---
title: "GSM and CDMA standards; GSM architecture"
subject: "Wireless Communication"
chapter: 8
key: "semester_7/wireless_communication:gsm-cdma-standard-architecture"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-cdma-standard-architecture
site: BCT Engineering
---

# GSM and CDMA standards; GSM architecture

Wireless Communication · Chapter 8: Wireless Systems and Standards

- Asked in: 2071 Magh, 4+4 marks
- Video: [GSM Architecture (Mobile Communication / Computation) Easiest Explanation Ever in Hindi](https://www.youtube.com/watch?v=oBiGDhnRl8M) — 5 Minutes Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-cdma-standard-architecture
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8

## Question

What is GSM and CDMA standard? Explain the architecture of GSM.

## Short answer

GSM is a 2G TDMA/FDMA standard with 200 kHz carriers, 8 slots, GMSK. CDMA (IS-95) is a 2G DS-CDMA standard with 1.25 MHz carriers. GSM architecture comprises the MS (ME+SIM), BSS (BTS, BSC), NSS (MSC, VLR, HLR, AuC, EIR), and OSS for network management.

## Answer

#### GSM and CDMA standards
- **GSM (Global System for Mobile communications)**: 2G digital cellular standard from ETSI (Europe, 1991). Uses **FDMA/TDMA** with 200 kHz carriers, 8 time slots per carrier, **GMSK** modulation, FDD (890–915 / 935–960 MHz in GSM-900), RPE-LTP 13 kbps speech, SIM-based subscriber identity, hard handoff. It became the most widely used 2G system (in Nepal, used by Nepal Telecom and Ncell).
- **CDMA (IS-95 / cdmaOne)**: 2G standard from Qualcomm/TIA (USA, 1993). Uses **DS-CDMA**: all users share a **1.25 MHz** carrier at the same time and are separated by codes (64 Walsh codes on forward link, long PN codes on reverse link), chip rate 1.2288 Mcps. Features frequency reuse 1, soft handoff, fast power control and RAKE receivers. It evolved to CDMA2000.

| Point | GSM | CDMA |
|---|---|---|
| Access | FDMA/TDMA | DS-CDMA |
| Bandwidth | 200 kHz | 1.25 MHz |
| Handoff | Hard | Soft |
| Reuse | N = 3–7 | N = 1 |

#### Architecture of GSM
```text
       Um          Abis          A
 [MS]~~~~~~[BTS]-------[BSC]-------[MSC]----[GMSC]--PSTN/
 ME+SIM     [BTS]------'  (TRAU)      |  |      ISDN
                                      |  +--[VLR]
 |<---- BSS ---------------->|        +--[HLR]--[AuC]
                                      +--[EIR]
                      |<-------- NSS -------->|
                [OMC] (OSS) monitors BSS & NSS
```

- **MS** = Mobile Equipment (IMEI) + **SIM** (IMSI, Ki).
- **BSS**: **BTS** – radio transceivers of a cell (Um interface); **BSC** – controls many BTSs (Abis), channel allocation, power control, handoff; TRAU – speech transcoding.
- **NSS** (via A interface): **MSC** – call switching, handoff, billing; **GMSC** – link to PSTN; **HLR** – permanent subscriber database; **VLR** – temporary data of visiting users; **AuC** – authentication and ciphering keys; **EIR** – IMEI check (stolen phones).
- **OSS**: **OMC** – operation, maintenance and network management.

## Check yourself

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

1. What does the BSC do?
   A. Controls many BTSs: channel allocation, power control, handoff
   B. Stores IMEI lists
   C. Links to PSTN
   D. Holds SIM key
   Answer: A. The BSC sits between BTS (Abis) and MSC (A). (Key point: BSC)

2. What does the MS consist of?
   A. Mobile equipment (IMEI) and SIM (IMSI, Ki)
   B. BTS and BSC
   C. MSC and VLR
   D. HLR and AuC
   Answer: A. Subscriber identity is in the SIM. (Key point: SIM contents)

3. Which entities form the NSS?
   A. BTS, BSC, TRAU
   B. OMC only
   C. MSC, GMSC, HLR, VLR, AuC, EIR
   D. MS, BTS
   Answer: C. BSS is BTS/BSC/TRAU; OMC belongs to OSS. (Key point: Subsystems)

## Related in Wireless Communication

- [GSM frame structure; GSM vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-vs-cdma) — Wireless Communication, chapter 8
- [GSM system architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-architecture) — Wireless Communication, chapter 8, asked 4×
- [CDMA vs LTE system architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-vs-lte-architecture) — Wireless Communication, chapter 8
- [CDMA merits/demerits; GSM frame efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-cdma-frame-efficiency) — Wireless Communication, chapter 7
- [GSM NSS components and frame hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-nss-frame-hierarchy) — Wireless Communication, chapter 8

## Related in other subjects

- [Multiplexing techniques and TDM standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-techniques-tdm-standards) — Telecommunications, chapter 3
- [Standardization levels and formulation of standards](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Alevels-of-standardization) — Metrology, chapter 3