Success report with Arris SB8200 and gigabit service

I just signed up for gigabit service and bought a new Arris SB8200 cable modem off Amazon a few days ago to use with it. I had some issues getting it all working, but I was eventually successful, so I wanted to post here to record that it can be done, and to help out other people that might be trying to debug a setup like this.

It wasn't working at all when I hooked it up with the self-install kit. I suspected that my apartment's sole cable wasn't connected at the apartment junction box, and when a tech came out to look at things, he confirmed that was true and hooked that up. He also installed a Commscope 6dB coupler between the modem and the wall plate.

This was enough to get some data flowing, though not at the performance I was hoping for. I'll spare the details of my back-and-forth with the tech about this, and cut to the things that mattered:

• I had hooked up my laptop to the modem with a Cat-5 Ethernet cable! Sure enough, that only got me 95 Mbps. I learned that all but one of my stash of old Ethernet cables is old-school Cat-5, and they're all going in the trash.
• Running speed tests from within my browser. Firefox gave me ~400 Mbps, and Chrome topped out at ~550 Mbps, both on speedtest.net and speedtest.xfinity.com. They just weren't fast enough to keep up with a gigabit stream of data on my 2016 MacBook Pro. The Ookla speedtest desktop app for Mac was able to keep up just fine, and gave me 940 Mbps down / 40 Mbps up.
• Running the speed test over WiFi. I have a Netgear R6700v2 WiFi router, and with that hooked up to the modem, I only got 450 Mbps on the speedtest app with my laptop sitting right next to the router's wifi antennas. This is clearly a limitation of wifi, though, and not the connection.
• Running the speed test over 2.4 GHz WiFi! This only gave 120 Mbps or so.

And, some things that didn't matter:

• I saw some tips suggesting that USB-based Ethernet adapters couldn't be trusted for speed tests, but I found that my Aukey 8-in-1 USB-C Ethernet adapter gave me the same top speed as a built-in Ethernet socket on a Mac Mini.
• The last channel under Downstream Bonded Channels on the modem's Status page had downstream power of 10 dBmV and hundreds of millions of correctable errors. Apparently this is the "OFDM channel" mentioned in the troubleshooting tips, and is normal.
• The modem event log had occasional T3/T4 timeouts, usually right after rebooting the modem. These didn't seem to be causing any trouble.
• The event log also showed the "SW Download INIT - Via Config file d11_m_sb8200_gigabit_c01.cm" / "SW upgrade Failed after download -Incompatible SW file" error that others have reported (1, 2, 3, 4), but apparently that doesn't prevent gigabit from working. For what it's worth, my modem has software version AB01.01.009.36.02_022720_183.0A.NSH.

I'm posting this in part because when I was having trouble getting good bandwidth while the tech was here, he gave me a hard sell on switching to a rented Comcast modem. He claimed that "in my eight years as a tech, I've never seen anyone using the Arris SB8200 for gigabit". This, despite it being the top recommendation for gigabit on the Xfinity My Devices page, and one of the top three on Amazon. When I asked him if he had any recommendations for modems that would work with gigabit, he said "gigabit only works with Comcast modems." So, in case anyone else has a tech make unbelievable claims like that, here's a counterexample.

I've also read plenty of stories on these forums about performance with this modem slowly degrading over time, so that might still be my fate. But for now it's nice to have speedy Internet.