Over Utilisation...The laymans story

[sniper007]

Originally Posted by kwikbreaks

When you moved from 20Mbps to 50Mbps you moved from DOCSIS1 to DOCSIS3. It sounds like your local 1 network was oversubscribed but the 3 network still isn't. If you don't actually need 50Mbps you can call retentions and ask to be downdraded to 10Mbs (retentions because it's easier - they're the only VM calls I ever make) - there will be no penalty for downgrading and you'll still be on the DOCSIS3 network. Eventually of course your DOCSIS 3 network may become borked too.

But initially I had a period of a month or so when I was still on 10mb and even that was bad. At the old house literally a street away it was fine. Sorry for not being too clued up on this all but can someone explain out of interest what the function of the CMTS, UBR and Cabinets are? Are the street cabinets simply access points to "turn on" different houses connections to the network? So a houses cabinet will always stay the same due to the physical connection underground from houses to the cab?

So if I have that right only the UBR can be changed by Virgin?

On that basis, when I moved house my cabinet I know for sure changed. Can a cabinet be over utilised or is this impossible? Should the question be, do cabinets each connect to different UBRs? So it was possible that a cabinet change made me go to a different UBR initially so even 10mb was bad, and 20mb, then when I changed to 50mb the UBR changed again to a different one which was ok?

So in conclusion it's complete pot luck and nothing to do with 50mb necessarily?

All I know is at the moment 50mb works and works fast, always. I'm happy until that changes!

[kwikbreaks]

There's obviously more to it than just this but my way of looking at it (which may be wrong) is...

The small street cabinets are just dumb splitters which join all the individual modem coax cables to one cable which goes on to other cabs and eventually back to what's called the optical node which is another big street box and serves a shedload of modems. That's where the cable RF tech gets converted to (I think) tcp/ip which goes by fibre back to another concentration point housing loads of fibre routers and then on again to VM's major infrastructure connecting to the internet backbone.

In the RF side of things there are RF amps and attenuator banks which are used to make sure the modems get signal levels they can handle and the optical node sees the correct level from the modems without them busting a gut to deliver it but none of that is relevant to congestion.

CMTS/UBR/Optical node are different names for essentially the same thing - the kit that interfaces coax to fibre.

DOCSIS1 and DOCSIS3 are different RF level technologies. Both can be carried on the same coax along with the TV channels. Either or both can be oversubscribed and as I said it sounds like your old docsis 1 was oversubscribed but the docsis3 isn't (yet).

[Ignitionnet]

Originally Posted by kwikbreaks

CMTS/UBR/Optical node are different names for essentially the same thing - the kit that interfaces coax to fibre.

Hmm.

http://en.wikipedia.org/wiki/Cable_m...ination_system
http://en.wikipedia.org/wiki/Hybrid_fibre-coaxial
http://www.cisco.com/en/US/prod/coll...0801ed384.html

Not entirely convinced you could say that this:



And this:



Are essentially the same thing.

One of them fits comfortably into a street cabinet, the other really won't. Check the links above for more info.

[kwikbreaks]

Thanks for the info and links. They do look tad different for sure...

Jesus all those words in the links are making my head ache - it's going to be a task for another day...

[Ignitionnet]

Originally Posted by kwikbreaks

The small street cabinets are just dumb splitters which join all the individual modem coax cables to one cable which goes on to other cabs and eventually back to what's called the optical node which is another big street box and serves a shedload of modems. That's where the cable RF tech gets converted to (I think) tcp/ip which goes by fibre back to another concentration point housing loads of fibre routers and then on again to VM's major infrastructure connecting to the internet backbone.

No, it hits the optical node which doesn't have to be a big street cabinet as the node is small, it converts the RF to light, nothing else is done just the conversion to optical transport. The RF is used to modulate a laser transmitter on the upstream and the optical signal is demodulated on the downstream path. Media conversion, nothing more.

The big street cabinets are telco / optical multiplexers.

Originally Posted by kwikbreaks

CMTS/UBR/Optical node are different names for essentially the same thing - the kit that interfaces coax to fibre.

No, the uBR is Cisco's brand name for their CMTS, Motorola refer to theirs as the BSR, and the CMTS is what manages the modems. It's these that package the IP data up in DOCSIS MPEG frames, address it to modems, and put it onto coax and demultiplex the upstream bursts.

Congestion is caused by having too many modems / too much demand on a particular CMTS port. The optical node sizes are very relevant to this as each optical nodal area is a 'broadcast domain' - within a single node the only option is to run more channels.

[kwikbreaks]

Ah good - that's saved me struggling - so the RF tech actually gets carried by fibre back to the CMTS/UBR and that's where it all gets converted to TCP/IP?

What governs which individual port a modem will be served by - the physical connection or is there something in the config?

[Ignitionnet]

Originally Posted by kwikbreaks

Ah good - that's saved me struggling - so the RF tech actually gets carried by fibre back to the CMTS/UBR and that's where it all gets converted to TCP/IP?

What governs which individual port a modem will be served by - the physical connection or is there something in the config?

Both. The physical connection governs which ports downstream and upstream a modem can connect to, configuration then decides which ones it does connect to.

That's why the DOCSIS 3 devices don't attach to the DOCSIS 2 network - they lock to it then are pushed by configuration to the DOCSIS 3 network.

[horseman]

Originally Posted by Ignitionnet

...
Congestion is caused by having too many modems / too much demand on a particular CMTS port. The optical node sizes are very relevant to this as each optical nodal area is a 'broadcast domain' - within a single node the only option is to run more channels.

Which then begs many more increasingly complex questions that attempt to speculate on why VM have some perversely protracted fix times in some areas?

For example:
1. Does Cisco licensing still depend on number of "channels" used per CMTS?

2. The number of "usuable" channels will depend on quality of RF spectrum. This must largely depend on combination of amount of domestic noise injected upstream and also total number of cm's to avoid laser clipping? Originally 10+ yrs ago the DOCSIS RFI spec ,number of street cabs, optical nodes would have been designed on basis of HPPN (homes passed per node) and an estimated "takeup" of say 10/20% subscribers? In some older Victorian/Edwardian built towns/city's (particularly those with higher student populations) 3 story town houses and similar have been progressively converted to self contained flats over the intervening period which appears in some cases to have resulted to congested tap boards and frequent "re-splitting" of original house drops. This must have some deleterious affect on both "noise" and upstream power by effectively increasing the HPPN and/or subscriber "takeup" per node?

3. If current RF spectrum is limited then the traditional approach is to re-segment. Presumably since most (or hopefully all) of the legacy FP lasers are replaced with WDM does this mean just splitting a coax trunk or would this still increase the risk of laser clipping?
3a. Since power for trunk amps/line extenders is injected at optical node this could imply co-requisite upgrades to power supply is also required in some cases from utility company? Interestingly as an aside note, I thought UK used 48v DC for trunk/amps,line extenders not AC?
3b. If dynamic load balancing is also restricted to within same PHY RF segment does this mean that either the FO has to be "uncombined" at CMTS or separate FO blown down to an additional optical node?

4. Presumably 3b is also dependent on the amount of "dark fibre" available again which could vary by area?. If not then "ducting" capacity/limitations could very well imply substantial civil engineering work?

5.Would upgrading to high density linecards increase either the capacity of CM's per channel,the number of channels/upstream ports or both? Again as well as more power efficient I thought these reduced the need to physically re-segment?

Re 2.

In most University cities, the number of students in the type of house you mention has been dropping drastically over the last 5-7 years, as they/their parents prefer them to live in the bespoke student accommodation supplied by Opal, Unite, et al, which has its own dedicated Broadband.

[horseman]

Originally Posted by Hugh

Re 2.

In most University cities, the number of students in the type of house you mention has been dropping drastically over the last 5-7 years, as they/their parents prefer them to live in the bespoke student accommodation supplied by Opal, Unite, et al, which has its own dedicated Broadband.

Halls of Residence / Campus usually JANET but here in Brighton still plenty (ab)using VM….

[Sephiroth]

Originally Posted by Hugh

Re 2.

In most University cities, the number of students in the type of house you mention has been dropping drastically over the last 5-7 years, as they/their parents prefer them to live in the bespoke student accommodation supplied by Opal, Unite, et al, which has its own dedicated Broadband.

Not according to my kids. The student house market is as bouyant as ever.

[Chrysalis]

not to mention the ever growing trend in cities to convert houses to flats/bedsits which bumps up population density.

Originally Posted by horseman

Halls of Residence / Campus usually JANET but here in Brighton still plenty (ab)using VM….

Re Unite/Opal/non-Uni owned halls - they are usually independent providers, not JANET/local MAN.

[Sephiroth]

Originally Posted by Hugh

Re Unite/Opal/non-Uni owned halls - they are usually independent providers, not JANET/local MAN.

So what? The 4 students to a hous market thrives.

Not in Leeds - lots are up for sale.