VDSL2: Turning Copper Into Gold
From the very first bang of the Internet explosion, telcos have been interested in finding ways to get more bandwidth out of their copper networks. It is an interest that has largely been fuelled by intensified competition from the cable operators, and the (resulting) need for telcos to start delivering the hot combination of video, data, and voice services: The Triple Play.
Today, an attractive Triple Play service package requires about 20 Mbps per household -- simultaneously enabling 1 or 2 high-definition television (HDTV) channels, a Video-on-Demand (VoD) service, high-speed Internet (HSI) access, and Voice-over-Internet-Protocol (VoIP) telephony.
Over the next 5 years, however, the required bandwidth per household is expected to increase to 50 Mbps -- an increase which is confirmed by the historically observed 20% compound annual growth rate (CAGR) for residential bandwidth consumption. Main drivers for this surge from 20 to 50 Mbps will be: the delivery of multiple HDTV channels, Femtocell backhaul, and the ever-increasing popularity of video sharing and community websites.
Not surprisingly, traditional asymmetric digital subscriber line (ADSL)-based access networks, which account for approximately two-thirds of broadband subscriber lines worldwide, will be under a lot of pressure to cope with this order-of-magnitude increase in bandwidth demand.
It makes us wonder: is this the end of the copper era? Or are there ways to fully unlock the bandwidth potential of DSL? In other words: is it possible to turn copper into gold?
Network Upgrade Mining
Three major architectures are being used by operators to deliver the increased bandwidths needed for tomorrow’s IPTV and Triple Play services: Fiber-to-the-Node (FTTN), Fiber-to-the-Curb/Building (FTTC/B) and Fiber-to-the-Home (FTTH). (See Figure 1.)
• Fiber-to-the-Node (FTTN): a fiber network runs from the central office to street cabinets, leveraging the Last Mile copper loops.
• Fiber-to-the-Curb (FTTC) / Fiber-to-the-Building (FTTB): fiber cables are trenched from the central office to a small enclosure near a building / a (bigger) building’s basement, respectively -- as from where the end-users are offered very high-speed broadband applications over the existing, in-building copper.
• Fiber-to-the-Home (FTTH): a fiber network is rolled out from the central office to every individual subscriber.
Which Is the “Right” Solution?
FTTH, with fiber running all the way from the central office to each home and every apartment, is the clear endgame. FTTH architectures are future-safe, enabling operators to deliver (more or less) unlimited bandwidth: 100 Mbps per household today, with technology evolutions set to increase this in the future. Their main drawback, however, is that (nationwide) FTTH deployments require a lot of time and money.
This is exactly where an FTTN or FTTC/B architecture comes into play. Even though this type of architecture is often regarded as a medium-term solution, it is often the right / most effective / most appropriate one for many service providers -- given budgetary and time-to-market constraints. Consider the following reasons:
Reason 1: Typically, in brownfield situations, an FTTN deployment will be three times cheaper than an FTTH architecture because the copper between the street cabinet and the subscriber is being re-used.1 Similarly, again in a brownfield environment, an FTTB deployment will be approximately 30% cheaper than FTTH since the so-called vertical fiber (i.e., the fiber inside the building) does not need to be installed. (See Figure 2.)
Reason 2: Construction of an FTTH network usually takes longer, given the fact that fiber has to be deployed all the way to the home.2 Many operators hence claim that connecting every single subscriber to fiber -- nationwide -- will take anywhere between 10 and 20 years, whereas operators leveraging an FTTN VDSL2 architecture did achieve nationwide coverage in less than 4 years.
Reason 3: All in all, in competitive environments where time and money are the major determining factors (in other words, pretty much everywhere), FTTN (as well as FTTC/B, for that matter) permits a less capital-intensive, and faster, deployment than FTTH. FTTN, FTTC, and FTTB allow to leverage to the largest extent possible the operator’s existing copper plant, and enable operators to start offering premium services quickly. They can thus start to generate revenue, all while getting more time to roll out their fiber network. As such, the implementation of FTTN/B/C architectures is a first key requirement for turning copper into gold!
No Fool’s Gold
Four Architecture/Technology Innovations to Unlock Copper’s Potential
Even though ADSL2plus, the latest ADSL standard, can reach up to 25 Mbps (depending on loop length and line conditions), operators will have to invest in newer-generation access technologies to deliver bitrates of 20 Mbps and beyond.
One option is leveraging very high-speed DSL (VDSL) technology, which can deliver up to 100 Mbps over copper. In order to achieve those high bitrates, however, VDSL(2) has to be deployed over shorter distances. Concretely, this means that VDSL(2) can perfectly be used to connect those subscribers close enough (let’s say within 1,000m) to the operator’s central office.
For end users further away from the central office, the operator will have to shorten the copper loops by replacing these partly with fiber. Leveraging VDSL2, 4 deployments models can be discerned (closely linked to the above discussion on FTTN, FTTC, and FTTB architectures):
• Option 1: Using FTTN/VDSL2 as a main strategy
A number of operators have rolled out VDSL2 (to street cabinets; sometimes combined with VDSL2 from the central office) nationwide. Biggest advantage: time-to-market. Most operators with a large-scale VDSL2 deployment have typically started their FTTH roll-out as well, with fiber being deployed in greenfield and strategic areas.
• Option 2: Using FTTB/VDSL2 as a complement to FTTH
As seen before, this reduces the cost of connecting apartments by up to 30% and allows for a more rapid deployment.
• Option 3: Using VDSL2 from the central office as a complement to FTTH
Some operators simply upgrade their central office ADSL subscribers to VDSL2 while starting their fiber roll-out. Obviously, only those subscribers close enough to the central office will be able to benefit from this, but it is a very straightforward thing to do: it is a mere replacement of line cards. The main benefit is that operators can immediately start offering premium services to a significant part of their subscriber base, and build a footprint. Many fiber operators are pursuing this strategy.
• Option 4: Using VDSL2 as a fall-back for FTTH
Even operators aggressively rolling out fiber are considering VDSL2 for areas that are too expensive to serve with fiber, or for areas that are not going to be served with fiber immediately.
Obviously VDSL2 does not offer the same promise of unlimited bandwidth as fiber does, which is why (as noted before) most, if not all operators, consider FTTH to be their long-term strategy.
However, VDSL2 is continuously evolving as well in order to extend the service life of the copper plant even more, and to allow operators to get the most out of their existing infrastructure. Two key evolutions in this regard are VDSL2 bonding (available today) and VDSL2 vectoring (expected to hit the market in 2011).
VDSL2 Bonding Boost
The bandwidth brought to the consumer over copper loops can be given a further boost through bonding: the practice of combining multiple wire pairs to increase available capacity, or extend the copper network's reach.
VDSL2 bonding typically combines 2 regular VDSL2 lines into a single, virtual "big pipe" that allows operators to double the bitrate for existing subscribers (since you're using 2 lines). Alternatively, it allows them to deliver the same bitrates over longer distances (covering subscribers that were previously out-of-reach, thereby also reducing the number of cabinets that need to be built to cover a given area).
North American operators have traditionally shown more interest in bonding than some of the other operators, due to the topology of their networks. That said, bonding can be used anywhere that the outside plant is capable of supporting it. Generally speaking, ideal bonding conditions exist when there are sufficient pairs available between the DSL line circuits and the home; it requires that you have (at least) 2 pairs available per subscriber, but in many countries this is actually the case for a significant part of the subscriber base.
A number of operators are currently trialling VDSL2 bonding in the field and are reporting good results, being able to extract much more bandwidth from their existing copper plant.
VDSL2 Vectoring Viability
VDSL2 vectoring works on a single pair and is based on the concept of "noise cancellation", much like the headphones people have started to use increasingly on planes, to reduce or cancel background/engine noise when listening to music or watching a movie.
VDSL2 vectoring calculates the interference between all pairs in a binder, based on the actual signals, and will use this information to generate a noise cancellation signal on each pair, effectively removing all crosstalk. The net gain is between 25% and 100%. (See Figure 3.)
Due to the amount of calculations involved, vectoring will provide the best results in nodes with a limited number of lines (FTTN or FTTB deployments, for instance). The only requirement is that all lines are under full control of a single operator, meaning that there can be no sub-local loop unbundling. Indeed, if the lines belong to multiple operators and are terminated on different nodes, then there's no way to collect all the signal and crosstalk data.
Vectoring is currently being tested in the labs, and is expected to be available for roll-outs in 2011.
There's Gold In Them Thar Hills!
The need for bandwidth enabling residential applications is expected to more than double in the next 5 years (from 20 Mbps today to 50 Mbps by 2015). Hence, operators are increasingly looking into various strategies that will prepare their access network for this evolution. FTTH is the clear end-game, supporting the delivery of 100 Mbps per household today and technology evolutions set to further increase this in the future.
Alternative deployment strategies and technology innovations exist, however, to also extract much more bandwidth from the existing copper plant.
A smart combination of copper/fiber deployments is a first step in the right direction, providing operators with a timely and cost-effective migration path to FTTH, all while leveraging the installed copper loops (and the past investments) to the largest extent possible.
Additionally, technology innovations such as VDSL2 bonding and vectoring are expected to significantly boost the capacity and/or reach of DSL-based access networks -- truly providing operators with the means to transform copper into gold!
1. That being said, in greenfield situations, FTTH is generally chosen, as its cost is then typically comparable to the cost per home passed for FTTN.
2. Again, in greenfield situations, operators will typically opt for installing fiber right away; installing new copper today makes sense only in exceptional circumstances.
About the Authors
Stefaan Vanhastel is Product Marketing Manager FTTx for Alcatel-Lucent. He is responsible for the global marketing of Alcatel-Lucent DSL products and technology. He was appointed to his current role in 2005. Prior to joining Alcatel-Lucent, Stefaan worked at a start-up company specialized in network and datacenter infrastructure management, and in the Broadband Communications group at the University of Ghent (Belgium). He has managed research projects focusing on network performance and network management. For more information, visit www.alcatel-lucent.com.
Wim Van Daele is part of Alcatel-Lucent's Corporate Media Relations team, working on the topics of fixed broadband access and end-to-end LTE. Wim has a Masters Degree in Applied Linguistics and recently obtained his PMI Project Management Professional (PMP) certificate. For more information, visit www.alcatel-lucent.com or email Wim at firstname.lastname@example.org.
Alcatel-Lucent is the trusted partner of service providers, enterprises, and governments worldwide, providing solutions that to deliver voice, data and video communication services to end-users. A leader in fixed, mobile, and converged broadband networking, IP technologies, applications and services, Alcatel-Lucent leverages the unrivalled technical and scientific expertise of Bell Labs, one of the largest innovation powerhouses in the communications industry. With operations in more than 130 countries and the most experienced global services organization in the industry, Alcatel-Lucent is a local partner with a global reach. Alcatel-Lucent achieved revenues of Euro 16.98 billion in 2008 and is incorporated in France, with executive offices located in Paris. For more information, visit Alcatel-Lucent on the Internet: http://www.alcatel-lucent.com.
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