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Standard User wand106
(newbie) Wed 24-Sep-14 10:37:25
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How far can you push a FTTC cabinet


[link to this post]
 
I don't mean physically.

I have a business customer who is sending their off site backups using their FTTC connection. It is a 80/20 line synced at 67/20.

Their ISP does not rate limit, and have no filtering, but you can clearly see after a few hours, some limiting going on, and using the TB broadband ping graphs you can see something is limiting, as the latency increases as the transfer rates goes down from 20mbps to about 10-12mbps (the min latency stays the same though)

so it got me wondering. Assuming your ISP has no rate limiting, and no capacity issues, you have 80/20 'truly unlimited' FTTC, and the far end (where you are downloading from) had capacity to give you all the bandwidth you need, what would happen? Would you really just get 70+mbps download all day long?
In the spec I'm sure it says for business lines if the cab gets congested you get a certain amount of bandwidth, say 20mbps, but if the cab is not congested would openreach be limiting you back or just let you get on with it?

Be interested in any real world observations, so far my only observation has been on the upload.

Cheers

W
Standard User ian72
(eat-sleep-adslguide) Wed 24-Sep-14 10:50:00
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
Theoretically you can get top speed all day long.

However, congestion isn't just at the cabinet or exchange. It could be anywhere in the BT network, ISP network, peering links or remote servers. There are so many places where contention could happen that dips in speed are inevitable.
Standard User Andrue
(knowledge is power) Wed 24-Sep-14 11:00:59
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
Good question. BT documents show that there is a concept of minimum bandwidth so that implies that cabinet to exchange backhaul is not 1:1. If that's the case then congestion could occur if enough users hit the cab hard enough at the same time. In practice though that seems unlikely to me.

Does the slow down appear to be time of day related or duration of transfer? If the former it could be congestion (and it could be occurring anywhere between your cab and the remote server). If the latter it implies throttling somewhere and I certainly don't think the cab or even the exchange would be doing that.

Lastly it seems unlikely that uploading would be causing congestion or throttling. A lot of ISPs don't even bother monitoring it. I think most data links are bi-directional so there should always be loads of spare bandwidth for uploading.

Edit: How far into the download does it start? One possibility is caching at the server. If it has enough spare RAM it could be sucking the data down very fast for the first few gigabytes then slowing when it runs out of memory and actually starts flushing the cache to disk. It seems unlikely though given that the slowdown occurs after a few hours. Most servers don't let pending writes pend that long smile

---
Andrue Cope
Brackley, UK

Edited by Andrue (Wed 24-Sep-14 11:06:42)


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Administrator MrSaffron
(staff) Wed 24-Sep-14 11:45:44
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
If you saturate the upstream of ANY connection, even FTTP you can get latency issues and slow downloads.

To be sounds more like you are describing standard TCP network behaviour and some traffic prioritisation for TCP ACKs on the CPE router might help things.

Each cabinet has one or more 1 Gbps fibre links back to the exchange, so no single user is going to cause that link problems.

The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User RobertoS
(elder) Wed 24-Sep-14 12:12:02
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
I think the slowdown will be roughly as suggested by another poster - the maximum allocatable buffer size per user at the backup site will have been reached, and the buffer filled.

My broadband basic info/help site - www.robertos.me.uk | Domains,site and mail hosting - Tsohost.
Connection - Plusnet UnLim Fibre (FTTC). Sync ~ 56.6/14.1Mbps @ 600m. - BQM

"Angels can fly because they can take themselves lightly." - G K Chesterton.
Standard User wand106
(newbie) Wed 24-Sep-14 12:51:30
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Re: How far can you push a FTTC cabinet


[re: RobertoS] [link to this post]
 
Thanks for the replies. Agreed if you saturate your upload then download will suffer, that is true for any connection, ADSL, FTTC, Fibre ethernet etc.

I have access to a 100mb symmetrical service, if I start an upload to the same box as the site backup is running on I get full bandwidth available, even if the FTTC upload appears to be throttled. Its a high end SAN I believe so it should be able to cope with the backups without having buffer or cache exhaustion, however comments about the per user buffer noted.

I was under the impression that a cab only has 1 x gigE fibre, and if that is filled then another cab is put in. I guess I am talking about the smaller size cab there, I have no idea what the big ones are capable of.

I have carried out a couple of tests using my own FTTC and I get a constant 70mbps down, but I have only done that for about 10mins as I am on a 100gb cap and don't want to exceed my cap.the cab local to me is also going to be fairly lightly loaded, not many properties served and no businesses.

So again ignoring backhaul and ISP throttle, 21CN congestion, the cab will give me full bandwidth until it runs out of capacity then?
Standard User Chrysalis
(legend) Thu 25-Sep-14 10:21:33
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
people need to stop thinking congestion happens cabinet side, doesnt. There is a lot of fibre pushed to the cabinets and most people are on 40/10 as well to boot.

Standard User wand106
(newbie) Thu 25-Sep-14 10:54:54
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Re: How far can you push a FTTC cabinet


[re: Chrysalis] [link to this post]
 
indeed, but the other contention (in the network, ISP centrals etc) is more understood, whereas cabinet contention was not (for me anyway) do we know if OR only push 1G to a cab, or if it gets busy / congested would they put another 1G in or another cab.
Standard User eckiedoo
(experienced) Thu 25-Sep-14 11:14:41
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
It may also be within the remote backup site, particularly if other customers are also running their own "remote backups" and/or "recoveries", overlapping and simultaneously with your customer, so may have little or nothing to do with the broadband connection per se.
Standard User Chrysalis
(legend) Thu 25-Sep-14 12:12:42
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Re: How far can you push a FTTC cabinet


[re: wand106] [link to this post]
 
I would think a congested cabinet is extremely unlikely, the VP at the exchange would probably fill up first or the isp's would see congestion first if average usage was high enough to max out a cabinet.

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