Originally Posted by gadge (Post 34176072)

sorry to but in bit of topic is it true tvdrive has been delayed?;) seems to be a bit up in the air at the moment.

Originally Posted by Escapee (Post 34176028)

I doubt if the bandwidth would be available, I know they used to run comms over the rails but I'm not really that clued up on it. Many years ago I knew a guy working on comms for BR, I think the technology then was a LF or HF signal. I also remember seeing some of this old equipment available at a radio rally aroundb that time. (thats how the discussion started)

It was a long time ago so perhaps I am confused and they were for use on overhead electrics. Any for of communication down the rails will be a problem, you have to consider loosing connections at points, and corrosion if not on a continuous welded section of track.

Also remember that the signal levels would need to be very high if using LF/HF in the same manner as ADSL, because the railway lines are not twisted pair:) With twisted pair the crossing of the wires cancels out any unbalanced signal. ie: interference picked up on the line by using the push-push effect. (I'm sure you could Google for an explanation)

BT's lines were designed for audio signals, getting good balance at HF frequencies (that ADSL uses) is not an easy task. The old Rediffusion HF cable TV systems used balanced twisted pairs desugned for the job, but they still had problems with balance and interference.
The use of digital technology these days goes some way to mask the inherent problem of using twisted pairs for broadband communications. Twisted pair in itself is not a bad thing, it has a higher velocity ratio than coax, and has less loss/km. The real problem is maintaining good balance at all the joints, the use of chock block type connections ruins the balance.
Whilst a network can be efficiently maintained for audio or low speed data (voice) using it for broadband is far different. ADSL technology masks the problems with error correction, so its very much like trying to make a silk purse out of a sows ear.

Using the railway lines would only offer one ADSL line shared between all the users on that track, even if it did work satisfactory.

Originally Posted by Lobster Ring (Post 34176119)

Many thanks Escapee.

I have noticed that the rails are no longer disjointed, they put a thick wire and connect disjointed lines/pints on the side, no idea why.

Why can't they apply the short-wave idea used on the powerlines?

Originally Posted by zinglebarb (Post 34139195)

so whats the timescale on this 21C? I knew this was on the cards but this is gonna take ages surely?

Originally Posted by murfitUK (Post 34139185)

What's the point in boasting about having cables capable of 100MB/s when my 4Meg service won't go above dial-up speed for 8 or 9 hours each day?

And BT's claim of 8MB being applicable to 7% of the UK... isn't ntl making similar false claims by advertising UPTO 4Meg when the network is obviously not capable of delivering. Might as well advertise a service upto 1 billion trillion gazillion MB/s and then when the complaints come in say "Ah, we didn't promise you would get that speed, we said upto..." which is the reason ntl complained about another service provider's adverts. Pot & kettle?

Originally Posted by gadge (Post 34176072)

sorry to but in bit of topic is it true tvdrive has been delayed?;) seems to be a bit up in the air at the moment.

Originally Posted by Escapee (Post 34176188)

I guess they could, but I think it would be limited bandwidth.

I'm not sure how their automatic signal system works, perhaps someone may know if its done via RF telemetry or some other method?


PS: I wonder how they managed with comms when a train was on the line, ie: shorting the two lines together via the axle. I guess it could work because the frequency used wouldn't see it as a short unlike 50Hz or DC.

Originally Posted by Elrond24 (Post 34186513)

Hi thought I'd butt'in I am not an S&T Engineer but can say the railway is bad environment any comms no matter how sophisticated.

My experience lies in Power Transmission, but the basis of signal telecommunication is that on Overhead Power Tranmission DC signaling is used and on DC Transmission AC signaling.

Bad - in that our rectifier,s induce 300 hz into all adjacent copper cable's, our HT cable surges (be it for our network or the REC), induce upward of 1,500v, mother nature via Lightning strikes at or close to running rails destroy most front ends, TOC induce further interference via chopper ccts in their trains, every night especialy in the London area track ccts(running rails) are disturbed due to engineering. I will not go on !!

The greatest disapointment that Network Rail cannot extend their existing Fibre optic system nationwide for commerial use.

Originally Posted by Escapee (Post 34186540)

So the signaling data is at 300Hz?

I remember many years ago that ntl actually came to an agreement with Network Rail or whoever it was at the time, to use the railway fibre/comms ducts in the South Wales valley's. As far as I'm aware they never actually made anything of this agreement, dont quote me but I think the agreement involved ntl using the ducts to run or upgrade the existing fibre routes and offer dark fibres for Network Rail to use. I dont remember the exact details, but had the information at the time from someone involved in the discussion.

The deal was struck (if it was ever signed) at a time when ntl in South Wales were called ntl but were in fact CableTel, It was around the period that CableTel bought ntl but before they bought any of the CWC/Comcasts/Diamond mob etc.

I believe that CableTel/ntl buying up most of the opposition early on had a big impact on the overall availability of cable in the UK.