[Gllug] Proposed broadband improvements

David L Neil gllug at getaroundtoit.co.uk
Sat May 15 22:43:11 UTC 2010

On 13/05/10 21:53, Jason Clifford wrote:
> On Thu, 2010-05-13 at 09:37 +0000, Chris Bell wrote:
>>     It was my understanding that DC volts are maintained by batteries at the
>> exchange to power a simple analogue phone connection through the copper
>> pair.
>>     How are analogue phones connected to the fibre?
> Who told you they were?
> The broadband service is terminated to fibre at the cabinet. That's not
> the PSTN service which, as I understand it, is simply jumpered on to the
> exchange as is already the case. There is no change to the PSTN service.
> All that changes is that the cabinet has something like a microfilter on
> each line.

Sorry Jason, when you say "jumpered" and "no change" are you saying that 
there will be two physical routes from the 'cabinet' to the exchange, ie 
fiber for broadband/digital and (still) copper for voice/analog?
(and thus a cabinet version of what jumpering means between the 
different and separate (digital=DSLAM and analog) racks inside a 
'traditional' DSL-enabled exchange)

Surely in the brave, new, world there will be ONLY a fiber link between 
the cabinet and the exchange, completely replacing existing/old copper 

So now (and I always appreciate the clarity of your explanations and 
translations of BT's technology choices, meanderings, and obfuscations) 
I'm wondering where the 'splitting' actually takes place, on the BT side?

If the voice/data split is carried-out at the cabinet, then I assume 
that is where the voice-over-IP part of 21CN 'starts'. Thus from that 
point (to the exchange, back-haul, 21CN nodes*, ISP, Internet, etc), 
there would be no difference between 'voice' (IP) packets (ex-analog at 
the premises) and 'data' (IP) packets (ex-broadband at the premises) - 
except their routing. Maybe?
*apologies am writing whilst off-line and can't remember what the new 
'centers', like 'super voice exchanges', are called

Returning to the OP's (Chris') original question: if 'everything' 
travels between the cabinet and the exchange over fiber, surely the 
cabinet's fiber transceivers are powered (not passive)?

That being the case, there must be a power supply at or to the cabinet. 
If that comes from London Electricity (et al) and there is a 
'discontinuity of supply', whither voice telephony, and 999 services in 

If there is no battery/UPS in the cabinet (and just stop to think of the 
physical security implications of having such installed, all over the 
place, for a moment!) then the limits of eng/imagine-ering would ask if 
perhaps there is a telco-power supply running within the ducts alongside 
the fibers, and presumably sourced/backed-up by the exchange - but then, 
stranger things have happened at sea!


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