VDSL2: Next important broadband technology - · PDF fileEthernet DSL access (EDA) DSLAM for ADSL2 and ADSL2plus. VDSL Efforts to standardize VDSL (currently de-noted VDSL1) got underway

DSL history

ADSL

While high-bit-rate DSL (HDSL) was stillin prototype phase, Stanford University andAT&T Bell Labs developed asymmetricalDSL (ADSL) technology from concept toprototype (1990-1992). Field technologytrials began three years later and ANSI is-sued the first standard for ADSL in 1995(T1 413 issue I); the second issue followedin 1998. The first ADSL recommendationfrom ITU-T (G.992.1), generally denotedADSL1, was complete in 1999.1 This rec-ommendation was based to a large extent onthe ANSI standards.

ADSL was originally intended for deliv-ering video on demand at a bit rate of 8Mbpsdownstream and 640kbps upstream. But itwas the popularity of the internet that madeADSL a major commercial success. In fact,ADSL is today mainly used as a form of high-speed internet access.

An option in the ADSL1 standard pro-vides for a downstream data rate of up to12Mbps. Moreover, plain old telephony ser-vice (POTS) or integrated services digitalnetwork (ISDN) technology can serve as theunderlying service (by not using the fre-quencies occupied by their respective ser-vices: 0.3-25kHz for POTS or 1-120kHz forISDN). A splitter filter can be used to sep-arate the POTS band from the ADSL band.This means ADSL can share the line with ei-ther POTS or ISDN service. Figure 1 showshow the frequency band is divided betweenPOTS/ISDN upstream and downstreamdata.

ADSL2

The second-generation ADSL standards(ADSL2 and ADSL2plus) were issued in2002 and 2003.2 - 3 The most important newfeatures of ADSL2 (G.992.3) were• an annex with extended upstream, which

made it possible to have an upstream datarate of up to 3Mbps; and

• an annex for extending the reach to morethan 5km.

ADSL2plus

The ADSL2plus standard (G . 9 9 2 . 5 ) dou-bled the spectrum for downstream data(ADSL and ADSL2 have a spectrum of1.1MHz; ADSL2plus has a spectrum of2.2MHz), giving even greater data rates onshort loops (Figure 1). ADSL2plus also de-fined a toolbox for sculpturing the down-stream transmission to meet different spec-

36 Ericsson Review No. 1, 2006

ADSL2plus is currently being deployed worldwide as the new mainstream

broadband technology for residential and business customers. But at the

same time, the industry is gearing up for the next step of the DSL evolu-

tion: VDSL2. This second version of the very high-speed digital subscriber

line (VDSL) standard from ITU-T promises to deliver 100Mbps symmetrical

traffic on short copper loops.

The greater bandwidth of VDSL2 gives telecommunications operators

the ability to offer advanced services such as multiple streams of interac-

tive standard and high-definition TV over IP over the existing copper

plant. TV services are fast becoming strategically important to telecom-

munications operators who must now compete head-to-head with cable

operators launching voice over IP (VoIP) and high-speed internet services.

The introduction of VDSL2 will have a major impact on the way access

networks are engineered. To make the most of VDSL2, operators will have

to move the DSL access multiplexers (DSLAM) out of the central office

environment and build a distributed network with smaller nodes that sit

typically less than 1500 meters away from end users. This puts more strin-

gent requirements on outside plant building practices. In many cases,

power and spacing might also be an issue because existing street-side

cabinets lack space and often solely contain passive equipment.

Although fiber to every home is the ultimate answer, it is not yet an eco-

nomically viable solution for overbuilding existing copper networks. This

is because fiber takes a long time to deploy and the cost of deployment

runs between USD 1,000 and 1,800 per subscriber. However, in Greenfield

building scenarios, fiber to the home (FTTH) is frequently seen as the best

way forward.

VDSL2: Next important broadband technology

Per-Erik Eriksson and Björn Odenhammar

AAL5 ATM adaptation layer 5

ADSL Asymmetric DSL

ANSI American National Standards

Institute

ATM Asynchronous transfer mode

BRAS Broadband remote access server

CPE Customer premises equipment

DELT Double-ended line test

DHCP Dynamic host configuration

protocol

DMT Discrete multitone

Docsis3 Data-over-cable service interface

specifications

DSL Digital subscriber line

DSLAM DSL access multiplexer

EDA Ethernet DSL access

ETSI European Telecommunications

Standards Institute

FDD Frequence-division duplex

FTTB Fiber to the building

FTTCab Fiber to the cabinet

FTTEx Fiber to the exchange

FTTH Fiber to the home

FTTN Fiber to the node

HDSL High-bit-rate DSL

HDTV High-definition TV

HFC Hybrid fiber copper

IEEE Institute of Electrical & Electronics

Engineers

IFFT Inverse fast Fourier transform

IP Internet protocol

INP Impulse noise protection

ISDN Integrated services digital network

ITU-T International Telecommunication

Union – Telecommunication

Standardization Sector

OC-3 Optical carrier 3

OFDM Orthogonal frequency-division

multiplexing

PHY Physical link

PMD Physical media-dependent

PMS-TC Physical media-specific –

transmission convergence

PON Passive optical network

POTS Plain old telephone service

PTM Packet transfer mode

PVC Permanent virtual connection

QAM Quadrature amplitude modulation

QoS Quality of service

SAR Segmentation and reassembly

(block processes)

SELT Single-ended line test

SHDSL Symmetrical high-bit-rate DSL

SNR Signal-to-noise ratio

STM Synchronous transfer mode

TPS-TC Transport protocol-specific –

transmission convergence

TV Television

US0 First upstream band

UTOPIA Universal test and operations PHY

interface for ATM

VDSL Very high-speed DSL

VLAN Virtual local area network

VoIP Voice over IP

BOX A, TERMS AND ABBREVIATIONS

Ericsson Review No. 1, 2006 37

trum capability requirements, in particularwhen ADSL2plus is put in a cabinet.

Figures 2-3 show the performance, dur-ing different noise conditions, of Ericsson’sEthernet DSL access (EDA) DSLAM forADSL2 and ADSL2plus.

VDSL

Efforts to standardize VDSL (currently de-noted VDSL1) got underway in 1995. ITU,ETSI and ANSI (T1E1.4) each carried outsimultaneous projects. In 1997, a group ofoperators belonging to the Full-service Access Network organization specified theend-to-end requirements for VDSL. Theprocess later stalled, however, due to discordregarding two competing line-code tech-nologies: • single carrier, which uses quadrature am-

plitude modulation (QAM); and • discrete multitone (DMT). Likewise, major efforts to complete theADSL2 and ADSL2plus standards movedwork onVDSL standardization to a backseat. As a result, proprietary implementa-tions of VDSL-QAM and VDSL-DMT weredeveloped and deployed in limited volumesin a few markets.

In 2003, eleven major DSL suppliersjointly announced their support for DMTline coding, in particular because it facili-tates greater interoperability and is morecompatible with existing ADSL installa-tions. This decision was also influenced byIEEE’s efforts to standardize Ethernet overVDSL as an element of the Ethernet in thefirst mile (EFM) standard defined in IEEE802.3ah. A clear objective of the EFM stan-dard was to adopt a single line code in co-operation with established DSL standard-ization bodies. This started a “VDSLOlympics” of sorts in which the performanceof VDSL-QAM was tested against that ofVDSL-DMT in independent labs run byBritish Telecom in the UK and TelcordiaTechnologies in the USA. VDSL-DMT out-performed VDSL-QAM and was thus adopt-ed by IEEE and ANSI. ITU-T SG 15/4 bycontrast included both QAM and DMT inthe VDSL1 standard but stipulated that • all future evolution of VDSL technology

would be based on DMT; and• a new standard, VDSL2, should be de-

fined.4

The scope of the VDSL2 standard is quitebroad. Its goals are to increase performanceover longer loops (longer than VDSL1), asan evolution from ADSL2plus, and veryshort loops, as an evolution from VDSL1.

VDSL1 occupies spectrum from 138kHzto 12MHz. The VDSL2 spectrum has beenexpended both upward and downward,using spectrum from 25kHz to 30MHz. The

Figure 1ADSL/ADSL2/ADSL2plus frequency allocation for POTS, ISDN and extended upstream

(Annex M) application.

Figure 2Capacity of ADSL2 over POTS on 0.4mm cable with 24 DSL disturbers.

key to increasing performance over longloops lies in the use of spectrum from 25kHzto 138kHz. Similarly, the key to increasingperformance over short loops lies in the useof spectrum from 12MHz to 30MHz.

The DSL industry successfully transi-tioned from ADSL to ADSL2plus, doublingthe spectrum while maintaining or improv-ing line density and cutting costs. The moveto VDSL2 from ADSL2, however, decreasesline density due to a significant increase inspectrum. A transition from VDSL1 toVDSL2 can take place without a loss in linedensity.

Perhaps the most important aspect of theVDSL2 standard is that it uses Ethernet asmultiplexing technology (64/65 encapsula-tion) in the first mile. The elimination ofATM in the first mile means the access ar-chitecture can be simplified into an end-to-end Ethernet access architecture that usesvirtual local area networks (VLAN) as theservice-delivery mechanism across the entireaccess network.6

Ericsson is a driving force behind the ar-chitectural transition to high-performancebroadband solutions that will propel broad-band from mere high-speed internet accesstechnology to a complete suite of IP-basedservices, such as IP telephony and IPTV.Ericsson introduced the first IP-based DSLaccess solutions in 2002 and has sinceworked to refine the architectures requiredto support the enhanced scope of service.5

VDSL2 technology

Using input from the ANSI and ETSI stan-dards, the ITU began drafting its VDSL2standard (G.993.2) in January 2004. Con-sensus for the standard was reached at ameeting in Geneva in May 2005. As withADSL/2/plus, the underlying modulation inthe VDSL2 standard is discrete multitone(DMT). VDSL2 is based on both theVDSL1-DMT and ADSL2/ADSL2plus rec-ommendations. Therefore, it is spectrallycompatible with existing services and en-ables multimode operability withADSL/2/plus.

DMT modulation

DMT modulation uses the same principle asorthogonal frequency-division multiplex-ing (OFDM).6 That is, it divides the usefulfrequency spectra into parallel channels,where the center of each channel is repre-sented by a modulated (QAM) subcarrier(Figure 4). One difference from OFDM is


Figure 3

Top: Downstream capacity of ADSL2plus over POTS with ETSI noise on 0.4mm cable.Bottom: Upstream capacity of ADSL2plus with ETSI noise on 0.4mm cable.


that each carrier in DMT can be loaded witha different number of bits, depending on thesignal to noise ratio (SNR). In OFDM, theconstellation size of each carrier is the same.Because each subcarrier is orthogonal to theother subcarriers, there is no interference be-tween subcarriers. The number of bits canbe varied between 1 and 15. The distancebetween subcarriers is 4.3125kHz. InVDSL2 a distance of 8.6125kHz may alsobe used. Inverse fast Fourier transform(IFFT) is used to generate the subcarriers.

Band plans

ADSL can be described as a two-band sys-tem where one part of the frequency spec-trum is used for upstream transmission andthe second part is used for downstreamtransmission (Figure 1). VDSL, on the otherhand, uses multiple bands for upstream anddownstream transmissions to enable agreater degree of flexibility with regards torate configurations and symmetry betweenupstream and downstream data.

Two band plans were defined (in 2000) tomeet operator requirements for symme-try/asymmetry (Figure 5). The first of these,Band Plan 998, better facilitates asymmet-ric services, whereas Band Plan 997 accom-modates symmetric services. VDSL1 sup-ports a bandwidth of up to 12MHz; inVDSL2 this can be extended to 30MHz. Tobe spectrally compatible with VDSL1,VDSL2 uses the same band plans below12MHz. VDSL2 can employ up to 4,096subcarriers. Depending on the band plan inuse, a subcarrier is designated for either up-stream transmission or downstream trans-mission. As in ADSL, the lower part of thespectra is allocated for POTS and ISDN ser-vice and a splitter filter is used to separatethe POTS or ISDN frequencies from theVDSL2 band. An “all digital mode” optionalso exists, where virtually all the spectra canbe used for VDSL2.

Duplexing

Today’s deployments of ADSL/2/plus usefrequency-division duplex (FDD) technolo-gy to separate the upstream band from thedownstream band. Given the physical prop-erties, however, it is not possible to create a“brick wall” transmission band. That is,there is always some spectral leakage be-tween the transmission bands. In ADSL, fil-ters or echo cancellers are used to suppressleakage between transmission bands. VDSL,on the other hand, uses a digital duplexingtechnique based on the “zipper” technology

Figure 4Discrete multitone (DMT): By employing DMT technique one can divide the useful spectra

into parallel channels where each channel is represented by a quadrature-amplitude-mod-ulated tone. The signal-to-noise ratio (SNR) value indicates the number of bits with which

each tone (1, 2, 3 … n) can be loaded.

Figure 5The band plans of VDSL1 and VDSL2.

invented at Telia Research in Luleå.7 Withthis technique, adjacent subcarriers carrydata in opposite directions (Figure 6). How-ever, requirements for spectral compatibil-ity with existing DSL technologies necessi-tate that several tones must be grouped intotransmission bands. One can preserve the or-thogonallity between the received signaland the transmitted signal echoed back intothe receiver by cyclically extending thetransmitted DMT symbols using a cyclicprefix and cyclic suffix and by synchroniz-ing the transmitters at each end to begintransmitting at the same time (a techniquecalled timing advance). Cyclic extension,which eliminates intersymbol interference(ISI) caused by the channel, reduces the datarate by 7.8%. Windowing, a technique forsuppressing side lobes, further reduces spec-tral leakage between transmission bands.Windowing is also used in OFDM (Figure 7).

Network evolution when

introducing VDSL2Going forward, the introduction of VDSL2technology will account for only a smallpart of the fundamental changes affectingnetwork architecture. Today, the lion’sshare of installed access lines is situated incentral office environments. Likewise, theaverage length of existing copper loops iswell beyond the “sweet spot” for derivingadded value from VDSL2. Many operatorshave thus already started to consider fiberdeeper into the access network – for exam-ple, fiber to the node (FTTN) and fiber tothe curb (FTTC) – shortening the lengthof copper loops to less than 1500 meters.

The second major shift is the introduc-tion of Ethernet as packet technology allthe way to the end user. ADSL2/plus em-ploys ATM in the first mile but IP/Ethernet-based DSLAMs deploy Ethernetin the second mile. Many legacy broadbandsolutions also use STM1/OC-3 on the ag-gregate side.

The shift toward Ethernet and FTTN/FTTC, and the introduction of additionalservices, will lead to a change in the serviceselection point in the network (currentlythe broadband remote access server,BRAS).

FTTN architecture

The push for an FTTN architecture forVDSL2 was initiated by operators in NorthAmerica, where long copper loops and theearly introduction of HDTV call for a steep

increase in broadband capacity. By situat-ing VDSL2 nodes close to subscribers, op-erators can boost capacity enough to supportmultiple HDTV streams to a householdwithout having to replace the entire copperinfrastructure with fiber. The capacity de-livered to each home is on a par with that ofa shared fiber architecture, for example, pas-sive optical network (PON) and hybrid fibercopper (HFC) alternatives (Table 1).

The FTTN architecture is also highly dis-tributed to accommodate a smaller numberof subscribers per site as DSLAMs are movedcloser to end users. A large number of dis-tributed nodes calls for an automated customer-activation process that enables op-erators to activate a new subscriber withouthaving to physically visit an FTTN location.Many operators eye the FTTN architectureas an attractive tool for effectively compet-ing with cable and fiber-access alternatives.

Challenges of outside plant

deployments

Moving the DSLAM out of the central of-fice poses several new technical challenges.The temperature and environmental re-quirements, for example, are much morestringent. Present-day installations primari-ly operate in a central office environment witha typical temperature range of -5°C to +45°C.Outdoor plant deployments, on the otherhand, must be able to operate in non-climate-controlled environments where temperaturescan range from -40°C to +65°C.8

Power is another area that will be af-fected. Most operators currently use -48Vpower solutions. In most outdoor deploy-ments, however, local power will not be anoption. Instead, one must consider remotepower (over the existing copper). In met-ropolitan areas, DSLAMs can be deployedin basements of multidwelling units withaccess to local power.

Because there are fewer subscribers persite, many vendors will have to revise the architecture behind DSLAM control me-chanisms. At present, numerous DSLAMsare optimized for large sites where a com-mon control blade supports an entire cabi-net (more than 1,000 subscribers). But witha maximum of, say, 200 subscribers per site,this architecture is not cost-effective.

Ethernet in the first mile

With VDSL2, Ethernet is the basic packettechnology all the way out to end users. Thischange affects the way in which networksare built. A direct benefit of the change is


Figure 7

The “zipper” principle of alternatingupstream and downstream transmissions

to avoid spectral leakage between trans-mission bands.


reduced overhead. The elimination of per-manent virtual connections (PVC), whichare an inherent part of ATM solutions, opensthe door to new architectures. Ethernet-based features, such as VLAN-per-service incombination with authentication usingDHCP Option 82, can greatly reduce thecosts of operating a network. The simplifiednetwork architecture makes it possible to in-troduce packet transfer mode (PTM) tech-nology. This, and increased requirements todeliver new services with the right QoS, willput new requirements on products and thenetwork architecture. Ericsson has previ-ously outlined many of the architecturalprinciples of IP/Ethernet-based access.5

Ericsson’s product offering

Ericsson has three years’ solid experience ofbuilding broadband architectures that canhandle advanced, high-quality services withsuperior performance. Its EDA product line,which was one of the first true IP DSLAMsto be released to the broadband market, iscurrently in operation in a large number ofcommercial deployments around the world.

From the outset, Ericsson designed theEDA to be a highly scalable and modularDSL solution based extensively on Ethernettechnology. This is precisely what is need-ed for a network in which VDSL2 is the maintechnology. In other words, for Ericsson, thefirst step toward VDSL2 is not very dra-matic: Ericsson designed its EDA to be adistributed system with software supportfor launching new services.

In 2005, to show operators how the tech-nology behaves and what services it sup-ports, Ericsson deployed EDA with VDSL2in live networks. Some features of EDAworth highlighting in the context of VDSL2are scalability, advanced single-ended linetest (SELT) and double-ended line test(DELT) capabilities, QoS, and greater ca-pacity. Vital objectives of the early deploy-ments were• to verify VDSL2 performance advantages

in real network conditions; and• to enable an early start of VDSL2 inter-

operability activities.

Scalability

The EDA VDSL2 nodes can be configuredto different sizes in steps of 12 lines. Oper-ators can thus cost-effectively build out nu-merous small nodes. This proven concepthas been in use since 2002 for deploying alarge number of 12-, 24-, and 36-line sys-tems (ADSL2plus).

Advanced SELT and DELT capabilities

Metallic line-test solutions tailored for cen-tral office use are expensive. What is need-ed instead is a software-based line-test solu-tion that makes key line measurements; inparticular because outdoor cabinets seldomhave space for test heads nor can the cost ofproviding this space be justified.

QoS

The driving force behind VDSL2 is TV ser-vice bundled with voice and data. This willbring an end to the current paradigm ofbuilding best-effort DSLAM solutions. Toeffectively deliver high-quality services,QoS provisions must be built into levels 1through 3 (L1-3). TV services can easily bedelivered using the VLAN-per-servicemodel coupled with features such as Ether-net overload protection and support for duallatency on the physical layer (PHY).

SERVICE BANDWIDTH REQUIREMENT

1-2 HDTV channels 12Mbps each

2-4 regular TV channels 3Mbps each

High-speed internet 8Mbps

VoIP 100kbps

TABLE 1. BANDWIDTH NEEDED TO PROVIDE IPTV SERVICES.

Figure 7

Cyclic extension. A cyclic prefix and cyclic suffix are appended to the DMT symbol. Afterinverse fast Fourier transform (IFFT), a cyclic prefix and cyclic suffix are appended to the

DMT symbol. Samples are copied from the end of the DMT symbol and added to thebeginning of the symbol, which represents the cyclic prefix, here denoted LCP. The cyclic

suffix is created by copying samples from the beginning of the symbol and adding them tothe end of the symbol, here denoted LCS. ß illustrates the portion of the symbol that is win-

dowed and added to ß samples from the end of the previous symbol, which overlaps withthe current symbol.

Greater capacity

EDA uses distributed processing power allthe way out to the line card. When com-bined with high-capacity Gigabit Ethernetuplinks, this feature provides considerablecapacity (packets-per-second throughput).The current rule of thumb for internet ser-vices is an average capacity of 25-50kbps peruser in the aggregation network. Lookingahead, however, it is estimated that capaci-ty must be increased at least 25- to 50-fold.What is more, apart from the challenge ofmerely increasing capacity, one must add ca-pacity for QoS-enabled traffic. Ethernet isthus the only truly cost-effective option forincreasing capacity. Notwithstanding, evenmore advanced Ethernet designs are re-quired to support the high capacity of QoS-enabled traffic.

Competing technologies

Some might argue that fiber to the home(FTTH) technology competes with VDSL2.In most cases, however, the two architec-tures are complementary. The level of FTTxdeployment is dependent on the specificbusiness case and is driven by telecommu-nications operators (as is the case withVDSL2). For operators wanting to migrateto FTTH, the current location of FTTxnodes is a suitable point for splitters or ac-tive fiber access nodes. Today’s main threatto telecommunications operators comesfrom the cable industry where new data-over-cable service interface specifications

(Docsis3) technology promises to deliverhigh-speed solutions that are comparablewith VDSL2.

VDSL2 capabilitiesExtended reach

Whereas the physical reach of VDSL1 is lim-ited to around 1500m on 0.4mm cable, thereach of VDSL2 can be extended to around2400m. As a first upstream band (denotedUS0) VDSL2 can use the same upstream fre-quencies as ADSL/2/plus. This extends thereach of VDSL2 compared with VDSL1. Theuse of US0 requires training sequences sim-ilar to those of ADSL to train equalizers andecho cancellers. For distances in excess of2000-2400m, ADSL2 remains the most ap-propriate choice of DSL access.

Profiles of different deployment modes

The VDSL2 standard is defined using setsof profiles, where each profile targets a spe-cific deployment. Figure 8 depicts the dif-ferent deployment scenarios anticipated forVDSL2. These include • fiber to the exchange (FTTEx): VDSL2 is

located at the central office; • fiber to the cabinet (FTTCab): fiber-fed

cabinets are located near customerpremises; and

• fiber to the building (FTTB): VDSL2 isplaced, for instance, in the basement of abuilding.

Profiles 8a-8b and 12a-12b apply to FTTEx;17a applies to FTTCab; and 30a, to FTTB(Figure 9). Each profile contains support forunderlying baseband services, such as POTSor ISDN.

Figure 10 shows the measured perfor-mance of the EDA VDSL2 with profiles 8a,12a and 17a during typical noise conditions.

Packet transfer mode

In all likelihood, when introducing VDSL2the industry will drop ATM in the first mile,replacing it with Ethernet (64/65 encapsu-lation). At present, the most common solu-tion for transporting Ethernet frames overDSL is bridged IP DSLAM, where Ethernetframes are assembled into ATM adaptationlayer 5 (AAL5) and encapsulated into ATMcells before they are sent to the DSL physi-cal link (Figure 11). The Segmentation andReassembly (SAR) block processes the Eth-ernet frames. The ATM cells are transport-ed over a UTOPIA (universal test and oper-ations PHY interface for ATM) L2 electri-cal interface to an application-specific in-


Figure 8

VDSL2 deployment scenarios: fiber to the cabinet (FTTCab), fiber to the exchange (FTTEx),and fiber to the building (FTTB).


terface called the ATM TPS-TC (transportprotocol-specific – transmission conver-gence). TSP-TC is also sometimes denotedATM-TC, for example, in the context of thexTU-C (xDSL transceiver unit – central of-fice).

A drawback of encapsulating Ethernetframes into ATM cells (Ethernet-to-AAL5-to-ATM cells) is that 64-byte Ethernetframes must occupy two ATM cells. This isbecause the payload size of the 53-byte ATMcell is only 48 bytes. Therefore, one ATMcell carries 48 bytes and the other cell car-ries only 16 bytes. Given the maximum sizeof an Ethernet frame, 1518 bytes, the ATMoverhead is 160 bytes or nearly 10% of thetransmission capacity.

IEEE 802.3ah has defined a specific Eth-ernet TPS-TC using the 64/65-octet encap-sulation for Ethernet applications withoutunderlying ATM. For VDSL1, ITU-T spec-ified a different generic packet transfer mode(PTM). In the ITU-T specification, TPS-TCis denoted PTM-TC.

The VDSL2 standard fully supports PTMbased on 64/65-octet encapsulation. TheIEEE 802.3ah task force defined PTM to en-capsulate Ethernet frames before they aremodulated in the DSL transceiver. The ITU-T SG15/Q4 has defined PTM forVDSL2 as well as for ADSL2/plus and sym-metrical high-bit-rate DSL (SHDSL). Fur-thermore, it has enhanced the 64/65 encap-sulation technique using a preemptionmethod, and added support for non-

Figure 10VDSL2 bit rates of different profiles. Real measurement in the presence of crosstalk from

20 VDSL2 systems.

Figure 9

VDSL2 profiles of Band Plan 997 (the VDSL2 profiles of Band Plan 998 are

similar).

Ethernet packets that are shorter than 64bytes in length. PTM thus makes it possi-ble to eliminate ATM as the layer-2 carrierover the physical layer (Figure 12).

Preemption mechanism

The standard 64/65-octet encapsulationtechnique has been amended with a pre-emption mechanism that allows high-priority frames to interrupt the transmissionof low-priority frames until the high-prior-ity frames have been sent. Transmission ofthe low-priority frames is then resumed. To

understand how this works, let us assumethat a 1518-byte packet with internet traf-fic is being processed when a high-priorityvoice packet arrives. The preemption mech-anism interrupts transmission of the 1518-byte frame, stores its current state, transmitsthe voice packet, and then resumes trans-mitting the packet with internet traffic.

Dual latency

Figure 13 shows a reference model of theVDSL2 transceiver. The TPS-TC serves asan adaptation layer between the transport


Figure 12

The logical blocks of xDSL connected toan Ethernet Network when ATM as layer-2

protocol is replaced with PTM. Thischange eliminates the segmentation-and-

reassembly (SAR) function, simplifying theprotocol stack.

Figure 11

Logical blocks for xDSL connected to anEthernet network when ATM is used as

layer-2 protocol (bridged IP-DSLAM) inthe physical layer (xDSL). Also shown are

the associated protocol stacks in the cen-tral office and customer premises equip-

ment (CPE).


protocols and the digital subscriber line.Ethernet frames or ATM cells are input intoTPS-TC. Among other things, the TPS-TClayer provides the transport mechanism, en-capsulates frames or cells, and decouplesinput and output rates.

The next station is the PMS-TC (physicalmedia-specific – transmission convergence)layer, which provides latency path func-tions. These functions determine the error-protection capability (together with Trelliscoding) and latency. Framing also takesplace in the PMS-TC.

Ordinarily, only one latency path is im-plemented in ADSL2/plus. This is not a lim-itation of the standard, however. Examina-tion of the latency path (Figure 14) showsthat an interleaver used together with Reed-Solomon code creates a powerful error-protection mechanism. However, the inter-leaver introduces delay proportional to itsdepth; that is, if there is only one latencypath and interleaving is used, then every ser-vice experiences the same delay.

Services such as video, which must be im-mune to short bursts of errors, must havegreat interleaver depth. As a consequence,they also experience significant delay. Videoservice is not sensitive to delay, however,provided there is little jitter. Voice andgaming services, on the other hand, do notrequire extensive error protection but theyare very sensitive to delay.

A dual-latency solution provides a secondlatency path in the PMS-TC. Data that mustbe protected uses the interleaved path whiledata that is sensitive to delay can use thepath without interleaving or with only aminimum of interleaving. Eventually, thedata streams from each of the latency pathsare multiplexed into a single bit stream thatis conveyed to the physical media-dependent (PMD) layer for modulation. Thenumber of bits taken from each latency pathand put into one DMT frame is determinedduring initialization. The output from thePMD layer is the analog signal delivered tothe analog front end. Operators have indi-cated that they want the dual-latency optionin VDSL2 in order to provide the QoS re-quired of triple-play services.

Power control for

improved spectralcompatibility

VDSL2 will mostly be used on short loops,pushing DSLAMs further out into the net-

work closer to user premises. It is anticipat-ed that FTTCab deployments will increasein all operator networks. However, servicessuch as ADSL/2/2plus, which are operatedfrom a central office environment and whichshare the binder used for VDSL2 in the cab-inet might experience severe degradation ofdownstream data performance due tocrosstalk from VDSL2 systems. Power con-trol can be used to shape the downstreamVDSL2 signal. This minimizes the impactof crosstalk from VDSL2 systems on centraloffice-based ADSL2/plus systems withoutpenalizing the downstream VDSL2 signal.

Upstream power backoff

When different sets of VDSL2 customerpremises equipment (CPE) are located at un-equal distances from the central office or thecabinet, the transmission of users closest tothe central office or cabinet disturbs the up-stream transmission of other users (near-farphenomenon, Figure 14).9 A simple reme-dy to this problem is to back off the up-stream power of users closest to the centraloffice or cabinet. An algorithm is thus used

Figure 13

Reference model of the transceiver. The dotted lines indicate an optional latency path.

to give every user within a given radius ofthe central office or cabinet the same up-stream capacity.

Loop diagnostics mode

To facilitate fault diagnosis, VDSL2 has aloop diagnostic mode and DELT similar tothat defined for ADSL2/plus. VDSL2 trans-ceivers can measure line noise, loop attenu-ation, and signal-to-noise ratio (SNR) ateach end of the line. The measurements canbe collected even when line conditions aretoo poor to enable a connection. In this case,as part of the loop diagnostic mode, themodem goes through each step of initial-ization but with improved robustness inorder to exchange the test parameters.

Impulse noise protection

Electrical appliances and installations atcustomer premises often generate shortbursts of noise of relatively high amplitude.These bursts, called impulse noise, are elec-tromagnetically coupled into the digitalsubscriber line, degrading performance andin some cases disrupting service. TheADSL2/plus standard introduced a parame-ter (impulse noise protection, INP) that al-lows operators to select the maximum im-pulse length that the system can correct.VDSL2 uses this same parameter. In effect,an INP value of between 2 and 16 can cor-rect errors from noise impulses ranging from250µs to 3.75ms in length.

Interoperability

Interoperability between vendors is a pre-requisite of mass-market technology. Inter-operability testing has thus had a vital rolein paving the way for today’s commercialsuccess of DSL: nearly 19 million digitalsubscriber lines were rolled out during thethird quarter (Q3) of 2005.1 0

The DSL Forum is the main driving forcebehind interoperability. The actual inter-operability tests are performed via “plugfests” which are run by several independenttest labs around the world. An advocate ofopen interfaces, Ericsson has been an activeparticipant at these events. For ADSL2plus,the TR-067 (formerly TR-048) specifies indetail how interoperability is to be tested.

Extensive interoperability testing hasopened the door to separate DSLAM andCPE markets in many countries.

VDSL2 interoperability

The initial VDSL standard was never fullyembraced by the market due to disagree-ments during standardization regardingmodulation scheme (DTM or QAM) andpacket technology (ATM or Ethernet).

Broad industry consensus and interoper-ability testing of VDSL2 is at the top of ven-dor agendas in 2006. A first meeting forchipset vendors took place at the end of Jan-uary at the University of New HampshireInterOperability Laboratory. Experiencefrom working with ADSL tells us it will takeat least 12 months to reach full interoper-ability in the market: for ADSL2plus theprocess from agreement on the standard tointeroperability in the market took nearly18 months.

The first step in the process is to gainlayer-1 interoperability between chipsetvendors. With that hurdle cleared, systemvendors can perform additional tests be-tween CPE and DSLAMs.

The DSL Forum is defining the frame-work for the VDSL2 tests in several impor-tant documents that will guide the indus-try forward. The two main documents cur-rently under development are:


Figure 14Near-far phenomenon. The signal from CPE X is stronger than the signal from CPE Y,

which is attenuated by loop y-x. As a result, crosstalk from CPE X leaks into the upstreamsignal from CPE Y. The solution is to back off the power of CPE X.


• WT-114: VDSL2 Performance Test Planthat sets performance targets (taking intoconsideration the specific profiles of dif-ferent regions); and

• WT-115: VDSL2 Functionality Test Planthat forms the basis for operators who arehomologating VDSL2 standards compli-ance.

Backward compatibility

The decision to retain ATM as multiplex-ing scheme helped keep migration fromADSL1 to ADSL2plus relatively simple be-cause the new IP-DSLAMs were backwardcompatible with the installed base ofADSL1 CPE. Therefore, migration on theline side did not affect the CPE side.

The migration from ADSL2plus toVDSL2, however, will require considerablymore planning at an early stage.ADSL/2/2plus always employs ATM on thecopper loop, but VDSL2 will mostly be de-ployed using Ethernet technology. Even so,VDSL2 chipsets generally allow for the con-figuration of ATM or IP on a per-port basis,thereby guaranteeing backward compati-bility with the installed base of ADSL andADSL2plus. This fallback feature comesinto play automatically during the initial-ization phase between CPE and DSLAM.

Conclusion

ADSL2plus is being deployed worldwide asthe new mainstream broadband technologyfor residential and business customers. Butat the same time, the industry is gearing upfor the next step of the DSL evolution:VDSL2. The greater bandwidth of VDSL2gives telecommunications operators theability to offer advanced services (such asmultiple streams of interactive standard andhigh-definition TV over IP) over the exist-ing copper plant.

Ericsson is a driving force behind the ar-chitectural transition to high-performancebroadband solutions that will propel broad-band from mere high-speed internet accesstechnology to a complete suite of IP-basedservices. The company introduced its first

IP-based DSL access solutions in 2002 andhas since worked to refine the architecturesrequired to support the enhanced scope ofservice.

The scope of the VDSL2 standard isquite broad. Its goals are to increase per-formance over longer loops (longer thanVDSL1), as an evolution from ADSL2plus,and very short loops, as an evolution fromVDSL1. Perhaps the most important aspectof the VDSL2 standard is that it uses Eth-ernet as multiplexing technology (64/65 en-capsulation) in the first mile. The elimina-tion of ATM in the first mile means that theaccess architecture can be simplified into anend-to-end Ethernet access architecture thatuses VLANs as the service-delivery mecha-nism across the entire access network. A di-rect benefit of this change is reduced over-head.

Because VDSL2 is based on both theVDSL1-DMT and ADSL2/ADSL2plus rec-ommendations, it is spectrally compatiblewith existing services and enables multi-mode operability with ADSL/2/plus.

Ericsson has three years’ solid experienceof building broadband architectures thatcan handle advanced, high-quality serviceswith superior performance. Its EDA prod-uct line is currently in operation in a largenumber of commercial deployments aroundthe world. In 2005, to show operators howthe technology behaves and what services itsupports, Ericsson deployed EDA withVDSL2 in live networks. Some features ofEDA worth highlighting in the context ofVDSL2 are scalability, advanced SELT andDELT capabilities, QoS, and greater capac-ity. Vital objectives of the early deploymentswere to verify VDSL2 performance advan-tages in real network conditions, and to en-able an early start of VDSL2 interoperabil-ity activities.

Interoperability between vendors is a pre-requisite of mass-market technology. Inter-operability testing has thus had a vital rolein paving the way for today’s commercialsuccess of DSL. Broad industry consensusand interoperability testing of VDSL2 is atthe top of vendor agendas in 2006.

1. ITU-T Recommendation G.992.1 (1999),

Asymmetric digital subscriber line (ADSL)

transceivers


Asymmetric digital subscriber line trans-

ceivers 2 (ADSL2)


Asymmetric digital subscriber line (ADSL)

transceivers – extended bandwidth ADSL2

(ADSL2plus)

4. ITU T Recommendation G.993.1 (2004),

Very high speed digital subscriber line

5. Begley, M.: The Public Ethernet – The

next-generation broadband access net-

work. Ericsson Review, Vol. 81(2004):1,

pp. 52-59

6. J.A.C. Bingham. “Multicarrier Modulation

for Data Transmission: An idea Whose

Time has come”. IEEE Communications

Magazine, 28(4):5-14 April 1990

7. F. Sjöberg, M. Isaksson, P. Ödling, S. K.

Wilson, and P. O. Börjesson “Zipper: A

Duplex Method for VDSL Based on DMT,”

IEEE Trans. Commun., vol. 47, pp. 1245-

1252, Aug. 1999

8. ETSI 300 019-1 class 3.4 environmental

requirements

9. K. Jacobsen “Methods of Upstream Power

Backoff on Very High-Speed Digital Sub-

scriber Lines”, IEEE Communications.

Magazine March 2001

10.http://www.broadbandtrends.com/Report

%20Summary/2005/BBT_3Q05DSLMkt-

Share_051250_Summary_TOC.pdf

REFERENCES

Documents

VDSL2: Next important broadband technology - · PDF fileEthernet DSL access (EDA) DSLAM for ADSL2 and ADSL2plus. VDSL Efforts to standardize VDSL (currently de-noted VDSL1) got underway