Oversubscription
Beginner
When more data can pour in than a shared path can carry. If everyone sends at once, everyone slows down.
Novice
The ratio of the total bandwidth of the links below a switch to the bandwidth of the single link above it. At 4:1, devices that all send toward the CPU at once each get a quarter of their own link speed.
Expert
Downstream lanes ÷ upstream lanes (same generation). Harmless for traffic that stays below the switch (peer-to-peer) or that is bursty and not simultaneous; a hard limit for host-bound streaming.