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The Complete

Executives Handbook

The Spectral

Compatibility Guide

for Carriersdeploying Mid-Band

Ethernet Services to

extend their Metro

Ethernet Service

Edge over Existing

Copper Facilities

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Executive Summary

Copyright 2007, Hatteras Networks Incorporated, Rev. 1.2 1 of 30

Spectral compatibility with existing services is an impor-tant factor in any deployment decision. It is simply vitalthat every access technology coexists with every otheraccess technology. Selective co-existence is simply notpossible in todays environment. The access network is a

very complex, heterogeneous environment that has beenundergoing constant upgrade and evolution for decades.You just never know what already exists in a binder.

Peaceful co-existence comes in two avors. First, a tech-nology needs to be exible in surviving and performingin any environment, no matter what other technologiesshare the network. This is the self-preservation aspect of

peaceful co-existence. The second aspect of spectral com-patibility is to do no harm. No technology should have anunnecessary adverse effect on another.

In fact it is for these reasons that spectral compatibilityguidelines were invented. Almost any technology can beused incorrectly, and used in ways that are unnecessarilyharmful to other technologies. This is actually a good thing

in that it allows the local carrier communities to choosehow spectrum, power and other resources are allocated tothe various technologies. If only all carriers had the sameperspective life would be so much easier! Unfortunatelycarriers have different priorities, and the exibility in thetechnologies allows them to be tuned to support thesevarious priorities.

This paper specically looks at the spectral compatibilityof symmetric services. There are a number of options fordelivering symmetric services T1s, HDSL, SHDSL, en-hanced-SHDSL, and VDSL2 to name a few. In this docu-ment we mostly focus on the two newest technologies forsymmetric services enhanced SHDSL and VDSL2 to seehow they compare. We actually investigate two symmetric

variants of VDSL2 one with the US0 cutover at 276 KHzand the other with the US0 cutover at 552 KHz. We referto those variants as VDSL2-276 and VDSL2-552.

The key results of this paper can be summarizedas follows:

1)Enhanced SHDSL has less impact is more spectral

friendly with ADSL, VDSL2-138, and ADSL2+ thansymmetric VDSL2.

2)When used in a realistic business environmentwith ADSL, HDSL, and/or T1 interferers, enhancedSHDSL offers much better performance at distancesbeyond 3500 ft than symmetric VDSL2.

3)When used in a realistic residential environmentwith ADSL, enhanced SHDSL offers better perfor-mance at distances beyond 3500 ft than symmetricVDSL2.

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4)Only when used in unrealistic isolation with noother technologies such as ADSL, HDSL, T1, etc. does symmetric VDSL2 offer the best performance.

These results may be surprising to a lot of people. In thispaper we present these results and the technical details

and reasoning why they are true. The results are pro-vided in enough detail where the experienced reader canreproduce the results on their own to verify the validityof these claims.

For those that dont know what US0 cutover meansthats ok, you dont have to. These are basically just twovariations of VDSL2 that are being investigated for sym-

metric services because they offer a higher upstream ca-pacity than normal VDSL2 (also referred to as VSDL2-138 you guessed it, the US0 cutover is 138 KHz!). Thelarger number (e.g., 552, 276, 138) implies a higher the-oreticalupstream capacity. As well see, theory doesntalways translate to reality.

Note thatVDSL2-552 is non-standard

this technologyis currently under investigation but is not part of anyinteroperable VDSL2 standard or chipset implementa-tion. Given that supporting this variation would requirea change to VDSL2 chipsets, even if a proposal is even-tually accepted into the VDSL2 standard in the future, itwill likely be several years before VDSL2-552 becomesavailable in cost-effective, standard, interoperable VDSL2

chipsets.

Enhanced SHDSL, on the other hand, is an internationallyaccepted standard, and is the physical layer for 2BASE-TL, the IEEE standard for long reach, symmetric Ether-net-over-copper. Enhanced SHDSL was selected by theIEEE 802.3ah committee as the underpinning of this keyEthernet standard because it was the best performing,most spectrally compatible, symmetric technology in ex-istence at the time. As we shall see in this paper, thosereasons still remain true today even considering therecent introduction of VDSL2.

Mid-Band Ethernet services based on 2BASE-TL and en-hanced SHDSL are already being massively deployed

throughout the globe, and provide the key to bringingEthernet services to the masses of business custom-ers that lack access to optical connectivity. 2BASE-TLand enhanced SDHSL provide the best performing, mostspectrally friendly, way to bring almost any customeronto your Ethernet network. Symmetric VSDL2 makes agreat complement for 2BASE-TL on short loops to maxi-mize throughput per pair, but does not have the perfor-mance or spectral compatibility of 2BASE-TL for mid orlong loop applications.

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Contents

Executive Summary ............................................ 1

Table of Contents ............................................... 3

Background and Introduction ............................. 5

Acknowledgements ............................................ 7

Spectral Compatibility of

Symmetric Broadband Alternatives .................... 8

Spectral compatibility with ADSL

Spectral compatibility with VDSL2-138

Spectral compatibility with ADSL2+

Quick summary on spectral compatibility

Performance of Symmetric

Broadband Alternatives .................................... 13

The effect of ADSL on symmetric technologies

The business environment

The dreaded T1

Advanced DSL Optimization Techniques. .......... 19

Effects of power back-off

Realistic DSM considerations

Conclusions ...................................................... 23

Appendix Simulation Details .......................... 25

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Well, the data in this graph is true. Were not here todispute that. Its an accurate, reective data point thatdeserves consideration. Unfortunately, the data in thegraph is both myopic and misleading. The graph is myo-pic in that it is exactly one case there are a lot of

other cases to consider where the results are signi-cantly different. Well present some of those later in thispaper. The graph is also misleading in that it uses the e-SHDSL technology incorrectly. Like other technologies,e-SHDSL offers several encoding options. This graphshows only the noisiest encoding option. When themore spectral efcient encoding options of e-SHDSL are

used, as well show later, the situation is completely re-versed e-SHDSL has signicantly less impact on ADSLthan VDSL2-552.

In the end, what this graph shows is that if you take onething at its best, and another at its worst, then the onetaken at its best will look better. True, but somewhatirrelevant. Well strive to present a more complete pic-

ture so that informed technical decisions can be made.

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Acknowledgements

The DSL and mathematical expertise required to under-stand the impact of various DSL technologies on one an-other is beyond my grasp. The performance and spec-tral compatibility data included in this document wasproduced and validated by two distinguished engineers

in the eld, Dr. Andrew Deczky and Jerry Radcliffe, towhom we owe a great deal of thanks.

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In this section we take a more complete look at the spec-tral compatibility of symmetric access technologies withtriple-play services. Triple-play services are the newbread-and-butter of the carrier community, and it is im-portant to ensure spectral friendliness with these very

high revenue services.Triple-play services will generally be delivered via ADSL,ADSL2+, or VDSL2-138. These will be the technologieswhose performance we will evaluate in the presence ofsymmetric technologies. In general, carriers have a verylimited radius for triple-play services because of the highbandwidth required by these services. The triple-play

radius is about 5 Kft for most carriers, so we will studythe impact of symmetric technologies on the asymmetrictriple-play technologies (ADSL and VDSL2) in the 3-5 Kftrange. We ignore the distances less than 3 Kft becauseno technology, symmetric or asymmetric, will interferewith delivering triple-play services at distances less than3 Kft. The distances at that point allow rates that are

signicantly higher than what can be achieved in the 3-5Kft range, no matter what the interference pattern.

When discussing VDSL2, there are many bandplans andvariations that could be included. For this paper, we willlook at Plan 998 (North American bandplan) with vari-ous US0 cutovers. In general, the North American plan,along with one VDSL2 variation, is depicted in Figure 2.

As can be seen from the diagram, the extended US0VDSL2 variant creates a region of overlap. VDSL2 (andADSL and ADSL2+) use the frequencies 138 thru 552

KHz for downstream trafc (D1), while VDSL2-552 usesthat region for upstream trafc (to try to improve sym-metric performance).

Spectral Compatibility with ADSL

Lets start with the graph we rst looked at in the previ-ous section. In that case, the goal was to measure the

spectral compatibility of symmetric technologies withADSL. The specic case under study was with 12 ADSLand 12 symmetric disturbers operating at 3.4 Mbps inthe same binder.

Spectral Compatibility of Symmetric

Broadband Alternatives

U0 D1 U1 U2D2

U0 D1 U1 U2D2

0.138 3.75 5.2 8.5 12

0.552 3.75 5.2 8.5 12

Basic VDSL2

Plan 998

VDSL2-552

Plan

Figure 2. North American Bandplan 998 and Extended US0

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With enhanced SHDSL at these distances and perfor-mance criteria, there are two possible encodings to use 16 TCPAM or 32 TCPAM. The latter option is generally

friendlier with other technologies because it ts moreinformation into each symbol, requires fewer symbols

for any given rate, and thus creates less interference.Figure 1 only showed the 16 TCPAM operation of en-hanced SHDSL. In Figure 3, we include the 32 TCPAMoption of enhanced SHDSL to show that, when enhancedSHDSL is used correctly, it is in fact friendlier with ADSLthan the symmetric VDSL2 variant.

As the graph shows, when comparing VDSL2-552 and

enhanced SHDSL as a symmetric technology running at3.4 Mbps, ADSL will generally run 500 Kbps faster withe-SHDSL in the binder rather than VDSL2-552 in thebinder.

Note that we show two curves for VDSL2, one uses a wa-terflled spectrum (VDSL2-552 WF) while the other uses

the standard spectrum (VDSL2-552 STD). A waterlledspectrum generally creates less interference than thestandard spectrum, but has lower performance. We in-clude both curves here because in many cases VDSL2has been analyzed so that the spectral impact is mea-sured via the waterflledVDSL2, while the performanceis measured using the standard spectrum. But both

cannot simultaneously be true - either performance orspectral compatibility is sacriced.

As an additional note that will be further detailed inthe next section, this graph is still slightly misleading.It turns out that VDSL2-552 doesnt even support 3.4Mbps upstream at 5 Kft with 12 ADSL and 12 VDSL2-552 disturbers in the binder. If you peeked ahead to

Figure 7, you will see that VDSL2-552 doesnt even sup-port 3.4 Mbps at 5 Kft with just 8 ADSL and 8 SELF dis-turbers with 12 of each the performance will be evenlower. So although the spectral compatibility graph is

Figure 3. Downstream ADSL performancewith symmetric at 3.4 Mbps

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shown above, the assumption that the symmetric tech-nology can achieve 3.4 Mbps is incorrect for the VDSL2-552 case.

But in the end, as with Figure 1, this is just one ex-ample. But at least in this example, both technologies

are shown in a manner where a fair comparison can bemade.

Spectral compatibility with VDSL2-138

Many carriers are looking at VDSL2-138 as the technol-ogy to deliver triple-play services. In this section, welook at a similar example to the previous section when

VDSL2-138 is used as the triple-play technology. Howwill VDSL2-138 be affected by adding symmetric tech-nologies to the binder? This case is shown in Figure 4.

As can be seen in the gure, enhanced SHDSL (both 16TCPAM and 32 TCPAM) offers signicantly better spectralcompatibility with VDSL2-138 than VDSL2-552. In quan-titative terms, the difference in the VDSL2-138 down-stream rate between the SHDSL-32PAM case and VD-SL2-552 STD case ranges from 1.5 2.5 Mbps for thewaterlled VDSL2-552, the difference is 1.7 5 Mbps.

Also note that in this case, the waterlled spectrum doesnot improve the spectral compatibility of VDSL2-552.There is sometimes a myth that a waterlled spectrumresults in better spectral compatibility. As we see here,

that is not always the case.

Spectral compatibility with ADSL2+

The nal comparison well make in this spectral com-patibility section is to measure the impact on ADSL2+,another technology often used for triple-play services.Just as with ADSL and VDSL2-138, enhanced SHDSL

offers better spectral compatibility with ADSL2+ thanVDSL2-552.

Note that in this case, there is no discernible differencebetween the waterlled and standard spectrum with VD-

Downstream VDSL2-138 Performance (12VDSL2, 12Symmetric)

13500

16000

18500

21000

23500

26000

3 3.5 4 4.5 5

Distance (Kft)

Rate(Kbps

)

VDSL2-552 WF

VDSL2-552 STD

SHDSL-16PAM

SHDSL-32PAM

Figure 4. Downstream VDSL2-138 performancewith symmetric at 3.4 Mbps

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SL2-552 their impact is almost identical.

Quick summary on spectral compatibility

The point here is not that VDSL2 is spectrally unfriendly.As we saw in Figure 1, any technology can be used ormisused depending on how it is deployed. The ADSL2+,

VDSL2-138, and ADSL downstream rates are all betterwith enhanced SHDSL providing the symmetric serviceinstead of VDSL2-552, but not always extraordinarily so.The difference is generally on the order of 10%. How-ever, we hope these examinations dispel the myth thatVDSL2-552 somehow offers better spectral compatibilitythan other technologies it clearly does NOT offer bet-

ter spectral compatibility with triple-play services.The simulations of this section were done with vanillaVDSL2-552 and vanilla enhanced SHDSL. We couldcontinue these investigations to look at some of themore advanced possibilities of DSL things like spectralshaping, DSM, power back-off, ... And we will talk aboutthose things a little bit later. Those advanced optimiza-

tion techniques can be applied to many xDSL technolo-gies to improve performance or spectral compatibility. Aswe saw with the differences between VDSL2-552 WF andVDSL2-552 STD, there is often a trade-off between spec-tral compatibility and performance. The promise of betterperformance does not always translate into reality whenrealistic deployments are considered. And with any of

these advanced tuning mechanisms, there is an increasein cost and complexity. Whether the overall trade-off is abenet to the carrier is really up to the carrier to decide,but - as well see - nothing comes for free.

Figure 5. Downstream ADSL2+ performancewith symmetric at 3.4 Mbps

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Performance of Symmetric

Broadband AlternativesIn addition to how symmetric technologies affect theasymmetric triple-play technologies used for video ser-vices, it is also important to look at how the symmetrictechnologies themselves perform in that environment.In this section we examine the performance capabilities

of various symmetric broadband alternatives in realisticoutside plant environments.

Just as in the previous section, there is sometimes amyth around the symmetric performance of VDSL2based on one specic case. For example, Figure 6 issometimes used to demonstrate the symmetric capabili-ties of various technologies.

As before, the data in this graph is completely true. Un-der the situation as depicted, the VDSL2 performanceis very impressive. Why even consider anything else?Well, unfortunately, the situation depicted in this graphis even more unrealistic than it is impressive.

This case assumes that the only disturber in the envi-

ronment is the symmetric technology under study (selfdisturbers only). This will hardly ever be the case in theoutside plant. In fact, in most deployment scenarios,asymmetric technologies will be more prolic than thesymmetric technologies in the binder. It is important toconsider the performance of the technology in a mixedenvironment, where ADSL, VDSL-138, and even other

symmetric disturbers are present in the binder. Thesimulations throughout this section deal with these re-alistic mixed disturber environments, and how varioustechnologies are affected by each other.

In this section, since were looking at performance data,we actually expand the distances under study to fullCSA distance (12 Kft). The distances beyond 5 Kft aregenerally not eligible for triple-play services, but theyare used to deliver high-bandwidth Internet access andother important services.

Figure 6. The MYTH: symmetric performancein SELF-only environment

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In our examinations of symmetric VDSL2 performance,we will always use the standard spectrum e.g. theone that provides the best performance. As we sawin the previous section, this performance may not beachievable while optimizing for spectral compatibility.

So in general, the performance of VDSL2 as shown inthis section is signicantly overstated compared to whatcarriers would actually deploy. Even with those over-statements, the VDSL2 symmetric performance is notall that great.

The Effect of ADSL

The rst case we consider is the case where the sym-metric technology has to co-exist with other asymmetrictechnologies. Figure 7 shows the performance of VDSL2and enhanced-SHDSL in an environment that has ADSLin addition to the symmetric technology being consid-ered. As you can see from the graph, in this realisticmixed environment, the better performing technology

depends upon the distance under consideration. For sit-uations less than 3500 ft, VDSL2 alternatives offer thebest performance. This is not surprising given VDSL2is designed as a high-rate, short distance technology.Enhanced SHDSL, however, has better symmetric per-formance than both VDSL2-276 and VDSL2-552 for thedistances between 4 and 8 Kft. After 9 Kft, VDSL2-276

can actually have slightly better performance, while theVDSL2-552 symmetric performance never exceeds thee-SHDSL performance at distances beyond 4 Kft. Ingeneral, the performance of all of these technologies inthe range 4 -9 Kft is almost identical, with VDSL2-552falling quickly after 9 Kft.

This may seem counterintuitive to many people. Af-ter all, why would traditional ADSL impact symmetric

VDSL2 so thoroughly? The reason is pretty simple byhaving a signicantly different bandplan, in particularUS0 cutover, VDSL2-552 and ADSL signicantly overlapin frequency. ADSL is trying to use the frequencies be-

Figure 7. Symmetric performance in a residential environment

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tween 138 KHz and 276 (or 552) KHz for downstreamtrafc, while VDSL2-276 (or VDSL2-552) is trying to usethe overlap frequencies for upstream trafc. This over-lap creates a signicant conict region which results invery high near-end crosstalk (NEXT).

The other interesting result is that VDSL2-276 actu-ally has the slightly better symmetric performance inthis particular mixed environment beyond 9 Kft. Thedifference between the performance of VDSL2-276and VDSL2-552 is that the region of US0 overlap withADSL DS1 is much smaller, which is much better for theperformance of both ADSL and VDSL2-138. So in this

mixed ADSL/symmetric environment, SHDSL has thebetter performance in the mid distances (4-9 Kft), whileVDSL2-276 can offer slightly improved performance atvery long or very short distances.

The Business Environment

Figure 7 looked at the performance when the symmetric

technology shares a binder with traditional asymmetrictechnologies, what may be found in a residential deploy-ment environment. The next step in the mixed disturberenvironment is to consider what happens when tradi-tional symmetric technologies, such as HDSL and T1,are included in the mix, to recreate a business deploy-ment environment.

In Figure 8, we introduce a very common traditional sym-metric disturber (HDSL) into the mix. Since the sym-metric technologies are traditionally not as numerous asthe asymmetric technologies (theres more residentialconsumers than business consumers), we introduce asmaller number of traditional symmetric disturbers.

As the gure shows, the impact of HDSL on the longer

reach VDSL2 performance is signicant. No longer isVDSL2-276 the highest performing symmetric technol-ogy at long distances. It is still the case that at veryshort distances, VDSL2-552 and VDSL2-276 offer the

Figure 8. Symmetric performance in a business environment

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best performance, but from 4 Kft on out, enhanced SH-DSL offers clearly better symmetric performance. Ingeneral the performance gain of enhanced SHDSL is10-50% over VDSL2-552. To put the improvements inclearer perspective, weve added Figure 9 that shows

the symmetric performance from 5 -12 Kft where thedifferences become more apparent.

The natural question to ask is, of course, how importantis this case? After all, ADSL is much more prevalentthan HDSL, so why worry even about it?

The answer is pretty simple its because the busi-ness environment is different than the residential en-

vironment. If the symmetric technology is to serve thebusiness customers, then this case is very important.In fact, it is probably more important than the simpleADSL case discussed earlier. Most businesses today areserved with some type of symmetric technology, withHDSL being one of the most prominent, delivering a T1or E1 service. If the symmetric technology under study

is going to be serving these same ofce parks and build-ings that are already getting some type of T1 service,then it will predominantly be deployed in binders withsome other symmetric technology. So if were talkingabout business services, this case is vital! And enhancedSDHSL is clearly the best performing technology whenco-existence with other business services is required.

The dreaded T1

There is one other type of disturber to bring into themix, the AMI-based T1 service. AMI as the rst technol-ogy used to deliver T1 services. The technology wasincredibly robust with very strong performance, but thatperformance came with a penalty the technology isvery noisy.

AMI-based T1s are rarely deployed today, but they stillexist in the outside plant, and will be serving businesscustomers now and for years to come. They are an ugly

Figure 9. Close-up look at symmetricperformance in a business environment

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Additionally, different repeater deployment guidelines forAMI-based T1s and HDSL-based T1s may cause changesat more places than just the endpoints new repeatersmight need to be deployed, and new repeater encasingsrequired. At the end of the day, its just complex and

costly to replace or move any existing service, especiallywhen there are better alternatives.

But such complex steps will be absolutely necessary ifVDSL2 is used for a symmetric business service. Thereal question iswhy? Why bother to go through all ofthat complexity and cost? There are already other cost-effective technologies that offer better spectral compat-

ibility and better performance in realistic business envi-ronments.

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Figure 11 shows the impact of adaptive power back-off,and how it improves the spectral compatibility of en-hanced SHDSL (at 3.4 Mbps) with VDSL2-138 (similarresults are obtained with ADSL and ADSL2+). First, ascan be seen, enhanced SHDSL has about the same im-

pact on VDSL2-138 as VDSL2-138 itself in this triple-playrange. When power back-off is applied, enhanced SH-DSL has less impact on VDSL2-138 than VDSL2-138 itself(except at 5 Kft where the impact is marginally different).Note that enhanced SHDSL with power back-off is alsomore spectrally friendly than VDSL2-276 (and VSDL2-552 which was shown in earlier graphs).

Figure 12 examines a similar case with enhanced SHDSLrunning @ 2.5 Mbps. In this case, enhanced SHDSL isalways more friendly with VDSL2-138 than VDSL2-138itself, sometimes signicantly so. So a 10 Mbps servicecan be deployed over 4 bonded pair in a way that impactsVDSL2-138 less than deploying more triple-play services!

As can be seen from the graph, enabling power back-offon enhanced SHDSL clearly has impressive spectral com-patibility. And this does not impact the longer reach per-formance all were doing is capping a maximum rate.

Note that similar power back-off mechanisms can be ap-plied to VDSL2; we havent done that analysis yet. How-ever, given that enhanced SHDSL has better symmetricperformance, it generally has more extra power that

Downstream VDSL2-138 Performance (12VDSL2-138 12Symmetric)

15500

17500

19500

21500

23500

25500

27500

3 3.5 4 4.5 5

Distance (Kft)

Rate(Kbps)

24 VDSL2-138

SHDSL-32PAM PBO @ 3.4M

SHDSL-32PAM @ 3.4M

VDSL-276 STD

Figure 11. Spectral compatibility with VDSL2-138with and without power back-off (3.4 Mbps)

Figure 12. Spectral compatibility with VDSL2-138with and without power back-off (2.5 Mbps)

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can be reduced, which should result in better improve-ments from power back-off with enhanced SHDSL thanwith VDSL2.

The last point to make in this section is on the simplic-ity of the power back-off as it applies to enhanced SHD-

SL. With VDSL2, there can be thousands of frequencies,each with thousands of parameters that can be twiddled,tweaked, and adjusted to change the performance char-acteristics. With enhanced SHDSL, the parameters andalgorithms are much simpler, resulting in a more reliable,cost-effective optimization mechanism.

These graphs should indicate to the reader that DSM

techniques are not restricted to VDSL2, and that whenapplied to enhanced SHDSL with the same vigor as theycan be applied to VDSL2, the spectral compatibility im-provements can be dramatic.

Realistic DSM Considerations

Multiple Input Multiple Output (MIMO) and Cross-Talk

Cancellation (CTC) are potential variations of DSM thatrequire coordinated signaling across multiple pairs. Notethat neither MIMO nor CTC is a required, or even impor-tant, aspect of DSM. DSM is about efcient spectrum use,and that can be accomplished without these variations.

Both approaches are similar in that they require veryspecifc circumstances to have a noticeable impact, but

when the stars align and the circumstances are just right,they can yield a signicant performance boost. One dif-culty with these techniques is of course getting the starsto align in the existing outside plant, when you often havelittle control and little knowledge of how the transmissionproperties of one pair affect the transmission propertiesof another.

So although these techniques can hold a lot of promise,there is still a great deal of difculty in the realization ofthat potential. A few quick examples of where the real-ized implementations of MIMO/CTC fall short are below.

1)MIMO & CTC have little effect when pairs are in dif-ferent binders. Unfortunately, carriers rarely know

which pairs are in which binders, so its impossibleto plan on improvements from cross-talk coor-dination. If the coordinated pairs are in differentbinders, theres no benet. If theyre not heavilyaffecting one another, then, again, theres no ben-et.

2)MIMO & CTC have little effect in the normal alien

environment. As we discussed earlier, todaysservices have to exist in a very mixed environment,with ADSL, ADSL2+, VDSL2-138, HDSL, SHDSL,T1, etc. If a carrier replaced everything in a binder

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with coordinated transmission, then there couldbe enormous benets. But in dealing with exist-ing technologies, and when it is a non-dominanttechnology in the binder, crosstalk coordination isalmost completely ineffective because the majority

of noise is not coordinated.3)MIMO implementations are NOT VDSL2 and DO

NOT have the same spectral properties. There isoften confusion as to the distinction between MIMOand VDSL2. To be clear, VDSL2 has nothing to dowith MIMO, and MIMO is completely different thanVDSL2. The proprietary MIMO implementations

available on the market today are not VDSL2 imple-mentations, will never interoperate with standardVDSL2, and have not been certied for deploymentin carrier networks.

4)Tight coordination lowers resiliency. Crosstalk co-ordination implementations do not handle changeswell they often take down the entire connection to

the customer. Because of the tightly coupled natureof the technology, changes cannot be handled on alocal basis. Noise on one pair has an effect on thatpair, which has an effect on all pairs coordinatedwith that pair. When everything is tied together,every problem becomes a global problem.

5)Cross-talk coordination needs to be a modem func-

tion. Cross-talk coordination has the potential toincrease throughput in the right circumstances. Butat the end of the day, its just another modem func-tion. DSL chip vendors are already implementingcross-talk and MIMO functionality into the next gen-eration xDSL silicon. As chip vendors integrate thisfunction in cost and space effective architectures,

some of the currently insurmountable obstacles inthe way of useful coordinated crosstalk implemen-tations could be alleviated.

So crosstalk coordination is a promising technology, but italso has its problems at least at present. Today it has avery high cost, and offers unpredictable performance re-sults that depends on how the pairs are correlated, what

alien disturbers are in the binder, and on whether T1/E1disturbers are there or not. A technology with high costand unpredictable results is almost impossible to acceptfor the enterprise environment, where carriers want toestablish simple deployment guidelines that can quicklyand reliably determine which customers can be reachedwith which services. With crosstalk coordination, you

dont know what you are going to get until you try. Ifcrosstalk coordination proves truly useful, it will becomea standard modem function, allowing cost-effective, in-teroperable implementations.

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Conclusions

This paper is intended to provide a factual basis bywhich symmetric technologies can be compared in bothperformance and spectral compatibility.

The spectral compatibility results show, that when usedproperly, enhanced SHDSL offers better spectral compat-

ibility with triple-play services than VDSL2-552, whetherthat triple-play service is offered with ADSL, ADSL2+, orVDSL2-138. This is true whether VDSL2-552 uses thestandard or waterlled spectrum.

Additionally, when power back-off is applied to enhancedSDHSL, it turns out that enhanced SHDSL is more com-

patible with these triple-play technologies than the tri- ple-play technology itself. As we have shown, withinthe triple-play range of 3-5 Kft, VDSL2-138 will performbetter with 12 VDSL2-138 and 12 enhanced SHDSL dis-turbers than with 24 VDSL2-138 disturbers only. This isa remarkable result!

Finally, in addition to how a symmetric technology im-

pacts triple-play services, we also examined how thesymmetric technology performs in more realistic mixedenvironments than are often investigated. At short dis-tances, VDSL2 will always offer the best performance.But at distances beyond 4 Kft, enhanced SHDSL tendsto perform better, signicantly so in the business envi-ronment.

We hope this paper sheds light on the reality of spectralcompatibility and symmetric performance. xDSL tech-nologies have been designed for specic applications,VDSL for short reach applications, SHDSL for mid tolong reach business applications. Each technology hasa place in the network, and can offer the best perfor-mance for its role.

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Appendix

This appendix details the simulations used in this whitepaper so that the educated reader can validate and re-produce these results.

Cable Models

Transmissions lines: 26AWG @ 70 C, Primary con-stants from Table 5/G.996.1(2001)

NEXT noise coupling: One Piece NEXT model, T1.417/A.3.2.1.1

FEXT noise coupling: Far end crosstalk model, T1.417/A.3.2.2

Insertion loss (DMT): ABCD parameter models using100 termination

Insertion loss (SHDSL): 135 termination for SHDSL

Transmitter Power Spectral Density (PSD) Masks

ADSL upstream: G.992.1/Figure A-3 reduced by 3.5dB

ADSL downstream: G.992.1/Figure A-2 reduced by3.5dB

ADSL2+ upstream: G.992.5/Figure A-3 reduced by3.5dB

ADSL2+ downstream: G.992.5/Figure A-2 reduced by3.5dB

VDSL2-138 upstream: G.993.2/Figure A-2, EU-32reduced by 3.5dB

VDSL2-138 downstream: G.993.2/Figure A-4, D-32reduced by (3.5+6)dBm to yield ~14.5dBm

VDSL2-276 upstream (standard): G.993.2/Figure A-2,EU-64 reduced by 3.5dB

VDSL2-276 downstream (standard): G.993.2/FigureA-4, D-64 reduced by (3.5+6)dBm to yield ~14.5dBm

VDSL2-552 upstream (standard): G.993.2/Figure A-2,assumed shape for future EU-128 reduced by 3.5dB

VDSL2-552 downstream (standard): G.993.2/Fig-

ure A-4, assumed shape for future D-128 reduced by(3.5+6)dBm to yield ~14.5dBm

VDSL2-276 upstream (waterflled): -39.9dBm/Hz from0 to 276kHz. Total power = 14.5dBm

VDSL2-276 downstream (waterflled): -51.2dBm/Hzfrom 276 to 3750kHz. Total power = 14.5dBm

VDSL2-552 upstream (waterflled): -42.9dBm/Hz from0 to 552kHz. Total power = 14.5dBm

VDSL2-552 downstream (waterflled): -51.2dBm/Hzfrom 552 to 3750kHz. Total power = 14.5dBm

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SHDSL-16PAM: Nominal Transmitter PSD fromG.991.2/A.4.1

SHDSL-32PAM: Nominal Transmitter PSD fromG.991.2/F.4

HDSL: G.991.2/A.3.3.1AMI T1: G.991.2/A.3.3.2

DMT Receiver Characteristics

Noise oor: -140dBm/Hz

Coding gain: 5dB

Implementation margin: 0dB

Margin: 6dB

Shannon gap: 9.75dB

Capacity calculation: using method of T1.417/A.3.6

Bit loading (ADSL, ADSL2+): 2 to 15

Bit loading (VDSL2): 1 to 15

Upstream tones:

ADSL/ADSL2+: 6 to 31

VDSL2-138: 7 to 31, 870 to 1205

VDSL2-276: 7 to 63, 870 to 1205

VDSL2-552: 7 to 127, 870 to 1205

Downstream tones:

ADSL: 33 to 128

ADSL2+: 33 to 256

VDSL2-138: 33 to 869

VDSL2-276: 65 to 869

VDSL2-552: 129 to 869

SHDSL Receiver Characteristics

Margin: 5dB

Coding gain: 5dBImplementation margin: 0dB

Required SNR (16PAM): 27.71dB

Required SNR (32PAM): 33.80dB

Folded SNR: calculation method via T1.417/A.3.4

We hope this information is useful to the reader in vali-dating the results of this paper.

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About Hatteras Networks

Hatteras Networks is headquartered in Research Trian-gle Park, North Carolina and was started several yearsago to develop high-performance telecommunicationsaccess platforms. Hatteras was formed to enable tele-communications service providers to deliver cost-effec-

tive, service-rich, high-speed access in the last mile tobusiness customers - one of the most difcult challengescarriers face today. Removing this critical bottleneck inthe access network is necessary to fuel future growthin broadband, multimedia and Internet services. Todate, capital-intensive ber-based solutions have beenthe most widely deployed means for meeting this chal-

lenge.Hatteras Networks provides standards-based Ethernetaccess solutions, which leverage the fully ratied Ether-net in the First Mile (EFM) standards from the IEEE andITU. With Hatteras Networks solutions, carriers can mi-grate from the complexity and expense of TDM-basedT1/E1 circuits, to the simplicity and availability of a pure

Ethernet access platform, all while increasing the band-width to, and revenue potential of each customer.

Hatteras is a founding member of the Ethernet in theFirst Mile Alliance, and was a leader in the developmentof the IEEE 802.3ah standards for delivering symmetri-cal Ethernet services natively over copper access loops.

Hatteras corporate commitment to standards and in-teroperability provides our customers with the certaintythat their capital investments are protected.

Carriers from around the world have deployed HatterasMid-Band Ethernet service solutions, enabling them todrive down access costs by eliminating the cost andcomplexity of ATM and T1/E1 solutions, and increase

revenue with higher bandwidth, value-added services.

Visit www.hatterasnetworks.com for additional informa-tion.

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Extending the Ethernet Service Edge

Hatteras Networks enables an emerging market seg-ment referred to as Mid-Band Ethernet. The Mid-BandEthernet service enables Carriers to deliver the MetroEthernet services over the existing copper infrastructure

to businesses whose application requirements fall withinthe bandwidth gap between T1/E1 and T3/STM-1. Whilethe Hatteras solutions deliver up to 45 Mbps over 8 cop-per pairs, the Mid-Band Ethernet service sweet spot is2-45 Mbps (the gap between T1/E1 and where ber de-ployment becomes economically viable).

Mid-Band Ethernet services are exactly the same asthose enabled by Metro Ethernet:

Transparent LAN Services (TLS)

Direct Internet Access (DIA)

Voice over IP (VoIP)

Ethernet Private Line

Storage Area Networks (SANs)

etc

Therefore, an essential requirement of Mid-BandEthernet equipment is the ability to transparentlyextend existing Metro Ethernet services beyond

fber.

Vertical System Group estimates that in the U.S. andEurope over 2.2 million T1/E1, Frame Relay and T3/STM-1 connections will migrate to Mid-Band Ethernetlinks over the next 5 years. Hatteras Ethernet ServiceEdge solutions enable this new market segment that isexpected to generate over $15B per year in service rev-enue for U.S. and European Carriers. The opportunity inother international markets is likewise compelling.

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