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The UK's First Software Defined Access Network

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Automating the Wide Area Network - Why you Need an SD-WAN

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Despite often being referred to as Fibre Broadband, FTTC, as the abbreviation suggests, only uses fibre optic cable as far as local street cabinets – what BT call PCPs (Primary Cross-connect Points). These are the dark green, metal, roadside cabinets that supply telephone and broadband lines to nearby businesses and consumers.

BT has delivered the first phase of FTTC as part of its commercial roll out in the UK, and is now working with local authorities and the government’s Broadband Delivery UK (BDUK) team to start filling in the gaps and delivering more FTTC (and some Fibre To The Premises – FTTP) to other areas.

There are now diverse circuit technologies with acronyms beginning FTT (Fibre To The…) which differ considerably in nature. Many are DSL based, and so still have copper as part of their delivery, but others are fibre all the way to the customer’s premises.

FTTC is defined as a Next Generation Access (NGA) technology, regardless of how fast the connection delivered is.

What is Bonded FTTC?

Is FTTC just VDSL2? What happened to VDSL after ADSL?

As explained above, FTTC involves moving the termination point of the DSL lines closer to the customer, shortening the run of copper and increasing the speed of the connection. It could easily have been called Copper to the Cabinet, just as ADSL could be called Fibre to the Exchange.

FTTC is often referred to as “fibre broadband”, in an attempt to show that it mixes what has long been held as a definition of broadband, with the hi-tech and desirable element, “fibre”. Sadly, fibre broadband has been over-hyped and sold as the answer to all internet access problems.

This page sets out the differences between FTTC as an access technology and older circuit types like ADSL, showing that it is also a DSL based copper access technology with many of the same issues and characteristics.

FTTC could have been named after the standard on which it is based: VDSL2 (defined in ITU-T G.993.2, finalised in 2005) the second generation of the VDSL standard (Very high bit-rate DSL). Like ADSL, it is an asynchronous technology needing a modem and a DSLAM at each end of the line.

Some ISPs do call the service VDSL, but because BT wholesale it as FTTC (and nothing beats the BT marketing machine!) the latter name has more traction in the business marketplace. Services marketed under the two different names are essentially the same, though.

To future proof their FTTC investment in the UK, BT chose VDSL2 as their FTTC access technology, instead of VDSL, because the newer standard supports a synchronous rate of 100mbps. VDSL only supports 55mbps downstream and 3mbps upstream.

How does FTTC work?

ADSL uses copper wiring all the way from the customer’s wall socket to the telephone exchange. VDSL2 aims to increase the speed of the line by using high frequencies, which only work on shorter runs of copper.

This is why the DSLAM – the exchange equipment the DSL line normally connects to – is essentially moved to the street cabinet, for FTTC connections. Runs of fibre optic cable are then made to each cabinet (new cabinets in fact, sited near the old ones) giving the rise to the name “Fibre to the Cabinet”.

FTTC Journey

FTTC modems on the customer’s site establish their connection with the “Street Side DSLAM”, which is located in the cabinet. Because in general end users aren’t that far from their cabinet, they benefit from faster broadband speeds. More on that later though, as it’s not always the case, and depends greatly on location and population density.

As described on the ADSL page, the UK infrastructure is one of telephone exchanges, cabinets and distribution points. As long as an exchange has ADSL equipment, anyone served by a connecting copper pair from the exchange can have an ADSL service, so long as they aren’t too far away, although the reach of ADSL is now very good.

With FTTC, not only does the customer need to be on an FTTC-enabled exchange, but also connected via an enabled cabinet. Not all exchanges have been (or will be) upgraded to FTTC, though, and to complicate things further a “parent-child” relationship exists between exchanges: a customer can be connected to one exchange for regular phone lines and ADSL, but its new FTTC cabinet can be served by a different exchange.

Higher frequencies – still DMT

As this chart shows, VDSL2 uses higher frequencies than ADSL on the copper wire, increasing the bandwidth available.

Higher Frequencies

As with the upgrade from ADSL to ADSL Max, and then to ADSL 2+, a wider range of higher frequencies is used by VDSL2. However, the same method of distributing bits into bins or channels remains. Discrete Multi-Tone (DMT) is a way of modulating the line so that multiple frequencies are used, avoiding noise and interference. DMT also addresses attenuation – the effects on connection speed of lengthening the copper line.

As with all other variants of DSL, the customers’ location is vitally important, and in particular their distance from the cabinet. Whereas ADSL’s key issue is the distance to the exchange, FTTC performance depends on the distance to the cabinet. It may seem that this should always be a plus, since surely every cabinet is closer to the customer than the exchange, but that isn’t always the case. In particular, because of the way DSL works, the fastest speeds are only available closest to the DSLAM. This chart shows the relationship between distance to the cabinet and the sync speed of the line.

FTTC (VDSL2) sync rate and copper line length

As you can see, there is a drop off where the attainable speed reduces more dramatically with the increase in the length of the copper line: loop loss (attenuation) increases with distance. This is where DMT comes in, ensuring only the frequencies that work are used. On those longer lines, the frequency spread and the bandwidth will look a lot more like ADSL again, despite the upgrade to a so-called NGA technology.

FTTC (VDSL2) sync rate and copper line length

How fast can FTTC go?

The standards defined for VDSL2 aren’t restricted to asymmetric speeds, and are split between different sub-profiles, for use in different deployments. In the UK, BT first released their Infinity product, along with the first FTTC wholesale products, capped at 40mbps downstream and 10mbps upstream. These were based on the 8a-d and 12a and 12b profiles. Shown below is a diagram of the different profiles, and how the frequencies are spread over the line.

How Fast can FTTC go

With the introduction of the 17a profile, BT boosted the top speed of connections to a capped figure of 80mbps downstream and 20mbps upstream. Because of the distance dependency, however, not every location will enjoy the benefit of the increased frequencies, as is it those increased frequencies that are lost through attenuation over longer copper pairs.

With higher bandwidths comes a further difficulty, though: interference between circuits, known as crosstalk. As more FTTC circuits are delivered throughout the country, crosstalk results reductions in overall sync rates, and therefore the speeds seen by customers. The severity of crosstalk depends on the number and density of circuits at various points in their delivery.

See this page for further information on crosstalk, and how vectoring will improve the situation. At present, vectoring only exists in testing, extra equipment may be needed to make it work in certain cabinets, and its roll out has not yet even been planned.

With all of BT’s DSL-based products, there is an initial phase, now completed for FTTC, during which a modem and faceplate filter are included as part of the installation, which must be undertaken by a BT engineer. As of late September 2014, FTTC’s initial phase is over, making it easier for providers like Evolving Networks to simplify the installation process and deploy alternative, better hardware.

FTTC with long lines

The distance from the customer premises to the street cabinet is is vitally important in determining whether or not the line will support FTTC. Unfortunately, BT supplies conflicting information on when they will and won’t supply an FTTC service.

For customers on long lines suffering less than 2mbps on an ADSL service, even an upgrade to 5mbps would be a useful boost. FTTC is often denied to those users, while customers connected to the same cabinet and marginally closer to it are allowed the faster speeds.

VDSL2 was designed to be able to serve long lines, unsuitable for VDSL. In this respect its purpose is essentially to replace ADSL, allowing old ADSL kit to be decommissioned in favour of VDSL2 hardware that can cope with both short and long runs of copper. There are still many areas where customers are not close to their street cabinets, a situation which will not change in the near future.

There are newer technologies, which in the future will be available to further shorten the length of copper wire, without having to build new street cabinets. These technologies will involve new nodes being deployed on poles and in other distribution points. This is discussed further here, when talking about Fibre To The Remote Node (FTTRN) and Fibre To The Distribution Point (FTTdp).

BT’s commercial FTTC roll out has now been completed. Further installations are being performed as part of BDUK, the government project to help plug some of the gaps in delivery, funded by central government and local councils across the country.

Each project is run separately by local projects such as Connecting Cambridgeshire and Superfast North Yorkshire, with individual local authorities having their own targets and installation timeframes.

Most of the money for FTTC has gone to BT as the primary supplier. There are question marks over how the tender process was run, as other major suppliers pulled out, essentially leaving BT as the only credible supplier.

Furthermore, in addition to the BDUK roll outs all having different FTTC coverage targets, determination of the likely bandwidths for each location is often vague, to the extent that it can be unclear whether FTTC will actually deliver any more bandwidth than the minimum 2mbps, itself now seen as the base level for any form of broadband. Timescales also vary and are generally broken into distinct phases, with some areas getting FTTC before others.

There is no central place where all the BDUK information is officially compiled, although has a good list, showing the varying targets and promises.

Determining how fast a service any given FTTC line will deliver has become an unnecessarily complicated process over the last few years.

With ADSL circuits, there is a simple estimate, based on length of the copper pair, giving a figure and an estimated range of possible speeds. When UK deployment began, this was the approach used with FTTC circuits, but more recently, BT have updated their checker to include a staggering eight different values in FTTC line speed predictions.

With lines split into Range A and Range B, and then split again into high and low figures for download and upload, assessing the speed that any new circuit is actually likely to deliver has become difficult indeed. The ranges are determined by the circuit’s predicted loop loss (attenuation) and read from a table of values which matches loss with predicted values. The High and Low figures are actually the 80th and 20th percentile values.

This in itself has important ramifications, because that means 20% of customers are likely to be below the Low value. There is a hidden third value that is even lower, which represents the 10th percentile value for downstream (in both Range A and Range B). This is the figure that BT considers sufficiently low that the customer has the right to terminate their new FTTC circuit within the first 90 days.

FTTC Speed Predictions

The customer could get any speed within a very wide spread. An 80mbps line could be 58% slower before being considered by BT to be too slow. A 30mbps line could be 72% slower. A line more like an ADSL2+ service (which has a top end value of 18mbps) could actually run a whopping 78% slower, at 2.8mbps.

To further compound matters, and the longer the line is, the greater the percentage drop in value from top to bottom will be.

It’s important to note that there is no BT check, either before or after a circuit is installed, as to whether the BT system thinks that a line is either Clean or Impacted. This makes it very difficult to give any level of certainty to potential customers over how their service should perform.

This range is for circuits that are deemed to be free of any copper line faults or wiring issues in the customer’s premises. This range is often very wide for both upload and download, and does beg the question of why a copper line fault may exist without being fixed.

This range is for circuits that may have copper line faults and/or internal wiring issues at the customer’s site. The term Impacted could be useful, as crosstalk is is sometimes mentioned as impacting on line speeds, but there is no official word from BT on whether either Range A or Range B takes into account the effects of crosstalk. You can read more about this here.

The trouble with all of the above ranges is that defining which lines will be Clean and which will be Impacted is impossible. There is no way of determining if a line is subject to crosstalk from neighbouring lines. There is no way of predicting if, in either range, a line will be at the High end, the Low end, or even lower.

BT’s FTTC products are capped, raising the question of how the 80mbps High value can be an 80th percentile figure, when bandwidths higher than this are not available: what are the top 20% getting if they can’t get higher than 80mbps? With the hidden 10th percentile figures taken into account, a line with a predicted top speed of 80mbps as its High Range A value could actually run as slowly as 33.89mbps. If it didn’t run lower than that after its 10 day training period, it would be considered a working circuit, delivering what BT intended.

Because of this, the scope for overly high customer expectations is extensive. Over-hyped marketing of FTTC as a saviour technology, coupled with many customers being more than 500m from their cabinet, is resulting in end users slowly discovering that FTTC is not always all it’s cracked up to be.

Reporting Faults with FTTC Lines to BT

Because of the complex nature FTTC service speed prediction, it’s vitally important to understand what BT consider to be an acceptable service: when a fault can be reported, and when it can’t.

There are 3 BT FTTC products, all with capped top speeds:

  • Up to 80mbps down, with up to 20mbps peak up
  • Up to 40mbps down, with up to 10mbps peak up
  • Up to 40mbps down, with up to 2mbps peak up

They all have the infamous “up to” qualifier, and all the speeds depend on the length of the copper line from the wall socket to the street cabinet. BT consider that environmental issues within an end user’s premises and crosstalk are both factors that may result in speed degradation. Given the broadband availability checker is used to set these expectations with customers in the first place, it would appear that crosstalk is not being taken into account with the Impacted Range B figures, and end users should expect further drops as more FTTC users go live.

BT say sync rate is a poor indicator of faults

BT now state that FTTC line rate (sync rate) is not a good indicator of a copper network fault. They say this because all lines are susceptible to external noise which can reduce the line rate, and because over time, as more FTTC subscribers are enabled, line rates will drop as a result of crosstalk.

Furthermore, BT do not clarify whether the Impacted Range B figures include the predicted effects of crosstalk on sync rates, leaving it ambiguous as to what they will consider a fault. What they do have for fault determination are some lower speed thresholds and a standard for significant line speed degradation over a short period of time.


Fault thresholds for FTTC lines

For lines that achieve a higher than 2mbps sync rate, any drop to below 2mbps is classed as a fault. This is not automatic, but will be accepted by BT for investigation.

If the sync rate of any FTTC line drops by more than 25% over a 14 day period, then a fault can be raised. Again this is not automatic, and not something that is tracked by the vast majority of ISPs.

A slower degradation of speed is not accepted as a fault by BT: if a line lost 25% of its sync rate slowly over the course of a month or two, then this would not be classed as a fault. This may be a hint that as new lines are slowly added, the effects of crosstalk are slow to manifest, so only sudden changes in line speed will due to other factors, and thus likely to be a genuine fault.

Bonded FTTC

Bonded FTTC lines can now deliver similar bandwidths to faster leased lines. Even FTTP currently tops out at 30mbps upload – if more upload bandwidth is required, Bonded FTTC will deliver it, as well as the added resilience that bonding broadband lines always provides.

To find out more about how Bonded FTTC could help your business, call 0330 55 55 333 or email us today.

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