When several queued packets compete for one output link, a router or host must choose which packet to transmit next. This packet scheduling policy affects delay, throughput and fairness.

Simple FIFO scheduling serves packets in arrival order. Priority scheduling can reduce latency for important traffic but can starve lower-priority queues. Fair-queueing approaches instead try to divide service among flows or classes so that one aggressive sender cannot monopolize the link.

For example, if a bulk transfer and an interactive call share a bottleneck, one FIFO queue can let a large burst delay the call. Separate queues with an appropriate scheduler can bound that interference.

Scheduling cannot create bandwidth; it decides how scarce transmission opportunities are allocated. It is therefore complementary to congestion control, which determines how much traffic endpoints inject into the network.