Product Application Note:
Using Return Step Attenuators to assist with DOCSIS 3.0 launches
Arcom Return Step Attenuators can significantly help MSO’s in the launch of DOCSIS 3.0. The increased datathroughput of DOCSIS 3.0 comes at a cost in the return path – less operating headroom, and an inherent increased susceptibility to impairments such as ingress, impulse noise, common path distortion,and micro-reflections. Return Step Attenuators (RSA’s) can offer great benefits in overcoming the challenges of running DOCSIS 3.0 in the return – and the solution should be kept in mind when planninga launch or in troubleshooting. RSA’s selectively add attenuation in the return band and allow the system operator to engineer the HFC network for optimized two way communications. One significantway that RSA’s can help is by minimizing the network effect of ingress and impulse noise generated from the home. There is a correlation between the tap value and the susceptibility of a subscriberlocation to introduce noise and ingress into the network. Conceptually it can be illustrated by focusing on the isolation between the subscriber and the network that is provided by the tap. In the exampleshown, the 4 dB tap provides only 4 dB of isolation between the subscriber and the networkwhereas the 30dB tap provides 30dB of isolation. A lower value tap will inherently allow more noise andingress into the network. To implement a step attenuator solution; in general, high value RSA’s are placed on lower value taps, and vice versa. Installing the correct value RSA on a tap allows every tap atreturn band frequencies to behave and offer the same isolation as a high value tap – thereby minimizing the effect of home generated noise and ingress on the network. Because of the addedattenuation, RSA’s also have the effect of lessening the spread of signal levels in the return path - thereby minimizing issues relating to overload and laser clipping. Today’s CATV networks are designed to...
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