On 16/10/2020 10:14, Dave Liquorice wrote:
On Fri, 16 Oct 2020 09:37:04 +0100, Andy Burns wrote:

The cabinet is about 3km from the exchange; I don't know
if that is a factor with fibre.

For FTTC it's the length and condition of the copper from the cabinet
to you that is important. With 37 Mbps down that equates to about 700
m.

58 km limit I believe.

That's certainly within the range of a single mode fibre point to
point link but I think Openreach are installing GPON (Gigabit Passive
Optical Network). This takes the fibre from the head end "line card"
and optically splits it 32 (or even 64) ways. This reduces the range
but it's still in the order of 10 km or more providing up to 1 Gbps
symetrical (if they wanted to).

yes, and no. You have described what the Open Reach engineer said
happened (optical splitting), but he said the fibre went 10 + miles to a
big exchange, and could do more than that. It is really probably about
the amount of *power* that can be sent down it - the splitter is a
different sort of loss from a long fibre, in that its probably a flat
loss across the band, not a load of frequency dependent phase shifts.

The limit of multimode is reached when all the different paths combine
to give a mess of incoherent phases, not because the signal is too low.
For example.

This may be of interest:

"As we all know, multimode fiber is usually divided into OM1, OM2, OM3,
OM4 and OM5 fiber types. When it comes to single mode fiber types, it
can be categorized into OS1 and OS2 fiber, which are SMF fiber
specifications. OS1 and OS2 are standard single mode optical cables
respectively used with wavelengths of 1310nm and 1550nm with a maximum
attenuation of 1 dB/km and 0.4 dB/km. OS1 fiber is a tight buffered
cable designed for use in indoor applications (such as campuses or data
centers) where the maximum distance is 10 km. OS2 fiber is a loose tube
cable designed for use in outdoor cases (like street, underground and
burial) where the maximum distance is *up to 200 km*. Both OS1 and OS2
fiber optic cable support Gigabit and 10G Ethernet links. "


Certainly undersea repeaters are over 100km apart if possible.

And thinking about the splitter, that represents only really a 20 db
loss for 100 way split...and provided the power is there that's no big
loss.

It probably represents no more than a third of the usable length being lost.

--
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