Noise on Cable and DSL Networks

[James Henry]

Originally Posted by jtwn

Haha, thankyou for recognition of the true great one James

Yes there there are noise issues on HFC networks, I never denied that, however its transparent to the end user to an extent compared to ADSL where your speeds depend on it. Am I right or am I right? I'm going to disallow any splitting hairs that you no doubt will try and do.

You are happy to acknowledge all the pluses of cable networks, and there are a few, over DSL without considering any of the negatives.

Both techs to an extent are transparent to noise issues. In the case of both technologies a certain amount of noise is acceptable and doesn't affect speeds in any way.

ADSL is based on a system of QAM / PAM carriers and bit loading is variable depending on noise in each frequency. From that point of view it's actually more robust than cable which depends on a single carrier whose modulation is usually fixed.

Cable is a single in theory enclosed network, however when noise on cable networks becomes an issue speeds drop, due to retransmits and errors, and service fails in a similar manner to a marginal DSL line. In addition to this contention becomes an 'issue' due to wasted bandwidth. If a node is being pushed hard speeds will drop for all customers on a node in a direct correlation to noise increasing - in a similar way to pushing an ADSL line at near 100% capacity occupancy results in a direct correlation between noise and performance.

So basically you are partially wrong, both with appropriate levels of noise on the line are unable to achieve required performance. ADSL is able to adjust to this by lowering bit distribution, while cable is unable to adjust and eventually falls off the 'wall'.

They both deal with noise issues in a different way, with the result that in noisy environments DSL is more robust due to its' ability to downgrade speed to compensate.

The difference is, as with everything else to do with cable and DSL, one is based on the line the other based on the node - an advantage for cable indeed. As DSL lines tend to be pushed near 100% capacity occupancy it's something that shows more on DSL, however it's all dependent on the configuration. Load a cable node heavily and you get the same result.

On the flip side, cable tends to be run with more headroom to compensate for noise, at least you'd hope so as there are guidelines. The noise tolerance of a single DSL connection is directly dependent on the amount of capacity occupancy being used. If you're running a line at 96% occupancy there may be issues.

For more information on this check TDMA - as a hint you use a timeslot up whether your transmission is errored or not, and if it's errored you have to repeat the process again and don't get your timeslot back.

Either way a crap comparison between two technologies that are difficult to directly compare. It all comes down to the same old crap about cable being a shared network and ADSL a dedicated line to the edge network.

Try telling the people in Southampton, St Albans, Bishop's Stortford and other areas that have had noise issues in the past that noise doesn't affect their speeds.

Summary: DSL is more subsceptible to noise affecting maximum speed achievable but to say that it's something transparent to cable is incorrect. It's only transparent while within tolerance and assuming a node isn't being heavily utilised. In addition to this DSL copes with marginal noise environments in a more robust way than cable. It slows down rather than getting packet lossy, laggy, then dying.

Edited slightly.

We appreciate technical articles and explanations, but please don't get personal with people.

[jtwn]

Thankyou for your explanation and confirming my statement.

Do note I did say 'its transparent to the end user to an extent', to an extent as I don't know why you would think I would not realise a ****ty line would mean problems. I didn't say transparent to cable end of sentence.

Its a nature vs problem scenario. Noise on ADSL lines is nature, where noise on cable is problem (to be fixed).

[James Henry]

Originally Posted by jtwn

Thankyou for your explanation and confirming my statement.

Do note I did say 'its transparent to the end user to an extent', to an extent as I don't know why you would think I would not realise a ****ty line would mean problems. I didn't say transparent to cable end of sentence.

Its a nature vs problem scenario. Noise on ADSL lines is nature, where noise on cable is problem (to be fixed).

Why would I have wanted to split hairs when you do it yourself? 'To an extent'. I could stick my own testicles in a vice, wind it shut and it would be painful 'to an extent'. Doesn't really mean much is a way to avoid quantising things.

Onto the rest of it.

Anyone who knows what they are talking about knows that noise is a fact of life and expected on cable networks as well.

Excessive noise is indicative of a fault on both platforms.

May I suggest you do some research on amplification. Maybe look into push-pull amplifiers, composite second order distortion, composite triple beat, common path distortion, intermodulatory products.

If you actually knew what you were talking about you'd know that the 'lining up' of HFC networks is a balancing act between power levels and distortion. In addition to this 'noise 'ingress from external sources is also both expected and allowed for in the SCTE specifications for cable networks. Oh and yes, distortion is considered 'noise' - cable networks by nature introduce 'noise' onto the networks themselves.

It's the balancing act between noise and distortion that was a major driver for HFC networks to replace pure coaxial networks. Reducing the number of amplifiers in a cascade results in a larger operating margin between power being too low, meaning noise floor - the noise caused from a combination of amplifier / optically induced distortions and ingress from external sources causes an unacceptable CNR (that's Carrier to Noise Ratio which you may not understand as your modem doesn't have it listed on its' diagnostics, google it) as it's too close to wanted signal for power, and amplification introducing unacceptable levels of distortion onto networks resulting in the noise floor being raised too high.

Cable networks course closed for this evening.

[jtwn]

You are reading far to into this.

Nobodys perfect James, we all make mistakes and judging by all your edited posts, you do too

[James Henry]

Originally Posted by jtwn

You are reading far to into this.

I'm sorry.

If you want to score points on technical matters you may want to obtain the knowledge and skills to back up your assertions. Trying to create absolutes where there aren't any with loaded questions doesn't impress.

That's about all I have to say on this to be honest. Hope the above is interesting reading for those who are actually interested in such things.

That's all anyone has to say on this, and please change your signature now to remove the personal references.