1085

CL03ECOM'87

Assessment of CroSs― connect Systems ■n Trangrt■ ss■ on Networks

Yukifuse Okaro, Satoru Ota and Takao I€wata

NII Electrical Cqrmunication Iaboratories

ABSTRACTIntroduction of synchronous multi,plexers(or diglital cross-connect systems) requires
thorough stu.lias on transmission network
topoJogy, as well as circrrit routing. In t}ris
pap*, transmission network designs related tocross-connect systems are di"cussed. An
assessment result is presented on theappJication of digital cross-connect systems andadd-drop type mrrlfiplcxers (ADMs) .

1. INTRODUCTION
Wjth the developrnent of digitaltransrnissjon techncrlogies and the ht:oduchjonof digitaf swJtches, dig:ital transrn:i.:ssion systems
are beJrg jntrroduced in virtually all parts oftelecommunications networks. They are taking
tie place of conventional analog carriers inlong-djs'tance networks, and of voice-frequency
systems transnitted over pair-cables jn
short-di.stance networks. When nuJtiplexedtansnission systems are used pojnt-to-pojnt,
g1g mu'ltiFlexers are acting merely as circuit
bundlers. However, when they domjnate thenetwork, muJtiplexers play an irnportant rol-e as
qfu'gtrir cross-connectors. To terrnina@ andcross-connect circujts more effi-ciently andeocnomically, synchronous multJplexers (i.e.,
digital access and cross-connecC sys'tems) havebeen developed. Cross-connecb systems wereti-rst i-ntroduced by NTT jn 1980. They have
DS2 (6.3Mb/s) interfaces witl: line faciliti.es.
The nr-irrirnum unit of cross-connection wittr
timeslot jnterchange (rSI) technjques is384Kb/s, or six telephone channels.
Cross-connect systems wj$l D4S interfaces are
now being developed. They brjrg about moregffi-icnf sisgrrit handljng, 3nfl alcA req\rire amore compJicated transmission network design.

2. DIGITAL cRoSS-coNNEcr SYSTEI'IS
In analog networks, each circuit
kansmitted over a muJtiplexed system has to be
demulti-plexed to a voice-frequency baseband
signal when switched. Test access andcross-connecti.on on per-chanrrel basis can beach.ieved at any appropriate distrjbuting frame.
In '&igital networks where t].e interface between
transnission and. swjtching is performed at themuJfi-plexed stage, digital cross-connect systems(i.e., synchronous muJtiplexers) perforrns thec{2sz1- 5/ 87 /0000-r 085 $l.00ol 987 |EEE

operations of multiplexing and operatircn. At
NTT. tie first cross-connect system, the M20,
was developed in 1977. It was Put jnto
commerqial use jn 1980. Presently, M40, havlng

a DS4 line interface, is bejng deveJoped(1), (2). It is expected to contribute to network
cost reduction as well as an ilcrease irt

operational flexibilitY .
In the cross-connect systemsr circuits caneaslly change transmj-ssion lines using timeslot

in@rchange (TSI) techniques. Cross-connection
beEween any two Ji-nes is possible. Besjdes
these cross-connect type mukiplexers, add-droP
type nukiplexers {ADMs) witfi lower cost andlindted funstions are applicable. Int-oducEion ofthese mulfiplexers requjres studies on circuitrouting as well as on transnr-ission network

toPoJogY.

The majn purpose of transmissircn network
optJmization has been to nirrimize the tctal
transnission ljne cost, witfi ljtH.e conqiderations
given to mr:Jfiplexers. Tfds is because mosttransnission facildty expenditures were foroutdoor plants, and, in many cases, ternr-inalequipment could be dealt with as addjfional costrequired at both ends of the ljle. However, tlte
assessmenE of both the cost and funstions ofmuJtlplexers is becorning more jmPortant becauseof the followirg circumstances.
One is ttre recent deveJoPment intransmission line technologies. The recent
advances jn transrnission technologies, such as
fiJcer optics, has conEibuted to the redustion ofline cost. Larger capacity transmjssion systems
have lower system cost, with lower line cost Per
channel, but higher muJtlplexer cost. Thus, tlte
development of ljne teChnologies results jn anincreases irr mdtlplexer cost
The other is the wide-spread use of dig&af
systems. Digital carrjers have been used jnnetworks, jn a point-to-pojnt manner, as
econonical alternafives to analog carriers.
However, they are now to be used as mairt
transrnission systems everywhere jl long- andshoft-distance networks. The shorter thetransrnission Jength becomes, the larger therelative cost of the nuJti-plexers becornes. lE
conventional outdoor plants (such ascondujts) 6as 4veil:ll€ when the new sys@m isiltroduced, muJfiplexer cost may be much larger

28.1.1

| 086than the ljne cost, especially in short- di.stancenstworks.

3. CROSS-CONNECTION SCHEME
Gi@nd+ions

attransrrission nodes, and, techrlical conditirons of
the transnr.issjon systems and muJtiplexers,
there should be many choices of transnission
neEwork topology and cross-connection schemes.
Transmjssion network topology is designed so as
to rdnimize the total investment on transrnissi.rcn'faciJjtires, such as conduits,
fiJrer cables,
repeaters, and muJti-pJexers. Tn many cases, thetopology may be deterrnined maidy by thegeographical consErai.nts and t.Jle already-bujlt
conduits routes for analog carriers. Given thetopology, the main problem is deterrnining (a)the routing of each point-to-pojnt circuit, and,(b) ttre cross-connection schemes atintermediate nodes.
As the number of nodes involved becomeslarge, ttre ratin of jntermediate muJtiplexers totermjnating multiplexers becomes large. For
examp.le, let us assume a network as shown in
Figure 1. If ttre jnter-node di-stance is longenough, jl is more econonr-icaL to cross-connecc
each qircrrit at a fajrly low digital stage jll nrost
irttermediate nodes. This will improve the ci:rcuit
fill rates in ttre transrnission lines. The
resultant decrease jl the number of requiredtansmjssion lines makes up for the increase jn
the number of tJle muJti_p.Lexers required forcross-connesEion. In actual networks, where thelalansg between invests on transrnission lines
and multlplexers is one of tJle importantconcerns, the choine of cross-connection
schemes becomes increasingly jmportant.
The design of cross-connection schemes foran entire network requires optimjzation
algorjthms that deal with combjnatorjal
prehtams (3) . The ajm is to 6nd the bestcross-connestion scheme that nr-inimizes ttre totalc<-rst of the 'transnr-ission lines and muJdplexers.
In general, cross-connection at a lower dig.rtal
stage resr:Jts jn excessive multi_plexer cost.
Contrarjly, cross-connection at a br_igher digital
stage resuJts in excessive Jjle cost, as shown
il Figure 2. An example of a cross-connecEion
scheme is shown in Figure 3. In ttrjs example, agroup of point-to-pojnt. cirgr"'irs between Nodesl and L4 through Nodes 2t 4,5,12, and 13, js
demuJtiplexed and cross-connected at the properstage at each jntermediate node. The
closs-connection schemg fo1 ell point-to_paintqi;srtire at each node is optirnized in terms ofthe total network cost. In tigure 3, stuff
muJti.plexers are assumed. Whenever digjtal
cross-connect systems are used, the same
design algorithm can be appJicable wifi somemodjfications of tlre parameters. The seLectircn ofa cross-connectircn scheme depends on tJ-e
inter-node qia'srrit demands, circuit routing,transrnbsjon lengttr, appJied transmission
systems, multiplexers, and so on.

28.1.2

4. HIERARCHICAL STRUCTURE
The conplexity in designi.ng digjtal paths
or cross-connection schemes drasticallyincreases as the nunber of nodes involved
increases. AJthough it is possible to design
dig&af patJrs in large networks wjth computerassj-stance, tie simpJicity of ttre network is
another important objective of designers. In
some cases, a large network can behrierarchically structured, as shown il Figure 4.
A transmission network is divided intosome subnetworks, or inEa-area short-distanceuetworks, and an ilter-area )ong-dietancenetwork that connects tie gateway nodes ofirtdividual subnebworks. If tlre network isdivjded into too many subnetworks, the cost. of

the jnter-area network becomes excessive, eventhough t}re cost. of each inka-area network
decreases. Consequently, there is a optjrnal
number of subnetwo.rk dirrision. In acbualnetworks, each area j.s deternined taking into
account the geographical crcndjtions,adnuinistrative areas, switchjng hierarchy, andso on.

In a transndssion link between an
intra-area node and a gate node, all outrcf-area
qivgrrir-q can be packed, assuming
cross-connection at tJle gateway nodes. If thenumber of clrcuits jn ttre link sum to a Jarge
number, no cross-connestion at the intermediate
jltra-area nodes may be necessarlr. At thegatevray node, circuits are cross-connected andbundled into trigher-order digital stages
accordjag to the area fur which they areheaded. At the destjnation ga@hray nodes, they
are cross-connected again and forwarded tothejr final destination nodes. In ttris scheme, a
Jarge numbga 6f qissrrir-c can be lsndlcfl sjgl
less cross-connecti.on points. This contjbutes
to a redusEion in tJ:e number of muJtlp.l-exers,
but at the expense of an jncrease jn g6s cirsr,rit
length. The detajled digital pattr design,
mentircned previously is necessary for anjnter-gateway network and for each irrdivjdual
subnetwork. Thus, the number of nodes
jrvolved is much less than that, of the origjnal
large network.
Besides the design of each network, theilteraction between these nelworks is anottrer
problem.

In a long-distance network that connecrs

gateway nodes, the number ef qi5srritq ltr31
traverse an jater-gateway link is generally

larger than that of intra-area ljlks.
Cross-connectircn systems appJied to gate nodes
will have tr.igher-speed bit-rate j-nterfaces wjthtransmjssjon lines. Cross-connect systems canbe reali-ed econonically if the cross-connecEion
unit is set to a larger circuit group. However,iI most networks, cross-connecti:on at each node
need to be executed at a smaller circr'it group.
ThG is because the number of point-to-paint1087

SCheme 3: Combined lo9士 ンュ 100p wn ADMs.
C量ctt are bi― rouも ed over One of loOps.

qlrgr'its is usually fairly small. To enlarge thecross--connecEion unit for the mdtlplexers at
gateway nodes, circuit pre-grouping witll a56all snil in each intra-area network is

For each scheme′ the crrclll十 rOuting and anecessary.
One solutircn is to define a set of nodes forthe circuA pre-grouping. They fcrm a circuit.
group that is cross-connected at the gateway

cross― connecい onscheme ■s designed′ and thetotal network costお evaluated. F■ gure 7 shows
which scheme is most econontcal′ w遍 respect
もo in十 or― nOde

nodes wjthout segregation. The segregati.rcn ofqissrri+s for each node js executed bycrossronnect systems appJied at the desEination
subnetworks. Other concerns are the choice oftransmission network topoJogy, qircuit routilg,
cross-connestion scheme, and multiplexer type.

distance and the amount ofdemand お r ■nter― node circlll● co As a generaltrend′ scheme l b∝ omes most e∞ nomixユ whenthe Lne lenⅢ becOme longer. This L because
g市 es the highest fil■ rate h the lines andthe shortest tdal dヒ ℃並 length.

工On the tter hand′
econorrucal as thescheme 3 becomes■nter― node distance becomesshortero This ls because cost hcrease due tothe long ctrctt length becOmes less criical.

5. CROSS-CONNECTION ASSESSMENT
From the viewpoint of allocation ofcross-connectlon nodesr. there are several
configurations. Some of wfr-ich are shown inFigure 5. In the hub configurati.on, thecross-connectinn function is centralized at a
specJfied node. In thjs scheme, the trade-off
between multjplexer redustion and circuit-length
irtcrease deterrnines the economics. In loop
conflgurations, add-drop type multiplexers(ADMs) can be applicable. These contrjbute tonetwork cost reduction, because the the ADMs
are less expensive. However, jn achral
networks, some nodes have more than two links
wjth different destjnations, as shown in Figure5(c) . ADMs are not applicable for these nodes.
The use of a cross-connectjon syst€m fgr'all ths
node is the conventional idea. The ]:ilt use ofADl"ls and cross-connect systems may provjde

pracHcal solutjon.
Another idea to cope with thJs problem isto syntheqi".e the requjred connecEi.on using aset of 1oops, as shown il Figure 5(d) . Theseloops are not necessarjly formed at the rateof transnission. They may be formed at a certajn
digital stage il large-capacity transrtGsion
systems. In each loop, ADMs are app1j.cable.
The first process in synttreqi-ing a network is
to find a set of logical loops that can prowjde
aI point-to-pojlt circuite. There are a large
number of sets that meet ghi5 gendir-ion. The
next problem is to fi-nd the best set t.]1at
mjrLiJnjzes the network cost, or nj-nirnjze the sum
6f prr'lfiFle;er and ljle costs. That jndudesdeternin-ing the loop for which eachpoint-to-po5-nt circua shou-ld be provided. Aheurj.stic algorjthm for ttr-is problem has been

The most econo― aL area of scheme 2 is sman′as shown h ご、 ure 7(a)′ when the ADM cost■ sassumed to be the same as that ofcross― connechon type mu■ ipbxers. However′shce the ADM cost■ s assumed to be cheaper′
the area grows and Scheme 2 co11la be mOst
eCOnOFrnal (eXcept when the hter― nodence…
■sOr■ nter― node cLrCuit demand■s fair■ y long′fah■ y sma■ ). These maps depend on thenumber of nodes hvolved and other detailed
parameters. In an actual network′ 上 isnecessary to base the dailed design on actual
parameters and constra■ nts′ such as
POint tO pQint d=cuit demands′ node anocatiOn′transmisson network topdogy′ he length′ andso on.

6. CONCLUSION
Recent developments h transnusmon line
technO10gおs and mu■ iple血 g ttchnA16gies
present a necessiy of thOrough study ontransnus田 on
netwOrkcOnfLguratiOns。
Introductlon of cross― connect systems g■ ves usa wme vanety Of poss量 )ill■ 。s ln network

planmung and operaton. Since cross― connection

schemes can noL be assessed at each node

hdependently′ stuЯ ios on an entte network arenecessary.
In this paper′ a few network configurations

related to crOss― cOnnect system and ADM
applicaton are evaluated.

ACknowledgement
The authors w■ sh to express thtt thanks
to Dr. Hmetosh直 Kimura′ and Dr. Tetsuya Mikiof NTT ILl― ical communca● めn Laborato」 bs′

deveJoped.

for thtt encouragement and valuable adtte.

A sirnple nine-node network model, shown
irt Pigure 6, is assumed to be able to evaluate

References(1)H. Ueda et al"A synchronous DS4

mmplexer wth cross― connect funcビ っn andthe fo.Ilowing three schemes:
Scheme 1: Tandem of cross-connect systems.
Circuits are bi-routed over tlle shortest
paths, and cross-conneqted at every

塾 impact on the network‖

1985 Global Telecom. Conf. PP.18.1.1‑18.1.5(2)■ . To賤 awa eし al"Large capa鋼
non― blebg 並gi■ スl cross― connect systemuttg h■gh sPeed oP面 にal lnks‖

int€rmediate node.
Scheme 2: Hub with a cross-connect systems.

1986 Global Teleom. conf. pp.33.1.1‑33.1.5
pathS・ n(3)Y. Okano et al‖ Deigttg
transnusmon networks‖…
1986 Global Teleoom. Conf.′ pp.25.2.1‑25.2.5

Circrril-s are bi-routed tirrouqh the central
node.

28.1.3

1088

circuit' d.ernand:2xDS2 b*ween every
two nodes

Fig.l Transnission network model

D S4ａM34ｂＣ

D S3

Cross-connection at DS3 at everv

intermediate node
Cross-connecEion designed by tlte algorjthm
Cross-conneclion at DS3 at evervintermediate node
M23

fi9.2

DS2

Cross-connecdon assessment

Fig.3 Example of cross-connection

Cross-connest, system\ \./□

Transnulssion line・ □□□F19.4 Hierarctrical structure and cross-connecti,on

28.1.4

1089(b)LOOp(a)Hub(d) Combjned Logjcal Loop(C)COmbined mp

Fig.5 Cross-connestion and topology
ADM/ ヘ(a)tandem of cross-connectsvstems(b) Hub wjth a cross-connect (c) Combined fogical loop
systemwjth ADM

Fiq.6 Cross-connectlcn assessment model︵へ∽ ∩ ︶ Ｏ ＣにＥ ００ コ﹁ ０︼日じ ０００ロー︼ωｐＣＨ︐︵Ｎ∽ ∩ ︶ Ｏ Ｃｏ 日 ００ Ｊ Ｊ ︒力 ＢｏＯ Ｏロ ー︼ωｐＣＨ

124816132 64 128

248163264 128

1nter― node Alヽ tance L (Km)

1nter― node distance L (Km)(b)ADM cost/ DCS COSt= 0.75(a)ADM cOSt/ DCS COSt= 1Fig.7 AppJication map

28.1.5