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[ryan2390]

Hi All,

I was having a discussion on a well known social media site earlier this week about whether or not an acquaintance should turn off his modem router at night. Assuming he had an ADSL or VDSL service I advised against it but then I was reminded that he has cable from VM and I wondered if DOCSIS has some sort of 'line management' like DSLAMS do? I had a quick look and couldn't see anything but thought it best to ask here.

I've been able to use a cable connection before and whilst I didn't have to live with it I found it acceptable. Central equipment was a tad unreliable and broke down a few times a year but I was led to believe this is related to who installed it in the first place. Anyway I digress.

I'm quite interested in hearing how reliable your cable connection is if you have one and if there is any sort of line management. Further I'm interested to know if there is the same speed falloff as with ADSL/VDSL and what the practicalities of the higher speeds in the latest EuroDOCSIS standard?

I apologise if I mixed some things up but I've only had a quick look on the web   

I look forward to the replies. 

[S.Stephenson]

I had virgin media a few years back, as far as I'm aware if you can get Cable you can get whatever speeds are available.

Only issues were quite bad contention at peak times, and trouble with latency occasionally but especially at peak times.

[burakkucat]

You will need to wait for Walter to return from his trip up to B4RN-land, for he has a Virgin Media connection. I sure he will be happy to provide information as a satisfied "cable user".

[kitz]

Im not aware that cable uses a DLM.  DLM was brought into use with the advent of radsl/maxdsl.  Prior to rate adaptive DSL you either synced at 500/1000/2000 or you couldnt.   

That said co-ax is subject to attenuation and therefore speeds will decrease the longer the cable is from the node due to packet loss. The cable companies use amplifiers at certain intervals in order to maintain the signal strength.   Off the top of my head I have no idea what the max recommended coax length would be for cable broadband but I would imagine there will be a guideline somewhere.

[niemand]

Ah this is my subject.

Cable does not have DLM as there's no way to compensate for the line conditions of any one subscriber at the moment. If an upstream channel due to interference is having issues with errors due to a low SNR the channels can fall back to a lower bandwidth, lower order modulation that needs a lower SNR to run error-free.

Speeds on cable do not decrease due to distance from the optical node. Packet loss is always a fault. Amplifiers are placed around the network to ensure that subscribers upstream and downstream signals are within the DOCSIS specification.

The modems themselves compensate for distance from the optical node by changing the power level they transmit at. Downstream power level is not changed, however there is a pretty big operating range, and there are ways to increase or decrease the power levels that reach a particular modem. The network has difference attenuation taps at cabinets that compensate for how far subscribers are from the cabinet. Those that are close are put on the highest attenuation taps to balance their signals, those farthest away on the lowest attenuation taps.

It's not like xDSL where the higher the signal power the better, pretty much, signals on cable can be too high as well as too low due to the amplifiers.

Either way cable is basically like a PON FTTP connection in that you either lock to the channels or you don't. You are synched at the full bandwidth offered to your node, subject to your modem being able to lock to all of them, and get a slice of it dictated by which tier you subscribe to. Your own line conditions are irrelevant to the speed you are synched at.

[niemand]

Practicalities of the new DOCSIS 3.1 standard.

It will allow more efficient use of the spectrum on the cable networks, so without any upgrades it's possible for Virgin Media to offer higher speeds.

With upgrades to the cable networks to increase their bandwidth, and by this I mean their RF bandwidth, it's perfectly feasible for VM to be offering speeds matching the 1Gb downstream, 100Mb upstream service BT are trialling to some of their footprint by next year if they so choose. The upgrades are ongoing and anywhere that has had them can, simply by changing the equipment at the customer end and some line card and software upgrades at the VM end, deliver the 1Gb/100Mb service I mentioned.

Even without going to the new standard VM should be knocking on the door of BT's final G.fast speeds, 500/50, by 2017 if they're minded to.

Virgin Media Ireland have just released a 360Mb/36Mb service over DOCSIS 3.0, not the newest standard. Comhem in Sweden offer a 500Mb/50Mb service over DOCSIS 3.0, too. VM UK could certainly match this next year, however whether there will be the competitive need to is debatable. For now Ireland and Sweden have way more FTTP delivered and in prospect than we do, and G.fast will still be in pilot phase in 2017.

[burakkucat]

Thank you for that description. It has filled a gap in my knowledge. 

[Weaver]

Could you tell me a little about upstream traffic shaping and pricing, Ignitionnet

[niemand]

Upstream throttling, applies to all traffic upstream.
Traffic management policy, applies to some types of traffic.
Pricing.

[WWWombat]

Packet loss is always a fault.

Do you mean this the way I think? That a lost packet should never happen, unless there is a cabling fault?

I'm not including packets dropped due to congestion.

I'd have thought that, even though cable works with all lines having in-spec SNR values, that noise is still possible, and can still knock out a packet. Are you saying this shouldn't ever happen?

[niemand]

The upstream has FEC to fix brief periods of burst noise.

Never was a bit too much though. The specification details a loss of no more than 1x10^-8 symbols if the channel is in spec.

If noise is causing more errors than that SNR has temporarily dropped outside of specification. If it fixes itself all good. If it remains too low and breaches thresholds the upstream channels will lower modulation order to try and stabilise. The cable plant should be sealed both from leakage and from ingress leaving the only source of noise as misbehaving CPE or an unsealed entry point into the network, which is bad. Obviously the extent to which this can be resolved varies but I've certainly seen VM noise busting on upstream where things from devices gone bad to unauthorised devices plugged into the network to bad installs / removals leaving tap points uncapped or not disconnecting cables from network when customer leaves.

That's as close as DOCSIS 3.0 and earlier get to rate adaption, the entire channel rate adapts. Depending on the profiles configured the likely course of events would be 64QAM -> 32QAM -> 16QAM or 16QAM -> 8QAM -> QPSK in areas not on 64QAM. This adaption is there to buy time and to mitigate transient issues as noise can only be found while it's impacting. There's no adaption on the downstream and BER is cliff-like when channels go outside spec.

[kitz]

Thankyou Ignitionnet for your informative posts.

Speeds on cable do not decrease due to distance from the optical node. Packet loss is always a fault. Amplifiers are placed around the network to ensure that subscribers upstream and downstream signals are within the DOCSIS specification.

Ahh, thanks.  I was thinking of co-ax in the days of old when it was used for LAN networking and what could happen if your cable was too long
I was kinda almost there when I said they used amplifiers to maintain the signal strength. 

[ryan2390]

Despite the fact that sometime this year we're getting FTTP as I'm sure most of you know I'd still like to try living with cable for a few months and see what it's like to actually use as I'm a bugger for noticing when something isn't quite right.

Given I play a lot of online games would be interesting to see if BT's recent claims on the tellybox about being better for gaming are founded. I've found that latency isn't the be all and end all. Hmm.

[niemand]

BT's comments are accurate. VM have higher upstream jitter.

[Weaver]

Where were the BT comments made folks?