Publié le 8 novembre 2012
You don’t have to look very hard in mainstream technical papers and journals to see the success of deploying vectoring in VDSL2 deployments. Compared to the cost of installing FTTH (fiber-to-the-home), reusing the copper twisted pair already in deployed in the field, and coupling it with the recent enhancements to DSL such as vectoring, service providers see a very attractive performance vs costing model by using DSL. By using vectoring, service providers can increase their ARPU (average revenue per user) by not only upselling to the next tier of internet service but also offering new services such as IPTV and OTT (over-the-top) multimedia thanks to the increased data rates Vectoring offers.
Defined by ITU-T G.993.5, vectoring manages the suppression of noise or more specifically, self-FEXT (far end crosstalk from the same technology), in a cable bundle(s). Vectoring uses intelligent signal processing to allow cancelling of self-FEXT either at the board level (i.e., 64 port line card), the system level (multiple line cards) or node level (one or more DSLAMs). The process is akin to that used by noise-cancelling headphones. Vectoring performs analysis of all the subcarrier frequencies of all the lines in the vectored group—considering that VDSL2-17a uses upwards to 4096 subcarriers, the computation requirements for using vectoring is immense. Vectoring is conducted at the access node (DSLAM at central office/exchange or remote terminal) within the vectoring control entity and management entity. Information is exchanged between the VTU-O (VDSL2 transceiver unit—Office/exchange) and VTU-R (VDSL2 transceiver unit—remote) in each vectored line to learn, track, and maintain the FEXT cancellation process.
Once self-FEXT is mitigated, there still may be other uncontrollable events or noise sources that can reduce the performance enhancement provided by use of Vectoring. A power failure or a customer turning their modem on or off are examples of uncontrollable events. These events necessitate the system to reevaluate the impact of self-FEXT across all ports. Noise interference can include crosstalk other technologies (T1, E1, HDSL, ADSL2+, etc), interference from AM radio or amateur/HAM radio, and impulse noise (REIN [repetitive electrical impulse noise], SHINE [single high amplitude impulse noise], and PEIN [prolonged electrical impulse noise]). With respect to impulse noise, the use of INP (impulse noise protection) or better yet the use of ITU.T G.998.4 (G.INP) for physical layer retransmission will help to mitigate impulse noise.
Existing field portable measurement solutions on the market may not be able to properly test in a vectored environment. Newer generation test solutions, such as EXFO's MaxTester 635, provide telecom service providers with a vector-friendly solution. What’s more, the MaxTester 635 offers comprehensive copper and DSL test tools to assist service providers in the detection of faults on cable pairs so that these faults can be fixed/removed to ensure vectoring in the cable bundle is not compromised. Poor balance and faults, such as bad splices, bridge taps, poor grounds/bonds, etc., can be the ingress/egress points for unwanted noise and must be fixed/removed for vectoring to be successful.