Cable Internet
Cable broadband (DOCSIS)
Broadband delivered over coaxial cable TV infrastructure — fast download, limited upload, shared with neighbours.
Cable internet uses the same coaxial cable infrastructure as cable TV, running the DOCSIS (Data Over Cable Service Interface Specification) protocol. DOCSIS 3.1 — the current standard — supports download speeds up to 10 Gbps and upload up to 1–2 Gbps in theory; real plans top out around 1–2 Gbps download and 50–100 Mbps upload.
How cable internet works (DOCSIS over HFC)
Cable ISPs operate a Hybrid Fibre-Coaxial (HFC) network. Fibre runs from the ISP's headend to neighbourhood nodes — typically serving 50–500 homes each. From the node, the last few hundred metres to each home is coaxial copper cable. DOCSIS modulates internet data onto specific radio frequency channels within the coax's available spectrum (roughly 5–1000 MHz on older plants, 5–1218 MHz on DOCSIS 3.1 Extended Spectrum deployments).
Downstream channels (from ISP to home) are allocated a much larger slice of the spectrum than upstream channels. Downstream occupies roughly 54–1000 MHz; upstream is squeezed into 5–42 MHz on older DOCSIS 3.0 plants (expanded to 5–204 MHz on DOCSIS 3.1 high-split). This physical spectrum asymmetry is why cable upload speeds are structurally limited — it is not a policy choice that an ISP can easily override without re-engineering the cable plant.
Node sharing and evening congestion
All homes on the same neighbourhood node share the node's backhaul capacity. During peak hours — typically 7–10 PM on weeknights — dozens to hundreds of simultaneous users saturate the node's upstream and downstream channels. ISPs manage this by splitting congested nodes (adding more fibre drop points to serve fewer homes per node), but the process lags demand in dense neighbourhoods. The result is predictable: cable speeds during off-peak hours can be 2–3× faster than during peak hours on the same plan.
Upstream vs downstream spectrum
DOCSIS 3.0 plants allocate approximately 38 MHz to upstream and 860 MHz to downstream — a ratio of roughly 1:23. Even DOCSIS 3.1 low-split plants expand this to only about 85 MHz upstream. A high-split or full-duplex DOCSIS 3.1 upgrade expands upstream to 204 MHz or higher, enabling upload speeds competitive with mid-tier fibre. Most US cable plants had not completed this upgrade as of 2026, which is why cable upload caps remain at 10–50 Mbps on most plans.
Cable vs fiber vs DSL comparison
| Technology | Download | Upload | Latency | Congestion | Availability (US) |
|---|---|---|---|---|---|
| Cable (DOCSIS 3.1) | Up to 1–2 Gbps | 10–100 Mbps | 10–25 ms | Shared node | ~88% |
| Fiber (FTTH) | 300 Mbps–5 Gbps | Symmetric | 3–10 ms | Dedicated | ~43% |
| DSL (VDSL2) | Up to 100 Mbps | Up to 20 Mbps | 15–40 ms | Dedicated | ~87% |
Typical cable speed tiers
Most US cable ISPs (Xfinity, Spectrum, Cox) structure plans in tiers. Common offerings include 100–200 Mbps, 400–500 Mbps, 800–1000 Mbps (gigabit), and in some markets 1.2–2 Gbps. Upload tiers are typically 10–20 Mbps for lower plans, rising to 35–100 Mbps on gigabit plans. Actual speeds during peak hours are typically 60–80% of the advertised maximum.
Owning vs renting your modem
ISPs charge $10–15/month to rent a modem/gateway. A quality DOCSIS 3.1 modem (Motorola MB8600, ARRIS SURFboard S33, Netgear CM1200) costs $80–150 retail and pays for itself in 6–12 months. Key rules: verify the modem is on your ISP's approved compatibility list before purchasing; ensure it is DOCSIS 3.1 if your plan exceeds 400 Mbps; confirm it supports the channel bonding count your ISP uses (32×8 channels minimum for gigabit plans).
Reading modem signal levels
Most cable modems expose a diagnostics page (typically at 192.168.100.1) showing signal quality metrics. Healthy targets are: downstream power between −7 and +7 dBmV, downstream SNR above 33 dB, upstream power between 38 and 48 dBmV, and zero or near-zero uncorrectable codeword errors. Elevated uncorrectable errors indicate a physical plant problem — corroded connector, damaged coax, or noise ingress — that warrants a call to your ISP's technical support line.
Frequently Asked Questions
Why is cable internet slow at night?
Cable networks are shared — all homes on your neighbourhood node share the available bandwidth. Peak usage between 7–10 PM saturates the shared upstream, reducing throughput for everyone on that segment. Fiber ISPs use dedicated lines and are not affected.
Can I use my own modem with cable internet?
Yes, and it often saves $10–15/month in modem rental fees. Buy a DOCSIS 3.1 modem from a reputable brand (Motorola, ARRIS, Netgear) that is approved by your ISP. Check the ISP's compatibility list before purchasing.