Congestion Control·Lesson 2 of 11
What is Congestion?
Congestion in Plain Terms
Congestion is what happens when a part of the network is asked to carry more data than it can handle. Every link and every device in a network has a maximum rate at which it can move data. As long as the traffic stays below that rate, everything flows smoothly. The moment traffic pushes past the limit, data starts piling up waiting to be sent — and if the pile-up keeps growing, some of it gets thrown away. That overloaded state is congestion.
The core imbalance
Congestion is simply demand greater than supply: more data arriving than the network can forward. Everything else — delay, loss, slowdowns — is a symptom of that one imbalance.
A sink filling faster than it drains
Picture water pouring into a sink. The drain can only let so much out per second. If you pour in slowly, the sink stays empty and water flows straight through. Pour in faster than the drain can cope and water starts to collect — the sink fills up. Keep pouring and it overflows onto the floor. A router is the sink, its queue is the water level, and the overflow is packets being dropped.
Where Congestion Builds Up
Congestion doesn't happen everywhere at once — it forms at the busiest point on a path. Data travels as that hop from one to the next. Each router holds incoming packets in a short waiting line called a (or queue) until it can forward them out the other side. When packets arrive faster than the router can send them on, that buffer fills up. Here's the sequence that turns a busy link into a congested one:
How a buffer overflows
- Packets arrive at a router faster than it can forward them
- The extra packets wait in the buffer, which starts to fill
- As the buffer fills, every packet waits longer — delay climbs
- The buffer reaches its limit and has no room left
- New arrivals are simply dropped — that's packet loss
The single slowest or busiest point on the path is called the . It sets the pace for the whole connection, exactly like the narrowest lane on a highway decides how fast all the traffic behind it can move.
What Sets the Limit?
A link's capacity isn't one single number — a few different limits can each become the ceiling. Understanding them makes it clear why congestion appears where it does.
Things that cap how much a network can carry
- Bandwidth — the maximum data rate of a link, often measured in megabits or gigabits per second
- Router processing speed — how quickly a router can examine each packet and decide where to send it
- Buffer size — how many waiting packets a router can hold before it must start dropping them
- Number of active senders — the more devices sharing a link, the less capacity each one gets
Bandwidth vs throughput
Bandwidth is the maximum speed a link could reach in theory. Throughput is the speed you actually get right now. Congestion is one of the main reasons throughput ends up lower than the bandwidth you were promised.
Spotting a Congested Network
You rarely see congestion directly — you see its effects. As a link gets more and more overloaded, three things change together. Comparing a healthy link with a congested one makes the pattern obvious:
| Healthy network | Congested network | |
|---|---|---|
| Router queues | Short — packets wait only briefly | Full — long waits or no room at all |
| Delay | Low and steady | High and climbing |
| Packet loss | Rare | Frequent |
| Throughput | Rises as you send more | Falls even as you send more |
That last row is the surprising one and the heart of the whole problem: past a certain load, sending more data actually delivers less. We'll unpack exactly why in the lessons on the causes and effects of congestion.
“"Congestion means the cable is physically full of data."”
Q:Where does congestion physically occur in a network?
A: At routers, in their packet buffers. When packets arrive faster than a router can forward them, the buffer fills, delays rise, and once it's full, packets are dropped. The busiest such point on a path is the bottleneck that limits the whole connection.
Quick Revision Cheat Sheet
Congestion: More data arriving than the network can forward
Where: At routers, when their packet buffers fill up
Bottleneck: The busiest point on a path — it sets the pace
Bandwidth: The theoretical max speed of a link
Throughput: The speed you actually get; drops when congested
Tell-tale signs: Rising delay, packet loss, falling throughput