Posts in fundamentals.
How SIP call transfer really works: the REFER method, blind vs attended flows, the Replaces header, NOTIFY progress reports, and why transfers fail.
G.711, G.729, Opus, and the other codecs that shape VoIP call quality. How they work, what they trade off, and why the same network sounds different with each codec.
Fax machines, fire panels, elevator phones, and security systems were built for analog circuits. Converting them to VoIP ranges from difficult to prohibited by code.
Lines, trunks, key systems, and line appearances don't map cleanly to VoIP. What replaced them, and why the old mental model causes so much confusion.
How VoIP trunk providers overbook capacity, what happens when they hit limits, how to spot overbooking as the cause of intermittent quality issues, and what to ask before signing.
How SIP over WebSockets enables browser-based VoIP, how the media path works, and how browser softphones connect to traditional phone infrastructure.
The mechanics of phone number porting: LOA, LSR, FOC process, how the NPAC routes calls after activation, why ports fail, and how to avoid common problems.
How SRTP encrypts VoIP media streams, the three key exchange methods (SDES, DTLS-SRTP, ZRTP), and why encryption failures cause confusing interop problems.
Symptom-based troubleshooting for common SIP problems: one-way audio, dropped calls, registration failures, and caller ID issues.
Capture and analyze SIP traffic with Wireshark and sngrep. Includes example traces for normal calls, NAT issues, and failed registrations.
How SIP trunks connect your PBX to the PSTN: authentication, codec negotiation, capacity planning, and SIP trunking vs. hosted PBX.
How SIP handles hold, transfer, forwarding, and conferencing at the protocol level, and why these features cause interop problems.
How SIP digest authentication works, the role of TLS and SRTP in protecting VoIP traffic, and practical defenses against toll fraud.
A field reference to the SIP response codes you will actually encounter when troubleshooting VoIP calls, registrations, and transfers.
Why NAT breaks SIP and RTP, causing one-way audio and registration failures, and how STUN, TURN, ICE, and SBCs work around it.
How SDP works inside SIP messages to negotiate codecs, IP addresses, ports, and media direction between two VoIP endpoints.
How SIP REGISTER works at the protocol level: authentication sequence, registration lifetime, and common failure modes.
Walk through every SIP message in a basic call from INVITE to BYE. Learn what normal looks like so you can spot problems fast.
A line-by-line walkthrough of a real SIP INVITE message, explaining what every major header does and how requests and responses relate to each other.
SIP handles call signaling, not voice audio. Understanding that distinction is the foundation for diagnosing every VoIP problem you will ever encounter.
How VoIP systems use DNS to find servers, why SRV and NAPTR records matter, and why DNS failures cause some of the most confusing phone outages you will encounter.
A systematic framework for diagnosing bad VoIP calls, walking through five network segments from endpoint to provider.
QoS can prioritize voice on your local network, but it has real limitations. Learn what DSCP marking actually does, where it stops working, and when you need it.
Deep dive into latency, jitter, and packet loss: how each metric works, how they interact, and what they mean for VoIP calls.
Voice packets cross ISP last miles, peering points, and transit networks you do not control. How each hop introduces latency, jitter, and loss.
Your local network is the one part of the VoIP path you fully control. Switches, cabling, Wi-Fi, and configuration that affect call quality.
A complete, plain-language walkthrough of everything that happens between tapping a contact on your phone and the other person hearing your voice. Every step, from microphone to speaker.
VoIP turns your voice into packets sent over the internet in real time. The step-by-step process from microphone to speaker, and why quality varies.
Voice traffic is uniquely demanding. Why latency, jitter, and packet loss affect real-time audio far more than web browsing, email, or file transfers.
All internet data travels in packets. How packets work, TCP vs UDP, and why the difference matters for VoIP call quality.
How the traditional phone system works: circuit switching, copper wires, and the PSTN explained as a foundation for VoIP.
How local networks differ from the internet, what the shared medium means for traffic, and why this matters when your office runs VoIP.