Hi, community.
This is the second post of this section. We talk here about Ultra-broadband technology.
xDSL: PRINCIPLES AND APPLICATIONS
Introduction to the Ultra-Broadband Technology
Advantages of VDSL2
· Crosstalk: interference caused by the coupling of signals DSL line pairs.
§ Near-end crosstalk (NETX): interference between upstream and downstream channels at the same end
§ Far-end crosstalk: (FEXT): interference between upstream and downstream channels at different ends
· VDSL2 can provide a higher bandwidth within 1 km (the theoretical rate can reach 100 Mbps), and has become the mainstream access mode of the last mile in an FTTx network. It makes high-speed Internet access possible and supports abundant services. However, VDSL2 occupies a high-frequency band, resulting in serious crosstalk between lines. As a result, the bandwidth of multi-wire interconnection decreases significantly compared with that of single-wire interconnection.
How to Eliminate CrossTalk
· NEXT: because the upstream and downstream frequency bands of the VDSL2 are different, near-end crosstalk is separated from normal signals by frequency bands and can be eliminated or greatly reduced by using a filter.
· FEXT: occurs between upstream signals or between downstream signals, and is in the same frequency bands as normal signals. Therefore, FEXT cannot be eliminated by using a filter
§ To reduce FEXT, ITU-T proposed the vectoring technology standard G.993.5
· Because VDSL2 adopts frequency division multiplexing (FDM), signals sent in the interference line pair and signals received by interfered line pair in NEXT occupy different frequency bands. Therefore, the impact of the NEXT can be eliminated or greatly reduced by using a filter.
· However, signals sent in the interference line pair and signals received by interfered line pair in FEXT occupy the same frequency band. Therefore, FEXT cannot be eliminated by using a filter. In addition, VDSL2 transmits data within a short distance (generally no longer than 1 km) at a high-frequency band (up to 30 MHz), resulting in more serious FEXT compared with other DSL technologies. Therefore, FEXT becomes the main factor affecting the performance of a VDSL2 system. FEXT decreases the SNR, reduces the line transmission rate, increases the bit error rate (BER), and even causes offline errors, severely affecting system stability and user experience.
· To eradicate the impact of FEXT on VDSL2, the ITU-T proposed the vectoring standard to resolve FEXT on VDSL2 lines and improve the multi-wire performance. In this standard, the crosstalk of one VDSL line is a vector in a set of crosstalk vectors for all other lines in the same bundle. The vectoring processing system performs matrix calculation based on the collected vector information and outputs vectorized crosstalk cancellation signals.
Vectoring Implementation Principle
· Vectoring improves VDSL2 rate by eliminating crosstalk
§ Obtain the crosstalk information on the line through the cooperation of CO devices and terminals and calculate the crosstalk cancellation scheme accordingly
Vectoring Acceleration Effect
· The highest rate supported by vectoring is the rate of VDSL2 single-wire pairs in an environment without crosstalk
§ The effective distance of vectoring is within 1.5 km, and the rate improvement in the 300-800 m range is the most obvious.
§ Vectoring can increase the rate by 50%-90% on the basis of VDSL2, reaching 90-95% of the theoretical VDSL2 rate
Vectoring Acceleration Effect
· G.fast (also called Giga DSL) achieves the FTTH access rate using existing copper lines, bringing copper access into the “gigabit era”.
FTTD: fiber to the door
§ Reverse power supply, 1-8 lines
FTTDp: fiber to the Distribution point
§ Reverse power supply, 8-16 lines
FTTB: fiber to the building
§ Local/remote power supply, 16-48 lines
· G.fastapplies to the following scenarios:
§ FTTD, FTTDp, and FTTB where the fiber length to end subscribers is usually less than 200 m
§ The twisted pair at the drop cable section can be reused to avoid complex fiber routing and greatly reduce the network deployment cost.
§ Provides services for generally no more than 50 subscribers.
§ Device installation is complex, and onsite power supply is difficult to acquire, requiring low power consumption devices to support some special power modes, such as remote power supply or PoE.
§ The installation environment, such as electric poles, corridors, and manholes, demands high environment adaptability and less maintenance. Generally, remote maintenance needs to be supported.
G.fast Key Technologies
· Currently, the ADSL2+ uses the highest frequency of 2.2 MHz to provide a maximum downstream rate of 26 Mbps, while VDSL2 uses the highest frequency of 30 MHz to provide a maximum downstream rate of 100 Mbps. The G.fasttechnology must use a higher frequency band to provide a higher access rate. Therefore, the G.fastspectrum resources must be expanded. In the initial phase, the highest G.fastfrequency is106 MHz, and can be extended to 212 MHz.
· ADSL/2/2+ and VDSL2 adopt the frequency division duplex (FDD) technology which uses different frequency bands to transmit data downstream and upstream at the same time. The disadvantage is that transmit signals generate echo which enters the receiver. When the frequency band is high, the impact is more obvious, and severely affects the performance of the receiver. In this case, the hybrid circuit needs to be used for echo suppression. G.fast does not distinguish the upstream and downstream frequency bands, and transmits data in the full frequency band in TDM mode in both directions. TDD allocates different timeslots for upstream and downstream data transmission. The transceiver sends and receives signals in different slots.
· As the frequency bandwidth and the number of subcarriers increase, the corresponding physical layers become more complex. To reduce complexity, the subcarrier spacing needs to be increased to reduce the number of subcarriers. To be compatible with the traditional DSL technology, the Giga DSL adopts a subcarrier spacing which is an integer multiple of 4.3125 kHz, where 4.3125 kHz is a subcarrier spacing used in traditional ADSL/2/2+/VDSL2 8a/12a/17a standards.
· During the upgrade from ADSL/VDSL2 to G.fast, some subscribers may want to retain the existing DSL services. To ensure smooth network upgrade, G.fastdevices also support traditional VDSL2/ADSL.
· Similar to VDSL2, the G.fast performance is also affected by crosstalk between lines. If the vectoring technology is not used, the G.fast rate will decrease severely.
Technical Application Comparison
We finish here with the main theme, the following link will take you to the first part of the post:
Thanks.
