Acknowledgement xxi

1 Introduction 1

1.1 Wireless Connectivity Alternatives 1

1.2 Goals 2

1.3 The Fundamental Problem 3

1.4 Audience 4

1.5 Recommended Reading 5

1.6 Can One Size Fit All? 6

1.7 Hardware History 6

1.8 On-the-move Connectivity Problems 7

1.9 Reference Implementations 8

1.10 Reference Microcontroller/OS Platform 9

1.11 Reference Cellular Communication Modules (CCMs) Family 11

1.12 A Few Words on Advice, Practices, and Efficiency 13

1.12.1 Best Practice or Good Practice 14

1.12.2 Efficiency Is a Large Umbrella 14

1.12.2.1 Spatial and Temporal Efficiency 14

1.12.2.2 Data Efficiency 15

1.12.2.3 Developmental Efficiency 17

1.13 3G, 4G, 5G, and 6G 17

2 Platforms, Tools, and Debugging 19

2.1 Importance of Toolchain Selection 19

2.2 An Expanded View of the Tool Chain 21

2.3 Computing/OS Platforms 21

2.4 Programming Language Choices 23

2.5 Running the Same Code on Development Computer and IoT Device 23

2.6 Running IoT Connectivity Code in a Debugger 24

3 Cellular Network Basics 27

3.1 Standards 27

3.2 What Do Cellular Networks Do? 28

3.3 Are Cellular Networks Wireless? 29

3.4 What Is a Cell? What Is a Sector? 30

3.5 Omnidirectional Cellular Coverage 33

3.6 Cell Towers 34

3.7 How Are Cellular Networks Identified? 37

3.8 How Are IoT Devices Identified? 39

3.9 eNodeB IDs and Cell # 40

3.10 Tracking Areas and Paging 40

3.11 Frequency and Modulation 42

3.11.1 Modulation 43

3.11.1.1 Radio Telegraphy 43

3.11.1.2 Amplitude Modulation (AM Radio) 44

3.11.1.3 Frequency Modulation (FM Radio) 45

3.11.1.4 Phase Modulation 47

3.12 Spectral Efficiency 48

3.13 Error Detection 49

3.13.1 Luhn Algorithm 50

3.14 Error Correction 52

3.15 LTE Modulation Techniques 54

3.15.1 Binary Phase Shift Keying (BPSK) 56

3.15.2 Quadrature Phase Shift Keying (QPSK) 57

3.15.3 Quadrature Amplitude Modulation (QAM) 57

3.16 Bandwidth and Latency 57

3.17 Range 58

3.18 Frequency Bands 59

3.18.1 Frequency Affects Range 60

3.19 Radio Access Technologies (RAT) and Categories 61

3.20 SIM Cards 63

3.21 What Happens When a Cellular Modem Switches On? 64

3.21.1 Network Selection, Cell Selection, Camping, and Cell Reselection 64

3.21.2 Network Registration 65

3.22 Handoff (Also Called Handover) 67

3.22.1 Maintaining Connectivity 67
3.22.2 Load Balancing 69

3.23 Multiple Access 70

3.24 Timing Advance 70

3.24.1 Why Is Timing Advance Useful? 73

3.24.2 How Accurate Are Distance Estimates Using Timing Advance? 73

3.24.3 Timing Advance Band Depth and Maximum Range 77

3.25 Expressing Power 77

4 SIM/USIM Card Basics 81

4.1 Mobile Virtual Network Operators (MVNOs) 81

4.2 Size 82

4.3 Native Versus Multi-SIMs or MNO Versus MVNO 84

4.4 Home Versus Roaming Access 85

4.5 SIM Factors Affecting Price and Coverage 86

4.5.1 How Much Do SIM Cards Cost? 88

4.5.2 Is There a Monthly Activation Fee? 88

4.5.3 Are There Fees for Activating and/or Deactivating SIM Cards? 88

4.5.4 How Much Does Data Cost? 88

4.5.5 Is the Monthly Data “Pooled”? 89

4.5.6 Are There Fees for Deactivated (but Not Terminated) SIMs Cards? 90

4.5.7 Is There a Not-yet-activated Fee? 90

4.6 Text Messages (SMS) 91

4.7 Usage Limits 91

4.8 Firewalls 92

4.9 Replacing SIMs and/or Network Providers 94

4.10 Access Point Name (APN) 96

5 Verify Cellular Connectivity 97

5.1 Preparation 98

5.1.1 Adequate Power 98

5.1.2 Activated SIM Card 100

5.1.3 Base Station in Range 100

5.1.4 SIM Card Authorization 100

5.1.5 Band Configuration 100

5.1.6 RAT Configuration 101

5.1.7 Automatic Registration 101

5.2 Try to Auto-register 101

5.3 What Can Go Wrong? 102

5.3.1 Operating System Interference 102

5.3.2 Communicating with Modem 103

5.3.3 Malformed AT Commands 103

5.3.4 Parsing Responses to AT Commands 103

5.3.5 Timing Problems 104

5.3.6 Unset or Incorrect Access Point Name (APN) 107

5.3.6.1 Pitfall: Failing to Explicitly Set the APN 107

5.4 Modem Configuration for Auto-registration 108

6 Let’s Move Some Data 111

6.1 Low-level Sockets or High-level Protocols 112

6.2 Verify ServerServer Is Running 116

6.3 Verify EchoServer Is Running 117

6.4 USB or UART? 117

6.4.1 Pitfall: USB Device Names Are Not Fixed and Should Not Be Hard Coded 118

6.5 AT Commands—A Troubled Past 119

6.6 Unsolicited Response Codes (URCs) 120

6.7 A Handy Modem Program 123

6.8 AT Commands Manuals 130

6.9 Communicating with the Cellular Modem 131

6.10 Getting EchoServer Information from ServerServer 134

6.10.1 Before Step 1: Enable Unsolicited Response Codes (URCs) 135

6.10.2 Step 1: Configure PDP Context for HTTP GET 136

6.10.3 Step 2: Configure a PDP Context 136

6.10.3.1 Test Commands 137

6.10.3.2 Read Commands 137

6.10.3.3 Write Commands 137

6.10.4 Step 3: Activate the PDP Context 139

6.10.5 Step 4: Allow/Disallow Response Header for HTTP GET 140

6.10.6 Step 5: Set HTTP(S) URL 141

6.10.7 Step 6: Send HTTP(S) GET Request 141

6.10.8 Step 7: Read HTTP(S) Response 142

6.10.9 After Step 7: Deactivate the PDP Context 143

6.10.10 Using Modem to Interact with ServerServer 143

6.11 Bouncing Data Off EchoServer 145

6.11.1 Before Step 1: Enable Unsolicited Response Codes (URCs) 145

6.11.2 Step 4: Open a TCP Client Socket Connection 146

6.11.3 Step 5: Send Mixed-case Text to EchoServer 146

6.11.4 Step 6: Read (Uppercase) Response from EchoServer 148

6.11.5 Step 7: Close TCP Client Socket Connection 151

6.11.6 Step 8: Deactivate PDP Context 151

6.12 No Problems Is Bad Luck 151

7 Cellular Connectivity Regions 153

7.1 How Geography, Topology, and Population Density Affect Connectivity 154

7.1.1 Geography and Topology 154

7.1.2 Population Density 154

7.2 Region Categories 156

7.2.1 Rural 156

7.2.2 Rural Town 156

7.2.3 Flat Farmland/Flat Arid 156

7.2.4 Mountainous 157

7.2.5 Suburban 157

7.2.6 Dense City 157

7.2.7 Interstate Highway 157

7.2.8 Uninhabited 158

8 Cellular Communication Modules (CCMs) 159

8.1 CCM Worldwide Market Share 162

8.2 Frequency Band Usage 162

8.3 Protocol Implementation 164

8.4 Similarities and Differences Across CCMs 165

8.4.1 Single or Dual AT Command Channels 165

8.4.2 Different AT Command Sets 166

8.4.3 Different Response Times for Similar or Identical Commands 167

8.4.4 Differing Response Formats 167

8.4.5 Differing Responses for Compound Statements 167

8.4.6 Different Timing Requirements 168

8.4.7 AT Commands Are Not Thread-safe 168

8.4.8 Support for Different Protocols 168

8.5 Consider the Whole CCM Family 169

8.6 CCM Firmware Bugs 169

8.7 CCMs Are a Lot Like Sensors: Imprecise and Not Entirely Reliable 170

9 AT Commands (A New Approach) 171

9.1 Purpose of AT Commands 171

9.2 Problems of AT Commands 173

9.2.1 Maximum Response Time for an AT Command 174

9.3 Traditional Solution to Executing AT Commands and Extracting Responses 175

9.4 Command Independent Processing (CIP) 179

9.4.1 The Central Observation Underlying CIP 180

9.4.2 Fundamental Elements of CIP 181

9.4.2.1 AtParams 181

9.4.2.2 AtCommand 181

9.4.3 AT Commands in CIP 182

9.4.3.1 Step 1: Define a Name for a Command 183

9.4.3.2 Step 2: Create a Set of Parameters for Each Command 183

9.4.3.3 Step 3: Store the Command Name and AtParams Object in a Map 184

9.4.3.4 Step 4: Create a Command Object 185

9.4.3.5 Step 5: Pass Arguments to the Command Object (if Necessary) 185

9.4.3.6 Step 6: Perform the Command 185

9.4.3.7 Step 7: Verify Success or Failure 186

9.4.3.8 Step 8: Extract Response Information 187

9.4.3.9 AT Commands with Parameters 192

9.4.3.10 Timing Out 194

9.4.4 Using CIP Across CCM Families and Across Manufacturers 196

10 CIP Design and Details 197

10.1 Pseudocode Conventions 198

10.1.1 Identifier Names 198

10.1.2 Angle Brackets 198

10.1.3 Constructors 199

10.1.4 Dot Operator 199

10.1.5 Unified Modeling Language (UML) 199

10.2 A Note on Objected-orientation and Threads 199

10.3 AT Command Basics 200

10.3.1 Echoing 200

10.3.2 Enable/Disable Response Codes 201

10.3.3 Short or Long Response Codes 201

10.3.4 Line Terminators 201

10.3.5 Housekeeping 201

10.4 Categories of Responses to AT Commands 202

10.4.1 Ok_only 203

10.4.2 Text_ok 203

10.4.3 After_colon 203

10.4.4 Ok_plus_after_colon 204

10.4.5 MULTI_RECEIVE, MULTI_SEND, and Multi_after_colon 205

10.5 Details of Command Independent Processing (CIP) 205

10.5.1 AtStep Purpose 205

10.5.2 AtStep Attributes 207

10.6 A “Factory Method” for Creating AtCommand Objects 208

10.7 Performing AT Commands 210

10.7.1 Why AT Commands Fail 212

10.7.2 Timing Out 212

10.7.3 Details of the Execute Method 217

10.7.4 Response Length 219

10.7.5 Hardware Timing 220

10.7.6 Combining Parameter Settings—Method Chaining 221

10.7.7 Assessing Success and Multiple Tries 221

10.7.8 Multi-line AT Commands—AtStep 223

10.7.9 A Second Example with Regular Expressions 225

10.7.10 Integrating AtStep into the Execute Methods 227

10.8 AT Commands for Multiple Modems 228

10.8.1 The Simplest Case 230

10.8.2 Connectors 231

10.8.2.1 All Connectors Are Also Threads 234

10.8.2.2 Connectors Are Created Using a Factory Method 235

10.8.2.3 Custom AT Commands Are Added in Static Blocks of Connectors 236

10.8.2.4 Where to Override Methods or Parts of Methods 237

10.8.3 An Asymmetrical Case—AtParamsNoOp 239

11 Geographical Coverage, Signal Strength, and Quality 243

11.1 Radio Access Technologies (RATs) 243

11.2 Cellular Network Coverage Maps 245

11.3 Signal Strength and Quality: RSSI, RSRP, RSRQ , SINR 246

11.3.1 RSSI and RSRP 247

11.3.2 Rsrq 249

11.3.3 Sinr 249

11.3.4 Using Modem to Report Signal Strength and Quality 250

11.4 Antenna Selection and Performance 251

11.4.1 Antenna Size 253

11.4.2 Passive Versus Active Antennas 255

11.4.3 Antenna Connectors 256

11.4.4 Antenna Placement 257

11.5 Antenna Testing 258

11.6 Geography and Signal Strength Must Be Considered Together 259

12 Network Selection and Registration 261

12.1 Network Registration 261

12.2 Radio Access Technology (RAT) 262

12.3 Network Frequency Band Selection 264

12.4 PLMN Selection 266

12.4.1 Manual PLMN Selection 267

12.4.2 Automatic PLMN Selection 267

12.5 How to Create Your Own User Preference List 268

12.5.1 Reading the UPL and OPL 269

12.5.2 Modifying the UPL 271

12.6 Once a PLMN Is Auto-selected, Is It Always Selected? 273

12.7 Forcing the CCM Back to the PLMN Preference List 274

12.8 A Mysterious PLMN Selection Behavior 275

12.9 Troubleshooting Registration Problems 276

12.9.1 New Modem, Never Registered 276

12.9.2 Old Modem, Previously Registered 278

12.10 Anomalous Behavior 278

13 Communication Protocols TCP, UDP, and PPP 281

13.1 Internet Protocol 281

13.2 Transmission Control Protocol (TCP) 283

13.3 Considering Data Consumption 285

13.4 User Datagram Protocol (UDP) 285

13.5 TCP Pros and Cons 287

13.6 Point-to-point Protocol (PPP) 290

13.7 AT Commands for Data Transfer Are Completely Unstandardized 292

13.8 PPP on Linux 292

13.8.1 Debugging PPP 295

13.9 Alternatives to PPP 298

14 Thin Air 301

14.1 A Most Dramatic Case 302

14.1.1 Watching the Server 304

14.1.2 Packets Not Getting to the Server 305

14.2 What Was Going On? Thin Air 306

14.3 Why Did Thin Air Persist Over Hundreds of Miles? 308

14.4 How to Detect Thin Air 312

14.5 What to Do About Thin Air 313

14.6 Minimizing the Size of a Thin Air Region 313

14.7 A Hybrid UDP Protocol for Detecting Thin Air 314

14.8 Reducing (or Eliminating) Thin Air by PLMN or Band Selection 315

14.8.1 The Most Direct Approach 316

14.9 Putting the Hybrid Protocol to Second Use 322

15 Time and Location (GNSS) 325

15.1 Clarifying Terminology 325

15.2 Time 326

15.3 Location 328

15.4 Obtaining Time Information 331

15.4.1 Real-time Clock 331

15.4.2 Cellular Modem 332

15.4.2.1 Additional Configuration 333

15.4.2.2 Local Time or UTC 333

15.4.2.3 Daylight Saving Time 335

15.4.2.4 Using Modem to Read the Clock 336

15.4.3 Get Time from a GNSS Receiver 336

15.4.4 Get Time from a Server 337

15.5 Sources of Location Information 337

15.6 Pros and Cons of CCM’s GNSS Receiver Versus Stand-alone GNSS Receiver 338

15.7 Cold Start, Warm Start, Hot Start 339

15.8 Assisted GPS 340

15.9 GNSS Antenna Selection 340

15.10 GNSS Receiver Placement 341

15.11 GNSS Accuracy and Precision 342

15.11.1 Improving Accuracy 345

15.12 NMEA Sentences 346

15.12.1 Using Modem to Read GNSS Sentences 347

15.13 Three Ways to Obtain Location Information 348

15.13.1 Simple AT Command Request for Location 348

15.13.2 Read Streaming Data from gpsd 350

15.13.2.1 For a Stand-alone GNSS Receiver 350

15.13.2.2 For a CCM’s GNSS Receiver 351

15.13.3 Read Streaming Data Directly from CCM’s GNSS receiver 355

15.14 Understanding gpsd JSON Output 356

15.15 Writing Software to Capture and Process gpsd Output 358

15.16 GNSS Data Streamed from a CCM 359

15.17 Nmea 0183 359

15.17.1 Talker Sentence Format 360

15.17.1.1 GSA Sentence Format 361

15.17.1.2 RMC Sentence Format 362

15.17.1.3 GSV Sentence Format 362

15.17.2 NMEA Checksums 364

15.17.3 CCM GNSS Receivers Only Stream Some NMEA Sentences 365

15.18 Some Additional gpsd Utilities 366

16 Establishing and Maintaining a Cellular Connection 369

16.1 Modem Selection 370

16.2 Foundational Tasks 371

16.2.1 State 1: Detecting CCM 373

16.2.1.1 Using Modem to Detect a CCM 373

16.2.2 State 2a: Initializing CMM 374

16.2.2.1 Viewing Modem’s Initializations 374

16.2.3 State 2b: Waiting to Retry 376

16.2.4 State 3: Set Mobile Network Operator 376

16.2.5 State 4: Checking Registration Status 376

16.2.5.1 Using Modem to Check Registration Status 377

16.2.6 State 5: Connecting 378

16.2.7 State 6: Manage Connection 378

17 Sending and Receiving Text Messages (SMS) 379

17.1 Why Send/Receive Text Messages? 380

17.1.1 Need to “Push” Information to an IoT Device 380

17.1.2 Serverless IoT Devices That Interact with End-users 383

17.2 Cost of Text Messaging via Cellular Modem 383

17.3 Application-to-person (A2P) Messaging Is Often Regulated 384

17.4 Overview of Sending/Receiving Text Messages 385

17.5 Sending Text Messages 386

17.5.1 Set the Message Format 386

17.5.2 Set Parameters for Sending 386

17.5.3 Specify the Destination Phone Number and the Text to Send 389

17.5.4 What If Sending an SM Fails? 390

17.5.5 Using Modem to Send a Text Message 390

17.6 Receiving and Reading a Text Message 391

17.6.1 Configure the CCM 392

17.6.1.1 Set the Message Format 392

17.6.1.2 Configure SMS Storage 392

17.6.1.3 Check for a Received Text Message 394

17.6.1.4 Using Modem to List Text Messages 395

17.6.1.5 Delete a Text Message 396

17.6.1.6 Using Modem to Delete a Text Message 396

17.7 SMS with Constrained Devices 397

17.7.1 Set the Message Format 397

17.7.2 Set Parameters for Writing to Mem- 2 397

17.7.3 Specify the Destination Phone Number and Text to Store 397

17.7.4 Send a Text Message Already Stored in Mem- 2 398

17.7.5 Verifying a Text Message Was Sent from Mem- 2 399

17.8 Integrating SMS into CIP 400

18 Power Saving Modes and Techniques 403

18.1 What Are Low-power CCMs (LP-CCMs) 404

18.2 Plenty of Power, Most of the Time 405

18.3 Low-power IoT Devices 407

18.3.1 Microcontroller Energy Consumption 407

18.3.2 Temperature Sensor Energy Consumption 408

18.4 Battery Capacity 408

18.5 Transmitter Power 409

18.6 Legacy (GSM) Power Consumption 410

18.7 Cellular Modem Energy Consumption 413

18.7.1 Additional Energy Consumption 414

18.8 Network Registration States—RRC_CONNECTED and Rrc_idle 414

18.8.1 RRC_CONNECTED (Without DRX) 416

18.8.1.1 Scenario 1—Sending a Location Packet 417

18.8.1.2 Scenario 2—Fetching an Over-the-air Update 418

18.8.2 RRC_IDLE (Without DRX) 420

18.8.3 Discontinuous Reception (DRX) 422

18.8.3.1 Discontinuous Reception in RRC_IDLE (iDRX) 423

18.8.3.2 Discontinuous Reception in RRC_CONNECTED (cDRX) 424

18.8.4 Registration Characteristics Summary 426

18.9 Latency 427

18.10 Using Low-power CCM—Cat M and NB-IoT and Cat 1 bis 429

18.11 Power Saving Mode (PSM) 431

18.11.1 How to Enable PSM 434

18.11.1.1 Using Modem to Enter PSM 436

18.11.2 Verifying PSM and Possible Problems or Surprises 437

18.11.2.1 Using Modem to Check PSM Status 439

18.11.3 Actual PSM Cycle Length 439

18.11.4 Exiting PSM 440

18.11.4.1 Using Modem to Exit PSM 441

18.11.5 Sending Data from PSM Inactive 442

18.11.6 PSM Effectiveness 442

18.11.7 Integrating PSM into CIP 443

18.12 Extended Discontinuous Reception (eDRX) 445

18.12.1 How to Enable eDRX 447

18.12.1.1 Using Modem to Enable eDRX 448

18.12.2 Verifying eDRX Cycle Length 449

18.12.2.1 Using Modem to Check eDRX Status 450

18.12.3 Disabling eDRX 451

18.12.3.1 Using Modem to Disable eDRX 451

18.12.4 Integrating eDRX into CIP 451

18.13 When to Use PSM, eDRX, or Both 454

18.14 Don’t Trust the Numbers 454

A Unified Modeling Language (UML) Primer 455

A. 1 Assumptions 455

A. 2 UML Syntax 456

A. 3 Visibility (Private, Protected, Public) 457

A. 4 Attribute/Parameter/Method Names and Types 457

A. 5 Class Attributes and Methods 458

A. 6 Aggregation 459

A. 7 Multiplicities 459

A. 8 Inheritance 460

A. 9 Interfaces 460

A. 10 Hidden Attributes 461

A. 11 Layout 462

A. 12 State Diagrams 462

B 3GPP AT Commands Used in This Book 465

C The Modem Utility 469

C. 1 Invoking the Modem Program 470

C. 2 Flags 470

C. 3 Commands 471

Glossary 479

Closing Notes 485

Index 487