Call Flow Diagrams
This page provides detailed call flow diagrams for different scenarios across 2G, 3G, and 4G/LTE networks, showing how calls are established, maintained, and terminated.
Table of Contents
- 2G/3G Circuit-Switched Call Flows
- VoLTE Call Flows
- CSFB (Circuit Switched Fallback)
- Roaming Call Flows
- Call to PSTN
2G/3G Circuit-Switched Call Flows
Mobile Originated Call (2G/3G)
A call initiated by a mobile user to another mobile or landline.
Key Points for 2G/3G Calls
Signaling Protocol: ISUP/BICC over SS7
Call Setup Phases:
- Service Request: Mobile requests call service
- Number Analysis: MSC determines routing
- HLR Query: Locate called party
- Resource Allocation: Traffic channels assigned
- Alerting: Called party rings
- Answer: Call connects
- Active: Circuit maintained
- Release: Resources freed
Circuit Characteristics:
- 64 Kbps dedicated bandwidth
- Guaranteed QoS
- Resources held for entire call duration
- Same path for all voice data
Mobile Terminated Call (2G/3G)
An incoming call to a mobile user.
Paging Process
Why Paging?
- Mobile's exact cell unknown
- Only Location Area is known
- Must broadcast page to find mobile
Paging Steps:
- MSC sends page to all BTS in Location Area
- BTS broadcast on PCH (Paging Channel)
- Mobile listening on PCH hears its identity
- Mobile responds on RACH (Random Access Channel)
- Call setup proceeds
VoLTE Call Flows
VoLTE Mobile Originated Call
Key Differences from Circuit-Switched
| Aspect | Circuit-Switched (3G) | VoLTE |
|---|---|---|
| Signaling | ISUP/SS7 | SIP |
| Setup Time | 4-7 seconds | 1-2 seconds |
| QoS Setup | Circuit reservation | Dedicated bearer (QCI=1) |
| Media | 64 Kbps circuit | RTP packets |
| Codec | AMR-NB | AMR-WB (HD) |
| Network | CS domain | PS/LTE |
VoLTE IMS Registration
Before any VoLTE call, the UE must register with IMS.
IMPU: IP Multimedia Public User Identity (tel:+15551234567) IMPI: IP Multimedia Private Identity (used for authentication) ISIM: IMS Subscriber Identity Module
CSFB Call Flow
CSFB (Circuit Switched Fallback) - Used when LTE device doesn't support VoLTE but needs to make voice call.
CSFB Issues
Problems:
- ❌ Long call setup delay: 4-7 seconds (vs 1-2 for VoLTE)
- ❌ Network switching: Drops from 4G to 3G
- ❌ Data speed drop: From LTE to 3G speeds
- ❌ Battery drain: Switching between RATs
- ❌ Poor user experience
Solution: Deploy VoLTE!
Roaming Call Flows
VoLTE Call While Roaming
Key Points:
- Signaling goes through home S-CSCF
- Media path can be optimized (direct or via home)
- Roaming agreements required
- Local Breakout: Media stays in visited network (more efficient)
Calls to PSTN
VoLTE to PSTN Call
Protocol Conversions
Handover Scenarios
VoLTE to VoLTE Handover (Same eNodeB)
Result: Seamless handover, no call drop
VoLTE to 3G Voice (SRVCC)
SRVCC (Single Radio Voice Call Continuity) - Hands over from VoLTE to 3G CS voice.
Use Case: Moving from LTE coverage to 3G-only area
Call Latching in VoLTE
"Call latching" refers to how a call is established and maintained on the VoLTE network using dedicated bearers.
Bearer Establishment Process
Bearer Types
Call "Latches" to Dedicated Bearer:
- Dedicated bearer created specifically for voice
- QCI=1: Highest priority, low latency
- GBR (Guaranteed Bit Rate): Reserved bandwidth
- Different from default Internet bearer
Summary Tables
Call Types Comparison
| Call Type | Network | Signaling | Media | Setup Time |
|---|---|---|---|---|
| 2G/3G CS | Circuit-Switched | ISUP/SS7 | 64 Kbps TDM | 4-7 sec |
| VoLTE | Packet (LTE) | SIP | RTP (AMR-WB) | 1-2 sec |
| CSFB | LTE→3G fallback | Mixed | 64 Kbps TDM | 5-8 sec |
| SRVCC | LTE→3G handover | SIP→ISUP | RTP→TDM | Seamless |
Key Protocols
| Protocol | Purpose | Used In |
|---|---|---|
| ISUP | Call signaling | 2G/3G CS |
| SIP | Session signaling | VoLTE, IMS |
| RTP | Real-time media | VoLTE |
| BICC | Bearer-independent signaling | 3G R4 |
| H.248 | Media gateway control | MGCF↔MGW |
| Diameter | AAA, policy control | PCRF, HSS |
Conclusion
Understanding call flows is essential for:
- Troubleshooting call failures
- Optimizing network performance
- Debugging signaling issues
- Planning network upgrades
- Understanding QoS mechanisms