Copper’s renewed power is no figment of the imagination
Enabling service providers to continue to deliver — over their existing copper lines — bandwidth intensive residential Triple Play and business services, will remain an important technology research track for many years to come.
And though many think that delivering high bandwidths over copper may be smoke and mirrors, we know differently. We know the future is in The Phantom — The Copper Phantom, that is.
DSL Phantom Mode technology is a bit of magic for the copper network. It creates a third (phantom or virtual) channel over 2 regular copper pairs. Combined with vectoring (to eliminate crosstalk) and bonding (to create 1 big DSL pipe), downstream speeds of 390 Mbps — at 400 meters — can be achieved using 2 copper pairs, commonly available in residential deployments. By adding more pairs, even higher speeds can be obtained; for instance, 910 Mbps at 400 meters using 4 pairs — which means more than 1 Gbps upstream and downstream combined. This is particularly relevant in business services environments and to support mobile backhaul.
Magic for Copper?
Broadband access over copper pairs, using the infrastructure deployed for telephony years ago, is an important market. The move from dial-up based access to DSL based services has enabled the Internet revolution. It is also clear that although fiber access is the end-game, copper-based access will be around for many years to come — as recent Dell’Oro Group (www.delloro.com) statistics show that copper-based broadband serves more than 250 million households around the world.
With the bandwidth growth expectations as shown in Figure 1, there is a very real interest to boost the attainable speed rates on copper lines — accommodating a wide range of consumer and business services, as well as wireless backhaul applications.
We can work in 3 dimensions to make the copper plant support higher bit rates:
1. Reduce the noise by crosstalk cancellation, commonly called vectoring.
2. Increase the number of physical (and virtual) transmission channels per user by bonding and phantom modes.
3. Increase the frequency bandwidth up to 30MHz (an approach that will not be further discussed in this article, as it is useful only for very short loops).
VDSL2 vectoring enables the full potential offered by VDSL2, currently limited by self crosstalk. Less crosstalk means that the signal-to-noise ratio and hence the capacity offered by VDSL2 can be improved significantly. And that’s good news given the future demand that is projected for VDSL2.
Let’s examine one example that illustrates the benefit brought by vectoring on the downstream scenario. With a good estimate of the crosstalk channels, we can predict and precompensate the crosstalk of each line, much as noise-cancelling headphones remove ambient noise. This allows for rates above 100 Mbps on copper loops in an FTTN scenario.
Moreover, we can exploit multiple wire pairs. The scenario in which multiple twisted-wire pairs are available for data transport to one end user is called a bonded scenario. In its simplest form, there is no extensive coordination between the pairs, other than what is required to multiplex the data over the pairs, and no mutual crosstalk reduction. The bit rate increases sublinearly (due to the increasing crosstalk) with the number of wire pairs. When combined with vectoring, the bit rate gain is linear.
Phantom Mode: Seeing Is Believing
Phantom mode transmission over 2 pairs exploits the difference between the common mode voltages of the 2 pairs. Phantom mode transmission is also possible over more than 2 wire pairs. To preserve the balance and avoid excess egress and ingress noise, typically hierarchical phantoms (or super-phantoms) are used in which case the number of pairs (N) is a power of 2 (N = 2n). The naming is introduced to make a distinction with the definition of common mode transmission, where the common ground of 1 twisted pair is used as a reference. (See Figure 3.)
In general, when N wire pairs are available, N normal (differential) channels and N-1 phantom channels can be used. Compared to a fully vectored bonding group without the use of phantom modes, the potential bit rate increase asymptotically goes to 200% for a large number of pairs (assuming that all pairs and transmission modes can achieve the same bit rate).
Ghostbusters?
Often, the phantom mode channel consists of weakly twisted or even untwisted pairs of twisted wire pairs. This makes the phantom mode signal more vulnerable to ingress of impulse noise, radio noise, alien noise, and crosstalk.
Phantom modes also tend to radiate more (a.k.a. egress) than regular differential modes. The applicability of the phantom mode is strongly related to the type of binder. In some binder types, only the individual wire pairs are twisted, and pairs are not twisted around each other. This has a negative impact on the ingress/egress.
Other binder types show twisting of wire pairs around each other (e.g., in a quad structure). This will limit ingress/egress, although the twisting is less efficient than the twisting of individual pairs. Also, the crosstalk between phantom modes will most likely be larger than the crosstalk between differential modes.
For all these reasons, crosstalk cancellation or vectoring needs to be applied when using the phantom mode. Otherwise, the extra bit rate gained over the phantom link is limited, and partially reduced by bit rate reduction over the differential modes.
Phantom Phenomenon
Bell Labs’ tests have successfully demonstrated the potential of DSL Phantom Mode technology, enabling service providers to continue to deliver — over their existing copper lines — bandwidth-intensive residential Triple Play and business services for many years to come.
Leveraging DSL Phantom Mode, Alcatel-Lucent has shown downstream speeds of 390 Mbps over distances up to 400 meters using 2 copper pairs, and obtained 910 Mbps using 4 pairs — which means more than 1 Gbps upstream and downstream combined. With further research currently ongoing to develop optimal deployment models and a specific set of customer premises equipment (CPE)-models, the technology is fully standards-based and has a bright future.