Hi everyone,
When I first entered the telecom industry, one of the biggest challenges was finding beginner-friendly material that explained how telecom networks actually work, especially when it came to roaming, service flows, signaling, and interactions between different network elements.
Most information is spread across standards, vendor documentation, training courses, and forum discussions. While these are excellent resources, they can be difficult for someone new to telecom to follow.
Over the years, while working with LTE, VoLTE, IMS, QoS, Roaming, and 5G technologies, I built a large collection of personal notes to help me understand these concepts. I eventually organized those notes into a handbook and recently published it as:
Understanding Mobile Network Architecture & Service Flows
The book is designed for:
-
Beginners entering the telecom industry
-
IT professionals transitioning into telecom
-
Students interested in mobile networks
-
Engineers looking to understand roaming and telecom services
The handbook focuses heavily on practical telecom concepts such as:
-
Mobile network architecture
-
LTE and EPC fundamentals
-
IMS and VoLTE basics
-
Roaming architecture
-
Roaming calls
-
Roaming SMS
-
Mobile data roaming and browsing
-
Telecom service flows
-
Interworking between network elements
-
5G Core fundamentals
Rather than jumping straight into complex standards and specifications, the book focuses on building a strong understanding of the fundamentals and explaining concepts in an easy-to-follow manner. Special attention is given to roaming architecture, roaming calls, roaming SMS, mobile data roaming, and the service flows that connect different network elements, helping readers understand how telecom services work in real-world networks.
If you’re new to telecom, transitioning from another IT domain, or looking for a structured telecom handbook, I hope this resource will help make the learning journey easier and provide a solid foundation in both mobile network architecture and roaming technologies.
Amazon Link:
View the Book on Amazon
I would appreciate any feedback, suggestions, or reviews from fellow telecom professionals.
Thank you.
Table of Contents
COPYRIGHT & DISCLAIMER 2
DISCLAIMER 2
Foreword 4
Chapter 1 — Cellular Network Fundamentals 10
1.1 What Is a Cellular Mobile Network? 10
1.2 The Three-Block Architecture: MS, RAN, Core Network 10
1.3 Network Evolution: 2G → 3G → 4G → 5G at a Glance 11
1.4 Control Plane vs. User Plane — The Core Separation 12
1.5 Circuit Switching vs. Packet Switching 12
1.6 Key Identifiers: MSISDN, IMSI, TMSI, MCC/MNC 13
Chapter 2 — GSM (2G) Architecture & Service Flows 14
2.1 GSM Architecture Overview 14
2.2 Mobile Station (MS) — ME and SIM 14
2.3 Base Station Subsystem (BSS) — BTS and BSC 15
Base Transceiver Station (BTS) 15
Base Station Controller (BSC) 15
2.4 Network Switching Subsystem (NSS) — MSC, HLR, VLR, AuC, EIR 15
2.5 GSM Interfaces & Protocols 16
Um Interface (MS ↔ BTS) — The Air Interface 16
Abis Interface (BTS ↔ BSC) 17
A Interface (BSC ↔ MSC) 17
Core Network Signaling — SS7 and MAP 17
2.6 Location Update (LU) — Full Roaming Flow 17
2.7 GSM Identity & Routing — MGT, Global Title Translation 19
2.8 Roamer-to-Roamer Voice Call — Step-by-Step Flow 20
2.9 Roamer-to-Roamer SMS — MO/MT Flow 23
2.10 2G Browsing — GPRS/EDGE & PDP Context Activation 24
Chapter 3 — CAMEL: Intelligent Network Services for Roaming 28
3.1 What Is CAMEL and Why It Exists 28
3.2 CAMEL Architecture — SSF, SCF, gsmSCF, SCP, SDP 28
Service Switching Function (SSF) 28
Service Control Function (SCF) / gsmSCF 29
Service Control Point (SCP) 29
Home Location Register (HLR) 29
CAMEL Application Part (CAP) 29
3.3 CAMEL Roamer-to-Roamer (R2R) Voice Call Flow 30
3.4 CAMEL Roamer-to-Roamer (R2R) SMS Flow 30
3.5 HLR Role in CAMEL Subscription 31
Chapter 4 — UMTS (3G) Architecture & Service Flows 32
4.1 3G Architecture Overview 32
4.2 User Equipment — UE and USIM 32
4.3 UTRAN — Node B and RNC 32
Node B 33
Radio Network Controller (RNC) 33
4.4 3G Core Network — CS Domain and PS Domain 33
4.5 3G Interfaces 34
4.6 Roamer-to-Roamer CS Voice Call in 3G — Full Flow 35
4.7 Roamer-to-Roamer SMS in 3G 37
4.8 3G Browsing — UMTS PS Attach & PDP Context 39
Chapter 5 — LTE (4G) Architecture, Roaming & Call Flows 41
5.1 LTE Architecture Overview — All-IP EPC 41
5.2 User Equipment (UE) and USIM in LTE 41
5.3 E-UTRAN — eNodeB and Flat RAN Architecture 42
5.4 Evolved Packet Core (EPC) — MME, SGW, PGW, HSS 42
Mobility Management Entity (MME) 42
Serving Gateway (SGW) 42
Packet Data Network Gateway (PGW) 43
Home Subscriber Server (HSS) 43
5.5 LTE Interfaces 44
5.6 LTE Roaming Architectures — S8HR vs. Local Breakout (LBO) 44
5.7 LTE Attach Procedure — Full Roaming Flow 45
5.8 LTE Bearers — Default and Dedicated EPS Bearers 47
5.9 LTE CSFB Roamer-to-Roamer Call — Full Flow 48
5.10 SMS over SGs — Delivery Without CS Fallback 50
5.11 NAS (Non-Access Stratum) — Role and Functions 52
Chapter 6 — VoLTE & IMS: IP-Based Voice and Messaging 53
6.1 What Is IMS and Why It Was Introduced 53
6.2 IMS Node Architecture — P-CSCF, I-CSCF, S-CSCF 54
6.3 IMS Registration Procedure — Full Roaming Flow 55
6.4 P-CSCF Discovery — ISIM, PCO, DHCP, DNS 56
6.5 VoLTE Roaming Architectures — S8HR vs. LBO 57
6.6 VoLTE Roamer-to-Roamer Call Flow — S8HR 58
6.7 Protocols Used in VoLTE 60
6.8 VoLTE SMS — Roamer-to-Roamer IMS SMS Flow 61
6.9 IP-SM-GW — Interworking IMS SMS with Legacy SMSC 62
Chapter 7 — 5G Architecture, Registration & Service Flows 64
7.1 5G Architecture Overview — Service-Based Architecture (SBA) 64
7.2 User Equipment (UE) and USIM in 5G 64
7.3 NG-RAN — gNodeB, gNB-CU, and gNB-DU 65
7.4 5G Core Network Functions — Complete NF Catalog 65
7.5 Network Slicing — NSSAI, Slice Types, NSSF Role 66
7.6 5G Interfaces — Uu, NG, N1–N12, N26 68
7.7 Service-Based Interfaces (SBI) — HTTP/2 and REST APIs 69
7.8 5G Signaling & Protocols 70
7.9 SUPI and SUCI — 5G Subscriber Identity and Privacy 70
SUPI — Subscription Permanent Identifier 70
SUCI — Subscription Concealed Identifier 71
7.10 5G Registration Procedure — Full Roaming Flow 72
7.11 PDU Session Establishment — Home Routed (HR) Architecture 74
7.12 5G User Plane Data Transfer — UL/DL Path 75
7.13 5G VoNR Roamer-to-Roamer Call Flow 76
7.14 5G Roamer-to-Roamer SMS (SMS over NAS) 77
Chapter 8 — Protocols Deep Dive 79
8.1 MAP (Mobile Application Part) — Role in 2G/3G 79
Key MAP Operations 79
8.2 CAP (CAMEL Application Part) — Intelligent Network Signaling 80
Key CAP Operations 80
8.3 SIP (Session Initiation Protocol) — IMS Call Control 81
Key SIP Methods 81
8.4 RTP & RTCP — Voice Media Delivery 81
8.5 SDP (Session Description Protocol) — Media Negotiation 82
8.6 Diameter — Authentication, Policy, and Charging 83
8.7 NAS (Non-Access Stratum) — LTE and 5G Control Signaling 83
8.8 GTP-U — User Plane Tunneling 84
8.9 IPSec/TLS — Securing IMS Signaling 84
Chapter 9 — Network Evolution Summary & Architecture Comparison 86
9.1 Radio Access Evolution 86
9.2 Core Network Evolution 87
9.3 Voice Evolution 87
9.4 Data Evolution 88
9.5 Roaming Architecture Evolution 88
9.6 Generation-by-Generation Comparison Table 89
Chapter 10 — Interview Preparation: 60 Q&A 91
10.1 GSM Architecture & Location Update 91
10.2 Roaming, CAMEL & SS7/MAP 92
10.3 LTE Architecture, Attach & Bearers 93
10.4 VoLTE, IMS & CSFB 95
10.5 5G Registration, PDU Sessions & SBA 96
10.6 Protocols — MAP, SIP, RTP, NAS, Diameter 98
Conclusion — The Continuous Evolution 100
Glossary of Key Terms 102
About the Author 107