Rod, at 12:12 on Mar 03 1996, you wrote to Bill Grimsley...

BG> Sure, but even fibre-optic has inherent line loss
BG> (attenuation), which must be reamplified every so often.

RS> Not in the sense of affecting the calls call by call. Once its digital,
RS> that that, what goes in one end, comes out perfect at the other end.

BG> Theoretically.

RS> Aint no theory, thats the whole POINT of going digital.
RS> Thats why you HAVE digital telephone exchanges etc.

Nope, FO digital square waves will eventually degrade to low-amplitude
sinusoidal waves, without regeneration.  Usually within 150Km too.

BG> However, I just called Alan Kennedy, my senior engineer mate with
BG> Telstra, and the reality can be rather different (although for the most
BG> part, there should be no appreciable loss along the entire length of FO).

RS> You are garbling the story considerably, 

Nope.

RS> the whole POINT of a digital system is that you DONT get increased 
RS> degradation of the data when it moves thru more repeaters. Tho you 
RS> certainly still need them just because you cant currently run a fibre optic 
RS> cable repeaterless for the 1000Km class distances we are talking about.

Exactly.  If you did, there'd be nothing but a tiny sine wave left.

BG> The actual FO design specs call for an exchange transmit level of
BG> -10dBm, with automatic regeneration once the level drops to -47dBm.

RS> Thats says nothing useful at all tho about whether a digital system
RS> gets any degradation at all. The whole POINT of it is that you dont.

Without occasional regeneration, you most certainly will.

BG> However, Telstra have set the system to regenerate if the
BG> signal reaches -40dBm.  This rarely, if ever happens, although the
BG> digital signal does degrade into sinusoidal waves after a time, and
BG> regeneration restores the square wave, as well as the -10dBm amplitude.

RS> Still says nothing useful about whether the digital DATA is ever
RS> degraded tho. That stuff just affects the repeater spacing. NOT
RS> any degradation of the actual digital data being sent over it.

Are you blind?  Read this next paragraph again...

BG> Without any regeneration along the FO, the signal will attenuate
BG> by 30dB (to the aforementioned -40dBm) after just 150km. That said,
BG> it is fairly safe to say that if handled correctly, the signal will
BG> arrive at the other end in much the same condition as it was sent.

RS> The point tho is that with the digital DATA there is NO degradation AT ALL.

Bullshit, Rod.  Not only does the digital signal degrade in amplitude, it
starts to take on sinusoidal characteristics.  The whole point of
regeneration is to reamplify the signal, and turn it back into a square
wave.

BG> Now comes the tricky bit.  The maximum allowable
BG> inter-exchange losses are 12dB on a normal copper pair,
BG> with a further allowable loss of up to 7dB to each subscriber.

RS> That NOT a fibre optic cable. The vast bulk of
RS> the inter exchange links are fibre optic now.

But not the exchange-to-sub links.  They're still copper.  And THAT'S where
the greatest attenuation occurs.

RS> Nope, digital doesnt work like that. Thats WHY you do
RS> digital in the first place. You can regenerate as often
RS> as you like with no added degradation each time you do it.

BG> Correct, but without regeneration, there is still an
BG> average loss of 30dB every 150km or so with FO cable.

RS> Yes, but that just affect the repeater spacing, has
RS> nothing to do with degradation of the digital data at all.

I'm not going to argue the point with you, Rod.  If you don't accept that
the digital signal degrades into sine waves which are corrected by
regeneration, that's your problem.

Regards, Bill

--- Msgedsq/2 3.20