@jetl 

Hi,

1000Mbit Ethernet (1000BASE-T) has no separate wire pairs for transmit and receive (i.e. upload and download), therefore the bandwidth will be shared.

A normal Ethernet cable (8 pin RJ45) has four wire pairs and in 1000BASE-T mode each of the four wire pairs provides a data rate of 250 Mbps. The combined data throughput over the four wire pairs is 1000 Mbps.

Furthermore, the main chip (SoC) of a consumer-grade router usually has only one or two LAN ports. The reason these routers still end up having anywhere from four to eight physical LAN ports is that there is a network switch built into the router. This works no different than those dedicated network switches that can be purchased as separate devices.
If we assume that the SoC's of less expensive routers have only one 1000 Mbps LAN port, which gets multiplied to four physical LAN ports via a switch chip, then you can imagine why two PCs connected to two different LAN ports of the same router will still be limited to a combined 1000 Mbps.

Regarding the low speed when uploading and downloading simultaneously on the same PC. I would have to test this myself with different hardware before drawing any conclusions.

  @woozle 

Hi Woozle,

I dont agree with you about the description of 1000BASE-T. Upload and download bandwidth will not be shared as full duplex means bidirectional. I can quote some citations for you:

        For 1000BASE-T, all four twisted pairs in the cable are used. Each pair transmits at 250 Mbps, and each pair is bidirectional, thus supporting full-duplex communication.

And the video here supports my statement too: https://youtu.be/vuD9mZtuV7U?t=454 . The youtuber has 1000M transmit + 1000M receive in total with 1GbE hardware.

But I agree with you that some less expensive routers have only one 1000 Mbps LAN port on SoC, that could be the reason why I got 1000M bandwidth in total.

Thanks for your clarification.

  @jetl 

Hi,

I need to apologize that I got this wrong.

I've just tested this myself with iperf3 and three PCs connected to a Gigabit router and, like you correctly said, 1000BASE-T can indeed transfer close to 1000 Mbps in both directions simultaneously. (there's some drop-off noticable, but it's not very significant)

Why the Archer C80 can't manage that, I don't know.

Do you get close to 1000 Mbps up/down simultaneously if you connect a PC to the LAN server directly, without the Archer C80 in between?