“Know your environment. Understand who’s running the game.”
☎️ Let’s Rewind
Once upon a time, calling someone meant a person literally plugged cables into a board to connect your voice to theirs.
Those were the manual switchboards, followed by automated telephone exchanges. That era used circuit-switched networks — where a dedicated line was held for your entire call.
Fast forward ⏩ — and now your voice zips through packet-switched networks, hopping across routers, firewalls, clouds, and code.
🛠 The Game Board: Old vs New
| Concept | Circuit-Switched World | Packet-Switched (VoIP) World |
|---|---|---|
| Call Routing | PSTN (Public Switched Telephone Network) | Internet (IP networks) |
| Central Device | PBX / Exchange | IP-PBX / Softswitch |
| Physical Wiring | Dedicated copper circuits | IP network, Ethernet, Wi-Fi |
| Call Setup/Teardown | Controlled by Telco systems | Controlled by SIP or similar |
| Call Audio Path | Fixed voice circuit | Dynamic RTP media stream |
🧠 Circuit vs Packet Switching — The Core Shift
🎚 Circuit Switching (Old School)
- Like renting an entire highway just for your car
- Fixed bandwidth, reliable, expensive
- Example: Landline call from your grandma’s rotary phone
📦 Packet Switching (Modern VoIP)
- Your voice is chopped into chunks (packets), each taking the best route
- More efficient, shared bandwidth
- Like a backpacker using public transport
- Example: WhatsApp call on patchy airport Wi-Fi
🔄 Enter the VoIP Equivalents
Let’s meet the players behind modern voice traffic:
🏢 IP-PBX (Internet Protocol Private Branch Exchange)
- The VoIP version of a PBX (Private Branch Exchange)
- Manages internal extensions, call routing, voicemail, IVR
- Runs on software, sometimes even cloud-based (like FreePBX or 3CX)
⚙️ Softswitch
- Software-powered switchboard
- Routes calls between networks — VoIP ↔ PSTN or VoIP ↔ VoIP
- Used by telcos and service providers (FreeSWITCH, Kamailio, OpenSIPS)
📱 SIP Endpoints
- Devices or apps that speak SIP (VoIP protocol)
- Can be a softphone app, VoIP desk phone, or even a smart fridge (yep)
🧩 Visual Time
📈 Circuit Switched Call Flow
User A ---[Analog Signal]---> PBX ---[TDM Circuit]---> Telco Switch ---[TDM Circuit]---> User B
|<------Dedicated voice circuit maintained throughout------>|
🌐 VoIP / Packet Switched Call Flow (SIP + RTP)
User A (Softphone)
|
|---[SIP INVITE (Signaling)]---> SIP Server
|
|<--[SIP 200 OK / ACK]----------|
|
|---[RTP Media Stream]----------> User B (Softphone)
🧠 Why This Matters
Knowing who's doing what behind the scenes helps you debug, design, and build better voice applications.
- Bad call quality? Might be RTP path or NAT issue
- Can’t make internal calls? Could be IP-PBX config
- Confused by latency? Maybe your packets are playing hopscotch through the cloud
🏁 TL;DR – Survived Another Round?
- Circuit-switched = old-school dedicated lines
- Packet-switched = modern, flexible, efficient
- IP-PBX = the office call controller
- Softswitch = the telecom traffic cop
- You’re not just making a call — you’re crossing systems, formats, and protocols
⏭️ Up Next in SIP GAMES:
"You’re Invited… to the Session" – A deep dive into SIP and how calls are actually set up.
🕹 Follow @sip_games to learn SIP without reading the RFCs.
