Hello everyone!
Welcome to the Access Network HCIA Learning Class.
Today, we will continue to learn about Vectoring.
G.fast Principle and Application
Why Not FTTH
FTTH Pain Point
The engineering of fiber home wiring, splicing, connections cost too much and too long.
Users are not happy with a home visit and engineering due to religion, home decoration damage reasons, etc.
FTTH(Fiber to the home) was initially seen as the only long-term solution to the bandwidth problem. But VDSL2 vectoring changed this perception. With a single innovation, the market shifted. Copper became a valuable commodity again as operators began using their copper assets to deliver fast broadband speeds faster.
The Need For G.fast
With demand for high bit-rate services straining access networks to their limit, the industry is looking for a solution to the next bandwidth bottleneck. G.fast promises to be that solution.
G.fast Key Technology
G.fast Technology Overview
Use DMT technology (backward compatible with VDSL2 CPE)
Extend the frequency to 106M in stage 1 (212MHz in future)
Changes from FDD to TDD to reduce complexity
Low power in Signal: -76dBm 4dBm, <4dmW (VDSL2: -60dBm, 25mW)
TDD window: < 1ms (US:D9 = any rate e.g. 1:8 or 1:1)
Comparison between TDD and FDD
TDD: (Time Division Duplex), use different time slots for downstream and upstream traffic.
Support flexible ratio for downstream and upstream. The length of the time slot is configurable.
Eg. Thexandy can be configured to be the Same to achieve symmetric transmission.
FDD(Frequency Division Duplex), use a fixed frequency band for downstream and upstream.
FDD requires the transmitter and receiver both on at the same time. The independent TX and RX will make the chipset more complicated with more power consumption and Large Size.
Crosstalk: The Killer of G.fast Speed.
Crosstalk is the dominant source of noise in G.fast which leads to significant performance loss.
Vectoring and G.fast Service Configuration
In electronics, crosstalk (XT) is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another.
VDSL2 performance is determined by the cable attenuation and the noise in the cable, while Crosstalk is the dominant source of noise in VDSL2 which leads to significant performance loss.
There are two types of crosstalk:
NEXT: Near-End Crosstalk
Interference between two pairs in a cable is measured at the same end of the cable as the interfering transmitter. It means the interference between upstream and downstream.
FEXT: Far-End Crosstalk
Interference between two pairs of a cable measured at the other end of the cable with respect to the interfering transmitter. It means the interference between upstream or the interference between downstream.
Crosstalk Challenges
Like VDSL2, the crosstalk also degrades performance when multiple G.fast lines occupy the same cable binder. 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 uses are at the root of the crosstalk challenges. At these frequencies, it is not uncommon to see crosstalk on a G.fast line that is similar in strength to 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 — 6 to 12 times that of VDSL2 17a — adds a factor of scale. A wider frequency range means more calculations per second for the vectoring engine.
VDSL2, Vectoring & G.fast Comparison
Access Network HCIA Course Series will be continuously updated.
Thank you for your participation and we'll learn together!
