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FTTC uses high frequencies over copper connections (between the cabinet and the customer’s premises). These higher frequencies make FTTC susceptible to a phenomenon known as crosstalk, which can dramatically reduce the bandwidth of the copper lines, and therefore of the connection as a whole.

Crosstalk can cause an FTTC circuit which originally delivered 80mbps to slow significantly – to 50mbps or even lower.

While 50mbps will undeniably sound fast to those still stuck on low quality ADSL lines (often delivering under 5mbps download) crosstalk is nonetheless widely considered to be the biggest issue facing FTTC and how it has been marketed and sold in this country. Businesses relying on that bandwidth to deliver internet communications to hundreds of users will be seriously impacted by such drops in bandwidth.

Bonded FTTC
The concept of maintaining bandwidth and quality

The concept of maintaining bandwidth and quality

One of our founding tenets at Evolving Networks was to break the cycle of poor broadband in the UK, introducing methods and systems to keep DSL lines working at their optimum levels, instead of allowing them to degrade over time.

Most businesses running ADSL connections have experienced the feeling that their ADSL line is slowing down over time. Daily drops in connection, reductions in available bandwidth, and rises in latency and packet loss have led to a growing feeling that broadband just isn’t that great after all.

However, in stark contrast to other ISPs, Evolving Networks is able to deliver broadband and keep it healthy, through a combination of constant monitoring of all circuits, advanced algorithms detecting changes in line quality, and the latest broadband chip sets, capable of customising key line metrics.

Other factors affecting broadband speed and stability

Before considering crosstalk, it is important to understand the other every-day reasons for poor performance of broadband lines, such as ADSL and FTTC. These are common to most ISPs’ networks.

Congestion and contention

Network contention and BT host link congestion are the principal causes of slow line speed.

ISPs buy host links from BT, which are expensive. In order to cut costs and offer lower prices to consumers, they cram as many customers onto those links as possible. This results in slower performance at various points in the day, when large numbers of customers are all trying to access the internet at once.

Likewise, inside an ISP’s network, over-subscription is common, with large volumes of customer traffic pushed through routers not designed to cope with them. Running the network “hot” like this causes generally slow performance for end users, and increases latency as packets queue up in the network.

If the ISP doesn’t have up to date routers with advanced packet queuing mechanisms, the problem is exacerbated with packets dropped entirely – packet loss of over only a few percent causes huge drops in performance, with multiple retransmissions needed to achieve complete receipt of the data stream.

Poor quality wiring

Any interference on the copper wiring delivering broadband will cause reductions in performance. It’s very common for old wiring in a property to cause significant drops in bandwidth on all forms of ADSL and FTTC.

Having high quality, undamaged internal cabling and DSL microfilters, and minimising internal extension leads all help keep interference down and line speed up.


Modems vary widely in terms of quality and DSL circuit and type compatibility. Poor quality modems will sync only at lower bit-rates and higher Signal-to-Noise Ratio (SNR) margins, meaning lower throughput for the end user.

Bad modems can also be unstable, dropping out frequently or needing reboots to regain connectivity.


Crosstalk is electromagnetic interference between cables running parallel to each other, which has a negative impact on line performance.

Crosstalk is not a new concept in the world of engineering and specifically applies to FTTC circuits, where the presence of a signal being transmitted on one cable causes degradation of the signal in a neighbouring circuit.

DSL circuits, and in this case FTTC, use copper cables known as pairs, or twisted pairs. Each circuit terminates in a wall socket, from which 2 thin copper wires, twisted together in a simple helix, runs back to the street cabinet.

Twisting pairs of wires is one of many techniques for reducing electromagnetic interference, whether from external sources, or from neighbouring cables. Shielding the cables is another method, as well as structuring the cables with cores of various materials.

The bundles of cables that run from houses and businesses to Distribution Points and Cabinets in the Openreach network are generally not individually shielded, rendering them susceptible to crosstalk, as increasing numbers of FTTC lines come into use.

The copper pair causing the interference is known as the Disturbing Pair, while the circuit being affected by it is termed the Disturbed Pair.

NEXT (Near End Crosstalk) is a measure normally associated with the specifications given for cables on how they deal with crosstalk. Minimum NEXT values are usually expressed in decibels per feet or decibels per 1000 feet. It is important to note that NEXT values vary with the frequency of the transmission. This is why FTTC, operating at higher frequencies, is so susceptible to crosstalk, yet is not a significant issue for ADSL.

FEXT, (Far End Crosstalk) is the term used to describe the interference between the two cables from the perspective of the furthest point from the transmitter. This can be a little confusing with FTTC as an FTTC user is both transmitting and receiving.

Essentially, groups of modems generating signals at the customer end of lines can cause FEXT close to the cabinet, and vice versa.

A solution to the problem is under development: Self-FEXT Cancellation, also known as vectoring.

Crosstalk reduces FTTC sync rates. It really is that simple. It may only reduce the rate by a few mbps, but its effect is unpredictable because of lack of information on the circuit at hand, and those around it.

For example, all of the following are typically unknown.

  • The exact route the cable takes to the cabinet
  • How many other cables run in the same bundle
  • What distance the cables run together for
  • How many other cables are carrying FTTC (this typically increases over time as more customers in the area upgrade)
  • Where in the bundle the circuit under consideration is in relation to those other FTTC circuits: if it is in the centre of the bundle, it will be likely to experience higher levels of crosstalk than if it is on the outer edge

Whether or not the other FTTC lines are in use or not is irrelevant. The simple presence of a signal on the circuit will cause crosstalk. As a result, busy periods will not result in worse crosstalk than quiet times – the effect depends on the number of subscribers in the local area and how their cables interact with the circuit at hand on their way to the cabinet.

Crosstalk was often mentioned as a potential risk for ADSL, but in practice it has not had any serious effect on the UK infrastructure.

This is because ADSL frequencies are quite low, and crosstalk affects higher frequencies more than lower. This chart shows the frequency ranges used by ADSL 2+ and VDSL2, the technology behind FTTC.

Frequency ranges used by ADSL 2+ and VDSL2

VDSL2 uses signal frequencies above 10MHz, approaching 30MHz – much higher than ADSL’s 1-2MHz.

Vectoring as a way of cancelling crosstalk

The G.vector standard (ITU-T G.993.5) was devised as a way of cancelling out the interference caused by crosstalk on VDSL2 lines. Vectoring is a noise cancellation technique that limits crosstalk on groups of VDSL2 lines, eliminating the bandwidth limitations that would otherwise be caused by interference between them.

Based on constructive interference (in which a signal is cancelled out by a precise mirror image of itself) signal noise cancellation is not a new concept, having been around for a while in applications such as noise cancellation headphones. Vectoring for FTTC relies on the same basic principle.

How does vectoring work?

Vectoring is a dynamic service, continually measuring and cancelling out crosstalk across affected lines. It is designed to restore near optimal performance to FTTC lines impacted by crosstalk.

By sending signals equal and opposite to the crosstalk interference (known as noise) the interference can be cancelled out, restoring a stable, fast sync rate – much faster than is possible without vectoring.

Vectoring can’t eliminate crosstalk noise completely, but it can reduce it to almost zero, which is sufficient to return the FTTC circuit to near expected bandwidth.

The technology is highly complex – much more so than headphone noise cancellation technology. The hardware needs to be able to cope with hundreds of lines at the cabinet, and with a much wider frequency spread than normal audio.

Working by processing each tone in the VDSL2 frequency spread individually (see how FTTC works using DMT) it is highly processor-intensive. Around 4000 operations per second are performed for each tone processed. When dealing with hundreds of FTTC lines together, this multiplies up to staggering numbers, in the trillions of operations per second arena.

As a result, vectoring requires additional computing power at the cabinet: only through recent developments in chip manufacture is vectoring possible.

This is one of the reasons why FTTC has been pushed by providers across the world before the crosstalk problem has been solved. Some improvements are delivered in the short term (FTTC impacted by crosstalk will typically be faster than ADSL) and speeds will increase again when services are upgraded to include vectoring.

The UK FTTC vectoring roll out

In the UK, vectoring on FTTC is only available in two BT trial areas: Barnet and Braintree.

Although now on its second round of testing in these areas, Openreach is still looking at exactly which technology will be used to eliminate crosstalk. Vectoring can be implemented in a number of different ways, and BT use both Huawei and ECI cabinets: there is still much testing to be completed before the roll out can continue in other parts of the country.

Once BT Openreach has made decisions on the viability of the available options, they will start to roll vectoring out to more areas. For now, though, we have to wait to see what options they choose, and whether there will be a price increase for something that merely enables the speeds that were promised in the first place.

UK FTTC vectoring roll out

BT’s decision to cap FTTC speeds

The decision by BT to cap FTTC bandwidths, initially at 40mbps (download) and subsequently at 80mbps, was made because of the expected effects of crosstalk, and because vectoring was not ready during the FTTC commercial roll out.

By way of comparison, the VDSL2 standard defines 200mbps as the download limit, although 250mbps is theoretically possible with the shortest runs of copper.

FTTC Speed Predictions

The first FTTC product BT Openreach released, using only the 12 MHz frequency profile, was capped at 40mbps download. However, the actual maximum throughput of this frequency is 68mbps. The difference is down to crosstalk.

Openreach’s current FTTC product, capped at 80mbps download, uses the 17 MHz frequency profile. This has a maximum download capacity of 100mbps, but, again, the reduced bandwidth cap was chosen because, as soon as multiple lines are installed, the top 20% of bandwidth disappears as a result of crosstalk.

In reality even more than that can go, and customers all over the country are already seeing their lines’ sync rates dropping even below the cap, because of crosstalk.

Openreach’s decision to cap download and upload speeds has helped reduce the impact of crosstalk, but, nonetheless, the effects are still being felt, and for most the reduction in capacity is significant.

Is my line affected by crosstalk?

Unfortunately there is no official way of finding out if any given FTTC line is being affected by crosstalk, or whether it’s likely to be affected in the future.

There is also no way of knowing for sure whether any drop in speed is due to a fault, or to crosstalk. See the FTTC guide for details on fault thresholds and how to predict the speed of FTTC lines.

The following graph shows estimated effects of crosstalk and vectoring on full capacity line speed.

Is my line affected by crosstalk?

The effects of crosstalk and vectoring on FTTC sync rates

Estimated effects of crosstalk and vectoring on full capacity line speed

Comparing this with the BT broadband availability checker’s graph of the Impacted and Clean ranges, and including the lowest 10% of speeds, it does look as if the Range B figures quoted may be based on the potential effects of crosstalk.

The details from BT are, though, ambiguous, and often contradictory, when it comes to how the speed of a line should be predicted, and how and when crosstalk may have an effect. You can read more about this on the FTTC page.

For now, we have to accept that line rates will drop over time, and that until vectoring is fully deployed, FTTC circuits will not run at full speed.

Addressing the issue

While currently crosstalk on FTTC cannot be addressed, Bonded connectivity and SD-WAN solutions from Evolving Networks offer long term, scalable solutions for organisations requiring dependable, high-capacity connections. Visit the Solutions section of this website for more details.

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