 DrDrewSo that others may surf.
join:2009-01-28
SoCal
kudos:8 | reply to bernardc
Re: [Speed] Download/upload ratio: Is it physics? said by bernardc:Thank you, DrDrew for all of your fine work here.
You're welcome, but as you can see there are many others here who can answer the questions just as well.
A large part of the asymmetry of cable systems comes from the limited upstream bandwidth (50 vs 800 mhz) and the noise "funnel" effect of having hundreds of customers connected to a handful of upstream receivers.
The noise from those customers is additive and to fight the effects of that noise, throughput is traded for connection reliability in the form of lower modulation rates. Lower modulation rates have lower "bits per hertz", basically lower throughput per Mhz of frequency space, but they also have lower SNR requirements for clean data transfer. A single customer or cable fault can easily add enough noise to affect every other customer attached to the same upstream channels.
Downstream channels don't have the noise funnel effect as it's a handful of channels split-out to hundreds of customers. If the signal source is clean, amplifiers are running well, and the cable is in pretty good shape, end users will get a pretty clean signal so high modulation rates can be used. A amp or cable problem will tend to only affect that branch downstream of the problem. A single customer, being at the end of the line, will have a very difficult time affecting the downstream signal quality of others,
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If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes. |
| Thank you DrDrew and Harald for the excellent information. Yes, there are some of your caliber, but they're not common. I was trying to preemptively filter most of the anticipated noise, and unintentionally ruffled a few feathers in the process.
Thanks again for this very informative discussion! |
