PSTN & Mobile Networks (2G-5G)

2. Mobile Network Evolution (2G → 5G)

2.1 Generational Timeline

The evolution of mobile networks follows a clear generational pattern, with each "G" representing a fundamental technological leap rather than incremental improvement. Each generation introduced new radio access technologies, modulation schemes, network architectures, and use cases that defined how people communicate and access information. The progression from 1G to 5G took approximately 40 years, with each generation having a lifecycle of about 10-15 years before being superseded.

What drives these generational shifts? Primarily, it's the insatiable demand for higher data speeds, lower latency, greater device density, and improved energy efficiency. Each generation required developing new international standards (like 3GPP specifications), allocating new radio spectrum, upgrading base station equipment, and deploying new core network infrastructure. Operators typically maintain multiple generations simultaneously during transition periods—today's networks often support 3G, 4G, and 5G concurrently, with devices automatically selecting the best available network.

Mobile Network Evolution Timeline 1G 1G 1980s Analog Voice AMPS, NMT ~2.4 kbps 2G 2G 1991 Digital Voice+SMS GSM, CDMA 64 kbps 3G 3G 2001 Mobile Internet UMTS, HSPA 2-42 Mbps 4G 4G LTE 2009 All-IP Network LTE, WiMAX 100-1000 Mbps 5G 5G NR 2019+ IoT, Ultra-Low Latency 5G NR 1-20 Gbps Key Technological Leap Analog → Digital → Broadband → All-IP → Ultra-Broadband + Low Latency

2.2 Detailed Generation Comparison

3. Cellular Network Architecture

3.1 Cellular Concept

Cellular networks divide geographic areas into cells, each served by a base station. This architecture enables frequency reuse, increasing network capacity while managing interference.

The cellular concept, pioneered by Bell Labs in the 1960s-70s, solved a fundamental problem: limited radio spectrum. Since radio frequencies are finite and valuable, how could networks serve millions of users? The solution was frequency reuse—using the same frequencies in geographically separated cells, with cell towers transmitting at lower power to prevent interference with distant cells using the same frequencies.

Cell towers don't actually create circular cells. The hexagonal cell pattern often shown in diagrams is an idealization. Real-world cells are irregular shapes determined by terrain, buildings, tower height, antenna configuration, and transmit power. Urban areas have small cells (macrocells 1-2 km radius, microcells 200-500m, picocells 50-200m, femtocells 10-50m) providing high capacity, while rural areas use larger macrocells (up to 35 km radius) for coverage over sparse populations.

Modern cellular networks use a three-layer architecture: Radio Access Network (RAN) consisting of base stations (eNodeB in 4G, gNodeB in 5G) and antennas that communicate with mobile devices over radio frequencies; Backhaul/Transport Network using fiber optics or microwave links to connect base stations to the core; and Core Network (EPC in 4G, 5GC in 5G) handling authentication, session management, mobility, policy enforcement, and connectivity to external networks like the Internet.

3.2 Core Network Components

4. Spectrum Bands & Frequencies

4.1 Frequency Spectrum Allocation

Different mobile generations use different frequency bands. Lower frequencies offer better coverage and penetration, while higher frequencies provide greater bandwidth and speed.

Radio spectrum is governed by physics: lower frequencies (below 1 GHz) have longer wavelengths that diffract around obstacles and penetrate buildings effectively, providing wide coverage areas but limited bandwidth. Higher frequencies (above 6 GHz) have shorter wavelengths carrying more data but requiring line-of-sight and struggling with obstacles. This creates a fundamental tradeoff between coverage and capacity.

Spectrum Licensing and Auctions: Governments allocate spectrum through regulatory bodies (FCC in USA, Ofcom in UK, etc.) via competitive auctions where operators bid billions of dollars for frequency licenses. The 2021 US C-band auction raised $81 billion, the largest in FCC history. Operators must carefully balance spectrum portfolio across low, mid, and high bands to deliver both coverage and capacity.

Licensed vs Unlicensed Spectrum: Most cellular networks use licensed spectrum with exclusive rights, preventing interference. However, technologies like LTE-U and 5G NR-U also leverage unlicensed spectrum (like WiFi's 2.4/5 GHz bands) to boost capacity. This coexistence requires sophisticated interference management through techniques like Listen Before Talk (LBT), where equipment senses the channel before transmitting.

4.2 5G Spectrum Strategy

5. Handoff & Roaming

5.1 Handoff (Handover) Process

Handoff is the process of transferring an ongoing call or data session from one cell to another as a mobile device moves. This ensures seamless connectivity without dropping the connection.

Handoff is one of the most complex and critical procedures in cellular networks. Without it, calls would drop every time you drove past a cell tower or walked between buildings. The network must balance competing objectives: handoff too early and you waste network resources with unnecessary transfers; handoff too late and calls drop when signal quality deteriorates beyond recovery.

Measurement and Decision: Mobile devices continuously measure signal strength and quality from their serving cell and neighboring cells, reporting these measurements to the network. The network analyzes these reports using handoff algorithms considering signal strength (RSRP - Reference Signal Received Power), signal quality (RSRQ - Reference Signal Received Quality), and signal-to-interference ratio. When measurements indicate a neighbor cell provides better service, the network initiates handoff.

Hard vs Soft Handoff: GSM and LTE use "hard handoff" (break-before-make) where the connection to the old cell terminates before establishing connection to the new cell, causing brief interruptions (typically 50-100ms). CDMA-based networks (including some 3G systems) use "soft handoff" (make-before-break) where devices maintain simultaneous connections to multiple cells during transition, improving reliability but requiring more network resources. 5G introduces sophisticated handoff procedures supporting ultra-low latency applications with sub-millisecond interruption times.

5.2 Roaming

Roaming allows mobile users to access services outside their home network by using visited networks. This involves authentication, billing, and inter-operator agreements.

Roaming is essential for mobile telecommunications, enabling the global connectivity we take for granted. When you travel internationally, your phone seamlessly connects to foreign networks without requiring new SIM cards or contracts. Behind this convenience lies complex technical and business arrangements between hundreds of network operators worldwide.

How Roaming Works: When your device powers on in a foreign country, it scans for available networks and selects one based on your home operator's roaming agreements. The visited network queries your home network's HLR/HSS (subscriber database) through SS7 or Diameter signaling to authenticate your identity and retrieve your service profile. Once authenticated, a temporary record is created in the visited network's VLR, and you can make calls, send messages, and use data.

Roaming Agreements and Wholesale Rates: Operators establish bilateral or multilateral roaming agreements defining technical parameters and commercial terms. When you use data while roaming, your home operator pays the visited operator a wholesale rate (typically per MB or per minute for calls), then charges you retail roaming rates. This multi-layered pricing explains why roaming was historically expensive, though EU regulations and competitive pressure have dramatically reduced costs in many markets.

Roaming Steering and Optimization: Your device doesn't necessarily connect to the network with the strongest signal. Home operators can steer devices toward preferred roaming partners offering better wholesale rates or quality. This steering occurs through preferred network lists in the SIM card and over-the-air updates. Advanced systems analyze real-time quality metrics and costs to optimize roaming partner selection automatically.

6. 5G Architecture Deep Dive

6.1 5G Network Slicing

Network slicing is a revolutionary 5G feature that allows operators to create multiple virtual networks on a single physical infrastructure. Each slice is customized for specific use cases with dedicated resources, SLAs, and performance characteristics.

6.2 5G vs 4G LTE Comparison

6.3 Key 5G Technologies

7. Real-World Applications & Future

7.1 Industry Use Cases

7.2 Future: 6G and Beyond