VeEX VePal CX350 User Manual Download Page 34

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Overview:

Historic video channel assignments limit the upstream frequencies between 5 - 42 MHz (North America) and 5 - 65 MHz (Europe). The upstream spectrum supporting return

path connectivity is frequently impaired by ingress of interfering signals, such as radio citizen band (CB), and other legitimate and non-legitimate RF emissions.

Reverse Ingress can be analyzed between 5 to 42 MHz (6 MHz channel plan) or 5 to 65 MHz (8 MHz channel plan).

Return Path Outages:

Return Path Noise and Ingress can be a result of:

Stationary impairments

Thermal noise

Intermodulation Distortion (IMD)

Frequency response problems

Transient impairments

RF Ingress

Impulse noise

Signal clipping

Multiplicative impairments

Transient hum modulation

Intermittent connections

Troubleshooting Techniques

1. The "divide and conquer" method normally works best. Establish the most distant point from the headend at which the signal is known to be good quality. Start

midway between this point and the affected subscribers to locate the problem amplifier

2. Systematically check the signal level at the return path receiver test points in the headend or hub until the problem node is found

Ingress Measurement Notes:

The spectrum analyzer’s ability to measure low-level signals is limited by the noise generated inside the spectrum analyzer. The sensitivity required to measure low-level

signals is determined by the measurement setup. The spectrum analyzer

Input Attenuator

and

Resolution Bandwidth

settings are the key factors that determine how

small of a signal the spectrum analyzer can measure.

Input attenuator

- When enabled, reduces the level of the signal at the input of the mixer to avoid compression which can produce unwanted frequency products. An

amplifier at the mixer’s output will re-amplify the attenuated signal to keep the signal peak at the same point on the analyzer’s display. However, in addition to amplifying the

attenuated input signal, the noise present in the signal and analyzer is amplified as well. This has the undesired effect of raising the displayed noise level of the analyzer.

Resolution bandwidth

- Affects how closely a small signal can be seen in the presence of a large one. By increasing the width of this filter, more noise energy is allowed to

hit the envelope detector of the analyzer. This also has the effect of raising the displayed noise level of the analyzer.

Maximum sensitivity

- For maximum sensitivity, both the input attenuator and resolution bandwidth settings must be minimized.

Note: Common Path Distortion (CPD)

Produced by poor contacts in the cable distribution network - these contacts create a diode or rectifier effect

which produce potentially harmful 2nd and 3rd order Intermodulation Distortion (IMD) products or beats.

These beats will occur every 6, 7 or 8 MHz in the reverse path depending on the channel plan used.

Although the magnitude of these beats are small, they increase at a node when several reverse paths are

combined.

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7.5 Cable Modem

Overview:

Historically, CATV was a unidirectional medium designed to carry broadcast analog video channels to the maximum number of customers at the lowest possible cost. The

DOCSIS interface specification basically enables the deployment of data-over-cable systems on a nonproprietary, multivendor, interoperable basis for transparent

bidirectional transfer of Internet Protocol (IP) traffic between the cable system head end and customer locations over an all-coaxial or Hybrid-Fiber/Coax (HFC) cable

network.

In its simplest form, the system consists of a Cable Modem Termination System (CMTS) located at the headend, a coaxial or HFC medium, and a Cable Modem (CM)

located at the customer premises. In addition, DOCSIS defines physical, signalling and protocol layers to support interoperability and evolutionary feature capabilities to

permit future value-added services.

The DOCSIS layers are briefly outlined as follows:

IP network layer

Data link layer comprising:

Logical Link Control (LLC) sublayer conforming to Ethernet standards

Link security sublayer for basic privacy, authorization, and authentication

Media Access Control (MAC) sublayer for operation supporting variable-length Protocol Data Units (PDU)

Physical (PHY) layer comprising:

Downstream convergence layer conforming to MPEG-2 (Recommendation H.222.0)

Physical Media Dependent (PMD) sublayer for:

Downstream based on ITU-T Rec J.83 Annex B with either 64 or 256 Quadrature Amplitude Modulation (QAM), concatenation of Reed-Solomon and

Trellis Forward Error Correction (FEC), in addition to variable-depth interleaving