Cable Access Technologies - Limitations and Specifications of the HFC Plant

Limitations and Specifications of the HFC Plant

The HFC network has the potential to offer tremendous bandwidth in the downstream or forward direction from the head end or hub to the customer. Depending upon the extent of the plant upgrade, the available bandwidth could be as much as from 54 to 860 MHz. Downstream channel bandwidths are determined by the individual country's video broadcast standards.

The historical broadcast video channel assignments limit the upstream or reverse direction from the customer to the spectrum between 5 to 42 MHz. This upstream spectrum is frequently hostile to return path connectivity due to the ingress of foreign interfering signals such as ham radio citizen band (CB), among other legitimate RF emissions.

Table 22-1 summarizes the specifications for the downstream direction, and Table 22-2 summarizes the specifications for the upstream direction.

A DOCSIS system must provide greater than 99 percent availability when forwarding 1500-byte packets at the rate of at least 100 packets per second. To achieve these criteria, certain CATV performance specifications are mandated on both the upstream and downstream spectrum.

Table 22-1 Downstream Cable Specifications

Downstream Parameter	Assumes nominal analog video carrier level (peak envelope power) in a 6-MHz channel with all conditions present concurrently and referenced to frequencies greater than 88 MHz
RF channel spacing (BW)	6 MHz
Transit delay, CMTS to most distant customer	Less than or equal to 0.800 ms
CNR in a 6-MHz band	Not less than 35 dB (analog video level)
C/I ratio for total power (discrete and broadband ingress signals)	Not less than 35 dB within the design BW
Composite triple-beat distortion for analog-modulated carriers	Not greater than -50 dBc within the design BW
Composite second-order distortion for analog-modulated carriers	Not greater than -50 dBc within the design BW
Cross-modulation level	Not greater than -40 dBc within the design BW
Amplitude ripple	0.5 dB within the design BW
Group delay ripple in the spectrum occupied by the CMTS	75 ns within the design BW
Microreflections bound for dominant echo	-10 dBc at less than or equal to 0.5 ms -15 dBc at less than or equal to 1.0 ms -20 dBc at less than or equal to 1.5 ms -30 dBc at less than or equal to 1.5 ms
Carrier hum modulation	Not greater than -26 dBc (5 percent)
Burst noise	Less than 25 ms at a 10 Hz average rate
Seasonal and diurnal signal level variation	8 dB
Signal level slope (50 to 750 MHz)	16 dB
Maximum analog video carrier level at the CM input, inclusive of above signal level variations	17 dBmV
Lowest analog video carrier level at the CM input, inclusive of above signal level variation	-5 dBmV

Good engineering, design, and maintenance practices for CATV plants ensure that these traditional video parameters can easily be met and maintained for operational systems. Parameters of primary concern, however, relate to signal level and noise.

Table 22-2 Upstream Cable Specifications

Upstream	Assumes all conditions present concurrently
Frequency range	5 to 42 MHz, edge to edge
Transit delay, most distant CM to nearest CM or CMTS	Less than or equal to 0.800 ms
Carrier-to-noise ratio	Not less than 25 dB
Carrier-to-ingress power (the sum of discrete and broadband ingress signals) ratio	Not less than 25 dB
Carrier-to-interference (the sum of noise, distortion, common path distortion, and cross-modulation) ratio	Not less than 25 dB
Carrier hum modulation	Not greater than -23 dBc (7 percent)
Burst noise	Not longer than 10 ms at a 1 kHz average rate for most cases
Amplitude ripple	0.5 dB/MHz (5 - 42 MHz)
Group delay ripple	200 ns/MHz (5 - 42 MHz)
Microreflections: single echo	-10 dBc at less than or equal to 0.5 ms -20 dBc at less than or equal to 1.0 ms -20 dBc at less than or equal to 1.0 ms
Seasonal and diurnal signal level variation	Not greater than 8 dB min to max

The greater challenge for the operator is to realize sufficient usable upstream bandwidth to achieve the systems throughput requirements for data or other services. The limited upstream bandwidth must often be shared with other services, ranging from impulse pay-per-view (IPPV), telemetry, and alarm gathering information from the active elements in the cable plant, as well as having to compete with interfering signals that radiate into the lower frequency range.

Because of the limited and often-hostile upstream bandwidth, the hardware design must implement diverse countermeasures to mitigate the effects of both fixed and transient harmful noise. In addition, the network designer must choose from the available remaining spectrum and often must implement bandwidth compromises for a DOCSIS deployment.

A combination of upstream signal quality measured by carrier-to-noise ratio (CNR), anticipated market penetration, services offered, and available upstream spectrum will ultimately dictate the physical configuration of the return-path physical layer.