SDG #208 Cityfibre / Vodafone Gigafast 900 still poor speeds. What's in the GPON ONT 801Gv2?

Gigabit fibre should be a set and forget upgrade, but this instalment follows an ongoing saga with a Cityfibre-delivered Vodafone Gigafast 900 connection that has never consistently produced the throughput it was sold on. Rather than accepting the usual “it must be your WiFi” explanation, the investigation moves to the one piece of hardware that sits between the fibre and the home network: the GPON ONT 801Gv2 supplied as part of the install.

The teardown is the interesting part for anyone who likes to know what their ISP has bolted to the wall. Inside a modest plastic enclosure sits a single board built around a GPON system on chip with its associated DDR memory and flash, a gigabit Ethernet PHY feeding the LAN port, the optical transceiver module that handles the actual fibre link, and the switching power supply section that runs from the external 12 V brick. Attention is paid to the thermal situation, the quality of the layout and component choice, and the debug and serial interfaces left on the PCB, which hint at how much of the unit’s behaviour is locked down by the operator rather than the hardware.

The practical takeaway is that a single gigabit Ethernet port is a hard ceiling on a service marketed at 900 Mbps, leaving very little headroom once overheads are counted, and that poor real world results are more likely down to provisioning, routing or the supplied router than the ONT itself. If you are diagnosing a slow fibre line, this is a useful reminder to isolate each link in the chain before blaming your own kit.