---
title: "Evolution from 1G to 2.5G (TDMA)"
subject: "Wireless Communication"
chapter: 1
key: "semester_7/wireless_communication:evolution-1g-2g-2-5g-tdma"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/evolution-1g-2g-2-5g-tdma
site: BCT Engineering
---

# Evolution from 1G to 2.5G (TDMA)

Wireless Communication · Chapter 1: Introduction

- Asked in: 2071 Magh, 4 marks
- Video: [1.2 - EVOLUTION OF COMMUNICATION -1G TO 4G & Towards 5G](https://www.youtube.com/watch?v=NUovkXWe15s) — LTE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/evolution-1g-2g-2-5g-tdma
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-1

## Question

Discuss the evolution from 1G to 2G, 2.5G in the case of cellular network based on TDMA.

## Short answer

1G (AMPS) used analog FDMA. 2G TDMA systems like GSM (8 slots per 200 kHz carrier) and IS-136 (3 slots per 30 kHz) provided digital voice and SMS. 2.5G evolved with GPRS packet data (up to ~171 kbps) and EDGE (up to 384 kbps) for always-on internet.

## Answer

The TDMA path of cellular evolution started from analog FDMA systems and led to GSM and IS-136, which were then upgraded for packet data.

#### 1G (analog FDMA)
- AMPS (USA), TACS (UK), NMT; analog FM voice on 30/25 kHz channels, one user per channel, circuit switched, no security, voice only.

#### 2G (digital TDMA)
- **GSM** (Europe): 200 kHz carriers, each divided into **8 time slots** (8 users per carrier), GMSK modulation, 9.6 kbps data, SMS, SIM, encryption.
- **IS-136 (USDC/D-AMPS)** (USA): keeps 30 kHz AMPS channels but puts **3 users** per channel using π/4-DQPSK; backward compatible with AMPS.
- **PDC** (Japan): TDMA, 25 kHz.

#### 2.5G (data upgrades on TDMA)
- **HSCSD** (High Speed Circuit Switched Data): one user gets up to 4 consecutive time slots; up to 57.6 kbps; circuit switched.
- **GPRS** (General Packet Radio Service): packet-switched, users share slots, up to 8 slots per user, up to ~171.2 kbps; adds SGSN and GGSN to the core; always-on internet.
- **EDGE** (Enhanced Data rates for GSM Evolution): 8-PSK modulation and adaptive coding on GSM carriers; up to ~384 kbps; also used by IS-136 operators to join the GSM path.

```text
AMPS ──→ IS-136 ───────┐
                       ├──→ EDGE ──→ 3G W-CDMA
TACS/NMT → GSM → HSCSD → GPRS ─┘
  1G        2G       2.5G
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does GSM share a 200 kHz carrier?
   A. Among 8 time slots, 8 users
   B. Among 3 users
   C. One user only
   D. By codes
   Answer: A. GSM divides each carrier into 8 time slots. (Key point: GSM structure)

2. What is notable about IS-136?
   A. It keeps 30 kHz AMPS channels with 3 users per channel using pi/4-DQPSK
   B. It uses 200 kHz carriers
   C. It is CDMA
   D. It is analog
   Answer: A. This gives backward compatibility with AMPS. (Key point: IS-136)

3. How do HSCSD and GPRS differ?
   A. HSCSD is packet switched, GPRS circuit switched
   B. HSCSD is circuit switched with up to 4 slots; GPRS is packet switched sharing slots, up to ~171 kbps
   C. Both are circuit switched
   D. Both are packet switched using 8-PSK
   Answer: B. GPRS adds SGSN/GGSN and is always-on. (Key point: HSCSD vs GPRS)

4. How does EDGE raise data rate?
   A. 8-PSK modulation and adaptive coding giving ~384 kbps
   B. Wider 5 MHz carriers
   C. OFDM
   D. Soft handoff
   Answer: A. EDGE improves modulation on existing GSM carriers. (Key point: EDGE)

## Related in Wireless Communication

- [Evolution of generations of cellular systems](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aevolution-cellular-generations) — Wireless Communication, chapter 1, asked 2×
- [Features introduced from 2G to 5G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afeatures-2g-to-5g) — Wireless Communication, chapter 1
- [Evolution of mobile communication up to 3G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aevolution-up-to-3g) — Wireless Communication, chapter 1, asked 3×
- [Advantages of TDMA over FDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atdma-advantages-over-fdma) — Wireless Communication, chapter 7
- [Improvements in 2G, 3G and beyond 3G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aimprovements-2g-3g-beyond) — Wireless Communication, chapter 1

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