White PaperTime DistributionStrategies in CellularNetworks
AbstractThis paper reviews the various methodologies currently available forensuring Time of Day (ToD) synchronization in ...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 1Contents1 Introduction .................
Time Distribution Strategies in Cellular Networks2 © 2013 RAD Data Communications Ltd1 IntroductionHaving overcome the cha...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 3Figure 1: Example of frequency lock (...
Time Distribution Strategies in Cellular Networks4 © 2013 RAD Data Communications LtdAny time distribution chain must ther...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 5Nonetheless, all of the above is just...
Time Distribution Strategies in Cellular Networks6 © 2013 RAD Data Communications LtdAlthough PTP is capable of both distr...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 7Figure 2: Example of centrally locate...
Time Distribution Strategies in Cellular Networks8 © 2013 RAD Data Communications LtdThe nice thing about PTP is that, con...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 9Figure 3: Example of centrally locate...
Time Distribution Strategies in Cellular Networks10 © 2013 RAD Data Communications LtdFigure 4: Example of Access located ...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 11an effective and economical source f...
Time Distribution Strategies in Cellular Networks12 © 2013 RAD Data Communications LtdHere, the fallback mechanism for the...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 13Figure 6: Details of a Joint GPS-PTP...
Time Distribution Strategies in Cellular Networks14 © 2013 RAD Data Communications LtdGPS on every site Centralized GM Dis...
Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 15References[1] 3GPP TS 25.402 version...
The RAD name and logo is a registered trademark of RAD Data Communications Ltd.© 2013 RAD Data Communications Ltd. All rig...
Time distribution strategies in cellular networks
Time distribution strategies in cellular networks
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Time distribution strategies in cellular networks

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This paper reviews the various methodologies currently available for ensuring Time of Day (ToD) synchronization in cellular networks. It also introduces RAD’s revolutionary Distributed GMTM scheme, designed to deliver superb ToD accuracy at a lower cost in LTE and small cell networks, by bringing Grandmaster functionality closer to the base station in a small form factor device.

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Time distribution strategies in cellular networks

  1. 1. White PaperTime DistributionStrategies in CellularNetworks
  2. 2. AbstractThis paper reviews the various methodologies currently available forensuring Time of Day (ToD) synchronization in cellular networks. It alsointroduces RAD’s revolutionary Distributed GMTMscheme, designed todeliver superb ToD accuracy at a lower cost in LTE and small cellnetworks, by bringing Grandmaster functionality closer to the basestation in a small form factor device.
  3. 3. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 1Contents1 Introduction ......................................................................................................................22 Using GPS for Time Distribution in Cellular Networks.................................................43 Backup to GPS using Sync-E ...........................................................................................54 Transition to IEEE 1588-2008 (PTP)..............................................................................55 Centrally Located PRTCs/PTP-GMs .................................................................................66 Access Located Distributed GMs....................................................................................97 Joint GPS-PTP ..................................................................................................................118 RAD’s Distributed GM Solutions...................................................................................129 Summary..........................................................................................................................13References..............................................................................................................................15
  4. 4. Time Distribution Strategies in Cellular Networks2 © 2013 RAD Data Communications Ltd1 IntroductionHaving overcome the challenge of precise frequency distribution, time distribution (or Time-Of-Day{TOD} as it is sometimes referred to) is the next hot thing when it comes to synchronization ofcellular base stations-and a worthy challenge it is indeed.3rd-generation cellular base stations, such as the UMTS-TDD and the TD-SCMA, require provisioning ofa time reference that deviates from the Universal Time Coordinated (UTC) by no more than 1.5microseconds [1]. Future LTE cellular networks (regardless of their duplexing method: FDD or TDD)will have even stricter requirements in order to enable new features, such as Multiple Input MultipleOutput (MIMO) and Location Based Services (LBS). End-to-end time accuracies here are likely to be inthe order of few hundreds of nanoseconds!Time synchronization is so challenging mainly because, unlike frequency synchronization, it cannotsolely rely on a stand-alone, specific physical phenomenon (such as the hyperfine energy leveltransitions of the Cesium element). Time synchronization, although usually based on accuratefrequency distribution, requires some additional things.Figure 1 below presents the conceptual difference between a frequency and a time lock from thepoint-of-view of a simple signal scope. With frequency lock (upper figure), the two clock signals arecompletely “standing waves” relative to each other, with some arbitrary fixed time (phase) offset inbetween. Since in a frequency lock we are only interested in having the locked signals pace at thesame rate, this arbitrary fixed time offset is of no importance. Nevertheless, with time lock, we wantboth signals to be completely time-aligned. That is, their signal rise events should occur in exactly thesame instant, which essentially means that the fixed offset must be zero.
  5. 5. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 3Figure 1: Example of frequency lock (upper) and time (phase) lock (lower)Thus, time synchronization is mandated by the following requirements:• A stable enough (good frequency stability) primary counter that counts time units based on agiven standard timescale (e.g., UTC – Universal Time Coordinates), relative to an arbitrarypredetermined epoch (e.g., 1 nanoseconds elapsed from 1stJanuary 1970, achieved by a 1GHz-driven frequency counter).• A method (protocol) that measures delay between the primary counter and the client thatrequires the time information (zeroing the fixed time offset).The first requirement is quite straightforward and easy to implement nowadays, using a GlobalNavigation Satellite System, or GNSS (e.g., GPS). A GNSS essentially disseminates the same time (upto some very small inaccuracies) to every point on earth. Thus, a decent GNSS receiver, backed with avery precise frequency reference (to maintain the progress of the time between GNSS updates orallow for holdover in case of a GNSS failure), can be used as such a primary counter at every point onearth. This apparatus is usually referred to as a Primary Reference Time Clock (PRTC). Contrary to theSDH/SONET/Sync-E Primary Reference Clock (PRC), whose stand-alone frequency source could have aresidual fractional frequency error of up to ±10-11compared to the UTC, a PRTC is always disciplinedto a GNSS (under normal operation), and, thus, its frequency output is always perfectly in-line with itstime output. We usually refer to this attribute as time-frequency coherency.
  6. 6. Time Distribution Strategies in Cellular Networks4 © 2013 RAD Data Communications LtdAny time distribution chain must therefore start with such PRTC. However, from this point on the“game” opens up and different strategies for time distribution exist, based on service-providerCapEx/OpEx preferences and GNSS geopolitical view.2 Using GPS for Time Distribution in Cellular NetworksThe challenge of time distribution to the base stations can be easily and quite effectively solved bydeploying a “PRTC” on each and every end-application. This essentially means installing a GNSSreceiver plus antenna on every base-station. Thus, assuming a clear sky view is available at each suchsite, each base station would directly get its time (and probably also frequency) reference directlyfrom the GNSS. This strategy is mainly used today in North America, where almost all time (andfrequency) supplied to cellular base-stations is GPS driven.Indeed, as long as it is operational, GPS is capable of delivering extremely accurate time reference inthe order of ±50 nanoseconds that is more than enough even for the most stringent cellulartechnology requirements. However, GPS (and GNSS in general) has its drawbacks.To begin with, putting a GPS antenna on every cellular base station has problematic consequences interms of both CapEx and OpEx. It complicates the initial installation process of the base station(additional antennas, wiring etc.), mandates having an unobstructed sky-view (a major problem forthe emerging small cell antenna technologies that are mainly targeting building walls and closedspaces such as shopping malls) and wastes expensive technician time whenever the outdoor antennarequires maintenance. But this is just the beginning…GPS is controlled by the U.S Department of Defense. Ever since GPS became fully operational in 1994,it has become such a prominent tool in our daily civilian lives that we often tend to forget this.Nevertheless, cellular service providers around the world (other than in North America) do take thatinto account and recognize that under certain circumstances, the GPS service could be summarilyterminated. Thus, relying on GPS has strong geopolitical factors attached to it and many countries inEurope and Asia are reluctant to place their strategic telecommunications assets in foreign hands.This is mainly the reason why new GNSS systems like the European Galileo project, the RussianGLONASS and the Chinese Beidou navigation system were initiated. Nevertheless, the only fullyoperational, GNSS system with full world coverage existing today – and for the foreseeable future – isGPS.
  7. 7. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 5Nonetheless, all of the above is just a prelude to the scariest problem of all, GPS jamming. Being apassive radio technology element, a GPS receiver can be easily jammed using a $5 piece of equipmentthat can be easily bought on Ebay. Such an active jammer can disrupt the operation of a base stationand even cause it to crash temporarily when it is operated somewhere nearby. The problem evenworsens in metro areas having a dense concentration of cellular base stations as well as movingvehicles. Some of these cars could have active GPS jammers, used by the drivers to block the car’sspeed/position log recordings. This, in principle, could cause occasional disruption to nearby basestations. GPS vulnerabilities have been at the center of a few recent conventions dealing withfrequency and time, as the European ITSF and American WSTS.3 Backup to GPS using Sync-EPutting aside the prohibitive cost issue of installing/maintaining a GPS antenna on every cell-site, abackup to GPS at each cell-site must be applied. Such a backup can be effectively realized bysupplying the base-stations with an accurate frequency source so that they will be able to keep theirtime ‘ticking’ at the right rate once GPS is lost. For networks that already employ and distribute it tothe end-applications, Sync-E would be a natural choice1. However, many cellular networks today (e.g.,wholesale networks) are not supporting Sync-E. Furthermore, the introduction of small cells and themassive role these small antenna technologies – expected to be mainly installed in denseurban/indoor locations – are going to play in 4G is driving the search for an alternative, less GPS-dependent, solution.4 Transition to IEEE 1588-2008 (PTP)The only time synchronization alternative today to GPS is IEEE 1588 (the 2ndversion of the standardtermed IEEE 1588-2008, or PTPv2, to be exact) [2]. With PTP, the time (and, possibly, also frequency)distribution is carried using dedicated packets that are exchanged between a PTP Grandmaster (PTP-GM) and a PTP slave device (PTP-slave). The PTP-GM is usually directly connected to a PRTC, receivingaccurate coherent time and frequency references, and uses the on-going packets exchange with thePTP-slave to convey the time (and frequency) information to it. It is the PTP-slave’s job to recover thetime (and frequency) information back from the received packets.1Though one needs to be certain the base-stations are capable of using the Sync-E ref. for the time holdoverwork, rather than just for controlling the frequency of the RF transmission.
  8. 8. Time Distribution Strategies in Cellular Networks6 © 2013 RAD Data Communications LtdAlthough PTP is capable of both distributing frequency and time, a specific service provider mightchoose, for various reasons, to take advantage of the existing physical layer’s frequency distributioninfrastructure (e.g., TDM or Synchronous Ethernet) and use the PTP service for time only. Everythingsaid in this paper is applicable to either case.Practices of distributing time using PTP in cellular networks can be divided into two main strategies:1. Small number of PRTCs/PTP-GMs at the cellular backhaul core/aggregation, each servicing alarge number of PTP-slave devices integrated within the base station or colocated with it.2. Larger number of PRTCs/PTP-GMs at the aggregation/access, each servicing a relatively smallnumber of PTP-slave devices integrated within the base station or colocated with it.5 Centrally Located PRTCs/PTP-GMsThe first strategy is more or less based on existing SDH/SONET and Synchronous Ethernet (Sync-E)frequency distribution principles. That is, a primary reference followed by a relatively long distributionchain of 10 and more nodes. This strategy is depicted in Figure 2. The advantages of this approachinclude lower total cost spent on PRTCs/PTP-GMs2(fewer of them are needed) as well as an easierand more efficient fault protection scheme (as each PRTC/PTP-GM is responsible for more PTP-slavesand has better visibility of the other slaves in the network not under its direct responsibility duringnormal operation).2A practical implementation is likely to integrate the PRTC and the PTP-GM within a single piece of equipment.
  9. 9. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 7Figure 2: Example of centrally located PRTC/PTP-GM time distributionThe main problem of this approach is the relatively high number of intermediate network elements(e.g., switches and routers) that will need upgrading to facilitate the PTP messages exchange in orderto bring the end-to-end Packet Delay Variation (PDV) to a minimum. Such PTP on-path supportmechanisms include the Boundary Clock (BC) and Transparent Clocks (TC). Meeting the most stringenttime distribution requirement (and giving Service Level Agreement {SLA} for it) would probablymandate that all intermediate network elements will offer some kind of on-path PTP support. Thisunderstanding was the main drive to the on-going development efforts for the new ITU-T G.8275.11stPTP Telecom Profile for time distribution with full network support. Current expected completiondate is middle or end of 2013.Even though many cellular service providers understand that at the end of the day they will probablyneed to implement some kind of network forklifting to support PTP, they do not necessarily want todo it from day one. Some would very much prefer to take a more gradual approach and delay therequired network modification to a date as close as possible to when they can realize a real paybackfor those services that require the precise time (e.g., LTE network MIMO or Location Based Services).In the meantime, they would go for a less expensive working solution, even though true SLA couldnot be guaranteed at any given moment.
  10. 10. Time Distribution Strategies in Cellular Networks8 © 2013 RAD Data Communications LtdThe nice thing about PTP is that, contrary to other sync distribution techniques such as Sync-E, it willbenefit from having more on-path network support but does not mandate it. Thus, different schemesof partial on-path support can be used in order to improve the level of performance while keepingCapEx under tight limits. These could later on be supplemented with more on-path network supportto yield an even better level of performance.A popular example for the use of partial support is depicted in Figure 3. Here, an intermediate BC isplaced at a strategic point in the time distribution path between the core-located PTP-GM and thePTP-slave in the base station. The job of the intermediate BC is to divide the PTP distribution chaininto two parts (e.g., core/aggregation and access). The BC will terminate the time information afterthe core/aggregation cloud, dealing with PDV introduced on that section only. The regenerated PTPflow would then traverse the access, terminated by the PTP-slave within the base station that willneed to mitigate PDV introduced by the access only. Such a scheme can allow better PTP end-to-endperformance3at the additional cost of just one PTP intermediate function (or two if a more securefault tolerant scheme is pursued). Nevertheless, as already stated, true SLA guarantee would still bevery difficult to deliver.As the time distribution following this approach is more ‘end-to-end’ in nature, the principles of theexisting ITU-T G.8265.1 PTP Telecom Profile for frequency only [3] could also be used here. This is thescope of the work currently unfolding in the ITU-T SG15/Q13’s group of timing experts. The aim is tostart working on a 2ndTime Telecom Profile for partial support (designated number G.8275.2) as soonas the work on the 1stfull-support one is finished.3Placing an intermediate BC would result in better overall end-to-end performance in many cases, but certainly not all. Themerits of this approach mainly depend on the PDV profile of the core-aggregation cloud. Taking into account this networksection is comprised of high capacity links (10GB), hardware driven network elements and high QoS for the PTP flows, thisapproach would probably work well. Moreover, the intuitive assumption that adding more PTP support, by placing moreintermediate BCs, would give even better performance might not always hold true. This is due to the inherent noiseaccumulation characteristics of BCs. Of course, when a BC is implemented in every node along the chain (full networksupport), PDV will no longer exist and performance would be optimal. TCs, on the other hand, do not have this problem andthe end-to-end performance will be directly proportional to the number of elements that support TC. As in the BC case, fullnetwork support will guarantee optimal performance.
  11. 11. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 9Figure 3: Example of centrally located PRTC/PTP-GM time distribution with intermediate BC(partial support)6 Access Located Distributed GMsAn alternative strategy to the centralized PTP-GM deployment would be to locate a relatively largenumber of distributed PRTCs/PTP-GMs in the access network, each servicing a smaller number (a fewdozens usually) of PTP-slave devices. The benefits of this approach are obvious. Positioning thePRTCs/PTP-GMs closer to the PTP-slaves would result in much smaller time distribution chains andwould dramatically cut the number of intermediate network elements that need to be enhanced withPTP on-path support. Furthermore, no timing distribution capability is demanded for the mobilenetwork preceding the distributed GMs. This is particularly important for mobile service providesleasing transport services from wholesalers. On the other hand, more PRTCs/PTP-GMs would berequired. An example for such a PTP deployment strategy is given in Figure 4.
  12. 12. Time Distribution Strategies in Cellular Networks10 © 2013 RAD Data Communications LtdFigure 4: Example of Access located PRTC/PTP-GM time distributionThe dramatically shorter time distribution chains together with the desire to meet, at the end of theday, the stringent cellular time accuracy requirement will probably drive many service providersadopting this strategy to incorporate full PTP on-path support from day one. Nevertheless, as thenumber of hops is now much lower, the gradual migration path concept for the end-to-end case wesaw in the previous chapter can be even more attractive here, by gradually adding on-path supportbetween the distributed GM and its PTP-slave devices.The traction of distributed approach to the cellular market is mainly conditioned on two factors:1. The new distributed GM would need to have a markedly reduced cost than its older “brother”,the big central GM.2. The means for backup are still required to protect against GPS failure.The latter point can be solved using Sync-E or any other accurate frequency source that can besupplied to the distributed GM unit. In cases where Sync-E is not applicable, PTP could also be used as
  13. 13. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 11an effective and economical source for backup. Such apparatus is described in details in the nextsection.7 Joint GPS-PTPThe ever growing quest for “cost-effective” and “good enough performance” solutions recently gavetraction to yet another time distribution strategy, which can be referred to as “Joint GPS-PTP”. Thenotion is quite straightforward. If we do not have Sync-E deployed in our network we can still have aplausible backup to revert to in case the GPS fails, by taking advantage of the central GM that mightalready be installed in our network. An example of this strategy is depicted in Figure 5.Figure 5: Example of Joint GPS-PTP
  14. 14. Time Distribution Strategies in Cellular Networks12 © 2013 RAD Data Communications LtdHere, the fallback mechanism for the distributed GMs is achieved using PTP. The distributed GMsreceive and terminate a PTP flow in addition to the time/frequency reference they receives from theGPS. As soon as the GPS fails, the distributed GM would fall back to work as a PTP-BC relying on thetime reference it receives from the central GM, until normal GPS operation is restored. The transitionis done in a hitless manner to prevent unnecessary transients from occurring. Furthermore, incontrary to the partial-support case, the very accurate GPS reference could be used to improve thebackup PTP service level of performance under normal GPS operating conditions4. Thus, on GPSfailure, an even better PTP time reference could be provided. An important implication is that the PTPtime distribution chain could be made far simpler, having a very limited partial on-path support oreven none at all (pure end-to-end).8 RAD’s Distributed GM SolutionsRAD’s solutions for mobile backhaul – the ETX-5300A Service Aggregation Platform and the new ETX-205A Mobile Demarcation Device – feature advanced timing synchronization functionalities in additionto their service demarcation and aggregation attributes. This combination allows backhaul operatorsand wholesale providers to reduce the number of network elements, together with their associatedcosts, that are require to ensure dependable, per-CoS service delivery. Both products are MEF CE 2.0-certified and feature a distributed GM with Sync-E holdover capabilities (as well as external frequencysource backup), while the smaller ETX-205A also includes a built-in GPS receiver. As depicted in Figure6, upon the loss of GPS, the system will automatically switch to “Sync-E holdover” mode if Sync-E issupported in the network. Otherwise, the system is designed to fall back to BC mode, taking its timeand frequency reference from a predefined centrally located GM.4Such improvements can include mitigation of inherit network asymmetries that directly affect the PTP level of performanceand could not be solved for otherwise.
  15. 15. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 13Figure 6: Details of a Joint GPS-PTP distributed GM solution9 SummaryDelivering accurate time to the cellular base stations will certainly be one of the major challengesfacing the cellular providers as they start to deploy their new LTE networks. Over the coming years,we will witness a constant struggle between the will to meet the very stringent time accuracyrequirement on one hand, and the need for a cost-effective migration path, on the other. In reality,accomplishing this challenging task will probably assume a variety of implementations based ongeographical location, CapEx/OpEx considerations and fault-protection perspectives. The differentattributes of the most prominent approaches discussed in this WP are summarized in Table 1.
  16. 16. Time Distribution Strategies in Cellular Networks14 © 2013 RAD Data Communications LtdGPS on every site Centralized GM Distributed GMsNumber of hopsbetween GM andslavesN/R High. Mandates fullBC/TC support in themobile backhaulSmall. Only thelast mileequipment.Reliance in GPS High. A GPS receiver isrequired on everybase-stationLow. One GPSreceiver covershundreds of base-stationsModerate. OneGPS receiver perdozens of base-stationsGPS backupprovisioningProblematic as manymobile network do notsupport Sync-E to thebase-stationAchievable usingSync-E or otheraccurate frequencysource at the coreAchievable usingSync-E or PTP(from the core)CapEx/OpEx High CapEx/OpEx toinstall and maintainthe GPS antennas onevery base-station(~1000$ per base-station)High CapEx due tothe required fullBC/TC support in thebackhaul net.Low. Smallernumber of GPSantennas and noneed for BC/TC inthe backhaul net.Applicability forsmall-cellsProblematic due to the“sky view”requirementApplicable (assumingfull PTP support)Ideal due to itsflexibility to placethe GM at theoptimal locationTable 1: Summary of time distribution strategies in cellular applicationRAD’s products comprise all the different synchronization ingredients and offer our customers a fullsuite of synchronization solutions to choose from. For more information, please contactmarket@rad.com.
  17. 17. Time Distribution Strategies in Cellular Networks© 2013 RAD Data Communications Ltd 15References[1] 3GPP TS 25.402 version 5.2.0 Release 5.[2] IEEE Std 1588™-2008, IEEE Standard for a Precision Clock Synchronization Protocol forNetworked Measurement and Control Systems[3] ITU-T Recommendation G.8265.1 (10/2010), Precision time protocol telecom profile forfrequency synchronization.
  18. 18. The RAD name and logo is a registered trademark of RAD Data Communications Ltd.© 2013 RAD Data Communications Ltd. All rights reserved. Subject to change without notice.Version 6/2013 Catalog no. 802593www.rad.comNorth America HeadquartersRAD Data Communications Inc.900 Corporate DriveMahwah, NJ 07430 USATel: (201) 529-1100Toll free: 1-800-444-7234Fax: (201) 529-5777E-mail: market@radusa.comwww.radusa.comInternational HeadquartersRAD Data Communications Ltd.24 Raoul Wallenberg St.Tel Aviv 6971923 IsraelTel: 972-3-6458181Fax: 972-3-6498250E-mail: market@rad.comhttp://www.rad.com

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