Community for discussing enterprise networks and the ensuing chaos that comes after inheriting or building one.
I've always wondered whether network interfaces that have these flashing lights flash as a gimmick or do they actually indicate the flow of traffic? Perhaps one flash per packet in or out? I wish I could remember what my call up modem looked like to make a historical comparison too.
TL-SG105E
It's an activity indicator, it doesn't necessarily correlate to anything other than transmission or receiving is occurring. I would also not assume that they work the same between brands. Some devices may have a separate link light, others may not. Some will have a combined tx/rx light, others separate. Some vendors they're configurable. Some vendors use them to indicate things like device boot state. Etc
It's not a gimmick though, they're still useful for that purpose.
They flash a lot longer then it takes to recieve one packet, but it's a useful indicator that tells you that there is some activity.
Right! Packets, of course, come in many sizes and a TCP handshake alone is what... Less than 1MB? And with a 1 Gbps connection, there is no way those LEDs flash once for each packet. They would probably look like static lights to the human eye if that was the case... Thanks! :)
An Ethernet frame's payload can't exceed 1500 bytes without using jumbo frames. So every internet routable packet is actually less than or equal to 1.5kB.
and a TCP handshake alone is what... Less than 1MB?
less than 1 KB
They were technically right... Less than 1MB. 😆
In 10MB ethernet days, they might have been directly driven by the signal on the wire. After that, they’re pretty much just “a flashing mechanism turned on when activity is present”
I was leaning towards this conclusion too. I wonder where my modems went... Already looked in the cellar.
I'm going off what I remember from a decade ago when working on embedded CPUs that have an Ethernet interface. IIRC, the activity LED -- whether a separate LED than the link LED, or combined as a single LED -- is typically wired to the PHY (the chip which converts analog signals on the wire/fibre into logical bits), as part of its transceiver functions. But some transceivers use a mechanism separate from the typical interface (eg SGMII) to the MAC (the chip which understands Ethernet frames; may be integrated into the PHY, or integrated into the CPU SoC). That auxiliary interface would allow the MAC to dictate what the LED should indicate.
In either case, there isn't really a prescribed algorithm for what level of activity should warrant faster blinking, and certainly not any de facto standard between switch and NIC manufacturers. But generally, there will be something like 4 different "speeds" of blinking, based on whatever criteria the designers chose to use
Your knowledge on this is amazing and I wish I also worked with such this earlier on. Thanks so much for sharing! :)
I went down some kind of a rabbit hole. I looked up my motherboard's NIC's data sheet and... Dam it! Why is tech so interesting!?
Source: https://datasheet4u.com/pdf-down/R/T/L/RTL8125BG-CG-Realtek.pdf
Something to be wary of when interpreting the datasheet:
- Act10 = LED blinking when Ethernet packets transmitted/received at 10Mbps.
- Act100 = ...
- Act1000 = ...
Bad wording on their part. What they really mean is: "LED blinking when Ethernet packets transmitted/received AND the link is currently in a XYZMbps link speed mode". The mode is negotiated once after you plug a cable in and usually does not change after that, regardless of how much data you try to send.
Technically each linkspeed/mode is a whole ethernet standard of its own, but we mostly gloss over that and pretend to end users that they're backwards compatible.
Very insightful! Are those the speeds that I can cat from /sys/class/net/[interface name]/speed? Assuming you know Linux, that is. Those negotiated speeds, are they hardcoded into the NIC and selected/negotiated based on what category cable I'm using and other such hardware related factors? Also, is there any "wiggle room"? As in, does it do a speed test to check the limits of the physical layer or does it just follow some vendor specifications?
It depends on the hardware
https://www.tp-link.com/us/business-networking/soho-switch-easy-smart/tl-sg105e/#specifications
Says your switch handles 7.4 million packets per second. How fast is your light flashing?
Darn it, I counted 7.3 million blinks, therefore my ISP must be throttling my speeds joke
I laughed hard at this. Thanks! XD
But in all seriousness, I'll record (for fun, because I'm damaged by this hobby...) the LEDs with my smartphone camera tomorrow, which has a slow motion function, and get back to you after either failing or succeeding to calculate the BPS (Blinks Per Second).