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Collaboration, Voice & AV · Intermediate

VoIP Bandwidth Calculation

Computing per-call bandwidth from codec payload, packetization period and Layer 2, IP, UDP and RTP header overhead.

Definition

Per-call bandwidth equals (payload bytes plus header bytes) times 8 times packets per second. A 20 ms packet carries 160 bytes of G.711 or 20 bytes of G.729, and the IP/UDP/RTP headers add 40 bytes; Ethernet adds 18 bytes (or more with 802.1Q). G.711 at 20 ms is about 80 kbps at Layer 3 and 87.2 kbps on Ethernet, while G.729 is about 24 kbps and 31.2 kbps respectively.

Example

Ten simultaneous G.729 calls over an Ethernet WAN handoff require roughly 312 kbps for voice in each direction.

Exam tip

Exams often ask for bandwidth at Layer 3 versus Layer 2; remember the 40-byte IP/UDP/RTP header and 50 packets per second at 20 ms.

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