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