VoLTE and IMS Architecture
VoLTE (Voice over LTE) represents a paradigm shift in mobile voice communication, moving from traditional circuit-switched networks to an all-IP packet-switched architecture using the IMS framework.
What is VoLTE?
VoLTE = Voice over LTE (Long Term Evolution)
Voice calls transmitted as IP packets over the 4G LTE data network, instead of using traditional circuit-switched channels.
Key Benefits
IMS (IP Multimedia Subsystem)
IMS is the architectural framework that enables VoLTE and other IP-based multimedia services.
IMS Core Components
IMS Components Deep Dive
CSCF (Call Session Control Function)
The CSCF is the heart of IMS, handling SIP signaling. There are three types:
1. P-CSCF (Proxy CSCF)
First contact point for the UE.
Functions:
- Entry point to IMS network
- SIP proxy: Forwards SIP messages
- Security: IPsec tunnels, authentication
- Compression: SIP message compression
- Emergency calls: Routes to emergency services
- QoS: Interacts with PCRF for bearer setup
Location: Usually in the visited network (where user is roaming)
2. I-CSCF (Interrogating CSCF)
Gateway to the home network.
Functions:
- Entry point to home network
- HSS queries: Find user's S-CSCF
- Routing: Routes incoming calls to correct S-CSCF
- Topology hiding: Hides internal network structure
- Load balancing: Selects S-CSCF based on capacity
Location: Edge of home network
3. S-CSCF (Serving CSCF)
Session control and registration.
Functions:
- Session control: Maintains session state
- User registration: Registers users with IMS
- Routing: Routes SIP messages
- Service triggering: Invokes application servers
- Charging: Generates CDRs
- Authentication: Works with HSS
Location: Home network
Analogy:
- P-CSCF: Security guard at building entrance
- I-CSCF: Receptionist who finds which office you need
- S-CSCF: Your actual office manager handling everything
HSS (Home Subscriber Server)
Master database for subscriber information.
Contains:
- User profiles: Subscribed services, features
- Authentication data: Keys for IMS authentication
- S-CSCF assignment: Which S-CSCF serves the user
- Service triggers: Application server information
- Roaming permissions
Functions:
- User authentication and authorization
- Service profile download to S-CSCF
- User location tracking
- Integration with application servers
Similar to: HLR in 2G/3G (evolved version)
PCRF (Policy and Charging Rules Function)
Policy and QoS controller.
Functions:
- QoS policies: Determines QoS for bearers
- Charging rules: Controls charging
- Deep Packet Inspection coordination
- Bandwidth management
- Service-based policies: Different QoS for different services
How it works:
- IMS requests bearer for voice call
- PCRF determines: QCI=1, priority=2, guaranteed bitrate
- Sends policy to PGW/SGW
- Network establishes appropriate bearer
Application Servers (AS)
Value-added services in IMS.
Types:
- SIP AS: SIP-based services (call forwarding, voicemail)
- OSA/Parlay AS: Third-party applications
- IM-SSF: CAMEL services integration
Examples:
- Call forwarding
- Call waiting
- Voicemail
- Conference calls
- Video calls
- Rich Communication Services (RCS)
Gateways
MGCF (Media Gateway Control Function)
- Controls MGW (Media Gateway)
- Interworks with PSTN/CS networks
- Converts SIP ↔ ISUP signaling
MGW (Media Gateway)
- Converts media between IP and TDM
- Transcoding
- Echo cancellation
BGCF (Breakout Gateway Control Function)
- Routes calls to PSTN
- Selects appropriate gateway
- Routing decisions based on number
LTE Network Architecture (EPC)
The Evolved Packet Core provides the transport network for VoLTE.
Complete LTE + IMS Architecture
EPC Components
eNodeB (Evolved NodeB)
LTE base station - the radio interface.
Functions:
- Radio resource management
- Header compression
- Encryption
- Scheduling
- Mobility (handover)
Simplified from 3G: No RNC needed!
MME (Mobility Management Entity)
Control plane - manages signaling.
Functions:
- Tracking: Tracks UE location
- Paging: Pages UE for incoming calls
- Authentication: Authenticates users
- Bearer management: Establishes/releases bearers
- Handover: Inter-eNodeB handover signaling
- Roaming: Manages roaming
Analogous to: MSC control functions (without media)
S-GW (Serving Gateway)
Local mobility anchor.
Functions:
- User plane anchor: During handovers
- Packet routing
- Buffering for paging
- Lawful intercept
- Charging support
P-GW (PDN Gateway)
Gateway to external networks.
Functions:
- IP address allocation
- Gateway to Internet/IMS
- QoS enforcement: PCRF policies
- Packet filtering
- Charging: Per-user, per-service
- DPI (Deep Packet Inspection) anchor
Analogous to: GGSN in 3G
HSS (Home Subscriber Server)
Same as IMS HSS - often integrated.
Functions:
- Authentication vectors (for MME)
- Subscriber profiles
- APN (Access Point Name) information
- QoS profiles
SIP Protocol
SIP (Session Initiation Protocol) is the signaling protocol used in IMS for establishing, modifying, and terminating sessions.
SIP Messages
Requests (Methods)
- INVITE: Initiate call
- ACK: Acknowledge response
- BYE: Terminate call
- CANCEL: Cancel pending request
- REGISTER: Register with IMS
- UPDATE: Modify session
- PRACK: Provisional acknowledgment
Responses (Status Codes)
- 1xx: Provisional (100 Trying, 180 Ringing)
- 2xx: Success (200 OK)
- 3xx: Redirection
- 4xx: Client error (404 Not Found)
- 5xx: Server error
- 6xx: Global failure (603 Decline)
SIP Message Format
INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
From: Alice <sip:alice@atlanta.com>;tag=1928301774
To: Bob <sip:bob@example.com>
Call-ID: a84b4c76e66710@pc33.atlanta.com
CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.com>
Content-Type: application/sdp
Content-Length: 142
v=0
o=alice 2890844526 2890844526 IN IP4 pc33.atlanta.com
s=-
c=IN IP4 192.168.1.100
t=0 0
m=audio 49172 RTP/AVP 0 8
a=rtpmap:0 PCMU/8000
VoLTE Call Flow
VoLTE Call Setup (Simplified)
Detailed Call Flow Steps
1. IMS Registration
Before making calls, UE must register with IMS.
2. Call Setup (Mobile Originated)
Step-by-step:
-
UE sends SIP INVITE to P-CSCF
INVITE sip:+15551234567@ims.example.com SIP/2.0 -
P-CSCF contacts PCRF for QoS policy
- PCRF determines: QCI=1, guaranteed bit rate
- Sends policy to P-GW
-
Dedicated bearer established
- Separate bearer for voice (QCI=1)
- Different from default Internet bearer (QCI=9)
-
IMS routes call
- P-CSCF → S-CSCF → I-CSCF → Destination network
-
Destination UE rings
- SIP 180 Ringing sent back
-
User answers
- SIP 200 OK
- Similar bearer setup for destination UE
-
Call connected
- RTP media streams start
- Voice packets flow directly between UEs (or via media proxy)
3. Voice Encoding
AMR (Adaptive Multi-Rate) Codec:
- AMR-NB: Narrowband (8 kHz) - 4.75 to 12.2 Kbps
- AMR-WB: Wideband (16 kHz) - 6.6 to 23.85 Kbps (HD Voice)
RTP Packets:
- Each packet: ~20ms of audio
- Voice Activity Detection (VAD): No packets during silence
- Comfort Noise Generation (CNG): Background noise
4. Media Path
Note: Media can flow through network or peer-to-peer (if supported)
VoLTE vs Circuit-Switched Voice
Comparison
| Feature | Circuit-Switched (3G) | VoLTE (4G) |
|---|---|---|
| Network | Circuit domain | Packet (LTE) |
| Technology | Dedicated circuit | IP packets |
| Call Setup Time | 4-7 seconds | 1-2 seconds |
| Voice Quality | AMR-NB (narrowband) | AMR-WB (wideband/HD) |
| Data During Call | Limited (HSPA) or none | Full LTE speed |
| Battery | Higher (dual mode) | Better (single RAT) |
| Capacity | Lower | Higher (spectral efficiency) |
| Latency | Higher | Lower |
| Network Switching | CSFB required | None |
Call Setup Time Comparison
PoI (Point of Interconnection)
Point where different networks interconnect for call routing.
Types of PoI
1. IMS-to-IMS PoI
- SIP-based interconnection
- Direct VoLTE to VoLTE
- Best quality
- No transcoding needed
2. IMS-to-CS PoI
- VoLTE to 2G/3G
- Goes through MGW (Media Gateway)
- Transcoding required
- ISUP/SS7 signaling
Call Scenarios
VoLTE to VoLTE (Same Operator)
UE A → eNodeB → IMS Core → eNodeB → UE B
(All IP, no conversion)
VoLTE to VoLTE (Different Operator)
UE A → IMS A → [PoI - SIP] → IMS B → UE B
(All IP, SIP-SIP interconnection)
VoLTE to 3G CS Call
UE A → IMS → BGCF → MGW → [PoI - TDM] → MSC → 3G UE
(IP→TDM conversion at MGW)
QoS in VoLTE
Dedicated Bearer for Voice
Default Bearer:
- Established when UE attaches to network
- QCI = 9 (best effort)
- Used for Internet, apps
Dedicated Bearer (for VoLTE):
- Established during call setup
- QCI = 1 (highest priority)
- Guaranteed bit rate
- Low latency
QCI Values for VoLTE
- QCI 1: VoLTE voice - 100ms delay budget, GBR
- QCI 5: IMS signaling (SIP) - 100ms, non-GBR
- QCI 9: Default bearer - 300ms, non-GBR
GBR = Guaranteed Bit Rate Non-GBR = Best Effort
Fixed Wireless Access (FWA) - JIO AirFiber
What is FWA?
Fixed Wireless Access uses cellular network (4G/5G) to provide home broadband instead of fiber cables.
Think of it like: Your home WiFi router, but instead of fiber/cable connection, it uses a 4G/5G SIM card to get internet from the nearest cell tower.
How JIO AirFiber Works
Components:
-
FWA Device (AirFiber Unit)
- Contains 4G/5G modem
- Has SIM card slot
- Built-in WiFi router
- External antenna (for better signal)
-
SIM Card
- Special data-only plan (unlimited/high quota)
- Fixed location (not for mobility)
- Higher data allowance than mobile SIM
-
Cell Tower Connection
- Uses same towers as mobile phones
- Typically higher gain antenna (better reception)
- Can use 4G or 5G bands
FWA vs Mobile - Same Bandwidth?
Yes, FWA and mobile phones share the same cell tower and bandwidth!
Key Points:
| Feature | Mobile SIM | FWA (AirFiber) |
|---|---|---|
| Tower Used | Same cell tower | Same cell tower |
| Frequency Bands | Same (e.g., 1800 MHz, 2300 MHz) | Same |
| Bandwidth Share | Shares total capacity | Shares total capacity |
| Data Usage | 2-10 GB/day typical | 100-300 GB/month |
| Priority | Equal (usually) | Equal (usually) |
| Location | Mobile | Fixed location |
Performance Degradation with More Users
Yes! FWA performance degrades when more users are active on the same tower.
Real Example:
Cell Tower Capacity: 100 Mbps
Scenario 1 - Low Usage (Morning):
├─ 5 mobile users: 5 Mbps each = 25 Mbps
├─ 2 FWA devices: 30 Mbps each = 60 Mbps
└─ Total: 85 Mbps used
→ Each FWA gets: 30 Mbps ✅ Good speed
Scenario 2 - Peak Usage (Evening 8-11 PM):
├─ 40 mobile users: 1.5 Mbps each = 60 Mbps
├─ 10 FWA devices: Competing for remaining 40 Mbps
└─ Total: 100 Mbps used (tower saturated)
→ Each FWA gets: 4 Mbps ❌ Slow speed
Why Degradation Happens:
- Shared Resource: All users share same tower capacity
- Peak Hours: 8-11 PM = streaming, gaming, downloads
- Cell Capacity: Fixed total bandwidth per tower
- Fair Scheduling: Tower tries to serve all users equally
- Congestion: More users = less bandwidth per user
Operator Solutions:
To Prevent Degradation:
├─ Add more cell towers (densification)
├─ Upgrade to 5G (higher capacity)
├─ Load balancing (redirect to less congested towers)
├─ Frequency refarming (allocate more spectrum)
└─ FWA-specific plans with QoS priority
Radio Frequency, Wavelength & Coverage
Understanding Frequency vs Wavelength
Physics Formula:
Relationship:
- Low Frequency = Long Wavelength = Better Coverage = Lower Speed
- High Frequency = Short Wavelength = Poor Coverage = Higher Speed
Frequency Bands by Generation
| Generation | Frequency | Wavelength | Coverage Radius | Max Speed | Penetration |
|---|---|---|---|---|---|
| 2G GSM | 900 MHz | 33 cm | 35 km | 200 kbps | Excellent (thick walls) |
| 2G GSM | 1800 MHz | 16.6 cm | 15 km | 384 kbps | Good |
| 3G UMTS | 2100 MHz | 14.3 cm | 10 km | 42 Mbps | Moderate |
| 4G LTE | 1800 MHz | 16.6 cm | 5-10 km | 100 Mbps | Good |
| 4G LTE | 2300 MHz | 13 cm | 3-5 km | 150 Mbps | Moderate |
| 5G NR | 3500 MHz | 8.6 cm | 1-2 km | 1 Gbps | Poor |
| 5G mmWave | 28000 MHz | 1.07 cm | 200-300 m | 5 Gbps | Very Poor |
Wavelength Calculation Examples:
2G (900 MHz):
λ = 300,000,000 / 900,000,000 = 0.33 m = 33 cm
4G (1800 MHz):
λ = 300,000,000 / 1,800,000,000 = 0.166 m = 16.6 cm
5G (3500 MHz):
λ = 300,000,000 / 3,500,000,000 = 0.086 m = 8.6 cm
5G mmWave (28 GHz):
λ = 300,000,000 / 28,000,000,000 = 0.0107 m = 1.07 cm
Why Low Frequency = Better Coverage?
Physics Explanation:
-
Diffraction (Bending Around Obstacles)
- Long wavelength (low frequency): Bends around buildings, trees
- Short wavelength (high frequency): Blocked by obstacles
-
Penetration (Through Walls)
- Long wavelength: Passes through concrete, brick walls
- Short wavelength: Absorbed by walls, glass, rain
-
Path Loss (Signal Weakening)
- Low frequency: Loses less energy over distance
- High frequency: Loses more energy quickly
Real-World Example:
Same tower location - different frequencies:
900 MHz Tower (2G):
├─ Signal reaches: 35 km radius
├─ Works inside buildings: ✅ Yes
├─ Works in rural areas: ✅ Yes
├─ Penetrates basement: ✅ Yes
└─ Speed: 200 kbps (slow)
3500 MHz Tower (5G):
├─ Signal reaches: 1.5 km radius
├─ Works inside buildings: ⚠️ Weak signal
├─ Works in rural areas: ❌ No
├─ Penetrates basement: ❌ No signal
└─ Speed: 1 Gbps (very fast)
Conclusion:
└─ Need 500 towers for 5G vs 20 towers for 2G (same area)
Why Different Frequencies Exist?
Trade-off: Coverage vs Speed
Use Case by Frequency:
| Frequency Band | Use Case | Example | Coverage | Speed |
|---|---|---|---|---|
| 700 MHz | Rural coverage, wide area | Village connectivity | 40 km | 50 Mbps |
| 900 MHz | Voice, basic data | 2G networks | 35 km | 10 Mbps |
| 1800 MHz | Urban 4G, balanced | City coverage | 10 km | 150 Mbps |
| 2300 MHz | 4G data capacity | Dense urban | 5 km | 200 Mbps |
| 3500 MHz | 5G mainstream | City hotspots | 2 km | 1 Gbps |
| 28 GHz (mmWave) | 5G ultra-fast | Stadiums, airports | 300 m | 5 Gbps |
Why Small Bandwidth at Higher Frequencies?
Misconception: "Small bandwidth" - Actually, higher frequencies have MORE bandwidth available!
Bandwidth = Range of frequencies available for data transmission
Low Frequency (2G - 900 MHz):
├─ Bandwidth: 10 MHz
├─ Channels: 2 channels of 5 MHz each
└─ Speed: Low (limited bandwidth)
High Frequency (5G - 3500 MHz):
├─ Bandwidth: 100 MHz
├─ Channels: 10 channels of 10 MHz each
└─ Speed: High (more bandwidth available)
Why Higher Frequency = More Speed:
-
More Spectrum Available
- At 28 GHz: Can allocate 400 MHz bandwidth
- At 900 MHz: Only 10-20 MHz available (crowded)
-
Wider Channels
- 5G: 100 MHz wide channels
- 4G: 20 MHz wide channels
- 2G: 200 kHz channels
Real Spectrum Allocation (India Example):
Frequency Band → Available Bandwidth:
700 MHz (Low):
├─ Total spectrum: 2×10 MHz (20 MHz)
├─ Crowded (TV, radio nearby)
└─ Max Speed: 50 Mbps
1800 MHz (Mid):
├─ Total spectrum: 2×20 MHz (40 MHz)
├─ Moderate congestion
└─ Max Speed: 150 Mbps
3500 MHz (High):
├─ Total spectrum: 2×100 MHz (200 MHz)
├─ Lots of free spectrum
└─ Max Speed: 1 Gbps
28 GHz (mmWave):
├─ Total spectrum: 2×400 MHz (800 MHz)
├─ Virtually unlimited
└─ Max Speed: 5 Gbps
Why Not Use Only High Frequency?
Problem with High Frequency:
├─ Poor coverage (need 100x more towers)
├─ Cannot penetrate buildings
├─ Blocked by rain, trees, walls
├─ Expensive infrastructure
└─ Limited use cases
Solution - Multi-Band Strategy:
├─ 700 MHz: Rural coverage, voice
├─ 1800 MHz: Urban data, balanced
├─ 3500 MHz: City hotspots, fast data
└─ 28 GHz: Stadiums, airports, ultra-fast
Tower Density by Frequency
Example: Covering 100 km² area
Cost Implication:
Cover 100 km² urban area:
2G (900 MHz):
├─ Towers needed: 10
├─ Cost: Rs 50 lakhs per tower = Rs 5 crores
└─ Coverage: Excellent
4G (1800 MHz):
├─ Towers needed: 50
├─ Cost: Rs 1 crore per tower = Rs 50 crores
└─ Coverage: Good
5G (3500 MHz):
├─ Towers needed: 500
├─ Cost: Rs 80 lakhs per tower = Rs 400 crores
└─ Coverage: Limited (city centers only)
Conclusion:
└─ 5G requires 50x more towers than 2G
└─ That's why 5G is deployed only in cities first
Summary
VoLTE Benefits
- ✅ HD Voice: AMR-WB codec (16 kHz)
- ✅ Faster Call Setup: 1-2 seconds vs 4-7 seconds
- ✅ Simultaneous Voice & Data: Full LTE speed during call
- ✅ Better Battery: No network switching (CSFB)
- ✅ Lower Latency: All-IP network
- ✅ Future-Proof: Foundation for 5G VoNR
IMS Core Functions
- CSCF: Call/session control (P/I/S-CSCF)
- HSS: Subscriber database
- PCRF: QoS and policy control
- Application Servers: Value-added services
- Gateways: PSTN/CS network interworking
Key Technologies
- SIP: Session signaling
- RTP: Real-time media transport
- AMR-WB: HD voice codec
- QCI 1: Dedicated bearer for voice
- LTE EPC: Packet core network
Network Evolution
Next Topics
- Call Flow Diagrams - Detailed call scenarios
- Circuit vs Packet Switching - Why packet-switching?
- 2G/3G Architecture - Legacy networks