Sckipio’s G.fast silicon to enable gigabit services
Sckipio’s newest G.fast broadband chipset family delivers 1.2 gigabits of aggregate bandwidth over 100m of telephone wire.
The start-up’s SCK-23000 chipset family implements the ITU’s G.fast Amendment 3 212a profile. The profile doubles the spectrum used from G.fast from 106MHz to 212MHz, boosting the broadband rates. In contrast, VDSL2 digital subscriber line technology uses 17MHz of spectrum only.
“What the telcos want is gigabit services,” says Michael Weissman, vice president of marketing at Sckipio. “This second-generation [chipset family] allows that.”
G.fast market
AT&T announced in August that it is rolling out G.fast technology in 22 metro regions in the US. The operator already offers G.fast to multi-dwelling units in eight of these metro regions. The rollout adds to the broadband services AT&T offers in 21 states.
AT&T’s purchase of DirecTV in 2015 has given the operator some 20 million coax lines, says Weissman. AT&T can now deliver broadband services to apartments that have the DirecTV satellite service by bringing a connection to the building’s roof. AT&T will deliver such connections using its own fibre or by partnering with an incumbent operator. Once connected, high-speed internet using G.fast can then be delivered over the coax cable, a superior medium compared to telephony wiring.
Michael Weissman“This is fundamentally going to change the game,” says Weissman. “AT&T can now compete with cable companies and incumbent operators in markets it couldn’t address before.”
Sckipio has secured four out of the top five telcos in the US that have chosen to do G.fast: AT&T, CenturyLink, Windstream and Frontier. “The two largest - AT&T and CenturyLink - are exclusively ours,” says Weissman.
In markets such as China, the focus is on fibre. The three largest Chinese operators had deployed some 260 million fibre-to-the-home (FTTH) lines by the end of July.
Overall, Sckipio is involved in some 100 G.fast pilots worldwide. The start-up is also the sole supplier of G.fast silicon to broadband vendor Calix and one of two suppliers to Adtran.
“Right now there are only two real deployments that are publicly announced - and I mean deployment volumes - AT&T and BT,” says Weissman. “The point is G.fast is real.”
Telcos have several requirements when it comes to G.fast deployment. One is that the technology delivers competitive broadband rates and that means gigabit services. Another is coverage: the ability to serve as high a percentage of customers as possible in a given region.
What the telcos want is gigabit services. This second-generation [chipset family] allows that.
Because G.fast works across the broader spectrum - 212MHz - advanced signal processing techniques are required to make the technology work. Known as vectoring, the signal processing technique rejects crosstalk - leaking signals - between the telephone wires at the distribution point. A further operator need is ‘vectoring density’, the ability to vector as many lines as possible.
It is these and other requirements that Sckipio has set out to address with its SCK-23000 chipset family.
SCK-23000 chipset
The SCK-23000 comprises two chipsets. One is the 8-port DP23000 chipset used at the distribution point unit (DPU) while the second chipset is the CP23000, used for customer premise equipment.
Sckipio is not saying what CMOS process is used to implement the chipsets. Nor will it say how many chips make up each of the chipsets.
As for performance, the chipsets enable an aggregate line-rate performance (downstream and upstream) of 1.7 gigabits-per-second (Gbps) over 50m, to 0.4Gbps over 300m. The DP23000 chipset also supports two bonded telephone lines, effectively doubling the line rate. In markets such as the US and Taiwan, a second wire pair to a home is common.
Vectoring density
Vectoring density dictates how many G.fast ports can be deployed as a distribution point. And the computationally-intensive task is even more demanding with the adoption of the 212a profile. “The larger the vector group, the more each subscriber’s line must know what every other subscriber’s signal is to manage the crosstalk - and you are doing it at twice the bandwidth,” says Weissman.
Sckipio says the SCK-23000 supports up to 96 ports (or 48 bonded ports) at the 212a profile. The design uses distributed parallel processing that spreads the vectoring computation among the DP23000 8-port devices used. “We are not specifying data paths between the chips but you are talking about gigabytes of traffic flowing in all directions, all of the time,” says Weissman.
The computation can not only be spread across the devices in a single distribution point box but across devices in different boxes. Operators can thus use a pay-as-you-grow model, adding a new box as required. “A 96-port design could be two 48-port boxes, or an 8-port box could [be combined to] become a 16- or 24-port design if you have a smaller multi-dwelling unit environment,” says Weissman.
Sckipio’s design also features a reverse power feed: power is fed to the distribution point to avoid having to install a costly power supply. Since the power must come from a subscriber, the box’s power demand must not be excessive. A 16-port box is a good compromise in that it is not too large and as subscriber-count grows, each new 16-port unit added can be powered by another consumer.
“You can only do that if you can do cross distribution-point-unit vectoring,” says Weissman. “It allows the telcos to do a reverse power feed at the densities they require.”
Dynamic bandwidth allocation
The chipsets also support co-ordinated dynamic bandwidth allocation, what Sckipio refers to as co-ordinated dynamic time assignment.
Unlike DSL where the spectrum is split between upstream and downstream traffic, G.fast partitions the two streams in time: the CPE chipset is either uploading or downloading traffic only.
Until now, an operator will preset a fixed upload-download ratio at installation. Now, with the latest silicon, dynamic bandwidth allocation can take place. The system assesses the changing usage of subscribers and adjusts the upload-download ratio accordingly. However, this must be co-ordinated across all users such that they all send and all receive data simultaneously.
“You can’t, under any circumstances, have lines uploading and downloading at the same time,” says Weissman. “All the systems that are vectored must be communicating in the same direction at the same time.” If they are not co-ordinated, crosstalk occurs. This is another crosstalk, in addition to the crosstalk caused by the adjacency of the telephone wires that is compensated for using vectoring.
“If you don’t co-ordinate across all the pairs, you create a different type of crosstalk which you can’t mitigate,” says Weissman. “This will kill the system.”
Sckipio says the SCK-23000 chipsets are already with customers and that the devices are generally available.
FSAN unveils roadmap plans
Part 2: Next-generation passive optical networks
The Full Service Access Network (FSAN) has outlined its vision for fibre access networks for the coming decade.
FSAN is an industry forum that includes over 20 operators and 70 members overall. The group identifies service requirements and develops optical access technologies that are passed to the International Telecommunication Union (ITU) for standardisation.
Source: FSAN
“One of the messages of the roadmap is that, in the immediate future, what FSAN wants to do is evolve the existing standards,” says Peter Dawes, FSAN NGPON co-chair.
The latest FSAN technologies to become standards are XGS-PON (10 gigabits symmetrical passive optical network) and the multiple wavelength TWDM-PON (time wavelength-division multiplexing passive optical network), also known as NG-PON2 (see chart).
PON status
XGS-PON is a single-wavelength PON standard that supports two rates: a 10-gigabit symmetrical rate and the asymmetrical 10 gigabits downstream (to the user) and 2.5 gigabits upstream originally introduced by XG-PON.
Peter Dawes
TWDM-PON uses four wavelengths to deliver up to 40 gigabits of symmetrical bandwidth and has an option for eight wavelengths overall. TWDM-PON also uses tuneable lasers enabling operators to move subscribers between wavelengths.
“FSAN operators see continued growth in PON deployment,” says Dawes. “There is still strong deployment of GPON and we are on the verge of needing 10-gigabit symmetrical services.” Other operators may delay and go straight to TWDM-PON, he says.
According to Dawes, operators are seeing a variety of applications that are driving the need for 10-gigabit access rates. One is the growing use of video and video conferencing. Another bandwidth driver for access networks is mobile applications such as connecting mobile antennas and mobile backhaul. In addition, there are digital home trends such as social networking and the moving of content to the cloud.
Mobile fronthaul can eat as much bandwidth as you can supply once you start to aggregate [radio] antennas
Operators are also keen to attach the labels ‘gigabit’ and ‘gigabit services’ to their broadband offerings as a marketing differentiator.
Other drivers for the move to the newer PON technologies include peer-to-peer services and business IP services, says Dawes.
Roadmap
FSAN’s plan to evolve the existing standards in the near term will take the group to 2021.
One obvious way the existing PONs can be evolved is to adopt 25-gigabit wavelengths. This would enable a 25-gigabit symmetrical extension to XGS-PON and a future TWDM-PON variant with up to 200 gigabits of capacity if the full eight wavelengths are used. “It is a case of looking for logical evolutions of these technologies,” says Dawes.
One application that could use such high capacities is mobile fronthaul, says Dawes: “It can eat as much bandwidth as you can supply once you start to aggregate [radio] antennas.”
After 2020, FSAN will investigate disruptive technologies as it defines future optical access schemes. R&D work, new modulation schemes and component developments including silicon photonics will all be assessed as to their suitability for future optical access schemes.
Meanwhile, FSAN says it will review its roadmap on a yearly basis and amend it as required.
See Part 1: XGS and TWDM passive optical networks, click here
Sckipio improves G.fast’s speed, reach and density
Sckipio has enhanced the performance of its G.fast chipset, demonstrating 1 gigabit data rates over 300 meter of telephone wire. The G.fast broadband standard has been specified for 100 meters only. The Israeli start-up has also demonstrated 2 gigabit performance by bonding two telephone wires.
Michael Weissman
“Understand that G.fast is still immature,” says Michael Weissman, co-founder and vice president of marketing at Sckipio. “We have improved the performance of G.fast by 40 percent this summer because we haven’t had time to do the optimisation until now.”
The company also announced a 32-port distribution point unit (DPU), the aggregation unit that is fed via fibre and delivers G.fast to residences.
G.fast is part of the toolbox enabling faster and faster speeds, and fills an important role in the wireline broadband market
The 32-port design is double Sckipio’s current largest DPU design. The DPU uses eight Sckipio 4-port DP3000 distribution port chipsets, and moving to 32 lines requires more demanding processing to tackle the greater crosstalk. Vectoring uses signal processing to implement noise cancellation techniques to counter the crosstalk and is already used for VDSL2.
G.fast
“G.fast is part of the toolbox enabling faster and faster speeds, and fills an important role in the wireline broadband market,” says Julie Kunstler, principal analyst, components at market research firm, Ovum.
G.fast achieves gigabit rates over copper by expanding the usable spectrum to 106 MHz. VDSL2, the current most advanced digital subscriber line (DSL) standard, uses 17 MHz of spectrum. But operating at higher frequencies induces signal attenuation, shortening the reach. VDSL2 is deployed over 1,500 meter links typically whereas G.fast distances will likely be 300 meters or less.
Another issue is signal leakage or crosstalk between copper pairs in a cable bundle that can house tens or hundreds of copper twisted pairs. Moreover, the crosstalk becomes greater with frequency. The leakage causes each twisted pair not only to carry the signal sent but also noise, the sum of the leakage components from neighbouring pairs. Vectoring is used to restore a line's data capacity.
G.fast can be seen as the follow-on to VDSL2 but there are notable differences. Besides the wider 106 MHz spectrum, G.fast uses a different duplexing scheme. DSL uses frequency-division duplexing (FDD) where the data transmission is continuous - upstream (from the home) and downstream - but on different frequency bands or tones. In contrast, G.fast uses time-division duplexing (TDD) where all the spectrum is used to either send data or receive data.
Using TDD, the ability to adapt the upstream and downstream data ratio as well as put G.fast in a low-power mode when idle are features that DSL does not share.
“There are many attributes [of DSL] that are brought into this standard but, at a technical level, G.fast is quite fundamentally different,” says Weissman.
One Tier-1 operator has already done the bake-off and will very soon select its vendors
Status
Sckipio says all the largest operators are testing G.fast in their labs or are conducting field trials but few are going public.
Ovum stresses that telcos are pursuing a variety of broadband strategies with G.fast being just one.
Some operators have decided to deploy fibre, while others are deploying a variety of upgrade technologies - fibre-based and copper-based. G.fast can be a good fit for certain residential neighbourhood topologies, says Kunstler.
The economics of passive optical networking (PON) continues to improve. “The costs of building an optical distribution network has declined significantly, and the costs of PON equipment are reasonable,” says Kunstler, adding that skilled fibre technicians now exist in many countries and working with fibre is easier than ever before.
“Many operators see fibre as important for business services so why not just pull the fibre to support volume-residential and high average-revenue-per-user (ARPU) based business services,” she says. But in some regions, G.fast broadband speeds will be sufficient from a competitive perspective.
“One Tier-1 operator has already done the bake-off and will very soon select its vendors,” says Weissman. “Then the hard work of integrating this into their IT systems starts.”
And BT has announced that it had delivered up to 330 megabit-per-second in a trial of G.fast involving 2,000 homes, and has since announced other trials.
“BT has publically announced it can achieve 500 megabits - up and down - over 300 meters running from their cabinets,” says Weissman. “If BT moves its fibre closer to the distribution point, it will likely achieve 800 or 900 megabit rates.” Accordingly, the average customer could benefit from 500 megabit broadband from as early as 2016. And such broadband performance would be adequate for users for 8 to 10 years, he says
Meanwhile, Sckipio and other G.fast chip vendors, as well as equipment makers are working to ensure that their systems interoperate.
Sckipio has also shown G.fast running over coax cable within multi-dwelling units delivering speeds beyond 1 gigabit. “This allows telcos to compete with cable operators and go in places they have not historically gone,” says Weissman.
Standards work
The ITU-T is working to enhance the G.fast standard further using several techniques.
One is to increase the transmission power which promises to substantially improve performance. Another is to use more advanced modulation to carry extra bits per tone across the wire’s spectrum. The third approach is to double the wire's used spectrum from 106 MHz to 212 MHz.
All three approaches complicate transmission, however. Increasing the signal power and spectrum will increase crosstalk and require more vectoring, while more complex modulation will require advanced signal recovery, as will using more spectrum.
“The guys working in committee need to find the apex of these compromises,” says Weissman, adding that Sckipio believes it can generate a 50 to 70 percent improvement in data rate over a single pair using these enhancements. The standard work is likely be completed next spring.
Sckipio says it has over 30 customers for its chips that are designing over 50 G.fast systems, for the home and/ or the distribution point.
So far Sckipio has announced it is working with Calix, Adtran, Chinese original design manufacturer Cambridge Industries Group (CIG) and Zyxel, and says Sckipio products are on show in over 12 booths at the Broadband World Forum show.