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My understanding is that gigabit Ethernet over twisted pair, 1000Base-T, is full-duplex by default. Does that mean that the actual bandwidth, can be 1 Gbit/sec in each way simultaneously, hence 2 Gbit/sec counting both ways?

In comparison, Wi-Fi is half-duplex on each channel, meaning a 800Mbit/sec connection can only achieve that number in one-way at a single time.

So the max bandwidth for wired Ethernet connection can actually double that of Wi-Fi, for the same claimed bandwidth?

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    Yes, Max bandwidth for a full duplex connection like typical Ethernet is at least double that of a half duplex connection like WIFI – davidgo Oct 12 '20 at 1:40
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    For wired Ethernet it is customary to specify the throughput speed which is in a single direction. For Powerline and Wifi ethernet the vendors have a tendency to add everything together, because that makes for bigger numbers that look better in the marketing material. E.g AC3200 Wifi actually means 4 simultaneous stream of maximum 800 mb/s each. You won't get 3.2 Gb/s throughput. – Tonny Oct 12 '20 at 11:39
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    In addition to @Tonny's caveat about marketing, know that Wi-Fi involves a lot of messiness and overhead. Even if your OS reports an 867Mb/s PHY links, you are not going to transfer anywhere close to 867 Mb per second no matter what, and further, those big PHYs only happen in ideal conditions. Lots more detail: duckware.com/tech/wifi-in-the-us.html – Reid Oct 12 '20 at 16:39
  • So if I want to upload a 1TB file and download another 1TB file at the same time, a wifi connection will take as least twice as long as a wired Ethernet, for the same claimed bandwidth? If that's true, I feel like the Ethernet marketing guys are so humble not to highlight this:) – QnA Oct 13 '20 at 16:29
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Full duplex means bidirectional communication. So yes it could in theory reach 1 gig up and 1 gig down, but the device transmitting up or down would still only reach a max of 1 Gpbs.

Reaching the theoretical max would also depend on the NIC and other hardware such as drive IOPS. Don’t hold your breath too hard if you don’t actually ever see the potential bandwidth being 100% utilized.

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    "ther hardware such as drive IOPS" - this is easier than you posit: even cheap consumer-grade spinning-rust drives will easily max-out a 1Gbps link. Somewhat counter-intuitively I've actually had slower IOPS on enterprise-grade gear compared to home-user equipment because the admins underprovisioned IO on the VM box - running 3-4 different DB VMs on the same SAS JBOD is not a good idea! – Dai Oct 12 '20 at 20:24
  • How does that work, please, DrZoo? Isn't "Gigabit" measuring both speed and bandwidth and plexity a different thing? If the network is already maxed out at 1Gb up and you fling another 1Gb down at it by duplexing, where will you get the extra bandwidth to "in theory reach 1 gig up and 1 gig down"? If that could be done, why would the marketers restrict themselves to "Gigabit" instead of trumpeting "Two Gigabit"? – Robbie Goodwin Oct 13 '20 at 9:14
  • @RobbieGoodwin full duplex utilizes simultaneous use of two physical twisted pairs. One pair receives and one pair transmits. Each pair will only get the maximum throughput of 1 Gbps. – DrZoo Oct 13 '20 at 15:44
  • @DrZoo Thanks and I'm sorry to point out that's a paraphrase, not an explanation. In other areas… such as radio or telephone… half- and full duplex have different measures than you're expounding. Can you be really clear? You're talking not at all about protocol but purely about bandwidth? Your half-duplex runs at 1Gb/s, both speed and bandwidth and your full-duplex at the same speed with twice the bandwidth? – Robbie Goodwin Oct 13 '20 at 21:19
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Expanding on DrZoo's answer, it depends on your applications utilizing the network connection. In layman's terms: e.g. if you do "up- and downloads" at the same time (e.g. via two independent sessions) you will "fully" utilize the full duplex but most of the times this is not the case.

Especially if you only use one session, there it's mostly either sending or receiving at "any given moment" (except for maybe the "occasionally send out the acknowledgement message" for a received data chunk while still receiving new data chunks but which doesn't eat much "bandwidth" anyway).

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