gotta ask, just because i went through this with spectrum recently. have you grabbed an extension cord and tried connecting the modem outside at the drop for awhile? i hear you that your neighbor has the same issue. but if youre in. say a development by the same builder, or were all part of a comcast upgrade at roughly the same time .. and well… you both recently upgraded to 1.2(?) because that would be the latter case after my gig upgrade and a few tech visits i ended up finding a splitter that only goes up to 800mhz or so (if that) inside a wallplate. TLDR: you might have a 5-1000mhz splitter. thats widely used by comcast still. MORE: OFDM is 1008mhz or so and you wouldnt notice the problem under, or maybe just UP TO gigabyte speeds (eg: downgrading to 500mb might mysteriously “fix” it). but you WILL notice this at 1.2gb. spectrum is future proofing and using 5-2500mhz splitters ANECODTAL: my modem locked with the 800mhz splitter, but it dropped , cycled and had horrible upload speeds. techs never tried or thought of this . the final boss tech took photos and even took the splitter back to show his boss. i guess multiple units had tickets after the gig upgrade and they had an “aha” moment. TECHNICAL: i would expect something more like multiplexing errors in this situation. forgive me because im 20 years out of the game (was an RF/install tech on analog CATV , and cable modems when those were brand new to Charter) and had to look it up but i think docsis 3.1 is dependent on 957–1151 MHz or 1008–1152 MHz its that 1008+mhz where now your splitter is acting like a 5-1000mhz filter. its not perfect like okay maybe 4-1003mhz gets through the filter maybe even more permissive if its a cheap one. but thats NOT a clean signal for that frequency band its more like bleed-through. sort of similar to traps (the little barrels theyd screw onto your line to block you from getting pay channels in the olden days) and how you STILL could sort of see and hear. a little bit of what was going on on cinemax at 3am and at least get the IDEA. :>
i touched on this in a longer comment in this thread because i think that docsis 3.0 goes up to 900-1002mhz if downgrading to docsis 3.0 (or downgrading to 500/700mb) “fixed” your issue, you probably have a 5-1000mhz splitter thats not just a rf splitter but also, a filter and its JUST leaky enough to allow 1002mhz through. or maybe the modems happy negotiating down to 900mhz. but maybe not quite enough for 1008-1100+ required by docsis 3.1 there will be anecdotal reports of a 5-1000mhz splitter “working just fine” maybe that ones a REALLY leaky filter thats also allowing 1008mhz. or also a case of negotiating a lower channel… gigabit speed and docsis 3.0 are about the threshold for the 5-1000mhz problems would manifest with docsis 3.1, gigabit speed(maybe) and then almost guaranteed at 1.2 gig service+ this idea of “sensitive channels” is extremely close to nailing it splitters fail as well. they’ll bleed through AND filter bands theyre not supposed to. but i didnt seize on that or inside wiring for OP because “the neighbor gets it too” im on a gigabit implementation that has to have +/- 1100mhz , and my own woes uncovered an 800mhz splitter inside a wallplate. it would lock. it would even run at gig somehow. just not very well. its a 5-2500mhz splitter now. a 5-1200mhz would also do (for now) everyone on your tap should be using multiplexed signals, and you should have a good 300mhz or so to play with and lock onto. but if every single one of you gets kneecapped at +/- 1000mhz, then theres a really congested 100mhz band and another 100-200mhz thats open for everyone but you cant lock on to it.
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