How a voice call actually works, one layer at a time, from a single packet to a group call. If you build on WebRTC or voice AI, these are the protocols underneath the API, and the reasons they behave the way they do.
Twenty milliseconds of speech becomes an 80-byte Opus frame inside a 120-byte packet. Why each header is there, and why one lost packet costs 20 ms over UDP and 80 or more over TCP.
Packets arrive unevenly, and a packet that misses its slot is as good as lost. How a jitter buffer trades delay for dropouts, what concealment can and can't hide, and what Opus FEC and RED buy against random loss and against bursts.
Alice's laptop doesn't know the address the internet sees it at, and the assistant can't reply to an address it was never told. How one STUN round trip finds it, why some networks need a TURN relay, and how ICE checks every path and picks one.
The path is found, and every router on it could hand Alice's voice to a decoder. How two machines that have never met agree on keys over a network anyone can read, what SRTP does to her packet and what it deliberately leaves alone, and how long a call takes from cold start, measured.
Alice's laptop sends fifty packets a second into a network that never answers. A few dozen bytes from the receiver every few seconds carry the loss, the jitter and enough timestamps to measure the round trip without synchronised clocks. Enough to back off, but not fast enough to back off well, which is why browsers add feedback for every packet.
Add Bob and Carol to Alice's call and someone has to copy her voice twice. Mesh, an SFU and an MCU put that copy in three different places, and the cost moves with it: upload, server bandwidth, or a decode and an encode per person. Then back to Alice's assistant, where the server is simply the other end of the call.
Packets arrive unevenly, and a packet that misses its slot is as good as lost. How a jitter buffer trades delay for dropouts, what concealment can and can't hide, and what Opus FEC and RED buy against random loss and against bursts.
Alice's laptop doesn't know the address the internet sees it at, and the assistant can't reply to an address it was never told. How one STUN round trip finds it, why some networks need a TURN relay, and how ICE checks every path and picks one.
The path is found, and every router on it could hand Alice's voice to a decoder. How two machines that have never met agree on keys over a network anyone can read, what SRTP does to her packet and what it deliberately leaves alone, and how long a call takes from cold start, measured.
Alice's laptop sends fifty packets a second into a network that never answers. A few dozen bytes from the receiver every few seconds carry the loss, the jitter and enough timestamps to measure the round trip without synchronised clocks. Enough to back off, but not fast enough to back off well, which is why browsers add feedback for every packet.
Add Bob and Carol to Alice's call and someone has to copy her voice twice. Mesh, an SFU and an MCU put that copy in three different places, and the cost moves with it: upload, server bandwidth, or a decode and an encode per person. Then back to Alice's assistant, where the server is simply the other end of the call.