WEB EXTRAS - February 2010
Upstream Cable Echoes Come in Two Flavors
CableLabs has established a “DOCSIS Proactive” working group with the goal of using the data in DOCSIS-based communications systems to improve plant operations. The group’s first task is to produce a recommended practices document that enables cable operators to mine the pre-distortion coefficients from the cable modem’s adaptive equalizer. That data can then be used to clean up transmission signals.
The author, Tom Williams from Arris, writes: “Cable lines typically have many small echoes, so-called micro-reflections, that will disrupt digital transmissions if not canceled. This is particularly true for highspeed upstream signals having a higher order modulation, such as 64 QAM, or having wider bandwidth, such as 6.4 MHz.
“The system chosen by DOCSIS uses pre-distortion, where a burst transmission is distorted prior to transmission and arrives at a cable modem termination system receiver with the plant’s distortion canceled. The idea is that by reading the cable modem’s pre-distortion coefficients using a network management system, technicians can tell what plant impairments a CM is compensating for, and then compute what may be wrong with the cable plant. By reading the data from many CMs, you can localize the problems using maps or connectivity data.” >>>More
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