HP-AN1264-3_Fundamentals of Synchronization Planning

8/14/2019 HP-AN1264-3_Fundamentals of Synchronization Planning

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Synchronizing Te lecommunicatio nsNetworks:

Fundamentals of Synchronization Planning

H

Application Note 1264-3

N

Primary Netw ork


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Table of Cont ent s

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

II. Synchronization Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 5

Major Methods for Synchr onization . . . . . . . . . . . . . . . . . . . . . . . . 5Ples iochronous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Hierarch ical Source -Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Mutual Synchron ization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Pulse Stu ffing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Pointe rs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Telecommunicat ion Synchronization . . . . . . . . . . . . . . . . . . . . . . . 6Primar y Reference Source s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

III. Synchronization Planning Concepts . . . . . . . . . . . . . . . . . . . . 8

IV. Synchronization Planning Requirements . . . . . . . . . . . . . . . 10

V. Carrier Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Carrier Network Synchronization Performance . . . . . . . . . . . . . 11Primar y Reference Source s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Intero ffice Timing Distribu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Intrao ffice Timing Distribu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Timing the BITS Clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Timing Distribution from the BITS Clock . . . . . . . . . . . . . . . . 15

Monitor ing and Ver ification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17SONET/SDH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

VI. Private Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Private Network Synchronization Performance . . . . . . . . . . . . . 19

Synchronization Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Intero ffice Timing Distribu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Intrao ffice Timing Distribu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Dynamically Changing Synchr onization Plans . . . . . . . . . . . . . . 23SDH and SONET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

VII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Reference s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24


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I. Introductio n

In recent years, the importance of synchronization has increase d due tothe exte nsive digitization of telecommunications network s and to theintroduction of SDH and SONET techno logies. These events ha ve placed

more deman ds on synchron ization planning, performance, and opera-tion. All networks require stringent synchronization planning to me etaccepta ble performance levels. Improper synchronization planning orthe lack of planning can cause severe per formance problems. Improperplanning can result in excessive slips, long periods of netw ork do wn-time, elusive maintenance pro blems, or high transmission error r ates. Aproper s ynchronization plan, which optimizes the pe rformance, must bemade for the e ntire digital network.

The role of synchronization planning is to determ ine the distribution of synchronization in a netwo rk and t o select the level of clocks an dfacilities to b e used to time the network . This involves the selection andlocation of master clock(s) for a net work, the distribution of primaryand seco ndary timing throughout a networ k, and an an alysis of thenetwork to en sure that acc eptable performance levels are achieved.

The first thr ee sect ions of this application not e provide the s ynchroniza-tion background for planning. Section II describes the major methodsused for synchronization. Basic planning concepts are p resented inSection III and planning requirements are covered in Section IV.

Synchronization planning of carrier netwo rks is the topic of Section V.A brief discussion of the performance of carrier ne tworks is given. Theuse, selection, and location of primary reference sou rces are covered.Interoffice and intraoffice timing distribution are explained next , with a

focus on the se lection of BITS clocks and d istribution of timing fromthem. Section V also discusses the mo nitoring and verification of network synchronization performan ce, followed by a br ief descriptionof the spec ial synchronization needs a nd con cerns of SDH and SONET.

Since private netwo rk synchr onization p lanning is very different thancarrier net work planning, it is considered separat ely. Section VI coversprivate networks. Private network performance is discussed , since itplays a major role in pr ivate n etwork synchronization planning. Thesources of synchronization, interoffice timing, and intraoffice timing aresubsequently covered. Special concer ns w ith dynamically changingsynchronization plans and SDH/SONET conclude the section on privatenetworks.

References are cited thro ughout this application note and included inbrackets [ ]. A complete listing of references is on page 24.

Additional supporting material on synchronization p erformance, clock performance, and clock functionality is given in [1].


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PointersThis method is used by SDH and SONET to transmitpayloads that are not necessa rily synchrono us to the

SDH/SONET clock. Pointers are used to indicate th ebeginning of a frame in the payload. Frequency differ-ences between SDH/SONET network elements orbetwee n the payload and the SDH/SONET equipment a reaccommod ated by adjusting the pointer value (Figure 4).Therefore, the payload does not need to b e synchronizedto t he SDH/SONET equipmen t. SDH/SONET equipmentis usually synchronized so th at the nu mber of pointer adjustments are kept to a minimum. Thisis desirable since eac h pointer adjustmen t willcause jitter and wander on the payload.

Telecommunications Synchronization

Most telecommun ication administrations use thehierarchical source-receiver method to synchro-nize the ir E1/DS1 network . The master clock for anetwor k is one or more primary reference sourc es(PRS). This clock reference is distributed thro ugha netwo rk of receiver clocks (Figure 5).

A node with the most stab le, robust clock isdesignated as a source node. The source nodetransmits a timing reference t o one o r more rec eiver node s. Receivernodes usu ally have equal or worse per formance than the sour ce node.The receiver node locks on to the timing reference of the source n ode

and then p asses the r eference on to ot her receiver nodes. Timing isthereby distributed down a h ierarchy of nodes.

Receiver nodes a re usually designed to accept two o r more reference s.One reference is act ive. All other alternate re ferences are standby. Inthe case where th e active reference is lost or is in error, the receivernode can sw itch references and lock to an alternate reference. Thus,each receiver node has acces s to timing from two or more sour ces.Most netw orks are engineered so tha t all receiver clocks are given twoor more diverse referenc es. In private netw orks, this may not bepossible due to limited conn ectivity between n odes.

Clocks are p laced into the hierarchy based upon p erformance levels.ANSI [2] designates pe rformance levels as stra tum levels: strat a 1, 2, 3,4E, and 4, in orde r of best performance to wor st. ITU [3] designates fourperformance levels: primary reference sou rce (PRS), transit node, localnode, terminal or customer pr emises equipment (CPE) nod e. In addi-tion, Bellcore [5] has defined a new level called stratum 3E, which isbetwee n ANSI stratum levels 2 and 3 in performance . Stratum 3E hasnot been adopted by any standards body.

H1 H2 H3

H1 H2 H3

H1 H2 H3

H1 H2 H3

Positive Stuff Byte

Start of STS Synchronous Envelope

STS-1 Frame

Frame n

125 microseconds

250 microseconds

375 microseconds

500 microseconds

Frame n + 1

Frame n + 3

Payload rate is slower than frame rate

P with Ibits

inverted

PNEW -P + 1

Frame n + 2

PrimaryReference

Source

Stratum 2Switch

Stratum 2Switch

Secondary

Secondary

Secondary

Secondary

Primary

Primary

Primary

Primary

PrimaryReference

Source

Stratum 3 DigitalCross-connect

Stratum 4PBX

Figure 5.Hierarchicalsource-receiveroperations.

Figure 4.SONET pointe radjustment.


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Stratum 1 or primary reference sources are master nodes for anetwork. Stratum 2 or transit node clocks are typically found in tollswitching and some digital cross-connect equipment. Stratum 3E isused in timing signal generators for use in networks with SDH orSONET. Local switching, most digital cross-connect systems, and somePBXs and T1 multiplexers have stra tum 3 or local node c locks. Most T1multiplexers, PBXs, channel banks, and echo ca ncellers incorporateStratum 4 or CPE c locks. Further discussion of the functionality androle of receiver clocks c an be found in [1].

Primary Refe rence Sources

A primary reference source (PRS) is a master clock for a network that is able to maintain a frequency accuracy of better than 1 10 11

[2, 4]. One class of PRS is a stratum 1 clock. A stratum 1 clock, bydefinition, is a free running clock [2]. It does not use a timingreference to derive or steer its timing. Stratum 1 clocks usuallyconsist of an ensemble of Cesium atomic standards.

However, a PRS does not need to be implemented with primaryatomic standards [2]. Other examples of PRS are Global PositioningSystem (GPS) and LORAN-C clocks. These systems use localrubidium or quartz oscillators that are steered by timing informa-tion obtained from GPS or LORAN-C. They are not consideredstratum 1 since they are steered, but are classified as PRSs. Theseclocks are able to maintain an accuracy within a few parts in 10 13

to a few parts in 10 12 .


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PBX PBX

PBX

Figure 6. Timingloop.

A synchron ization plan is the de termination of the distribution of synchronization in a n etwork. It involves the selection an d location of amaster clock or clocks, the distribution of primary and seco ndary

timing, and the selection of the clocks and reference facilities. Toachieve the best performance and most ro bustness from a synchroniza-tion network, several rules and procedu res must be followed whendeveloping a synchronization plan. Some of the most importa nt rulesare a voiding timing loops, maintaining a hierarchy, following the BITSconcep t, using the best facilities for synchronization referenc e trans -port, and minimizing the casca ding of the timing reference.

Timing loops oc cur wh en a clock use s a timing reference t hat is trace-able to itself (Figure 6). When su ch loops oc cur, the r eference fre-quency becomes unstable. The clocks in a timing loop will swiftly beginto op erate at th e accu racy of the clocks pull-in range. This will result inthe clock exh ibiting performance many times worse th an it does in free-run or holdover mode. Therefore, it is importan t that the flow of timingreferences in a networ k be designed such that t iming loops cann ot formunder an y circumstan ce. No combination of primary and/or second aryreferences should resu lt in a timing loop. Timing loops can always beavoided in a proper ly planned netwo rk.

Maintaining a hierarch y is important to achieve the b est po ssibleperformance in a netwo rk. Under ideal or stress co nditions, passingtiming from better to w orse c locks will maximize per formance. Syn-chronization will still be maintained in normal operation if timing ispassed from a wor se clock to a bette r clock. Only performance maysuffer slightly, since a better clock is more immune to short-term

networ k impairments a nd will accumulate less timing error. It is only inthe case where an upstream clock enters holdover or free run that anon-hierarchy caus es major problems. In this case, the poor er perform-ing upstream clock in holdover may have a frequency accura cy worsethan the d ownstre am clock can lock to. The downstream c lock will notremain locked and will also go into holdover. This results in multipleclocks being in holdover and ex cessive slips in the netw ork.

Most administrations follow the Building Integrated Timing Supply(BITS) concept for synchronization distribution (Figure 7). In the BITSmethod, the best clock in an office is designated to receive timing fromreferences outside the office. All other clocks in the office are timedfrom this clock. In many ca ses th e BITS clock is a timing signal genera-

tor, whose sole purpose is for synchronization. Other adm inistrationsrely on clocks in switches o r cross -connec t systems for the BITS. TheBITS clock should be th e clock that is bes t performing in stress andholdover and is the most robust. By use of the BITS concept, theperformance of the o ffice will be dictated by the BITS clock, since o nlythe BITS clock is subjected t o stress on its timing reference.

III. Synchronizatio n Planning Conce pts


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Figure 7. BITS withdedicated timingfacilities.

Using the be st facilities to transpo rt a synchr onization reference isrequired to minimize slips. The best facility may be defined as the

reference with the fewest impairments. This refers to a reference tha thas the least average number of severely errored secon ds (SES) [1] andthat is free from exces sive timing instabilities (jitter and w ander).References that are pa yloads o n SDH/SONET should not be us ed fortiming, since the y are subjected to p ointer proces sing, which addsexcessive wande r and jitter to the referenc e. Similarly, references thatare transmitted by ATM Constant Bit Rate services will exhibit largeamounts o f wander and sho uld not be used for timing.

Cascading of timing references throu gh a netwo rk sho uld be minimized(Figure 8). Timing performance will always degrade as t iming is passe dfrom clock to clock. The more clocks and facilities in a synchronizationchain, the greater the accumu lated degradation will become, and the

larger the frequency offset grows . Each facility will add impairments towhich the clocks in the chain must reac t. Therefore, for best perfor-mance, synchronization chains should be kept short.

PBX PBX

PBX PBX Figure 8.Excess ive cascading.

PBX PBX

PBX


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IV. Synchronization Planning Requirement s

The synchronization plan for a netwo rk must m eet several requirements,ranging from m eeting performance objectives a nd s atisfying serviceneeds t o being easy to maintain and administer.

Several performance o bjectives h ave been established by ITU, ETSI, andANSI to control slip rates in ca rrier networks . ITU, ETSI, and ANSIallow th e long-term frequency inaccura cy at the output of a digitalsystem clock to be no worse than 1 10 11 [2, 3, 9]. Short-term r equire-ments of 1 to 10 microseco nds of time error in a day must a lso be met atthe output of each network clock.

In private ne tworks, ANSI requires that the first customer premisesequipment (CPE) in the synchronization chain have no more tha n 4.8milliseconds of time error pe r day. This corre sponds to 40 slips per day,but it is only an interim requirement. In the next few years, this require-ment is expec ted to ch ange of 18 microseconds of daily time error.

Service requirements for s ynchronization dep end on the app lication.ITU has es tablished a slip rate thresho ld of one slip in every five hoursfor an acceptable end-to-end c onnection that will support the s erviceneeds of digital data, video, facsimile, and mo st e ncrypted services. Insome net works, a tighter requirement of one slip per day is needed tosupport encryption. Signalling System 7 networks also typically requireno mo re tha n 1 slip per d ay. All these service requirements w ill beachieved if the 1 10 11 long term frequency accuracy and 1 to 10 micro-second daily timing error requirements are met.

SDH and SONET services r equire good shor t-term stability. ANSI

requires that the b and-limited shor t-term noise at t he outpu t of anyclock not exceed 100 nanoseco nds. In addition, the band-limited sh ort-term no ise from a BITS clock should not exceed 17 nanoseconds. Thisimplies that t he netw ork ope rate at a stratum 3E [5] or local clock performance levels.


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V. Carrier Net wo rks

Carrier network s require stringent synchro nization planning if perfor-mance goals are to be ach ieved under n ormal operating conditions. Areceiver clock normally operate s with a frequency offset comp ared to

the so urce clock to which it is locked [1]. This frequency offset acc u-mulates in a chain of clocks. It is a goal of carr ier synchronizationplanning to ensure that these a ccumulated frequency offsets do no texceed th e required 1 10 11 frequency accuracy.

Since synchro nization performance is dominated by rece iver clock an dfacility performance, the major focus of synchronization planning forcarrier networ ks lies in the determination of timing distribution andthe selecti on of clocks and facilities used to time the net work. Theselection and determination of the number an d locations of masterclock or clocks for the networ k are largely based on performance goalsand maintenance issues.

Carrier Network Synchronization Performance

The synchronization performanc e in a carrier netwo rk is charact erizedby three compon ents: the accuracy of the master clock, the perfor-mance of the facilities distributing the reference, and the pe rformanceof the receiver clocks obtaining a reference over the facility [1].

The inaccuracies of the mast er clock usu ally contribute a small portion of the timing inaccura cies in a synchronization network. Primary referencesources (PRS) typically have long-term acc uracies on the or der of a fewparts in 10 13 to a few parts in 10 12 . This performance is usually 10-100times better than t he performance o f receiver clocks locked to the PRS.

Synchron ization performance is dominated by a co mbination of thefacility and rece iver clock performance [1]. In n ormal oper ation, areceiver clock must lock to a s ource c lock by extracting timing from afacility that has sh ort disruptions or erro r bursts. The number of errorburst events can range from an a verage of 1 to 100 events per da y,depend ing on facility type, mileage, and oth er factors .

These cons tant de gradations will adversely affect the distribution of the t iming reference. The re ceiving clock will react to each e rror. It isallowed to mo ve up to 1 microsecon d in response to e ach error o n itstiming reference. Accumulation of facility errors and the resultingphase e rror in the re ceiving clock can result in the rec eiver clock operating with a frequency inaccuracy of a few par ts in 10 12 to a fewparts in 10 10 . Therefore, receiver clocks con tribute a much largerportion of timing errors and slips in a n etwork than t he PRS will.


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Primary Refe rence Sources

Almost all carriers with digital networks re ly on primary referencesources ( PRS) for timing since ITU recommends tha t network s operatewith a long-term a ccuracy of 1 10 11 [3]. A PRS is placed in a ne twork as sh own in Figure 9. The PRS times all the equipment in the location inwhich it resides. This equipment, in turn, will time the rest o f thenetwork.

The slip rate contribution of a PRS is usually negligible. A network which derives timing from two P RS clocks w ill experience at mostfive slips per year, caused b y the inaccuracy of the two clocks. This is

negligible compar ed to the p erformance of receiver clocks. Receiverclocks typically operate with a d aily performance that is 10 to 100 timesworse t han the PRS to which they are slaved. Therefore, it has been t hetrend of telecommunication networ k operato rs to rely more heavily onPRS clocks a nd to use mu ltiple PRS clocks to time their networ k. Thisreduces the casc ading of timing in the synchronization netw ork.

The choice of PRS technology used by an adm inistration dep endslargely on reliability, maintenance, and control. Performanc e is u sually asecond ary conce rn. LORAN-C or GPS tec hnologies h ave been usedextensively in North America. However, since GPS is controlled by theU.S. Depart ment of Defense a nd LORAN-C is not global, many othe radministrations rely on Cesium clocks.

The locations in which the PRSs are used are de termined by network topology. PRSs are usually placed in locations that will minimize thecascading of timing in the network. In this manner, the b est perfor-mance ca n be ac hieved in the n etwork. Additional sites that may requirethe us e of PRSs are international switching locations. It is at theselocations that one administration interfaces with an other an d all signalsare tran sferred plesiochronously. It is important, therefore, to guaranteethat these locations opera te with the 1 10 11 frequency accuracynecessary for plesiochronous operation.

Figure 9 Primaryreference clockconfiguration.

Sw itch SDH NEDigitalCross-ConnectE1 STM-N

P S P

P

S

S

P STiming toOther

Locations

Timing toOther

Locations

Clock Distribution Unit

PrimaryReference

Source


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Interoffice Timing Distribution

The determination of the distribution of primary and seconda ry timing

through a network depends on the network topology and performancerequirements. Interoffice t iming is configured such that timing loops ar eavoided, the h ierarchy is maintained, casc ading is minimized, andperformance goals are met. Most administrations use traffic-bearinglines betwee n offices to p ass timing. Some rely on d edicated timing-onlyfacilities between offices for improved maintenance, reliability, andperformance.

Facilities with the best error pe rformance, in terms of lowest nu mber of daily severely errored second s (SES) and least down time, should bechosen to carry synchronization. Error rates typically correlate with thelength of the facility. Therefore, short facilities are preferred. Satellitefacilities ar e not used for timing distribution due to diurnal wander [1].

References that are carried as traffic in Constant Bit Rate ATM, SDH, orSONET are also not used to car ry the reference due to ex cessivewander.

Most DS1/E1 references passed b etween locations are carried onfacilities that use asynchronous multiplexing (e.g., DS3 multiplexing).These mu ltiplexers u tilize pu lse stuffing method s (s ee Section II, PulseStuffing) to t ransmit the reference without retiming or changing thelong-term frequency of the s ignal. The inclusion o f asynchronou smultiplexing in the reference signal will not significantly affect synchro-nization pe rformance or planning as long as jitter levels caused b y themultiplexing remain controlled.

The amount o f reference cascading that can be allowed from the PRS toall other clocks in the network depends on the facility and BITS clock performances an d the performance objectives of the networ k. Thedesign objective for most netw orks is 1 10 11 long term frequencyaccurac y or 1 microsecond per da y of timing inaccur acies (Section I).To achieve this level, BITS clocks must h ave rearran gement maximumtime interval error (MTIE) per formance of less than 1 microsecond,facilities can only have a few errors per we ek, and t iming cannot becascade d to more tha n two to four offices. If very good BITS clocks a ndfacilities are u sed, more cas cading can be allowed and a fewer numberof PRSs are re quired in the ne twork.


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Intraoffice Timing Distribution

The BITS configuration is the recommen ded me thod for intraofficetiming (see Figure 7 on page 9). By using a BITS, the best clock controlsthe office, cascading is minimized, and a dministration is easier. In theBITS method, all equipment in the location receives two diverse r efer-ences from the BITS in a star arrangement.

The stratum level required in a BITS clock is largely determined byperformance requirements and the size of office. Stratum 2 and transitclocks are typically used in large toll offices, wher eas st ratum 3 or localclocks ar e us ed in smaller locations. If there is SONET or SDH equip-ment in the location, it is recomme nded th at the BITS clock be at least astratum 3E or local clock, respectively [5]. This is to minimize thenumber o f pointer adjustment e vents occurring on traffic through theSDH/SONET syste m.

Determination of which clock is best for a given stratum level is basedon performance and ma intenance issues. The most important pe rfor-mance p arameter distinguishing clocks at the s ame strat um level is theMTIE d uring rearrangement [1]. This performanc e pa rameter will havethe greatest impact on d aily performance of any function of the BITSclock.

Timing the BITS ClockThe BITS clock rec eives two o r more re ferences from other locations.If dedicated facilities are used to time th e BITS clock, the facilities willterminate directly on the BITS (see Figure 7 on page 9). When traffic-bearing facilities a re u sed, the reference facility that is ch osen typically

does no t terminate on the BITS clock. In these cas es, a device called abridging repeater is used (Figure 10). A bridging repeater is an in-linedevice that taps onto a ny DS1/E1 signal and pr ovides a copy of thatsignal. In this manner, any traffic-bearing line coming into an office canbe use d as a timing reference for the BITS clock.

DigitalCross-Connect

Multiplexer

ClockDistribution

Unit

ChannelBank

P

P

S

S

TSGBITS

Bridging Repeaters Swi tch BITS E1/DS1 Traffi c

CompositeClock

Square W aveSignal

P

S

Figure 10.Switch BITS clock.


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In order to receive timing not ter minated on the clock, the BITS clock also requires exter nal timing capabilities. Exter nal timing is the feature of having ports on the device dedicated for synchron ization only. Some

equipment, however, can only extract timing from traffic that term inateson the equipment. If such equipment is cho sen for the BITS and thereference line does not terminate on it, the carrier must ded icate two (ormore) tr affic ports to be used for synchronization only.

An important factor in supplying timing to the BITS clock must be thatdiversity is maintained among the references. Diversity must be achieved atall levels. The timing source should be diverse, coming from a differentlocation and different equipment, over diverse facilities. In ad dition, pow erto all devices in the synchro nization path , including facility multiplexers an dbridging repeaters , should be d iverse.

Timing Distribution from the BITS ClockThe BITS clock is used to provide synchronization to all equipment in itslocation. There are several methods used for intraoffice timing distribu-tion: dedicated DS1 and E1 synchronization lines, synchron ous c lock insertion units, composite clock lines, and square and sine w ave signals.

DS1 and E 1 signals are typically used to t ime digital switches and cross-connec t systems in an office. If a timing signal generator (TSG) is used asthe BITS, the TSG will supply these s ignals directly (Figure 11). If adigital switch or c ross-connect system is used as the BITS clock, thedevice will typically only transmit traffic and not synchronization signals.Therefore, with a switch or cross-connect BITS, a bridging repeater or aclock distribution unit (Figure 10) is used to sup ply DS1 and/or E1 timingsignals.

Bridging repeaters are tapped into any outgoing DS1/E1 traffic-bearingline from the sw itch or cross-connect BITS clock. These b ridging repeat-ers can now sup ply timing (traceable to the BITS clock) to oth er clocksin the office.

Sw itch M ultiplexerChannelBank

P

P S

S

CompositeClock

TSGBITS

SquareWaveSignal

Bridging Repeaters

Figure 11. TSGBITS clock.


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Clock d istribution un its (CDU) are devices that a ccept a DS1/E1reference and s upply multiple references. They esse ntially act like amultiplicative device giving multiple references out for one incoming

reference. A clock distribution u nit is distinguished from a TSG in thatthe CDU does not include a clock and d oes not retime the line. It has noreference sw itching or holdover functions that ar e normally associatedwith a c lock. If the input line to a CDU is lost, the CDU will cut off signals to all outgoing lines.

The DS1/E1 BITS referen ce from the TSG, bridging repea ter, o r CDU isconnec ted to the ex ternal timing input o f all equipment in the location.The equipment which doe s not have extern al timing capability canreceive timing directly from the BITS clock by either dedicating trafficports for synchron ization use on ly or by using a synchronous clock insert ion unit (SCIU) (Figure 12). An SCIU is a device wh ose o nlyfunction is to reclock a DS1/E1 line that is passed through it. Other thanreclocking, the SCIU does not affect the line being sent through it. Withthe use of the SCIU, BITS clocking can be injected on traffic-bearinglines and sent to equipment that does not have ext ernal timing. Theadvantage of the SCIU is that a star arra ngement can b e maintainedwithout re quiring the use of extra DS1/E1 ports in the re ceiving equip-ment.

In some equipment, the extern al timing input do es not a ccept anexterna l DS1 or E1 s ignal, but r athe r a 2.048 MHz, 1.544 MHz, or8 KHz square or s ine wave t iming signal. For this equipmen t, TSGs andCDUs will often have square and sine wave timing signal options. Thesecan be used to directly time the equipment. Many administrations,

however, do not use this option and will either time such equipment byan SCIU, or looped-time it [1] from other equipment in the location.

Switch PBXSCIU

P S P

Traffic

P

BITS

Figure 12.Synchronous ClockInsertion Unit.


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A composite clock is anoth er means commonly used to time equipmentin a location. A composite clock s ignal is used to synchro nize equip-ment tha t has DS0 interfaces, such as channe l banks with DS0

datapo rts an d DDS equipment. The c omposite clock signal is a 64 KHzbipolar square w ave with an 8 KHz signal riding on it in the form of abipolar violation every 8 bits. Most TSG and CDUs will provide comp os-ite clock signals (Figures 10 and 11). Receiving equipment that usescomposite clock signals requires on ly one timing reference. It sh ould benoted th at not all networks require composite clock signals.

Monit oring and Verificat ion

A recent t rend in telecommunication synchronization is the use of monitoring systems t o verify synchronization pe rformance [6]. Thetiming performance of an office ca n be verified by compar ing thetiming of a DS1 or E1 line coming from tha t office with the timing from

a kn own, highly-accurate reference ( Figure 13). Monitoring can be donereal-time and con tinuously by using a bridging repeater to tap into theDS1 line to be mo nitored and continuously measuring the line re lativeto a primary reference source (P RS).

Verification is nee ded t o dete ct an d reso lve timing degradations beforethey impact service. There are numerou s source s of timing degrada-tions, such as maintenance activities, circuit re-provisioning activity,clock diagnostics, and excessive facility errors [6]. These degradationsare not easy to detect or diagnose using standar d alarm mechanisms.Monitoring allows for instant detection and simplified d iagnostics o f synchronization pro blems.

Switch ComputerCounter

Data

PrimaryReference

Source

Lines toM onitor

M onitoring System

Figure 13.SynchronizationMonitoring System.


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SONET/SDH

SDH and SONET use a pointer mecha nism (see s ection II, Pointers, on

page 5) to identify the beginning of frames in their payload. If thepayload timing differs from the SDH/SONET network element, or if thetiming of two SDH/SONET network elements d iffers, adjustments aremade in the pointers. Pointer adjustments c ause significant amou nts of

jitter and wander. Therefore, ANSI [2] recommends that timing is neverpassed as payload through an SDH/SONET facility. It recommendsinstead th at timing is ext racted from the o ptical signal (Figure 14). Inthis case , the timing is derived from t he p revious SDH/SONET netwo rk element (NE) in the facility.

SDH and SONET also place further re strictions on the s ynchronizationplan since BITS clocks in offices w ith SDH or SONET equipmentshould have at least a strat um 3E or local clock. This is to minimize thenumber o f pointer adjustment e vents occurring on traffic through theSDH/SONET syste m.

Further discussion on t he impact of SDH and SONET to synchroniza-tion can be found in [7].

SONET NESwitch

P

P

S

S

S SP

DS1Traffic

OC-N OC-NDS1

Traffic

P

TSG BITS

SONET NE

E1Clock

E1 Clockderived from

STM-N

Figure 14. Timingdistribution usingSONET lines .


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VI. Private Net works

Synchron ization planning for digital private netw orks is extremelydifficult since the private networ k to pology is typically complex a nduncons trained. The widesprea d use of stratum 4 clocks in these net-

works can lead to severe performance de gradations and force excessivecascading of the timing reference. Private net works are also in amultivendor, mu lticarrier environment which make s synchron izationplanning complex. Proper planning requires the know ledge of the entiredigital private network. All digital equipment and digital services mustbe included in the plan.

An a dditional complexity with p rivate network s synchro nizationplanning versus c arrier netw ork planning is that most reprovisioning inthe private networ k affects the synchronization plan. This is because of the limited connect ivity in a private network . A new plan is need ed if any digital service, facility, or equipment is added, deleted, or changedin a private netw ork. This is usually not the case in carr ier networks.

Private Network Synchronization Performance

The majority of private networks rely on stratum 4 CPE clocks for theirtiming. Since stratum 4 CPE clocks perform 100-1000 times worse thanstratum 4E or bett er clocks, private netw ork synchronization perfor-mance is dominated by the performance of stratum 4 clocks.

Under stres sed co nditions, stratum 4 CPE clocks pe rform very differ-ently than other netwo rk clocks. When a stratu m 4 clock expe riences ashort interruption, it will declare the r eference unu sable and will switchits reference to a b ackup t iming source, either anoth er timing referenceor its interna l oscillator. During the reference switch, the c lock willtypically produce a large, fast phase hit of 10 to 1000 microseconds.This phase hit is often large enough to c ause mu ltiple slips and occurson all outgoing lines of the CPE.

Downstream clocks are una ble to remain locked to a reference withsuch a phase hit. To the down stream de vice, the phas e hit is indistin-guishable from a facility error. As a result, the d ownstre am clock willswitch its reference, cause ano ther phase hit, and the error eventpropagates. Therefore, one erro r on a facility at the top of the synchro-nization chain can cause all lines and no des in the synchronizationchain to have erro rs (Figure 15).

Since transmission facilities carrying the timing reference can havefrom 1 to 100 error burst s per day, the performan ce of private networksusing stratum 4 clocks is typically poor. It can be 1,000 times worse inperformance t han is seen in public network s, operating at an effectivelong-term frequency accuracy of 1 10 9 to 1 10 7. Slip performance of dozens of slips per day per CPE is not unusual.

CPE

NetworkTimingSource

NetworkTimingSource

CPE

CPE CPE

P P

P SError Burst

Phase Hit

Phase HitPhase Hit

P

Figure 15.Cascading errors inprivate networks.


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Synchronization So urces

Private networks t ypically do not o wn and operate their own PRSs, butrather rely on the service-provider for timing. Many services provided bythe carr ier has timing that is traceab le to a PRS. The private netw ork

can re ceive timing from the se services.

It must be noted that no t all services pro vide timing. The private net-work o perator should check with the service provider to see if timing isprovided. In general, in North America, switched s ervices are timed,wherea s point-to-point services may or may not be timed.In Europe , both switched an d po int-to-point services usu ally havetiming.

To optimize synchro nization, the pr ivate n etwork should rece ive timingfrom as many PRS-traceab le sources as poss ible, even if the sourcescome from d ifferent carriers. This will provide the best pe rformance inthe private networ k. In normal operation, the frequency accuracy of

carrier n etwork equipment is typically 100-1000 times b etter than that of a CPE [6]. Therefore, the synchronization pe rformance o f a CPE w illimprove if it rece ives timing directly from a carrier sou rce, as oppose dto ca scaded timing from anoth er CPE. The us e of multiple PRS-trace-able timing sources improves pe rformance, minimizes cascad ing of timing, and increases t he diversity of timing sources in the netw ork.

PBXPBXPBXPBX

PBXFreeRunPBX

Location 2 Location 3

Location 1

P

P

S

S

Figure 16.Locatio n 1 PBX isNetwork Maste rClock.

If the private networ k does n ot have any service that provides PRStraceab le timing from the carrier, the private network must designate amaster c lock for the networ k (Figure 16). In this case, the privatenetwor k is a digital island and does not interface digitally with any PRStraceab le equipment. Therefore, the private netw ork does n ot need to

be timed from a PRS. The designated maste r clock is the clock with the best free-run performan ce. The clock should also becentrally located in the network so that c ascading of timing canbe minimized. This master clock, usually a digital PrivateBranch Exchange (PBX) or multiplexer, is set to free-run. Allequipment in the network obt ains timing traceable to themaster clock.

The hazard of operating a private network tra ceable to aclock that is not a PRS is when ne w services are added tothe n etwork. A new digital, PRS-traceable s ervice ca n beadded to t he netwo rk later. The synchronization distribution must bechanged so tha t the private network rec eives timing from the new

service. If a new s ynchronization plan is not developed and mastertiming for the network remains as a free-running CPE, exce ssive slipswill appear an d make the n ew service unusable.


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PBX

Location 1

Stratum 4PBX

NetworkTimingSource

Stratum 4Multiplexer

PBX

Location 2

Stratum 4PBX

Stratum 3Multiplexer

Ideally, primary and se condar y sources s hould be d iverse, espec iallysince most CPEs do not incorpora te holdover in case of loss of alltiming references. However, in private net work d iverse re ferences maynot always be pos sible. A location may be connecte d to only one otherlocation. In many case s, access of private net work s ervice providers isnot d iverse. Also, in North America , having service from multiplecarriers does not ens ure diversity, since one carrier may lease linesfrom another.

Figure 18.Locatio n 2Multiplexer hasnon-hierarchicaltiming.

CPE 2CPE 1

P

PP

SS

CPE 4CPE 3P

Figure 17. EitherCPE 2 or CPE 3cannot havesecondary.

Interoffice Timing Distribution

Timing is passe s betwee n private network locations when a location isconnec ted to th e private network by point-to-point services that do n othave carrier timing. This is typical in da ta net works. The rules fordetermining how timing is passed betwe en private network locations issimilar to c arrier netw ork r ules: use the b est (t ypically, shorter) facili-ties, minimize cascading, and avoid timing loops. Minimization of cascading is especially important in private network s since CPE ca s-cade phas e hits throughout a network. The shorter the synchronizationchains are, the less str atum 4 CPE equipment is impacted .

The thre e major difficulties in interoffice timing in pr ivate network s ar elack of seconda ries, non-hierarchical situations, a nd diversity. Privatenetwor ks typically have limited connec tivity. Each location is con-nected to only a few other locations. Sometimes, it is impossible toprovide a location with a se condary without forming a timing loop(Figure 17). In this situation, it is better to con figure the ne twork without a seconda ry than it is to run the risk of forming a timing loop.The pe rformance of a CPE in a timing loop is typically 10-100 timesworse than it is in free-run or holdover.

Non-hierarchical situations also a rise due to lack of conne ctivity (Figure18). Often, they are unavoidable. As discussed in Section III, the net-work will operate w ith non-hierarchical situations. These situations canbe avoided b y introducing a TSG in the timing source location.


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Intraoffice Timing Distribution

In private ne tworks, wh en a location relies on a CPE for timing, as

oppose d to a TSG, the use of the BITS concept is often infeasible orundesirable (Figure 19). Most CPEs do n ot have e xternal timing op-tions. Therefore, in order to use the CPE a s a BITS, two ded icated DS1ports (for synchro nization uses on ly) and tw o bridging repeaters arerequired. This is usua lly more e xpensive than the c ost o f a TSG. Inaddition, if the CPE is not ma jor equipment (such as a P BX or a mu lti-plexer), the CPE is typically reprovisioned o ften and ma y not make areliable t iming sour ce.

In situations where a BITS clock is feasible, the private network shoulduse a BITS that has a stratum 4E or bett er clock, if available. If theprivate network is to meet t he future ANSI requirement of no morethan 18 microseconds of daily timing error, a str atum 4E or betterBITS clock is re quired. Stratum 4 BITS clocks typically have dailytiming errors that ran ge from hundr eds of microseconds to te ns of milliseconds.

Figure 19. No BITSin private Network.

PBXPBX

P

P

P

P

P

EchoCanceller

EchoCanceller

EchoCanceller

Multiplexer

Multiplexer

Multiplexer

P

P

S


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P

P

P P

NetworkTiming

Source #1

NetworkTiming

Source #2

P

P

P

P

P

NetworkTiming

Source #1

NetworkTiming

Source #2

Dynamically Changing Synchronizatio n Plans

Several CPE manufacturers incorporate a proprietary, dynami-

cally-changing synchronization plan feature in the ir equipment.With this feature, the s ynchronization distribution continuallychanges among a netwo rk of equipment mad e by the manufac-turer. One or more netw ork timing sources a re designated forthe netwo rk. From these source s, the CPE automatically deter-mines how to pass timing through the networ k, with the goal of minimizing the casc ading of timing (Figure 20A). If a CPE losesits timing reference (Figure 20B), the entire synchronizationdistribution in the netwo rk will change to resupp ly all CPEs witha timing source w ith minimum casc ading from the n etwork reference.

In most cases, it is preferable for the private network to avoid the useof dynamic synchronization plans. Such a plan w ill lead to an excessive

number of reference switching events. Since mos t CPEs with th isfeature have stratu m 4 clocks, excessive reference switchingcauses large numbers of error bursts and very poor perfor-mance. In addition, the use of a dynamic plan does not a llowthe private networ k to follow the BITS clock concep t andmakes th e netwo rk more d ifficult to administer.

SDH and SONET

As carr iers introduce SDH and SONET into their netwo rks,many private network services will be provisioned a s payloadon SDH or SONET systems. In these situations, all services fora private net work location may be s ubjected to SDH/SONET

pointer adjustments and the associated jitter and wander(Figure 21). Typically, the ca rriers SDH or SONET equipmen t will notbe on the p rivate netwo rk operat ors premises and, therefore, thatoperato r w ill not h ave acce ss to the SDH/SONET timing signal that isderived from the optical carrier. The private network operato r must re lyon the SDH/SONET traffic signal for timing. References passed aspayload throu gh SDH/SONET can have significant amounts of wande r.A DS-1 or E1 signal, mapped and t rans por ted though SDH/SONET, canexperience tens o f microseco nds of wan der pe r day [8]. This situation iscurrent ly under stu dy in standa rds bodies and affected private network operato rs are h andled on a c ase-by-case bas is by the service provider.

Figure 20B.Dynamic Synchro-nization Plan:Network timingsource #1 fails.

Figure 21. Privatenetwork se rviceprovided bySONET.

Figure 20A.Dynamic Synchro-nization Plan:Network timedfrom networktiming Source #1.

PBXPBXPBXSONET

NE

DS1

Traffic

Private NetworkLocation

Netw ork ServingOffice


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Data Subjec t to ChangePr in ted in U.S .A. Ju ly 1995

Hewle t t -Packard CompanyCopyrigh t 1995

United States :Hewlett-Packard CompanyTest and Measurem ent Organization5301 Stevens Creek Blvd.Bldg. 51L-SCSant a Clar a, CA 95052-80591 800 452 4844

Canada:Hewlett-Packard Canada Ltd.5150 Spectru m WayMississauga, OntarioL4W 5G1(905) 206-4725

Europe:Hewlett-PackardEuropean Marketing CentreP.O. Box 9991180 AZ Ams telve enThe Netherlands

Japan:Hewlett-Packard Japan Ltd.Measureme nt Assistance Center9-1, Tak akur a-Cho, Hac hioji-Shi,Tokyo 192, Japa n(81) 426 48 0722

Latin America:Hewlett-PackardLatin American Region Headquarters5200 Blue Lagoon Drive9th FloorMiami, Florida 33126U.S.A.(305) 267 4245/4220

Australia/New Zealand:Hewlett-Packard Australia Ltd.31-41 Joseph Str eetBlackbur n, Victoria 3130AustraliaMelbourne Caller 272 2555(008) 13 1347

Asia Pacific:Hewlett-Packard Asia Pacific Ltd.17-21/F She ll Tower, Time Square,1 Matherson Street, Causeway Bay,Hong Kong(852) 599 7070

VII. Conclusions

Synchronization planning for telecommunication network s has be enpresent ed. Stringent planning must be done for all networks if perfor-mance ob jectives and service needs are t o be met. In carrier networks,

the major focus of synchronization planning for carrier netwo rks lies inthe de termination of timing distribution and the selection of clocks an dfacilities used to time the net work. Careful attention must be mad e tothe s election of BITS clocks and re ference facilities, and to the minimi-zation of reference casca ding. In private net works, the major goal of synchronization planning is to reduc e errors cau sed and p ropagated bypoor CPE clocks. This requires limited use of stratum 4 CPE clocks,limited cascad ing, use of as ma ny carrier timing sources as p ossible, andthe us e of BITS architectures.

References

[1] Synchronizing Telecommunications Networks: Basic Concepts,Hewlett-Packard Application Note 1264-1.

[2] American National Standard for Telecommunications, Synchroniza-tion Interface Sta ndar ds for Digital Networks, ANSI T1.101-1994.

[3] ITU-T Recommendation G.824, The Control of Within DigitalNetworks w hich are Based on the 1544 kbit/s Hierarchies.

[4] ITU-T Recommendation G.811, Timing Requirements at t he Out putof Primary Reference Clocks Suitable for Plesiochronou s Operation o f Internationa l Digital Links.

[5] Clocks for the Synchronized Network: Common Generic Criteria,Bellcore Technical Advisory, TA-NWT_001244, Issue 2, November 1992.

[6] J. E. Abate, e t al, AT&Ts New Approach to the Synchron ization of Telecommunication Networks, IEEE Communications Magazine,Vol. 27, No. 4, April 1989.

[7] Synchron izing Telecommu nications Netwo rks: Synchron izing SDHand SONET, Hewlett -Packard Application Note 1264-2.

[8] G. Garner, Total Phase Accumulation in a Network of VT Islands forVarious Levels of Clock Noise, Contribution to ANSI T1X1.3, Number94-094, September, 1994.

[9] European Telecommun ication Standards, The Contro l of Jitter andWander Within Synchron ization Netw orks, Draft ETS DE/TM-3017.

HP-AN1264-3_Fundamentals of Synchronization Planning

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