What is DOCSIS 3.1?
DOCSIS 3.1 is the version of the Data Over Cable Service Interface Specification that followed DOCSIS 3.0. It changed how data is carried on the coax so providers could offer multi-gigabit cable internet over their existing networks. The specification allows up to 10 Gbps downstream and faster upstream than 3.0, though the speed a customer actually gets is set by the plan. DOCSIS 4.0 came next, mainly to raise upstream speeds.
What changed from 3.0
- OFDM downstream and OFDMA upstream: instead of many separate 6 MHz channels, data rides on wide blocks made of thousands of small subcarriers, a technique also used by Wi-Fi and LTE.
- Wider channels: a single OFDM downstream channel can span up to 192 MHz.
- Denser modulation: up to 4096-QAM, carrying more bits per symbol on clean lines.
- Stronger error correction (LDPC), which makes the denser modulation usable on real-world cable plant.
Backward compatibility
DOCSIS 3.1 modems also bond the older DOCSIS 3.0 channels, so spec sheets list both, for example "32x8 plus 2x2 OFDM/OFDMA". A 3.1 modem works on a 3.0 network at 3.0 speeds, and a 3.0 modem works on a 3.1 network but can't use the OFDM channels.
Matching the modem to the plan
- A faster modem doesn't make the plan faster. A DOCSIS 3.1 modem on a 300 Mbps plan still delivers about 300 Mbps.
- Many gigabit-class plans require a DOCSIS 3.1 modem. Check the provider's approved-modem list before buying.
- A modem with a single 1 Gbps Ethernet port caps throughput at 1 Gbps. Plans faster than that need a modem with a 2.5 Gbps or faster port, plus a router and NIC that can keep up.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.5 covers cable modems and objective 2.7 covers cable internet service. DOCSIS versions aren't named in the objectives, but installs and troubleshooting go more smoothly when you know a customer-owned modem must match the provider's DOCSIS version and channel requirements.