With the arrival of fibre and broadband wireless, the demise of copper in the ground was predicted. However, it now appears that copper is entering into the realm of the broadband spectrum world. The latest development is G.fast – offering speeds approaching that of optic fibre, with researchers hinting that more technology is on its way.
G.fast technology makes gigabit-level aggregate bit rates possible by adding spectrum to copper lines. Current VDLS2 lines use spectrum up to 17 MHz.
G.fast widens this spectrum to 106 MHz and anticipated developments will use frequencies up to 212 MHz and take bit rates to 1 Gbps and beyond.
The high frequencies of G.fast can’t be sustained over long distances so a good approach is to use a mix of technologies.
G.fast is ideal for applications that bring fibre closer to the home and use very short copper loops to cover the last few metres. DSL technology remains the best technology for longer copper lines.
Over the years many versions of digital subscriber line technologies were developed. On a good copper pair, ADSL can deliver up to 24 Mbps over a distance of up to 5 km.
As various versions of DSL were developed the speed went up but the distance between the subscriber and the exchange was greatly reduced.
Currently VDSL2 will deliver 100 Mbps over a maximum distance of 1 to 1,6 km.
To overcome the distance problem many Telco erected street boxes fitted with DSL modems, feeding the street box with fibre. Fibre to the home (FTTH) has also changed this scenario.
The development of G.fast was coordinated with the Broadband Forum’s fibre to the distribution point (FTTdp) system architecture project. ITU and the Broadband Forum have been collaborating to ensure that G.fast solutions can be quickly placed into FTTdp deployments.
On 5 December 2014 ITU members achieved final approval of G.fast, a new ITU broadband standard designed to deliver access speeds of up to 1 Gbit/s over existing telephone wires.
The standard answers to service providers’ need for a complement to fibre to the home (FTTH) technologies in scenarios where G.fast proves the more cost-efficient strategy.
Despite G.fast’s leap forward in sophistication over DSL access technology, it maintains the installation simplicity of ADSL. G.fast customer equipment, in line with that of ADSL, will arrive in a box containing only a G.fast-compliant modem and filters to separate audio from the digital signal to enable standard analogue telephone calls on the same copper pair.
Innovations target crosstalk challenges
No technologies come without its challenges. With G.fast, interference among multiple active lines in a copper cable significantly reduces performance compared to the case where a single line is active. The impact of this interference, known as far-end crosstalk (FEXT) is unpredictable and causes G.fast not to reach its full potential.
Nokia Bell Labs studies indicate that the effects of crosstalk are much greater with G.fast than they are with VDSL2. The very high frequencies that G.fast operates on are at the root of the crosstalk challenges.
At these frequencies, it is not uncommon to see crosstalk on a G.fast line to be similar in strength as the actual signal. One challenge is to create a compensating signal that eliminates crosstalk without exceeding the Power Spectral Density (PSD) mask. More advanced algorithms are required to compensate for these high crosstalk levels.
The broad frequency range used by G.fast is six to twelve times that of VDSL2 17a adding to the problem. A wider frequency range means more calculations per second for the vectoring engine.
In concept, vectoring is comparable to the noise-cancelling technology used in headphones. It produces a clean signal for each line by measuring the crosstalk from all other lines and generating anti-phase signals to cancel the crosstalk signals out, resulting in almost zero noise.
This concept sounds simple, but its execution can be highly complex, depending on the environment in which it is deployed.
G.fast comes to South Africa
South Africa is amongst the first countries in the world to trial G.fast and deployment is imminent. Nokia and Telkom’s wholesale division, Openserve, recently conducted a trial of G.fast technology in an office complex in Pinelands, reaching fibre-like speeds over existing copper infrastructure in buildings.
This trial is a dry-run for Openserve’s commercial deployment of G.fast later this year, meaning it will soon join the top ten companies worldwide deploying the technology.
Daniel Jaeger, head of Central, East and West Africa at Nokia, said that the results of the trial demonstrated an aggregate bandwidth (upstream and downstream) of 900 Mbps on short copper loops, and speeds of 500 Mbps downstream and 250 Mbps upstream on an existing copper line at a distance of 150 m.
Nokia’s G.fast technology uses the last few hundred metres of existing copper located in a building to deliver ultra-broadband access to customers, and it will allow Openserve to meet demands for fibre-like broadband speeds without significantly increasing operational cost.
The built-in vectoring technology reduces cross-talk interference between copper lines, hence improving data speeds.
Openserve CEO Alphonzo Samuels said that they completed testing Nokia’s G.fast technology in the Telkom labs, and are pleased with the results.
He said that the technology is a great alternative in scenarios where the length of the copper tail is 150 m or less. Moreover, the business case is convincing as G.fast provides a way to maximise the usage of existing network infrastructure.
G.fast is an attractive solution for people living in gated complexes where units are served by copper cable and where body corporates are not allowing the internal roads to be dug up to lay fibre or where overhead cabling is not aesthetically acceptable.
It also offers opportunities to provide high speed broadband in older office buildings where fibre reticulation is not practical or cost effective.