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A voice call's wire bandwidth is far above its codec rate because each 20 ms packet carries 78 bytes of RTP, UDP, IP and Ethernet headers around a small payload. G.711 at 64 kbps needs about 95 kbps on Ethernet, and G.729 at 8 kbps needs nearly 40 kbps — a five-fold overhead ratio, which is why low-bitrate codecs save less than their headline figure suggests.
VoIP bandwidth
payload = codec rate x interval / 8; frame = payload + 12 RTP + 8 UDP + 20 IP + 38 Ethernet (+ tunnel bytes); per call = frame x 8 x (1000 / interval)
Yes, substantially, because header overhead is amortised over more payload. The cost is added latency and losing more audio when a packet is lost.
Headers dominate. Going from 64 to 8 kbps codec rate only takes wire bandwidth from about 95 to 40 kbps, not to 12.