Each generation is a new radio and a new core, built to fix the last one's limits. Learn them as a ladder.
The ladder at a glance
| Gen | Era | Radio tech | Switching | Peak speed (typical) | Headline feature |
|---|---|---|---|---|---|
| 0G / 1G | 1980s | AMPS, NMT, TACS (analogue, FDMA) | Circuit | none (voice only) | First mobile phones, no encryption |
| 2G | 1991 | GSM (TDMA+FDMA), IS-95 / cdmaOne (CDMA) | Circuit | 9.6–14.4 kbit/s | Digital voice, SMS, SIM card, encryption |
| 2.5G | ~2000 | GPRS | Packet added | ~40–115 kbit/s | First mobile data (always-on) |
| 2.75G | ~2003 | EDGE (8-PSK) | Packet | ~236–384 kbit/s | Faster data on GSM |
| 3G | 2001 | UMTS / WCDMA, CDMA2000 | CS + PS | 384 kbit/s – 2 Mbit/s | Video calls, mobile internet, MMS |
| 3.5G / 3.75G | ~2006 | HSDPA / HSUPA / HSPA / HSPA+ | CS + PS | 7–42 Mbit/s (HSPA+) | Mobile broadband |
| 4G | 2009–10 | LTE (OFDMA down, SC-FDMA up), LTE-Advanced | Packet only | 100 Mbit/s – 1 Gbit/s | All-IP, VoLTE, carrier aggregation |
| 5G | 2019 | NR (New Radio, OFDM) | Packet only, service-based core | 1–20 Gbit/s (peak) | eMBB, URLLC, mMTC, slicing |
| 5G-Advanced | 2024+ (3GPP Rel-18/19) | NR evolution | AI/ML in the network, better uplink, RedCap, NTN | ||
| 6G | ~2030 (research) | sub-THz, AI-native | target Tbit/s | ISAC (sensing), AI-native, satellite integration |
3GPP (3rd Generation Partnership Project) writes the standards in Releases: Rel-99 (3G) · Rel-5 (HSDPA, IMS) · Rel-8 (LTE, EPC) · Rel-10 (LTE-A) · Rel-15 (first 5G) · Rel-16/17 (5G phase 2/3) · Rel-18 (5G-Advanced). ITU-R / IMT-2000, IMT-Advanced, IMT-2020 define what may be called 3G / 4G / 5G. GSMA is the operators' industry body.
2G — GSM in detail
Architecture
Phone (MS+SIM) — BTS — BSC — MSC/VLR — GMSC — PSTN / other PLMN
| |
(A-bis) HLR / AuC / EIR / SMSC
| Node | Role |
|---|---|
| MS (Mobile Station) = ME (Mobile Equipment) + SIM | The phone and its identity card |
| BTS | Base Transceiver Station — the antenna site |
| BSC | Base Station Controller — manages many BTSs, handovers, radio channels |
| BSS | Base Station Subsystem = BTS + BSC |
| TRAU / TC | Transcoder (speech ⇄ 64 kbit/s) |
| MSC / VLR | Switching + visitor database |
| HLR / AuC / EIR | Home database / authentication keys / equipment blacklist (IMEI) |
| SMSC | SMS centre |
| NSS | Network Switching Subsystem = MSC + VLR + HLR + AuC + EIR |
| OMC / NMS | Operations & Maintenance Centre — monitoring |
Interfaces: Um (air), A-bis (BTS–BSC), A (BSC–MSC), B (MSC–VLR), C (MSC/GMSC–HLR), D (VLR–HLR), E (MSC–MSC), F (MSC–EIR), Gs (SGSN–MSC for combined attach).
Radio basics (GSM)
- Frequencies: 900 / 1800 MHz (Europe), 850/1900 (Americas). Channel width 200 kHz.
- FDMA + TDMA: frequency split into carriers, each carrier into 8 time slots (about 4.6 ms frame). A voice call uses one slot.
- Frequency hopping against interference. Timing advance (TA) compensates distance — max range ~35 km.
- Channels: BCCH (broadcast), CCCH/RACH/PCH/AGCH (common), SDCCH (signalling, and SMS), SACCH, TCH (traffic).
- Cells and Location Areas: a cell = one antenna sector; a LA / LAI groups cells so paging is done only there. Moving between LAs triggers a Location Update.
- Handover: the BSC (or MSC) moves a call between cells (intra-BSC, inter-BSC, inter-MSC).
- Authentication (A3/A8): the SIM answers a random challenge (RAND → SRES) using key Ki; Kc (cipher key) is derived. A5/1, A5/2, A5/3 encrypt the radio link (A5/1 is breakable).
- Triplets: RAND + SRES + Kc, delivered by the AuC → HLR → VLR.
GPRS / EDGE data (the 2.5G packet overlay)
| Node | Role |
|---|---|
| PCU | Packet Control Unit — in/near the BSC, handles packet radio |
| SGSN | Serving GPRS Support Node — the "VLR/MSC" for data |
| GGSN | Gateway GPRS Support Node — the exit to the internet / corporate networks |
| Gn / Gp / Gi | SGSN–GGSN / roaming (inter-PLMN) / GGSN–internet interfaces |
| GTP | GPRS Tunnelling Protocol — carries user data between SGSN and GGSN |
| APN | Access Point Name — selects which network/service the data session uses (e.g. internet, ims, corp.example) |
| PDP context | The data session: IP address + QoS + tunnel |
EDGE = Enhanced Data rates for GSM Evolution; adds 8-PSK modulation to triple the bit rate in the same radio.
2G variants
- CDMA / IS-95 / cdmaOne — US/Asia rival to GSM. CDMA2000 1x / EV-DO are its successors. No SIM (RUIM), spread-spectrum.
- iDEN, TETRA, GSM-R — private/professional mobile radio; GSM-R is railway GSM; TETRA is used by police and emergency services.
3G — UMTS in detail
Why
2G was built for voice. 3G aimed for multimedia and real data with WCDMA (Wideband Code Division Multiple Access, 5 MHz channels).
Architecture
Phone (UE+USIM) — NodeB — RNC — MSC/VLR (voice, SMS) — HLR/AuC
└ SGSN — GGSN (data) — Internet
| 3G term | 2G equivalent | Note |
|---|---|---|
| UE (User Equipment) / USIM | MS / SIM | |
| NodeB | BTS | Radio site |
| RNC (Radio Network Controller) | BSC | Handles soft handover, power control |
| UTRAN | BSS | UMTS Terrestrial Radio Access Network |
| Core: MSC/VLR, GMSC, SGSN, GGSN, HLR | same | Core reused from 2G/GPRS |
| MGW / MSC-Server | MSC | R4+ splits MSC-Server (control) and MGW (media) |
| Interfaces | Uu air, Iub NodeB–RNC, Iur RNC–RNC, Iu-CS RNC–MSC, Iu-PS RNC–SGSN |
Radio concepts
- CDMA: all users share the same frequency, separated by spreading codes (scrambling + channelisation).
- Soft handover — the phone is connected to two NodeBs at once; no break.
- Power control — fast closed-loop control keeps everyone at the lowest power (otherwise the loudest user drowns the rest — the "near-far problem").
- Cell breathing — as load rises, coverage shrinks.
- Security: mutual authentication (the network also proves itself) using AKA (Authentication and Key Agreement): quintets RAND, XRES, CK, IK, AUTN. KASUMI (UEA1 / UIA1) cipher.
- HSPA family: HSDPA (fast download, 14 Mbit/s), HSUPA (fast upload), HSPA+ (MIMO, 64QAM, dual-carrier, up to 42 Mbit/s).
- Other 3G flavours: TD-SCDMA (China), CDMA2000 EV-DO (US).
Sunset
Operators are shutting down 2G and 3G to re-use spectrum for 4G/5G — but 2G/3G devices (smart meters, alarms, car modems) and old SS7 infrastructure keep SS7 alive longer than expected. Many "VoLTE-only" networks keep 2G just for roamers.
Remember this: 2G = digital voice + SMS. 2.5G/EDGE = first data. 3G = WCDMA + HSPA mobile broadband with the same core logic. 4G is the break: no circuit domain at all.