As a CATV or cable modem signal is transmitted downstream through the branched cable topology, the signal becomes attenuated and weaker at each branching and with cable distance. What is initially a strong signal requires amplification to adequately reach the subscriber. Noise or interference in downstream frequencies that enters the cable system at some distance from the headend only affects customers past that point. It does not affect all subscribers, as it does not originate at the headend. However, when noise is present it is amplified along with the desired signals. Additionally, each amplifier adds its own noise and distortion to the signals. Noise and interference in the downstream are typically viewed as ghosting or static on video signals. If a cable plant is designed to adequately transmit video signals, it is likely to be of high enough quality to transmit DOCSIS signals. Thus, although noise in the downstream is an issue on the cable plant, it is upstream noise that is the primary concern for DOCSIS deployment.
In the upstream path the additive nature of noise and interference has an increased impact. As noise and interference occur at one or more subscriber sites and travel upstream they are gradually combined with other noise through a process sometimes called noise funneling. What might be an insignificant electronic hum at one subscriber site can be combined with hum from hundreds or thousands of other subscriber sites to become destructive by the time it reaches the headend. In particular, unterminated cables at the subscriber premises can inadvertently receive radio frequency (RF) signals and funnel those into the network. Although each non-terminated cable might only receive a whisper of undesirable RF signal, the cumulative signal after the effect of noise funneling is massive. HFC plant segmentation can help reduce the effect of noise funneling by reducing the number of combined signals traveling on a given upstream path. Because it is impossible to eliminate noise funneling it is vital that the cable IP network upstream noise and that interference levels be characterized and constantly monitored or network outages can occur.
Some common upstream impairments on the cable plant and their mitigation strategies include the following:
• Ingress noise—Inadvertently present signals, often from broadcast sources such as amateur, short wave, and citizens' band (CB) radio users. Often a result of bad grounding and poor connections littered through the cable plant. Can be mitigated by the identification of problem frequencies and avoidance of them for upstream transmission. Also mitigated by the use of intelligent cable modem termination system (CMTS) devices that can identify ingress locations and use frequency changes to dynamically avoid this type of noise as it occurs. Most often, a technician must visit the worst ingress locations, and physical changes must be made to the network. Often these locations are also sources of signal leakage and can be located by looking for small (or large) signal leakage (a process known as Cumulative Leakage Index [CLI] testing). Leaks on the order of five microvolts per meter can be sufficient to allow disruptive ingress if there are strong signal sources nearby.
• Impulse noise—Caused by bad grounding and loose connections in the cable plant. A classic source is power line arcing. Impulse can be partially mitigated by the use of forward error correction (FEC) and interleaving error correction algorithms. Again, a technician might need to visit the worst locations and make physical changes to the network. In some cases, defective CATV network components such as power supplies must be serviced or replaced.
• Reflections—The result of unterminated cable drops, usually at the customer premises. Caused by signals literally bouncing off the end of poorly terminated or unterminated cables. Can be mitigated through equalization coefficients. Technicians can also make changes to the network to reduce severe reflections. Reducing reflections is important to cable operators as reflections typically result in disturbances in analog video quality, as quickly noticed by subscribers.
• Laser clipping—Caused by laser nodes receiving a signal at power levels beyond their supported range. Mitigated by appropriate amplifier alignment and levels in the upstream path. Can also occur in the downstream path and cause modem instability. Most easily monitored with quadrature amplitude modulation (QAM) analyzers.
• Common path distortion—Often caused by corroded connections and other cabling issues. Best resolved by physical repair to the damaged cable network infrastructure. Can most easily be located and repaired by starting at the subscriber premises and sampling sequentially along the network towards the headend in a process called divide and conquer.
Other common types of upstream impairments include the following:
• Unwanted signal
• Interference from electrical and magnetic sources (electromagnetic interference [EMI])
• Thermal noise/RF amplifier noise (unless severe, usually no impact on DOCSIS networks)
• Co-channel ingress
• Composite second order distortion (generally a downstream impairment but can impact the upstream)
• Discrete second order distortion
• Composite triple beat distortion (generally a downstream impairment)
• Cross modulation
• Intermittent or poor connections
• Poor in-channel frequency response
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