Fun fact. DOCSIS 1.0 through 3.0 cable Internet uses MPEG-2 Transport Streams to deliver the IP packets. It has to, because the QAM specification (ANSI/SCTE 07) is built around 188 byte TS packets.
And in those early days many cable providers didn't even implement the Baseline Privacy Interface, so you could sniff the entire neighborhood's downstream traffic with a modified modem or possibly even a DVB-C capable TV card.
There was literally a register bit in the cable modem MAC to enable promiscuous mode, and you could just set it: https://pastebin.com/18702Ziq
Sadly the Broadcom modem hardware I used to experiment with then seemed to lack the ability to get at the raw MPEG-TS packets, so I didn't manage to repurpose it as a TV receiver. The idea was to tune to a TV channel and stream it over multicast RTP on the LAN.
As used modems were very cheap, I wanted to have a whole bank of them, to demodulate every carrier on the CATV system. All fed into a giant, noisy 24 port 100Mb Ethernet switch that had IGMP snooping support. That was back in the 2000s...
You would have loved the old ATI/AMD demodulator evaluation board that I have. Does both ATSC 1.0 and QAM. TS output is DVB-ASI (which requires a pretty expensive DVB-ASI to USB converter).
I’ve always wondered if that was done to allow mixed video and DOCSIS channels, shared hardware on either end, or just to ensure that TVs and STBs can quickly and safely skip DOCSIS channels that they won’t be able to decode anyway.
If I remember correctly, the 188-byte packets are a result of using ATM AAL-5 frames, each 53-byte frame having 5 bytes of ATM header, 1 byte of AAL-5 framing, and 47 bytes of payload (47x4 = 188).
