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Old 29-08-2008, 21:57   #13
Ignitionnet
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Join Date: Jun 2008
Location: Leeds, West Yorkshire
Age: 45
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Re: fibre optic is ist worth it?

Quote:
Originally Posted by eth01 View Post
Hrm. The further away to the exchange you live, the slower your connection will be (this does generally apply to cable and ADSL customers) ...

The more it has to travel, the more you lose....
Obiously those amplifiers, both coaxial/RF and erbium doped fibre in the cable network are pointless.

Basingstoke for example has no uBRs of its' own, its' nodes are aggregated at a hubsite in Basingstoke then go to uBRs in Reading over DWDM optical backhaul. This is 20km as the crow flies, and has no effect at all. Distance is completely irrelevant to cable so long as distance to the uBR is within spec.

The issue with cable is nothing to do with distance, it's about how many amplifiers are in the network. Amplification introduces distortion, which is why fibre is introduced into networks, optical amplification is much cleaner than RF amplification and offers a much larger range, in addition fibre removes the need for so many RF amplifiers meaning less distortion and higher quality signals and in addition reduces segment size meaning far less funneling effect of noise on the return path.

In addition on QAM / modulated digital signals over RF there is a very very small margin where speeds are 'slowed' then BER (Bit Error Rate) grows unacceptably high and modems lose sync. It's not a gradual thing it's a 'cliff' effect as the 1s and 0s get too mangled to be decoded and the Reed Solomon encoding / FEC (Forward Error Correction) is unable to correct the increasing errors.

Unlike DSL cable is not rate adaptive. Bit allocations and modulations cannot be dropped and speeds cannot be slowed to accomodate poorer quality signals. You either have signal quality that can handle the modulations and channel specifications the uBR provides or you don't, in which case your service becomes unstable.

The whole point of cable is that customers are not affected by distance from headend / hubsite due to the active components in the access networks.

Easy concepts really.

---------- Post added at 20:57 ---------- Previous post was at 20:50 ----------

Quote:
Originally Posted by Earl of Bronze View Post
The VM network backbone is fibre, but that terminates at you local ring (A&B Cabinet I think its called). The remaining network for upto 1500 houses is copper.... Damn, I can't believe I remember this stuff from when I started as an installer in 1998.
Nearly! But for broadband the fibre feeding the nodes is usually a 'star' network rather than a ring, IE a single fibre pair feeding each area. Rings tend to be confined to the telco / PDH / SDH network due to it being a tad more vital and justifying the additional expense of the extra fibre.
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