DOCSIS Glossary

DOCSIS Glossary

The DOCSight dashboard shows technical measurements from your cable modem. Here's what they mean in plain language.

Term	What it means
DOCSIS	The technology that makes cable internet work. Your internet signal travels through the same coaxial cable as TV.
Downstream / Upstream	Downstream = download direction (Netflix, browsing). Upstream = upload direction (video calls, sending files).
SNR (Signal-to-Noise Ratio)	How clear your signal is. Think of it like radio reception: high SNR means a clear signal, low SNR means static and interference. Measured in dB.
Power Level	How strong your signal is. Too weak or too strong and your internet suffers. Measured in dBmV.
Uncorrectable Errors	Data packets that were lost and couldn't be recovered. A few are normal, but many of these cause buffering, dropouts, and slow speeds.
Correctable Errors	Data packets that arrived damaged but were automatically repaired. Lots of these are a warning sign.
QAM / Modulation	How much data is packed into each signal. Higher QAM (like 256QAM or 4096QAM) means more speed. If your modem drops to a lower QAM, something is wrong with the signal.
QPSK (4QAM)	Quadrature Phase Shift Keying — the lowest modulation level in DOCSIS, encoding just 2 bits per symbol. If your upstream drops to 4QAM, the signal quality is severely degraded. DOCSight displays this as "4QAM" for consistency with other modulation labels.
Channels	Your cable connection uses many frequencies at once (like lanes on a highway). Each lane is a "channel." More healthy channels = more bandwidth.
SC-QAM	Single Carrier QAM. The classic DOCSIS 3.0 channel type. Each channel is a narrow 8 MHz (EU) or 6 MHz (US) slice of spectrum carrying one data stream. A typical modem bonds 24-32 of these for download. Simple and robust, but less efficient than OFDM.
OFDM	Orthogonal Frequency Division Multiplexing. The DOCSIS 3.1 downstream technology. Instead of many narrow SC-QAM channels, one OFDM channel spans up to 192 MHz and packs thousands of tiny subcarriers inside. This is far more efficient (up to 4096QAM or even 16384QAM) but more sensitive to interference. In DOCSight, OFDM channels typically appear with high channel IDs (e.g. 200+).
OFDMA	Orthogonal Frequency Division Multiple Access. The upstream counterpart to OFDM, also DOCSIS 3.1. Works the same way but in the upload direction, spanning up to 96 MHz. Allows multiple modems to share the same frequency block without collisions.
Mixed Mode (3.0 + 3.1)	Most modern modems use both technologies at once: SC-QAM channels for DOCSIS 3.0 and OFDM/OFDMA channels for DOCSIS 3.1. DOCSight detects and labels each channel type automatically. If your modem shows both, that's normal and expected.
T3 / T4 Timeout	Error codes in your modem's event log. T3 means a single request to the CMTS (cable headend) failed. T4 means total loss of communication. Multiple T4s indicate a serious upstream problem.
CMTS	Cable Modem Termination System. The equipment at your ISP's end that your modem talks to. Think of it as the "base station" for cable internet.
vCMTS	Virtual CMTS. A software-based version of the CMTS running on standard server hardware instead of dedicated appliances. In Germany, Vodafone uses Harmonics CableOS (vCMTS) and Casa Systems (traditional/Remote PHY CMTS) across their network. A Fritz!Box showing "CMTS Hersteller: unbekannt" typically indicates a Harmonics vCMTS.
Remote PHY	Architecture where the physical RF layer (PHY) is moved to a node closer to the customer while the CMTS logic stays centralized. Used with Casa Systems in parts of the German cable network.
Rückwegstörer	German term for "return path interference." When a faulty device on the same cable segment injects noise into the upstream, it degrades service for all connected modems.
Node Split	When an ISP divides a cable segment into smaller sections to reduce congestion. Often promised, rarely delivered on time.
Shared Medium	Cable internet is shared among all users on the same segment. This is by design and works fine when the segment isn't overloaded.

Understanding Health Status

Status	What it means
🟢 Good	All values within ideal range. Your connection is healthy.
🟡 Marginal	Some values outside ideal range. You may not notice issues yet, but worth monitoring.
🔴 Poor	Critical values detected. You likely experience dropouts, slow speeds, or instability. Contact your ISP.