5.2Performance & traffic design

For a network designer, it is not simply a matter of "more bandwidth is always better." You must estimate the bandwidth and device capacity that are necessary and sufficient to meet actual performance requirements (response time, voice quality, etc.), and use quantitative calculations such as queueing theory as the grounds for that estimate. This section covers what the performance metrics mean and how to translate calculation results into design decisions.

5.2.1Performance metrics (throughput, bandwidth, latency, jitter, packet loss)

• Bandwidth is the theoretical maximum data volume a link can carry (bps). Throughput is the actual effective data volume that flows, and is always less than bandwidth due to protocol overhead, congestion, or device processing capacity. Increasing bandwidth does not necessarily increase throughput proportionally—if the bottleneck lies elsewhere (e.g., device CPU), the increase has no effect.
• Latency is the time from when data is sent until it reaches the recipient. Jitter is the variation (fluctuation) in latency. For real-time communication such as voice and video, not just the absolute latency but a large jitter is perceived as audio dropouts or jerky video. Packet loss is the fraction of packets lost in transit, mainly caused by congestion or physical-layer degradation.