Broadband Access Technology Comparison

The goal of this document is to provide a handy, convenient and accurate comparison of the available broadband technologies including DOCSIS 3.0 (and 3.1), vectored VDSL technologies and G.fast variants, and G.hn which is used as an access technology (G.Now and Korea Telecom GiGa Wire™) and the most recently announced MoCA Access™.

There is much information but no singular repository or document which fairly and accurately represents all the broadband technologies, thus MoCA contracted with Peter White and his team at Rethink Research to produce such a document. The firm gathered and analyzed the data which already existed in their database, from discussions and demonstrations at tradeshows and conferences, publicly available documents and by lengthy interviews with companies such as Nokia, MaxLinear, Arris, Sckipio and Adtran. Rethink Research also spoke with a few key operators and Professor John Cioffi.

Throughout the information gathering process, an attempt was made to better understand attenuation, QoS layers, latency, security, real world performance, and whether the networks can operate with or without a home gateway. Additional attention was given to speeds the protocol can support upstream as well as downstream. This typically meant establishing whether the protocol is TDD or FDD based, and if the FDD spectrum or the TDD timings, are fixed, programmable and/or dynamic.

This paper also captures other relevant details such as whether vectoring is required or built in, if system is point-to-point (PtP) or point to multipoint (PtMP), how many clients it can support in the real world, and where in the spectrum the usable bandwidth resides and took that into consideration when addressing actual throughput.

G.fast, VDSL and their variants

VDSL has been around a long time. There are significant variants which use 8.5 Mhz, 17.7 MHz and 35.33 MHz and a second version of VDSL (VDSL2) which supports longer distances and use vectoring, speeds up to 360 Mbps have been achieved when the DSLAM connection is under 100 meters from the home falling all the way back to 50 Mbps as far as 1 kilometer from the home.

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VDSL2 mostly adhered to the 17.7 MHz version (at a top rate of 130 Mbps) and the 35 MHz version (VPlus or 35b). These are often used as operators migrate to G.fast. In some cases telcos replace VDSL with a combination of 35b alongside G.fast, each in different parts of the network. The selection is made based on loops lengths, especially if no greater benefit than 35b’s 300 to 360 Mbps is possible.

Unlike coaxial cabling, unshielded twisted pair is not protected from interference. These pairs tend to have been installed for VDSL in large bundles so the likelihood of crosstalk interference is high.

Vectoring was invented to record and cancel such crosstalk. The calculations for this reside either in the same device in a separate component within or close to the DSLAM, or a nearby processor which can take on the crosstalk workload for multiple DSLAMs. Typically this resides in a low volume ASIC or FPGA, which could adversely affect cost.

The closer G.fast and VDSL components are located to the home, the fewer cables there are in a single binder creating crosstalk. This actually makes managing crosstalk easier. Sckipio has a distributed crosstalk cancelling system across multiple DPs (distribution points) which cancels crosstalk on up to 24 lines each. They talk to other G.fast DPs to share crosstalk information on other pairs in the same binder.

G.fast itself comes in two flavors, using baseband frequencies up to 106 MHz and the more recently introduced systems reaching 212 MHz. When these sit alongside 17a or 35b VDSL devices, the crosstalk has to be coordinated between the two types of systems (as one is FDD and the other TDD), but also part of the spectrum (0 to 17.7 MHz or 0 to 35 MHz) can have no G.fast signals resident at all as it will impede performance.

G.fast is organized by time, not frequency. In principle this means a system could dynamically allocate more time in one direction when needed and later change to accommodate more traffic in the other direction depending upon load. All broadband lines have had to accommodate changes to user habits, where there are more apps which upload data and which are increasingly creating large file sizes. An example might be UHD video from onboard cameras. It is these applications and greater need of bandwidth which is changing the asymmetric nature of broadband to one which needs to be dynamically reversible.

But this is not easily done despite G.fast chips adopting TDD. Crosstalk from the client (far end cross talk or FEXT) and crosstalk from the DP (near end cross talk or NEXT) are also affected by attenuation. To cancel them, a complex calculation related to their declining power at various points on the line is needed.

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A recent development standardized under the ITU, is called Collective Dynamic Time Assignment (cDTA). It allows the DP to adjust up/down ratios in response to traffic demands automatically, squeezing more out of the theoretical two-way capacity of a line. Sckipio insists this actually allows all of the capacity in either direction when needed, which it claims enlarges the area over which G.fast can be effective. It remains true that if crosstalk cancellation was not needed these complexities would not be required. Fundamentally if twisted pair (TP) is used, crosstalk cancellation (vectoring) remains necessary.

DOCSIS

A recent change to DOCSIS has been the move from using 6 MHz channels – the equivalent of a traditional analog TV signal – as the core building block for data. These had to be multiplexed in numbers as high as 32 separate channels to provide sufficient bandwidth in DOCSIS 3.0. In DOCSIS 3.1, the system relies on basic OFDM using sub-carriers between 20 KHz and 50 KHz. These can be multiplexed together with up to 192 MHz of spectrum (32 channels) for one user at a time.

DOCSIS 3.0 tops out at 1.2 Gbps real world downstream. Upstream capacity is capped at 200 Mbps.

DOCSIS 3.1 has a theoretical upper limit of 10 Gbps downstream and two (2) Gbps uplink. Real world performance is closer to 7.3 Gbps downstream and 1.4 Gbps upstream as maximum upstream capacity, when using 4 separate OFDM clusters of 3840 sub- carriers using up 768 MHz. Real world services will be configured way below that for some years although the standard has room for maneuver, but the price of CPE and backhaul are the limiting factors. Comcast currently offers a DOCSIS 3.1 service with one Gbps download and 35 Mbps upload.

In DOCSIS 3.0 environments, typically only 300 Mbps home gateways are supported in the U.S. Anything requiring faster speeds, such as enterprise, services tend to use fiber all the way to the building. DOCSIS 3.0 spectrum begins at 111MHz and can go all the way up to 1002 MHz.

DOCSIS 3.1 can use up to 1.2GHz of spectrum if the plant allows it and theoretically all the way up to 1.8 MHz. Usually the lion's share of any spectrum (about 80 percent) is allocated to downstream traffic. It would be more effective of course, if both upstream and downstream could use the same big block of spectrum. If DOCSIS 3.1 is ever configured to use all available spectrum on its plant, it will mean that any MoCA connections, especially MoCA Access, will have trouble coexisting with it, using just 500 MHz of the highest frequency spectrum (which has the highest attenuation) available to it. Increasingly, DOCSIS will be unable to coexist with other coax technologies. Currently, G.hn cannot coexist with DOCSIS without the use of a special RF front end.

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Similarly it makes little sense to look in detail at the latest DOCSIS Full Duplex technology, which is still in laboratory trials and by no- means deliverable with any certainty regarding price or timeframe. Full Duplex relies on receivers which treat all communication in the reverse direction as if it were interference, so that both directions can have the full capacity of a DOCSIS channel. This is the cable dream, to push 10 Gbps in each direction through multiple bonded channels, at some future point.

G.hn

Korea Telecom offers a version of G.hn branded as GiGAWire™ which is marketed in South Korea and worldwide. The chips were formally branded as G.Now by Marvell, but the product line has since been acquired by MaxLinear. It was initially available at 500 Mbps performance but is now capable of 1 Gbps. KT claims to have installed 100,000s of MDU lines with this technology.

The Wave 2 G.hn chips which emerged during 2016 use a full 200 MHz of spectrum to deliver 2.0 Gbps PHY rate, of which about 1.5 Gbps is usable. G.hn uses up to 4096 OFDM subcarriers in coax and the spectrum can be shifted up into RF frequencies with an RF front end to avoid baseband and other RF segments.

When operating over twisted pair however, G.hn does not have full crosstalk cancellation. Consequently, speeds are somewhat lower and KT does not use it at any faster speed than 1.2 Gbps across both directions (1 Gbps down and 200 Mbps up).

G.hn is TDD so any performance numbers for it are an aggregate of up and down speeds. It is installed at distances as short as 50 meters in twisted pair but also as long as 150 meters where performance falls below 1 Gbps due to attenuation. G.Now chips have interference mitigation when used in the powerline version, and also mitigation of near-end crosstalk (NEXT) when used over twisted pair, but not full crosstalk cancellation. Typically one fiber supports 24 point to point twisted pair users in the Korean installation. These replaced pre-vectored VDSL units and were designed simply to plug into the VDSL slot.

MoCA Access

Though known for its home networking technology, the Alliance recently announced MoCA Access. It is based on the MoCA 2.5 specification, announced in 2016, and is capable of 2.5 Gbps upstream and 2.0 Gbps downstream throughput. MoCA Access is point- to-multipoint serving up to 63 modems (clients). It is designed to co-exist with legacy services such as TV, DOCSIS, and cellular (4G/5G) technologies. The operating frequency range is 400MHz - 1675MHz.

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The network controller in an MDU installation, for instance, is typically found in the basement and serves multiple locations in multiple apartments on the same coaxial loop. This might be a few as four apartments or up to 63, though with multiple connections per home this will likely support far less homes, perhaps 16 to 24.

Like all cable technologies MoCA Access has multiple power settings. The basic mesh network can survive 45 dB of path loss, has two more powerful setting of 55dB or 65dB path loss to ensure that it can deliver its full 2.5 Gbps at up to 1,000 foot connections when delivered through RG-6 cable.

Modulation Scheme

Modulation schemes vary between basic DMT modulating between 2,000 and 8,000 carriers on TP wiring, up to the 4096 QAM modulation of DOCSIS, which has options above at 8K and 12K. Higher orders of QAM are available on Coax simply because it is protected, and therefore there is less noise. But the maximum modulation scheme is no great indicator of overall performance, and all of these systems fall back gracefully to more robust modulation schemes which carry less data, once they meet interference, ingress noise or attenuation. The data rate is a combination of the modulation scheme and the number of carriers it uses.

Latency

Latency is a function of processing before and after sending, plus the time it takes for signals to get from A to B. When storage buffers are included there can be major problems. G.fast talks point-to-point as a single agency channel so there is little need for any scheduling, and so has far less processing which helps keep latency to a minimum. Each direction has a time to talk in a TDD environment which builds in a certain amount of delay. In point-to-point environments like G.fast, there is no MAC overhead to process. A MAC is a protocol to arrange traffic in a point to multipoint system.

All of the access technologies referenced in this document use interleaving and FEC (Forward Error Correction) and this creates a varying amount of pre- and post-processing. These technologies allow broadband systems to go closer to their maximum possible throughput, lowering interference and making the signal more tolerant of errors. But they come with a processing overhead which can affect latency. If each of these jobs has to be taken in turn and storage buffers used, then it can have a massive effect on latency.

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DOCSIS 3.0 has suffered from this hugely in what everyone came to call “bufferbloat.” DOCSIS created queues in storage when the TCP protocol pushes the network to the point where it drops packets. CableLabs introduced Active Queue Management to eliminate this but it is not installed on all DOCSIS systems.

High latency is particularly worrisome among gamers who rely on fast reaction in real time. Many games require latencies below 100 ms and some as low as 20 ms.

TDD or FDD

The only access technologies addressed in this document that are FDD are VDSL and DOCSIS 3.0, both of which have a hardwired amount of spectrum for each direction. This is an historical accident, due to early broadband apps being primarily downstream. Both of these technologies are integrating TDD or have publicly promised to introduce technology which will change this. DOCSIS is at the largest disadvantage, as real world maximum upstream speeds on DOCSIS are seen as its weakest point.

Max Upstream speed in Mbps

The maximum upstream speed limitations are sometimes artificial and sometimes a result of the way the technology has been designed. G.fast has been architected to remove those limitations which were characteristic of VDSL with 90 percent of capacity switched upstream dynamically using cDTA. Few G.fast chips currently have this feature.

For G.hn, up and downstream traffic is dynamically allocated for each link. This suggests that upstream and downstream is more or less the same with the configuration managed from a neighboring network processor. However the only known operational G.hn access service at Korea Telecom limits upstream traffic to 200 Mbps.

MoCA Access can operate at 2.5 Gbps downstream, and each individual client can talk upstream at 2.0 Gbps, with the aggregate upstream rate across multiple devices reaching 2.5 Gbps in total. This means it is well placed for the increasing upload patterns in broadband access.

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Security

Each of these systems rely on different security architectures. The point –to-point protocols tend not to have onboard encryption, so physical security is far more important or encryption has to be built into network processors or a VPN has to be used across them. All the point to multipoint services suffer from consumer concern that their traffic may end up on someone else’s device and be readable, and distribution services have to be built into the MAC to distribute packets. Early levels of DOCSIS only had a privacy system called Baseline Privacy Interface, upgraded in DOCSIS 3.0 and beyond to encrypt data between the CMTS and cable modem using a 56-bit DES encryption, which can go to 128-bit AES. It has automated key refreshes.

G.hn has point-to-point security within each domain, so each transmitter/receiver pair use their own encryption key, not shared by other devices in the same domain. G.hn uses the concept of relays, where one device takes a message from a node and delivers to another, but this only requires the reading of headers, which are in the clear, not the entire message which is encrypted.

MoCA uses an encryption system similar to Wi-Fi’s WPS called MPS. This sets up a PKI key via a button push at installation. This key set up is protected by an Elliptic Curve Diffie-Hellman protocol designed to pass keys securely and assumes it is being watched and recorded. This process uses a shared secret 256-bit key and the nodes then extract a protected channel key. This is also similar to how DTCP IP sets up keys for security and is more advanced than the other architectures simply because it was designed so much later.

QoS

VDSL2 and G.fast transceivers (and most of the other technologies) are always used in conjunction with a network processor. In telco networks, this is where QoS has to be isolated for different traffic classes. Both are point-to-point protocols and do not have native QoS services, which have to be arranged from a network processor prior to delivery.

G.hn and MoCA Access are both point-to-multipoint with native QoS. While the policy for this usually still resides in a network processor or bandwidth manager, it is supported with multiple service layers actually on the network. For G.hn, this means seven classifiers with four queues per port. For MoCA Access, it is eight QoS service levels with four levels of priority which is similar to DOCSIS which has support for four service types.

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Summary

The chart below is a comparison of the relevant features and capabilities of the previously identified access technologies. Obviously there are pros and cons to each and implementation decisions are likely to be based on a variety of considerations including performance (down and upstream), latency and attenuation over distance and type of wire.

The following charts compare the various access technologies based on a variety of features and capabilities, and are divided into twisted pair and coax for readability.

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Twisted pair

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Coax

*** (8k and 12K optional) *** Up to 4 OFDM channels with 3840 sub-carriers of 50K each *** (1794 MHz Optional)

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