FIFO (First In, First Out) is the simplest queuing mechanism. Packets leave in arrival order. This is the default when no QoS is configured.
How FIFO works:
- All packets enter one queue
- First packet in is first out
- New packets tail-drop when full
FIFO problems:
- Large file transfers fill the queue
- Small voice packets wait behind data
- Bursty traffic causes oscillation
- Drops affect all flows equally
When acceptable: High-speed links that rarely congest. Homogeneous traffic with similar requirements.
FIFO runs in time for enqueue and dequeue with space for packets.