[j-nsp] acceptable/good laser receive power in case of different interfaces

[Jonathan Lassoff]

On Sun, Aug 7, 2011 at 2:03 PM, Martin T <m4rtntns at gmail.com> wrote:
> Lane,
> while browsing the specifications of the optical modules listed in
> this "Optical Interface Support—EX 3200 and EX 4200 Switches.pdf"
> file:
>
> http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/reference/specifications/optical-interface-support-ex-series.pdf
>
> ..all the modules have minimum and maximum launch power which differ
> from each other quite a lot. What does this mean? Shouldn't the launch
> power be consistent? In addition, what is a "Maximum Receiver
> Sensitivity"?

They're different underlying hardware. Most likely, Juniper is OEMing
transceivers from big manufacturers like Finisar and sticking their
stickers on them.

The different transmit powers are simply because they're different
hardware and standards.

I would guess that "Maximum Receiver Sensitivity" is just the max
recommended optical power that can be stuck into it. However, the
numbers listed in there seem pretty low for that, IMO. "Minimum
Receiver Sensitivity" makes sense, but the maximum possible power is
usually pretty high.

> In addition, there isn't some sort of connection between Rx power and
> Tx power, is there? I mean for example in case the received signal is
> low, the transmit signal of the SFP/XFP is increased automatically? As
> far as I know and as Lane confirmed, the Tx signal should be always
> consistent..

Not really. Each link between two transceivers is usually made up of a
duplex of fibers (two strands), so each TX / RX pair on either end is
its own optical system. One side transmits, and the other receives.
The other fiber strand does the same thing, but in the opposite
direction.

There's not usually a connection between RX and TX power, in my
experience, though I've heard of some GMPLS systems that will signal
the RX power to the transmitting device so that it can alter its
transmit power to compensate for over/underpowering the far end.

--j