📘 11-01: Performance Concepts & QoS¶
📌 Key Metrics¶
| Metric | Definition |
|---|---|
| Bandwidth | Maximum theoretical capacity of a link |
| Throughput | Actual achieved data rate (always ≤ bandwidth) |
| Latency | Delay for data to travel (often round-trip time) |
| Jitter | Variation in latency between packets |
| Packet loss | Percentage of packets dropped |
Bandwidth = the pipe's size; throughput = what actually flows through it.
📌 What Hurts Real-Time Traffic¶
VoIP/video are most sensitive to latency, jitter, and packet loss. A late retransmitted voice packet is useless — so these apps use UDP and rely on QoS.
📌 Quality of Service (QoS)¶
QoS prioritizes important traffic during congestion.
| Mechanism | Purpose |
|---|---|
| Classification/marking | Tag traffic — DSCP (L3) or CoS/802.1p (L2) |
| Queuing | Priority/low-latency queue for voice |
| Traffic shaping | Buffer/smooth traffic to a rate |
| Traffic policing | Drop/remark traffic over the rate |
Shaping buffers excess; policing drops it.
📌 Bottlenecks¶
A bottleneck is the point of least capacity that caps end-to-end performance (a slow link, overloaded router, or saturated server). Monitoring locates it for targeted upgrades.
Tip
Key idea — Know bandwidth vs. throughput and latency/jitter/loss. Use QoS (mark with DSCP, prioritize voice, shape/police) to protect real-time traffic and find the bottleneck.
See also: Network Monitoring, TCP vs. UDP
Previous: Switch & Port Hardening | All Notes | Module 11 Exercise | Next: Network Monitoring