Throughput

When a network is slow, bandwidth is often the first thing people check.

Bandwidth looks sufficient, but HTTPS downloads are slow. Cellular throughput jumps up and down. Wi-Fi signal looks acceptable, but API calls occasionally time out. Packet captures show retransmissions, duplicate ACKs, RTOs, and then the sending rate drops. The server CPU is fine and the application did not change, but TCP suddenly becomes conservative.

Congestion control is often the reason.

A device can sit next to a router and report 866 Mbps, while real application throughput is only a few hundred megabits or less. BLE has the same trap: the PHY may say 2M, but GATT notifications do not deliver 2 Mbps of application data. The gap is not always marketing abuse. Bandwidth, rate, and throughput are different layers.

The common mistake is treating a larger advertised rate as a promise that application data will arrive at that speed.