lineside output selector 207 selects one of each of the three STS-1 signals output from the west-side STS add signal switch unit 196, and a corresponding one of the ine -hree outputs of the westside 1- -side path overhead insertion units 206, to 206 3 f to supply the selected one to the west-side line-side interface unit 208. The westside line-side outout selector 207 selects the above one -0 Erom the west-side STS C.L the three STS-1 signals output: add signal switch unit 196 when circuit setting is performed at the level of the STS signals, or selects 1-he above corresponding one of the three outputs of the west-side line- side path overhead insertion unit 206, to 206 3 when circuit setting is perflornied at the level of the VT signals.

The east-side drop signal selector 200 selects one of one of the three outputs (STS-1 signals) from the east-side STS drop signal switch unit 197, and a the three outou rom the east- corresponding one o-L -s f side 7r drop signal switch unit 199, to supply the selected one to -he VT path protection switch 220 in Fig. 27D. The east-side drop signal selector 200 selects the above one of the three outputs (STS-1 signals) from the east-side STS drop signal switch unit 197 when circuit setting is performed at the level of the STS signals, or selects the above corresDondin( one of the three outputs from the east-side VT drop signal switch unit 199 when circuit setting is performed at the level of the T7T signals. The westside drop signal selector 205 selects one of one of the three outputs (STS-1 signals) from the west-side STS drop signal switch unit 202, and a corresponding one of the three outputs from the west-side VT drop signal switch unit 204, to supply the selected one to the VT path protection switch 220 in Fig. 27D. The west-side drop signal selector 205 selects the above one of the three outputs (STS-1 signals) from the west-side STS drop signal switch unit 202 when - circuit setting is performed at the level of the STS signals, or selects the above corresponding one of the three outputs from the west-side VT drop signal switch unit 204 when circuit setting is performed at the level of the VT signals.

The VT path protection switch 220 has a function of selecting one of a respective one of the three outputs (east-side drop signals) from the east-side dror) signal selector 200, and a corresponding one of the three - ide drop signals) from the west-side drop outputs (wes-IL.s signal selector 205, to supply the selected one to the tributary interface (which is not shown). This selection can be made for each VT signal. The tributary interface contains a path protection switch at the STS signal level. When the protection switch 220 does not perform the above selection at the VT signal level, the above path protection switch at the STS signal level operates.

That is, the selection is performed at the STS signal level.

The control unit 216 controls the above operations of the VT path protection switch 220, in accordance with an instruction from outside.

Figs. 28A and 28B are diagram illustratipg an example construction of the VT path protection switch 220 in Fig. Although. three STS-1 signals are dropped to the tributary side from each of the east side and the west in the embodiment of Figs. 27A to 27D, Fig. 28A indicates only a portion of the VT path protection switch 220, which relates to a pair of one east-side drop signal and one west-side drop signal.

In Fig. 28A, reference numeral 231 denotes a microcomputer i_nterface which is connected to the control unit 216 in Fig. 27C, 232 denotes an east-side demultiplex unit, 233 denotes a west-side demultiplex unit, 256, to 2506 23 each denote a VT path switch unit provided for each of twenty-eight VT channels, 252 denotes a multiDlex unit ' 253 denotes a path overhead insertion unit, 254 denotes a VT/STS selector, and 255 denotes a west-side STS signal fix control unit.

The demult-iclex units 232 and 233 on the eastside and west-side in Fig. 28A, receive STS SPEs for STS-1 signals of the east- side and of the west-side Erom the eastside drop signal selector 200 and the west-side drop signal selector 205, respectively. Then, the demultiplex units 232 and 233 demultiplex the STS SPEs into a plurality (twenty-eight) of VT signals, which are supplied as east- side and west-side drop signals to t-,-e respective VT path switch units 2561 to 25623 In each of the VT pathswitch units 256, to 256,3f rerence numeral 234 denotes a condit:f 1-ing ef L-Lon sett memory, 235 denotes an eastside VT path;,JS detection unit, 242 denotes a west-side VT path AIS detection unit, 236 denotes an east-side major error (Bit Interleaved Parity-2 Major Error) detection unit, 213 denotes a west-side major error (Bit Interleaved Parity 2 Major Error) detection unit, 237 denotes an east-side nLinor error (Bit Interleaved Par-ity-2 Minor 'Er-cr) detection unit, and 244 denotes a west-side minor error (Bit Interleaved Pari'lCY-2 Minor Error) detection unit, 238 denotes an east-side unequipped VT signal detection - i V-i signal unit, and 245 denotes a west-side vacant detection unit, 239 denotes an east-side 7-1.7 path tes a west-side T,7T path monitor, 240 monitor, 246 de7,.o- denotes a path protection switch counter which records a history of the path protection switch, 247 and 250 each denote a perforipance monitor, 248 denotes an east-side timing counter, 249 denotes a west-side timing counter, 5 and 251 denotes a path switch.

The east-side and west-side timing counters 248 and 249 count differences in bit phases of the received signals in i-Ite east side and west side, and cont-rol tile path switch 251 so that the phases of the VT signals in the east-side and the west-side coincide.

The setting condition of the path protection - switch is set in the condition setting memory 234 from the external terminal 215 through the control unit 216 and the microcomputer interface 231. The contents of the setting condition contains: for example, whether the add-drop multiplexer is connected to a communication network of a ring construction or a linear cons t-, uc tion; manual mode or not; forced setting or not; east- side drop signal or west-side drop signal when the add-drop multiplexer is connected to a linear network (PROVISIONING EAST OR WEST); whether the setting is automatically reverted to the original setting after trouble is cleared (REVERTIVE), waiting time for protection before the original setting is restored (W_k7T TO RESTORE), and the like. The east-side and west-side VT path AIS detection unit 235 and 242 detect east-side and west-side VT path AIS'es (Alarm Indication Signals). The east-side and west-side major error (Bit Interleaved Parity-2 Major Error) detection units 236 and 243 detect a major error (error exceeding a predetermined level) 4Ln VT signals of the east side and west side, respectively. The minor error (Bit Interleaved Parity-2 Minor Error) detection units 237 and 244 detect a minor error (not exceeding the predetermined level) in VT signals of the east side and west side, respectively. The east-side and west-side unequipped VT signal detection units 238 and 245 detect unequipped -code (which is trans Mitted whern a -V7T signal is unequipped or not used):Ln the VT signals of the east side and -west side. The east-side and west side VT path monitors 239 and 246 suoply VT signals of the east side and west side to be monitored, to the performance monitors 247 and 250, the east-side and west-side major error detection units 236 and 243, the minor error detection unit 237 and 244, and the unequipped VT signal detection units 238 and 245, respectively. The east-side and west-side performance monitors 247 and 250 monitor bit error rates in the vT signals of the east side and west side, respectively, to - be read out through the microcomuuter interface 231 and the control unit 216 from the external terminal 215. The east-side and west-side timing counters 248 and 249 count deviations in bit Dhases in the VT signals of the east side and west side, respectively, and control the VT path switch 251 so that bit phases in VT signals of the east side and the west side coincide. The VT path switch 251 selects VTI signals of the east side or the west side, in accordance with the above setting in the condition setting memory 234, and in particular, when -he add-drop multiplexer is connected - _ion --o a communicat network of a ring (RING) construction, the detect.Lon results of the above east-side and west-side VT path AIS detection units 235 and 2412, east-side and west-side major error detection units 236 and 243, the nano-r error detection units 237 and 244, and east-side and west-side unequipped VT signal detection units 238 and 245, for each VT signal. The selected VT signals are supplied to the tributary side. In addition, a condition that no pointer is detected in the VT signals of the east side and west side, can be added to t.11e above inform aticn based on which the select--on of VT signals is made in int _he VT path switch 251. In this case, a loss-of-po- t-er "or the VT signals of detection units may be provided f in addition to the k-he east side and the west side, construction of Fig. 28A.

The multiplex unit 252 in Fig. 28B multiplexes the VT signals selected by the VT path selection switch 2511 to form a multiplexed data for an STS SPE for the STS-1 signal. The path overhead insertion unit 253 generates a STS SPE for an STS-1 signal by inserting path overhead into the above multiplexed data. The VT/STS selector 2541 can be set to the STS mode or the VT mode under control from the external terminal 215 through the microcomputer interface 231 and the control unit 216. In the VT mode, the above STS-1 signal formed by the multiplex unit 252 and the path overhead insertion unit 253 is selected, and in the STS mode, the east- side STS-1 signal per se, which is supplied from the east-side drop signal selector 200 is selected. The selected STS-1 signal is supplied to the tributary side interface unit. The westside STS signal fix control unit 255 is provided on the path for the STS-1 signal of the west side, which is supplied from the west-side drop signal selector 205. when it is required to fixedly select the STS-1 signal of the east side in -he above-mentioned STS level path protection switch in the tributary-side interface unit, all -one- data is written by the west-side STS signal Lix control unit 255, in the STS-1 signal of the west side. Since the STS level path protection switch in the tributary-side interface unit determines this STS-1 signal containing all -one- data, as an AIS signal, it substantially fixedly selects the STS-1 signal of the east side.

As can be understood from the above explanations, the above construction indicated in Fig. 27A to Fig. 28B and the tributary-side interface unit (not shown) containing the STS level path protection switch (not showr), realizes an embodiment of the present invention.

STORAGE MEDIUM STORING PROGRAM OF PROCESS E)ECUTION Any of the processes, as exDlained above, can be executed by a programmable machine such as a com.Duter, the program which instructs the programmable machine to execute the process can be stored J= a con, puter-readable storage medium such as a floppy disc, a CD-ROM, or a t ROM.

Claims

CLAIMS: -45-

1 An add-drop multiplexer com-prising:

a first demultiplexing unit (51) for receiving at least one of a plurality of first signals, and demultiplexing each of the at least one of the pluirality of first signals into a plurality of second signals which are contained in each of the at least one of 4--he 6 - fJ,.-s-. signals; Plurality oL a second demultiplexing unit (52) for receiving at least one of a plurality of third signal--f corresponding to the at least one of the plurality cl 2 first signals, and demu,-L--4-plexing each of the at least E third signals into a plurality one of the plurality of - L of fourth signals which are contained in each of the at least one of the plurality cl- third signals; a lower multiplicity level signal selecting unit (53) for selecting one of the outputs of the f-irs-, and second demultiplexing units (51, 52); a multiplexing unit (55) for receiving the selected one of the cutouts of the first and second demultiplexing units (51, 52), and multiplexing the plurality of second signals in the selected one, to generate at least one fifth signal corresponding to th-e at least one cl Lhe Dlural-L-Y of --s- signals; and a selecting unit (56) for selecting one of tIne at least one of the plurality of first signals, the at least one of the plurality of third signals, and the at least one fifth signal.

2. An adid-drop multiplexer according to claim 1, wherein the selecting unnit (56) comprises, a higher/lower multiplicity level selecting.

unit (57) for selecting one cl' the at least one of the plurality of first signals and the at least one fi-fth signal, and a higher multiplicity level signal selecting unit. (59) 1-or selecting one of the at least one of the plurality of third signals and the signal selected by the higher/lower multiplicity level selecting unit (57).

3. An add-drop m,-,!-tiulexer according to clai_m. 2, further comnrisina, a lower multiplicity level signal selection control unit (54) for monitoring the outputs Of the lirst and second demultiplexing units (511, 52), and cont-rolling the lower multiplicity level signal 47- selecting unit (53) so that the lower multiDlicity leve-, signal selecting unit (53) selects the output of a specific one of the first and second de =,".!tinlex-Jng units (51, 52) according to the monito-red outputs, and a higher multiplicity level signal selection control uni-t (58) for monitoring the at least one third signal and the output of the higher/lower mu.ltiplicity ting unit (57), and controlli_ng - level select the higher multiplicity level signal selecting unit (59) so that the higher multiplicity level signal selecting unit (59) selects a specific one of the at least one third signal and the output of the higher/lower multiplicity level selecting unit (57), according to the manitored outputs.

4. A computer-readable storage medium storing an operating program which, when run on a programmable machine, causes the programmable machine to become an add-drop multiplexer as claimed in any preceding claim.

5. An add-drop multiplexer substantially as hereinbefore described with reference to and as illustrated in Figures 25 to 28.