Muni fiber: L1 or L2?

Sat Feb 2 14:19:08 UTC 2013

On Feb 1, 2013, at 22:54, Owen DeLong <owen at delong.com> wrote:

>> If you have multicast and everyone is watching superbowl at same time,
>> you're talking up very little bandwidth on that 2.mumble GPON link.
>
> Meh. Since everyone seems to want to be able to pause, rewind, etc.,
> multicast doesn't tend to happen so much even in the IPTV world these
> days.

Most of the time this is handled with a sliding buffer on the DVR at
the customer prem (TiVo time shifting style) unless you're talking
VOD.

On Feb 1, 2013, at 19:44, Leo Bicknell <bicknell at ufp.org> wrote:

> My limited understanding is that fiber really has two parameters,
> loss and modal disperson.  For most of the applications folks on
> this mailing list deal with loss is the big issue, and modal disperson
> is something that can be ignored.  However for for many of the more
> interesting applications involving splitters, super long distances,
> or passive amplifiers modal disperson is actually a much larger
> issue.
>
> I would imagine if you put X light into a 32:1 splitter, each leg
> would leg 1/32nd of the light (acutally a bit less, no doubt), but
> I have an inking the disperson characteristics would be much, much
> worse.
>
> Is this the cause of the shorter distance on the downstream GPON
> channel, or does it have to do more with the upstream GPON channel,
> which is an odd kettle of fish going through a splitter "backwards"?
> If it is the issue, have any vendors tried disperson compensation with
> any success?

I'd expect dispersion to be dispersion, in my limited optical
education I've only heard that this is influenced by distance, not
power level, so the signal would disperse the same amount whether its
7km of trunk + 100m of drop, or 100m of trunk + 7km of drop. 1310 and
1410 aren't particularly close so no need to worry about CMD causing
cross channel interference.

Quick googling shows this isn't an issue in 2.5G GPON plants which
have an 16000ps-nm CMD tolerance, but 10G (XGPON or whatever the
latest name is) will only have an 1100ps-nm tolerance which might add
up fast depending on the fiber in the ground (Anyone have any good
references on common fiber CMD/PMD at different wavelengths? Most of
the references I found were focused around 1550)

How the receiver in a GPON would respond to rapidly shifting
dispersion/power levels due to upstream TDMA isn't something I'm
familiar with. You could compensate for the power level with
attenuators, but if you needed DC on every customer that's going to
get expensive quick unless you can do it on the trunk side just to get
the worst offenders back into your receivers window.

~Matt