Before you can trust a bandwidth number, you must earn every symbol behind it. Below we build them from absolute zero — each one gets a plain meaning, a picture, and a reason the topic needs it. The parent note (PCIe lanes, links, and bandwidth) assumes all of these; here we make none of them assumed.
Picture a single wire whose voltage jumps up and down over time. Each moment it "means" a 1 or a 0.
Look at the amber trace in the figure: every time the line is up it's a 1, every time it's down it's a 0. If those up/down changes happen fast, we push more bits per second.
Why the topic needs it: Everything called "bandwidth" is ultimately a count of bits per second. If you don't own "bit per second," every later number is a mystery.
In the figure, the top cyan wire and the bottom cyan wire are mirror images. Any electrical noise (grey wiggle) hits both wires equally, so when the receiver subtracts one from the other, the noise cancels and the clean signal survives.
Why the topic needs it: The parent note says "one lane = TX pair + RX pair." A pair means two wires. Without this idea, "pair" is just a word. This is the hardware reason PCIe is noise-tolerant at multi-GHz speeds. See PCIe Electrical Signaling for the deeper electrical story.
The figure shows one lane = 4 physical wires total: a TX pair (amber, arrow pointing right) and an RX pair (cyan, arrow pointing left). Both arrows fire simultaneously — that's full-duplex.
Why the topic needs it: The lane is the atom of everything. Bandwidth is "per-lane rate × number of lanes," so if the lane is fuzzy, the whole formula is fuzzy.
Why the topic needs it: Step 5 of the parent's derivation multiplies a single-lane rate by N. That Nis the link width. Also crucial: the physical slot (the plastic connector) can be wider than the electrical link actually wired to it — see Motherboard Lane Allocation and PCIe Bifurcation.
Why this tool and not Gbps here? Because payload depends on the encoding scheme, which differs between generations. GT/s pins down the physical wire speed cleanly; we then multiply by an efficiency to get real payload. Mixing them up is the #1 source of "why is my drive slow?" confusion.
Why the topic needs it: GT/s counts all wire bits; efficiency is the fraction that is real data. Without it you'd overstate Gen 1/2 bandwidth by 25%. Deeper mechanics live in 8b10b Encoding and 128b130b Encoding.
Why the topic needs it: SSD makers quote MB/s and GB/s; PCIe math starts in Gbps. The ÷ 8 bridges the two worlds — see NVMe Protocol and GPU Bandwidth Requirements.
