G.fast breaks through the gigabit barrier
G.fast is a new technology capable of delivering gigabit speeds — and potentially, with “XG-FAST” technology, multi-gigabit speeds — over traditional copper telephone lines. Designed for use on short lines, G.fast further extends the bit rate increases provided by VDSL2 vectoring technology, giving service providers a cost-effective means to complement and accelerate fiber-to-the-home (FTTH) deployments. By capitalizing on the ultra-broadband capabilities of G.fast, service providers can deploy FTTH services without actually having to bring fiber all the way into the building or home.
Bell Labs G.fast trials set speed records
Recent lab trials by Alcatel-Lucent showcased the ultra-broadband capabilities of copper. A new technology prototype from Bell Labs (dubbed XG-FAST) achieved a world-record speed of 10 Gbps over a distance of 30 meters by using two pairs of copper lines. Reproducing a real-world fiber-to-the-distribution-point (FTTdp) deployment, the prototype achieved speeds of 2 Gbps aggregate or 1 Gbps symmetrical over 70 meters using a single copper pair. These real-world conditions validated the key G.fast use case: At 70 m, service providers can use existing copper infrastructure to bring fiber speeds (1 Gbps symmetrical) into the home.
Bell Labs achieved these speed records by using frequencies up to 500MHz — significantly higher than the frequencies currently planned for the G.fast standard. But the trial results confirm that copper broadband can still be pushed to greater limits, and that hybrid fiber-copper can complement full fiber deployments for decades to come.
G.fast brings gigabit speeds to copper lines
The high cost and intense effort required to extend fiber infrastructure have slowed progress toward comprehensive FTTH deployments. Wary of the challenge involved in digging up streets and yards and rewiring every household, service providers have built out their fiber access networks gradually, as dictated by market demand and budgetary constraints. Meanwhile, demand for ultra-broadband access has accelerated, spurred on by new applications, increasing competition and ambitious government broadband plans.
Service providers now have their sights set on the gigabit speeds enabled by G.fast, which represent a significant leap forward from VDSL2. G.fast allows service providers to extend fiber to small DSLAMs or micro-nodes installed at the last distribution point before the customer premises. Providers can deploy these compact distribution units in a variety of indoor and outdoor locations. Each unit typically serves between 1 and 16 end customers and is connected to customer premises using copper lines with loop lengths of approximately 100 meters.
The short loops used for FTTdp deployments provide an ideal environment for delivering ultra-broadband speeds. For example, vectored VDSL2 uses 17 MHz of spectrum and delivers an aggregate data rate of up to 150 Mbps on each line. But short copper loops can support much higher data rates. The wider spectrum used by G.fast (up to 106 MHz in phase 1 and 212 MHz in phase 2) brings aggregate bit rates of 500 Mbps to 1 Gbps within reach. As shown by the Bell Labs trials, taking frequencies higher can create even more speed.
This wide spectrum is only effective with short loop lengths. On long loops of several hundred meters, the attenuation of the copper is too great to support the high frequencies used by G.fast. As a result, VDSL2 vectoring remains the preferred technology for longer loops.
Vectoring remains essential
Vectoring 2.0 will play a key role in enabling service providers to get the full benefit of G.fast. The high frequencies used by G.fast create strong crosstalk between neighboring copper pairs— significantly stronger than that created by VDSL2 technology. This crosstalk takes away much of the capacity boost offered by G.fast. Service providers must use vectoring to cancel this crosstalk and allow each line to perform to its potential.
Vectoring brings significant performance gains. Applied to VDSL2 lines, vectoring can improve performance by a factor of 2. When used with G.fast, vectoring can improve performance tenfold.
Figure 1 illustrates the performance gains produced when vectoring is applied to G.fast. The blue bars show the bit rates delivered by G.fast on single lines of varying length and type (with different physical properties such as gauge, twist and isolation material), where no crosstalk is present. The red bars show how these bit rates drop (typically 50%–90%) when a second line is added. The green bars show how the bit rates improve (near single-user capacity) when vectoring is activated.
Figure 1: Vectoring 2.0 makes G.fast faster
In most cases, G.fast vectoring delivers an average data rate slightly lower than the single-user rate. The small difference can be attributed to gain scaling, which must be applied with G.fast to ensure compliance with power spectral density constraints. In some cases (as with the 100 m loop in Figure 1), the vectored rate actually exceeds the single-user rate. This happens because vectoring converts energy from crosstalk into useful energy that increases the line’s signal-to-noise ratio.
Vectoring is not required for every G.fast application. If each endpoint is serviced by a cable well separated from other cables, a service provider may be able to deliver the full G.fast bit rate without vectoring. In most cases, G.fast applications will involve lines from multiple subscribers deployed in close proximity to one another. These applications will require vectoring to deliver the best possible performance.
Homes, MDUs and more: Applications of G.fast
For residential applications, service providers will deploy G.fast from distribution points close to end customers. This proximity will enable them to bring fiber deeper into the network and maintain the short copper loop lengths that G.fast requires.
Proximity will also enable service providers to take advantage of the reverse power capabilities of G.fast distribution units. These capabilities allow the units to draw power from customer premises equipment using the same telephone line that carries the G.fast signal. This helps service providers avoid having to make appointments with utility providers and dig up streets to lay cable.
For multi-dwelling units, service providers will deploy G.fast nodes inside the building and connect them to individual apartments using existing copper lines. Crosstalk is likely in these multi-line, multi-user applications. Service providers will need to use vectoring to eliminate it.
Bringing higher bit rates to homes and multi-dwelling units will help service providers improve their managed IPTV offerings and handle over-the-top video more effectively. G.fast will complement and accelerate FTTH deployments by supporting more simultaneous streams and recordings, and more signals to devices like smartphones and tablets.
G.fast will also provide the high bit rates required for Wi-Fi and LTE backhaul. When combined with support for network timing reference and time of day protocols, G.fast is a smart choice for mobile backhaul.
Looking beyond bit rates
While its applications and performance potential attract attention, G.fast presents some other compelling advantages to service providers. These advantages include:
- Flexible downstream/upstream capacity ratios: With its use of TDD, G.fast offers service providers the freedom to configure any ratio between 90% down/10% up and 30% down/70% up.
- Support for coax deployments: G.fast runs effectively in cases where it can be inserted in a P2P coax cable dedicated to a single user.
- Customer self installation: G.fast facilitates self installation with mechanisms such as fast rate adaptation (FRA) and a powerful retransmission scheme. These mechanisms ensure that G.fast can be used on any in-home network.
- Low-power modes and power consumption reduction techniques: G.fast provides mechanisms to minimize power consumption when the line is inactive, in stand-by mode or operating under difficult thermal conditions. It also provides mechanisms that can scale power consumption in step with actual data throughput.
- Simple distribution unit installation: G.fast distribution points will outnumber cabinets 10 to 1 and serve small groups of customers. Distribution units designed for easy installation and maintenance will allow service providers to visit each site once and return only if necessary.
Addressing new challenges
Service providers will have new challenges to address as they embrace G.fast. For example, they will need to ensure that their G.fast deployments can coexist with FM radio, digital audio broadcast and VDSL2 services. G.fast facilitates coexistence by providing a highly configurable power spectral density (PSD) mask. Service providers can use the PSD mask to notch out frequencies that could potentially harm any of the coexisting services. To allow coexistence with VDSL2, providers can configure a starting frequency that allows them to spectrally separate the two technologies and avoid crosstalk.
Reverse powering of distribution units will also present challenges. Distribution units will need to draw sufficient power and operate efficiently regardless of how many users connect to them or how much load is placed on them. And they will need to manage heat dissipation when they are subjected to high levels of activity under sun load or in locations with insufficient airflow.
Although not a new challenge, crosstalk is much greater — more like cross-shouting — with G.fast. Adoption of G.fast will require new approaches to crosstalk cancellation. In the downstream, the linear gain-scaled precoder provides the right balance between the precompensation signal and transmit power for each line. In some cases, transmit power is reduced on one line to ensure adherence to PSD constraints on other lines. The approach to crosstalk signal reduction must address the needs of all lines simultaneously.
Getting ready for G.fast
G.fast technology is evolving quickly. The G.fast standard achieved consent at the International Telecommunications Union in December 2013. The first G.fast chipsets are expected later this year, with ratification of the standard to follow in December 2014. G.fast field trials will come in 2015, and volume deployment-ready G.fast products will appear by 2016.
Service providers should take G.fast into account as they plan their FTTH deployments. G.fast will enable them to get to FTTH faster and avoid having to rewire every home and multi-dwelling unit. Providers can begin clearing the path to G.fast developing strategies for managing coexistence with VDSL2. They can start the move to G.fast in earnest by deploying VDSL2 in FTTdp configurations. When G.fast becomes available, service providers can replace VDSL2 micro-nodes with G.fast micro-nodes and begin shipping G.fast modems to customers.
Inventing the Future at Bell Labs (video) Going the last mile: A history of the copper telephone line Infographic
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