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Scientists Learn from ADSL to Boost Fibre Optic Broadband Speeds

Tuesday, April 24th, 2018 (9:34 am) - Score 2,409
fibre optic cables and uk network server setup 2017

A team of Dutch scientists from the Eindhoven University of Technology (TU/e) have borrowed a technology from existing copper based ADSL and VDSL (FTTC) networks in order to boost speeds over pure fibre optic (FTTH/P) lines, such as by implementing adaptive modulation techniques.

Generally speaking ordinary consumers and businesses aren’t going to need to worry about the limitations of optical fibre cables, at least not for a very.. very long time. In the UK we can already see existing home packages that run at Gigabit speeds (few can make full use of such speeds today) and if it were actually needed then you could potentially push several Terabits per second (Tbps) down a single fibre.

Nevertheless scientists are constantly finding new ways of manipulating laser light or other areas of fibre optic networking in order to improve the performance of data transfers over such cables, which is where PhD student Robbert van der Linden from TU/e comes in.

Essentially what Robbert has done is to devise a way of increasing the capacity of a fibre optic network without introducing expensive techniques. At present many Passive Optical Networks (PONs) connect perhaps 16 to 64 households to one fibre entering the street or district, which saves money, space and power because you only need one transceiver at the central data station per street or district.

Robbert van der Linden said:

“In theory, every connection to a single home in a PON may be equally long. In practice however, the situation often is less optimal: some of the users are very close to the central data station of the network, and others are a fair distance away. Or there might even be an extra power splitter between them and the main PON.

This leads to differences in signal qualities between the different users: the closer, the higher the signal quality. Since providers want to guarantee a minimum data rate for everybody, current networks are over-dimensioned, leading to non-used overcapacity.”

In trying to solve this Robbert and his team, which worked alongside broadband equipment maker Genexis, developed a solution that borrows from the existing adaptive modulation technologies used in wireless, ADSL / VDSL and coax based broadband ISP networks. This enables them to utilize the over-capacity, where it is available.

The adaptive modulation technique enabled the team to match the signal parameters to the available properties of the data channels, creating something of a “smart fibre” network.

Robbert van der Linden said:

“A normal signal consists of two states: a bit is either a one or a zero. We have increased the number of states four or even eight levels. A higher number of levels implies more information per symbol, so four levels contain two bits of information, and eight levels contain three bits.

This means that by implementing this fairly simple type of modulation, you can immediately transfer twice or even three times as much information in the same amount of symbols. This comes at a cost though: you need to invest more effort in decoding the signals.

In contrast to for example ADSL/VDSL, the PON is shared among the users. This allows us to allocate shorter timeslots to those users nearby the central station that are able to decode the symbols with a large number of levels, thereby increasing also the performance for a user further away that can only decode symbols with a low number of levels.

We have shown for a practical case of a field-installed network that it is feasible to reach an increase in throughput of 74 percent in the case of 4 levels, and up to 115 percent with eight.”

Further performance improvements have also been found by tweaking the different signal levels a little, such as by positioning the levels with unequal intermediate spaces (i.e. easier to decode and able to handle a worse signal quality) and the use of multiple wavelengths and clock rates. “Currently, a new standard is being deployed which will indeed make use of four to eight different wavelengths. We have demonstrated that by using only three different clock rates, already an increase in throughput of 180 percent can be achieved,” said Robbert.

fibre_2_levels_to_8_levels
Diagram of 2 levels turning into 4 and 8 levels

The proposed techniques have the advantage of being relatively simple and familiar, which Karl Tran of Genexis anticipates will mean they “can be implemented in a reliable and cost-effective way.” A related research paper on the project notes that new chips are already being designed to take advantage of this work and the good news is that only some users will need to buy a new modem to benefit.

Put another way, even if only your neighbours that are part of the same PON upgrade, your own capacity is still likely to increase as well. This is because the improved techniques are designed to shorten the timeslots the upgraded users need for data transfer, which of course still leaves more time for the other users to get their share of data. Handy.

Leave a Comment
4 Responses
  1. Avatar CarlT

    I must look into this one some more – use of QAM and AM have been part of optical transmission for a while so be good to look into what’s new.

  2. Avatar Sometimes but not always

    And could take 20 more years for OpenRoach testing that?

  3. Avatar nt

    OFDM-PON was ruled out for NG-PON2 and is basically the same thing as what’s used in VDSL but applied to light (yeah ok it’s DMT but that’s essentially OFDM).

    The “P” in PON — passive — rules out a lot of the fancy DSP features since they suck down power.

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