5.7. Quality of Service (QoS)

💡 First Principle: Networks treat all packets equally by default—your CEO's video conference gets the same treatment as someone streaming Netflix. When the link gets congested, both suffer equally. QoS changes this by prioritizing important traffic over best-effort traffic. But how does a router know which traffic matters? That's where classification, marking, and queuing come in.

Think of it like an airport security line with priority boarding. Everyone's going through the same bottleneck, but some passengers get to go first. QoS doesn't create more bandwidth—it decides who uses available bandwidth first when there isn't enough for everyone.

Why QoS matters for voice and video: Voice traffic is incredibly sensitive to delay and jitter. A 200ms delay makes conversation awkward. Packet loss causes audio dropouts. QoS ensures voice packets get through immediately, even if that means delaying file downloads. Without QoS, a user starting a large file transfer can make VoIP calls unusable for everyone on that link.