How it works
A radio-over-IP gateway or a VoIP phone cuts the voice into packets. Each packet carries the codec's bits for one packet time plus headers: RTP 12 bytes, UDP 8 bytes and IPv4 20 bytes, or IPv6 40 bytes, so 40 or 60 bytes in all.134 At 20 ms, the default audio packet time, a channel sends 50 packets a second.2
The rate is bytes per packet × 8 × packets a second. G.711 at 20 ms makes 160 + 40 = 200-byte packets: 80 kbit/s at the IP layer and 87.2 kbit/s on Ethernet, which adds 18 bytes of frame header and check; G.729 makes 60-byte packets: 24 kbit/s and 31.2 kbit/s, the same figures as Cisco's tables.8 On the wire Ethernet also sends a preamble and a gap, 20 bytes more. LTE compresses the 40 bytes of IP/UDP/RTP headers to a few bytes with ROHC; the box counts 4.7 WireGuard adds 60 bytes over IPv4 and 80 over IPv6.9
Codec rates: G.711 and G.722 64 kbit/s, G.729 8 kbit/s, GSM full rate 13.2 kbit/s,2 iLBC 15.2 kbit/s,5 Opus anything from 6 to 510 kbit/s6 and the AMBE+2 vocoder of DMR, dPMR, NXDN and P25 Phase 2 2.45 kbit/s, or 3.6 kbit/s with error correction.10 A push-to-talk channel is half-duplex: it carries one direction at a time, and nothing while nobody talks.
What radio over IP is and where gateways fit: radio over IP explained. Monthly data of a PoC radio: PoC data calculator.
