Dynamic bandwidth allocation

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.