Extending FTTH buildouts with G.fast - enabling a Gigabit experience to buildings where fibre cannot easily reach

In their fibre network expansion plans, network operators rely on a mix of fibre-fed G.fast in buildings as well as VDSL2 vectoring if FTTH cannot be rolled out economically. Which technologies are best suited for which situations?

Network operators agree that their primary objective should be to bring fibre access services to every home and business. However, if, for whatever reason, it is not possible to have a fibre-optic connection to every premises in the short term, many providers are taking a different approach – leveraging the latest technologies to bring a fibre-like experience over existing copper infrastructure. By leveraging technologies like G.fast to support Fibre-to-the-Building (FTTB) and VDSL2 with vectoring, operators able to the delivery of data rates from hundreds of megabits per second to almost 2 gigabits per second over existing infrastructure. Deutsche Telekom for instance now puts its focus on FTTH, but has been leveraging the fiber optic connection up to the central offices and outdoor cabinets (FTTC) and VDSL2 vectoring from there to its subscribers. Many other regional and local operators generally follow a similar multi-track approach that brings fibre where they can, but leverages copper where they must. For economic reasons, they often continue to expand their existing VDSL2 vectoring infrastructures and install FTTB and FTTH fiber optic connections in newly developed residential and commercial areas.

VDSL2 vectoring has become very widespread in the market in recent years. Network operators of all sizes use the technology to accelerate existing VDSL2 access. Network operators benefit from VDSL2 vectoring connections with profile 17a in the frequency spectrum up to 17 MHz, achieving data transmission rates between 50 and 100 Mbps. Vectoring with profile 35b has been available in Germany since last year; this variant uses the frequency spectrum up to 35 MHz. This allows data transmission rates of up to 300 Mbps to be achieved.


G.fast: Data transmission rates as on optical fibers

The G.fast transmission method offers significantly higher bandwidths than VDSL2 and is the first copper transmission technology to achieve data rates that are otherwise only possible using fibre optics. G.fast is suitable from short to very short distances. With the first generation of G.fast, cumulative data rates (upstream + downstream) of almost 1,000 Mbps can be achieved on very short lines - using a frequency spectrum of up to 106MHz. The second G.fast generation – available today – allows the use of frequency spectrum up to 212MHz. This doubles the achievable total data rate to up to 2,000 Mbps on very short lines. In most cases, network operators use G.fast in FTTB architectures, i.e. for data transmission on the in-building copper infrastructure.


Data transmission technologies in comparison

But which technology represents the optimum for the respective application? DZS-KEYMILE has conducted an extensive series of measurements of data transfer rates across line lengths in both G.fast Profile 106a and 212a. Also tested were network nodes with vectoring profile 17a and with profile 35b. The aim of the measurements was to determine which technologies were best suited for specific applications. DZS-KEYMILE uses G.fast in its FTTB DPUs (Distribution Point Units), for example in MileGate 2042, a network node optimised for FTTB installation and providing eight G.fast interfaces with profiles 212a and 106a.

FTTB G.fast - Data Rate vs. Loop Length

Compared to VDSL2 vectoring, significantly higher data rates can be achieved with G.fast up to a distance of 500 m (see Figure 1). The measurement results up to 50 m deserve special attention. Here G.fast with 212 MHz achieves aggregated data transmission rates of 1.8 Gbps. For a distance of 250 m, the transmission rates are reduced to around 700 Mbps; in this range, the data transmission rates of G.fast at 106 MHz and G.fast at 212 MHz are also approaching. A brief look at the measured maximum values for very short lines, which are quite common in the FTTB environment: In the range of less than 100 m, G.fast with 106 MHz reaches almost 1,000 Mbps; while G.fast with 212 MHz it is about 1,400 Mbps. At less than 50 m even almost 2 Gbps are possible.

From a distance of more than 500 m, the data transmission rates of G.fast and VDSL2 vectoring are similar. In other words: VDSL2 vectoring with profile 17a and with profile 35b are the more suitable methods for longer copper lines. VDSL2 profile 35b achieves higher data transmission rates than VDSL2 profile 17a up to a distance of approx. 700 m.


Frequency spectrum separation for VDSL2 and G.fast

A further differentiation between VDSL2 and G.fast - in addition to the performance on the copper line - can be seen in the modulation and duplexing processes used in each case. In modulation, both VDSL2 vectoring and G.fast use DMT (Discrete Multi Tone): Both split the frequency range into individual transmission channels (subcarriers), carrier signals then transmit the data.

FTTB G.fast - Modulation / DuplexingUnlike modulation, there are clear differences in duplexing: VDSL2 uses FDD (Frequency Division Duplexing), which divides the available frequency spectrum into individual ranges and uses them for downstream and upstream. Broadband network operators determine the division by band plans and profiles. The advantage of dividing the frequency spectrum into individual ranges for downstream and upstream: Down- and up-stream data can be transmitted in parallel on the same cable.

In contrast to this, the separation of G.fast is done time-related by TDD (Time Division Duplexing). Downstream and upstream alternately use the entire frequency range. G.fast first transmits all downstream data in specially defined time spans and then switches to upstream. The advantage of this procedure is that network operators can freely define the ratio of downstream and upstream. Not only an asymmetrical (e.g. high downstream and low upstream rate), but also a symmetrical distribution with, for example, 500 Mbps each for downstream and upstream is possible.


Technically possible: parallel operation of G.fast and VDSL2

If G.fast and VDSL2 are to be used in the same cable, there are some things to consider: It does not matter whether the network operators are the same or different. VDSL2 and G.fast are spectrally incompatible. This becomes a problem if, in the case of FTTB, VDSL2 is routed from an outdoor cabinet into a multi-dwelling unit and a G.fast node is present or is to be used in this building. VDSL2 vectoring with profile 17a uses the frequency spectrum up to 17 MHz, profile 35b the frequency spectrum up to 35 MHz and G.fast with 106 MHz the frequency spectrum from 2.2 to 106 MHz. Due to this overlap in the frequency range, VDSL2 and G.fast signals interfere with each other.

However, both technically and in practice, parallel operation of G.fast and VDSL2 in the same cable is possible. To do this, network operators must configure the G.fast devices so that the G.fast transmission does not use the VDSL2 frequency range. To prevent "crosstalk", G.fast transmission in coexistence with VDSL2 profile 17a only starts from the frequency range of approx. 20 MHz - or from approx. 40 MHz in coexistence with VDSL2 profile 35b. In addition, the frequency ranges can overlap if the VDSL2 node is located at a greater distance from the G.fast node. This is due to the fact that with increasing distance, the carriers of the higher frequencies of the VDSL2 signal no longer contribute to data transmission anyway, so there is no longer any influence by G.fast.

FTTB G.fast Rate/Reach - 212a/106a

But there is no way around it: co-existence with VDSL2 means a loss of power for G.fast transmission. If network operators use G.fast 106 MHz together with VDSL2 profile 35b, only the frequency range of approx. 40 to 106 MHz remains for data transmission. This was also one of the main reasons for developing the G.fast 212 MHz technology. In the frequency range extended from 106 MHz to 212 MHz, coexistence with VDSL2 results in a reduction of the data rate that is of little relevance in percentage terms.

The possible parallel operation of VDSL2 and G.fast, and thus of vectoring and fibre optics, has also attracted the attention of the German Federal Network Agency (Bundesnetzagentur), because in this scenario the interests of Deutsche Telekom and those of the alternative network operators collide. With its decision, the Federal Network Agency wants to ensure that different broadband technologies such as G.fast and VDSL2 work side-by-side and in the same building. The decision specifies the exact parameters in tables, which the network operators must refer to in individual cases. The fact is: the lower G.fast frequencies are capped in coexistence with VDSL2 vectoring, but G.fast 212 MHz still offers sufficient bandwidth over the upper frequencies to enable competitive data rates even in these cases, which - particularly important for network operators and their customers - are actually achieved realistically.


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DZS-KEYMILE ist ein führender Anbieter von Telekommunikationssystemen für den Breitbandzugang. Das Portfolio umfasst Access-Lösungen für alle FTTx-Netzarchitekturen: glasfaserbasiert für aktive und passive optische Netzarchitekturen und kupferbasiert mit VDSL-/Vectoring-/G.fast-Technologie. Darüber hinaus entwickelt DZS-KEYMILE Software-defined Networking (SDN)-Lösungen für die Integration programmierbarer, Software-definierter Netztechnologien in die Zugangsnetze. Das Unternehmen mit Hauptsitz in Hannover gehört seit 2019 zu DASAN Zhone Solutions (DZS).