Interface TX Power should allow negative values

[yzguy]

NetBox version

v4.2.6

Feature type

Data model extension

Proposed functionality

Update the validations for Interface TX Power to allow negative values

Use case

We run 400G ZR optics in our devices and in some cases we need to lower the transmit power to a negative because it goes through an amplifier or the path is short enough

Database changes

Possible database validation change

External dependencies

No response

[jeremystretch]

Forgive my ignorance but it's been a while since I worked with optics. How can transmit power be less than zero? I'm guessing you're referring to a deduction from a default power level. Can you provide a real-world example?

Edit: Decibels are a thing. 🤦

[sleepinggenius2]

In NetBox, the transmit power is specified to be in units of dBm. The reference power level is 1 mW, so a negative dBm value just means that it it transmitting below 1 mW, i.e. -10 dBm is 0.1 mW, -20 dBm is 0.01 mW, etc. For long reach optics, you will typically see positive values, but for short reach optics or short distances, you will often see negative values. Looking at a link for one of our customers, it is a 10GBASE-LR optic currently running at -2.1 dBm to get from our PoP to the customer's site ~2.5 miles away.

Name : Te0/2
Administrative Speed: 10000
Administrative Duplex: full 
Administrative Auto-MDIX: on
Administrative Power Inline: N/A
Operational Speed: 10000
Operational Duplex: full 
Operational Auto-MDIX: on
Media Type: SFP-10G-LR

                                           Optical   Optical
           Temperature  Voltage  Current   Tx Power  Rx Power
Port       (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
---------  -----------  -------  --------  --------  --------
Te0/2        33.2       3.17      34.7      -2.1      -5.8

[bctiemann]

Ah, OK. That seems pretty reasonable then.

[yzguy]

Just more info about our use case, we are running 400G ZR optics (QSFP56-DD-400G-ZR-M-HP) in Juniper PTXs, and originally we had them going through just a Passive Mux and turned up their tx-power to max (+1dBm) but the loss was too much, so we moved to using an amplifier. We have to turn down the tx-power in this case because 0-1dBm is too much.

The use of tunable optics is increasing throughout the industry so this would be great.

> show configuration interfaces et-0/2/0
speed 400g;
mtu 9216;
optics-options {
    wavelength 1561.42;
    tx-power -6;
}

It would be cool to be able to specify a wavelength for an interface too, the "wireless" section has one, but not one for the non-wireless section. I considered using the wireless section but it felt a little dirty :P