RE: 3PMD Proposal is not enough
Your perceptions are fair and I will attempt to respond to each. I know you
are intimately familiar with these issues, so much of this explanation is
for the benefit of others who may not be.
The US market has been a 62.5 um market. True. There has been reluctance to
switching to 50 um in the US up to now, even though 50 um offered extended
distance capability up to 550 m for GbE. My view is that the added advantage
of the extra distance was insufficient to persuade the customers to switch.
We know that 300 m is a critical "magic number" for the great majority of
building backbones. Neither the installed 62.5 or 50 um fibers can fulfil
this need for serial transmission at 10 G. The next generation of 50 um
fiber does fill this need. Since this is a critical coverage requirement for
customers planning on 10G networks, customers are willing to switch core
sizes to gain this advantage. Our sales force confirms this with reports
from their contacts with customers.
50 um fiber does not couple as much power from LEDs as 62.5 um. We account
for this in the applications and structured cabling standards. This loss of
power translates into smaller budgets which reduce the number of connection
supportable, or in some cases reduce the link distance. For example, short
wavelength (850 nm) LED-based applications such as 10BASE-FL, when operating
at minimally-compliant levels (worst case power budget), will typically not
have sufficient power to support 2km on 50 um fiber. Depending on the
connections, the worst case supportable distance is usually between 1 and
1.5 km. But long wavelength (1300) LED-based applications like 100BASE-FX
easily still achieve 2km with margin. This is due to the lower attenuation
of the fiber at 1300. There is no penalty for VCSEL-based applications like
1000BASE-SX, since coupling efficiency is not an issue for lasers.
50 um fiber is more bend sensitive, both to macro-bends (loops in the cable)
and micro-bends (microscopic deflections due to fiber buffer and coatings).
The macro-bend sensitivity causes some small amount of power loss from
highest order modes excited by LEDs. 50 um macro-bending losses are
generally on the order of 0.5 dB higher than 62.5 um fiber for overfilled
(LED) launches. Macro-bending is not an issue for laser launches. Since the
next generation 50 um fiber is aimed at 300 m link lengths, this small
additional loss is inconsequential to the support of LED-based systems,
which are virtually all specified to support much longer lengths on 50 um
fiber. There is more than adequate power budget to cover this loss.
Micro-bending loss, which is induced by the cabling process, was more of an
issue for earlier cabling processes (1980s). Today major cablers around the
globe produce 50 um cables without problems. The loss specification in the
structure cabling standards specifies cabled (as opposed to un-cabled) fiber
loss. Therefore, compliance to the structured cabling standard specs assures
product that meets the specifications of the applications standards. The
IEEE link model uses the worst case values in calculating link distance
capability. So whatever micro-bending loss exists, it is accounted for. As a
testament to the minimal impact for cabling 50 um relative to 62.5 um, the
specs for cable attenuation at 850 nm for both 50 and 62.5 um fiber are the
same in the structure cabling standards. Both are specified at 3.5 dB/km.
We at Lucent looked hard at all of these issues before deciding that 50 um
offered the best trade-off of all parameters and market acceptance for next
generation MMF. Our customers both in the US and abroad are telling us that
we made the right choice.
From: Gair Brown [SMTP:gdbrown@xxxxxxxxxxxxx]
Sent: Friday, June 23, 2000 11:38 AM
To: George, John Emanuel (John)
Subject: Re: 3PMD Proposal is not enough
50 micron is more bend sensitive (notably so for the cabling from
wall outlet) and more loss sensitive at connections. The community
bought into those facts years ago when we went 62. Those points are
issue for some folks. Especially if they have had no problems with
There always are better technical solutions, but they don't always
on like we think they will. A lot of customers have a mindset of
I've had no problems, why should I change?"
The other side of the coin is - "maybe I just buy a better grade of