Network Congestion
Network Congestion
What happens when more traffic wants to use a link than the link can carry. It is the reason the internet slows down in the evening — too many people, not enough capacity, all at the same time.
Network congestion occurs when the demand for bandwidth on a particular link exceeds its capacity. Like a motorway at rush hour, the road has a fixed number of lanes; when too many cars arrive at once, everyone slows down. On a network, the "cars" are packets, and when they arrive faster than a link can forward them, they pile up in buffers, wait their turn, and — if the queue overflows — get dropped entirely.
What congestion does to your connection
| Symptom | Why congestion causes it |
|---|---|
| Lower throughput | Your share of a saturated shared link shrinks as more users compete |
| Higher latency | Packets wait in queues before being forwarded |
| Packet loss | When buffers overflow, excess packets are discarded |
| Buffering / lag | Real-time apps suffer most from the added delay and loss |
Where congestion happens
Congestion can occur at several points, and identifying which one matters for the fix:
- Inside your home: too many devices saturating your own connection — a big download starving a video call. This is the one you can directly manage.
- The local node / last mile: in shared technologies like cable, neighbours share capacity at the local node, so the whole street can slow at peak times.
- Provider backhaul: if the link from your node back to the core is undersized, congestion builds during busy hours.
- Peering and transit: a congested interconnection between networks can slow traffic to specific destinations.
How networks respond
The internet has built-in congestion control. TCP constantly probes for capacity and backs off when it detects loss or delay, so flows automatically slow to share a congested link rather than collapsing it. The downside is bufferbloat: oversized buffers absorb the overflow but add large, variable latency, which is why a saturated connection can feel laggy even while still moving data. Modern queue management — SQM and QoS — fights this by keeping queues short and prioritising latency-sensitive traffic.
Congestion vs throttling
It is easy to mistake one for the other. Congestion is accidental — a shared resource simply runs out at peak demand, and it clears when demand falls. Throttling is deliberate — a provider intentionally limits speed. The tell is timing and scope: congestion follows the clock and hits everyone on a shared link during busy hours, while throttling tends to target a specific customer, plan, or traffic type regardless of time. Comparing a peak-hour and an off-peak speed test on a wired connection is the simplest way to spot congestion.