On Feb 1, 2013, at 22:54, Owen DeLong <owen@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@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