Calculate aggregate SD-WAN throughput capacity across multiple underlay links.
SD-WAN combines multiple underlying transport links (MPLS, broadband, LTE/5G, satellite) into one logical connection, but the effective usable capacity is less than the simple sum of link speeds: a path quality factor accounts for real-world loss, jitter, and the SD-WAN controller steering some traffic away from degraded paths, and a fixed control-plane overhead — consumed by encapsulation, path monitoring probes, and orchestration traffic — is subtracted from the total.
Effective SD-WAN capacity
effective = (sum of link bandwidths × path_quality_factor) − control_overhead
Real-world links rarely deliver their full rated speed continuously — congestion, loss, and jitter on broadband or cellular links, plus the SD-WAN controller's own quality-based steering (which shifts traffic away from a degraded path rather than using its full nominal capacity), mean actual usable throughput runs below the simple arithmetic sum.
Continuous path quality probing (measuring latency/jitter/loss on every link to make steering decisions), tunnel encapsulation headers, and orchestration/telemetry traffic to the SD-WAN controller all consume a small but constant slice of aggregate bandwidth, independent of how much user traffic is flowing.
Only if the added links have comparable quality — adding a high-latency, lossy link contributes less usable capacity per Mbps of rated speed than adding another reliable link, since the path quality factor effectively discounts unreliable paths more heavily in real deployments than this simplified average-factor model shows.