Cisco Certified ExpertRequirements for Cable Plant Quality
Because noise and distortion can severely impact the operation of two-way cable IP networks, a set of cable plant quality requirements has been published as part of the DOCSIS cable specification. These requirements set stringent guidelines for plant quality in the upstream and downstream directions.
Cable plant quality is measured in several ways depending on the noise or distortion type. Because of the variable noise tolerances of standardized modulation types (discussed later in this chapter in the section "Modulation Types"), plant and signal quality requirements vary between modulations. In Table 13-2, operational settings are detailed. These operational settings should be considered the absolute minimum. Operation of DOCSIS equipment outside of these ranges is not recommended. The frequencies used for upstream transmission are typically noisier than those used for downstream transmission, and thus more noise tolerant modulation types are incorporated into the standard for upstream transmission.
Table 13-2 DOCSIS Requirements for Cable Plant Quality
|
System/Channel |
DOCSIS Specification |
|
Carrier to noise (upstream) |
25 dB |
|
Carrier to ingress (upstream) |
25 dB |
|
Carrier to interference (upstream) |
25 dB |
|
Carrier to noise (downstream) |
35 dB |
|
Carrier hum modulation (downstream) |
-26 dBc (5 percent) |
|
Carrier hum modulation (upstream) |
-23 dBc (7 percent) |
|
Carrier to composite second order and |
-50 dBc |
|
composite third order distortions |
|
|
(downstream) |
|
|
Amplitude ripple (downstream) |
0.5 dB in 6 MHz |
|
Amplitude ripple (upstream) |
0.5 dB/MHz |
|
Group delay (downstream) |
75 ns in 6 MHz |
|
Group delay (upstream) |
200 ns/MHz |
Operating near minimal DOCSIS settings can provide satisfactory service to data-only subscribers; other service and technologies might be less tolerant. Voice services, because of their extensive availability and quality of service (QoS) requirements, require the cable operator to design to superior standards. Specifically, when deploying voice services the cable plant should meet full DOCSIS specifications as listed in Table 13-2. In many cases, the operator might want to design to even better plant quality to add a safety margin or buffer in case of partial plant failure.
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