I have a pair of GLC-LH-SMD devices which are 1000BASE-LX/LH
In the field they will be a couple of kilometers apart , but whilst I test the setup I only have 5m patch leads.
I looked at the specs and Cisco state they are
Transmit Power -3 to -9.5 dBm, Receive power -3 to -20 dBm
So this seems to me to mean that you can connect them with any length cable as the Transmitter maximum is the same as the receiver maximum (-3 dBm)
Yet if I connect them to each with a five meter patch lead to each other (in the same switch) – or if I connect one to itself I get a high RX power warning
Dec 19 22:44:44.946: %SFF8472-5-THRESHOLD_VIOLATION: Gi1/0/1: Rx power
high warning; Operating value: -1.4 dBm, Threshold value: -3.0 dBm.t
C3750#sho int g1/0/1 tra
ITU Channel not available (Wavelength not available),
Transceiver is internally calibrated.
If device is externally calibrated, only calibrated values are printed.
++ : high alarm, + : high warning, - : low warning, -- : low alarm.
NA or N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).
Optical Optical
Temperature Voltage Tx Power Rx Power
Port (Celsius) (Volts) (dBm) (dBm)
--------- ----------- ------- -------- --------
Gi1/0/1 27.4 3.30 -5.1 -1.4 +
C3750#sho int g1/0/2 tra
ITU Channel not available (Wavelength not available),
Transceiver is internally calibrated.
If device is externally calibrated, only calibrated values are printed.
++ : high alarm, + : high warning, - : low warning, -- : low alarm.
NA or N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).
Optical Optical
Temperature Voltage Tx Power Rx Power
Port (Celsius) (Volts) (dBm) (dBm)
--------- ----------- ------- -------- --------
Gi1/0/2 33.3 3.30 -5.5 -1.9 +
C3750#
If I connect the transceiver XMT to its own RCV I get
Optical Optical
Temperature Voltage Tx Power Rx Power
Port (Celsius) (Volts) (dBm) (dBm)
--------- ----------- ------- -------- --------
Gi1/0/1 31.4 3.30 -5.5 -1.9 +
I don't see how a transmit power of -5.5 dBm is being received as -1.4 dBm so my question is , Is it real – how am I receiving more power than I transmit? Can I control the transmit power ? (other than inserting the lead but not clicking it into place – which actually seems to work as shown by the receive level below
Optical Optical
Temperature Voltage Tx Power Rx Power
Port (Celsius) (Volts) (dBm) (dBm)
--------- ----------- ------- -------- --------
Gi1/0/1 31.8 3.30 -5.1 -13.0
And more importantly is this a problem ?
I can see that the 1000BASE-ZX has a much higher transmit power than a receive power – so this would need an attenuation cable
Best Answer
Possible? Yes under the right set of circumstances. To begin with, you would need to be on a short fiber run (per your description, check) and very few insertion points (again by your description, check). Beyond that, any interference created would have to be nearly perfectly constructive.
To understand the last, you need to know that part of the signal is reflected back at the end of the cable. When this reflected signal reaches the original source end of the cable, again part of it will be reflected back. If the wave forms align correctly, you can then have a form of constructive interference.
Let's put this in VERY simplistic and unscientific percentages to illustrate the effect. You transmit on side A of the cable at 100%. Some signal is lost along the way, but being a short cable, 98% reaches side B. A tenth of that, 9.8%, is reflected back toward A. 9.6% reaches side A, and a tenth of that, or 0.96% is reflected to constructively interfere perfectly with the new 100% source, becoming 100.96%. As this happens in a continuous cycle, the gain can be significant.
Again, that is simplistic and not reflective (pun intended) of the real life math/physics, but it was meant more as an illustration of the effect, not the full details. In real life, the numbers change depending on a wide range of factors and there are limits to the medium.
Of course, you could just have faulty hardware or a bug in the software was well.
As in manual adjust the transmit power of the transmitter up or down? No. Many will adjust to some degree automatically, but you won't have control over it.
However you can reduce the signal strength in a number of ways (or a combination of the below):
This last has been alluded to by others in their answers, but this will likely permanently damage your cable. I believe the basis for this solution is in mandrel wrapping, which is a technique used in multi-mode fiber to limit or eliminate higher modes allowing for better signal loss measurements. This also results in the attenuation of the signal, and has often been used in a pinch for that purpose.
However mandrel wrapping done correctly doesn't damage the cable as it follows specific standards as well. Your typical mandrel will be about 2cm in diameter, with some variance depending on the core size and jacket thickness of the MM fiber in question. Wrapping around a smaller diameter object can introduce cracks in the cable, allowing light to escape; the smaller the diameter, the more damage and the more signal loss.
Additionally, by it's nature, SM fiber doesn't have multiple modes, so any real loss is a result of damaging the cable.
Yes. Optics can and will burn out. Just like your eyes, if the receiver is exposed to a light source that is too bright for too long, it will cause damage. The brighter the source, the shorter the exposure necessary.
You should take some action to attenuate the signal.