In a message written on Sat, Oct 19, 2013 at 07:33:19PM -0700, Chris Costa wrote:
What are the opinions/views on attenuating short, 1310nm LR cross-connects. Assume < 20m cable length and utilizing the same vendor optics on each side of the link. Considering the LR transmit spec doesn't exceed the receiver's high threshold value do you pad the receiver closer to the median RX range to avoid potential receiver burnout over time, or just leave it un-padded?
With any optics, you need to go to the specifications. I assume here you mean 10GbaseLR, although I will point out that "LR" is ambiguous as there is also for instance OC192-LR. I'm going to pick on Juniper specs, just because they were the easiest to find with Google: http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/refer... And similar for 1000baseLX, the similar technology for GigE: http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/refer... Note that for both 10GbaseLR and 1000baseLX the transmitter power range is entirely inside the allowed receiver range. They were designed this way on purpose, to never need a pad. An in-spec optic can never over drive the receiver, even with zero loss. Answering your question, I would never pad them. Compare with for instance a 10Gbase-ER or 1000baseEX, 40km over single mode optics. In both cases an in-spec can exceed a receiver. 10Gbase-ER can transmit up to +4.0dBm, while the receiver needs -1.0dBm or below. When connecting them "back to back" a 5dB attenuator is required to keep the receiver in-spec. For any real connections (over a fiber path more trivial than a jumper) a light meter should be used, the value checked, and an attenuator that places the circuit 1-2dB inside of the safe zone of the receiver should be used. -- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/