---
title: "GSM system architecture"
subject: "Wireless Communication"
chapter: 8
key: "semester_7/wireless_communication:gsm-architecture"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-architecture
site: BCT Engineering
---

# GSM system architecture

Wireless Communication · Chapter 8: Wireless Systems and Standards

- Asked in: 2080 Baisakh, 8 marks; 2079 Bhadra, 8 marks; 2073 Magh, 7 marks; 2070 Magh, 8 marks (asked 4 times)
- Video: [GSM Architecture | MS, BTS, BSC, MSC | VLR, HLR, AuC, EIR, OMC | BSS, NSS, OSS | Mobile Computing](https://www.youtube.com/watch?v=HIQ8Z2xlCS8) — Dr. Ankit Verma
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-architecture
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8

## Question

Draw and explain the GSM system architecture, mentioning the operation and functions of each component (block) in the architecture.

## Short answer

GSM architecture comprises Mobile Station (ME+SIM), Base Station Subsystem (BTS, BSC, TRAU), Network and Switching Subsystem (MSC, GMSC, HLR, VLR, AuC, EIR), and OMC. The BSS manages radio transmission, handoff, and transcoding; the NSS handles call switching, subscriber location, and security; OMC performs monitoring and maintenance.

## Answer

GSM (Global System for Mobile communications) is a 2G digital cellular system using TDMA/FDMA with 200 kHz carriers (8 time slots each), GMSK modulation and FDD (890–915 MHz uplink, 935–960 MHz downlink for GSM-900). Its architecture has four parts: **Mobile Station (MS), Base Station Subsystem (BSS), Network and Switching Subsystem (NSS) and Operation Support Subsystem (OSS).**

```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
```

#### 1. Mobile Station (MS)
- **Mobile Equipment (ME)**: the handset hardware – radio, speech codec, display; identified by the **IMEI**.
- **SIM card**: holds the **IMSI**, secret key **Ki**, A3/A8 algorithms, PIN and user data. Any ME works with any SIM, so the subscription is portable.

#### 2. Base Station Subsystem (BSS)
- **BTS (Base Transceiver Station)**: radio transceivers and antennas for one cell; does modulation/demodulation, channel coding, interleaving, encryption, frequency hopping and measurements over the **Um** (air) interface.
- **BSC (Base Station Controller)**: controls many BTSs over the **Abis** interface; allocates radio channels, sets transmit power, manages frequency hopping and handles handoffs between its BTSs.
- **TRAU (Transcoder and Rate Adaptation Unit)**: converts 13 kbps GSM speech to 64 kbps PCM for the MSC.

#### 3. Network and Switching Subsystem (NSS)
Connected to the BSC by the **A interface**.
- **MSC (Mobile Switching Centre)** – the central switch. Sets up, routes and releases calls; handles handoff between BSCs/MSCs, billing records, and mobility management. Works with HLR/VLR to locate users.
- **GMSC (Gateway MSC)** – the MSC that connects the GSM network to PSTN/ISDN/other networks; for incoming calls it asks the HLR where the subscriber is.
- **HLR (Home Location Register)** – permanent database of every subscriber of the operator: IMSI, MSISDN, subscribed services, and the current VLR/MSC area of the user.
- **VLR (Visitor Location Register)** – temporary database attached to an MSC; holds data of subscribers currently roaming in its area (copied from HLR), TMSI and location area. Reduces queries to the HLR.
- **AuC (Authentication Centre)** – stores each subscriber's secret key Ki and runs the A3/A8 algorithms to generate authentication triplets (RAND, SRES, Kc) for authentication and ciphering.
- **EIR (Equipment Identity Register)** – list of IMEI numbers: white (allowed), grey (monitored) and black (stolen/blocked) handsets.

#### 4. Operation Support Subsystem (OSS)
- **OMC (Operation and Maintenance Centre)**: monitors and configures all network elements; handles fault, performance, configuration, security and billing management, and subscriber administration.

#### Interfaces
| Interface | Between | Purpose |
|---|---|---|
| Um | MS – BTS | Air interface (TDMA, GMSK) |
| Abis | BTS – BSC | Traffic and control links |
| A | BSC – MSC | Speech/data and signalling |
| SS7 (MAP) | MSC–HLR/VLR/EIR | Mobility database queries |

**Call flow example (mobile terminated)**: PSTN → GMSC asks HLR → HLR asks current VLR for roaming number → GMSC routes to the serving MSC → MSC pages the MS via BSC/BTS → authentication with AuC data → channel assignment → call connected.

## Check yourself

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

1. Which are the four parts of GSM architecture?
   A. MS, BSS, NSS and OSS
   B. MS, BTS, HLR, EIR
   C. BSC, MSC, VLR, AuC
   D. SIM, ME, BTS, PSTN
   Answer: A. GSM = Mobile Station, Base Station Subsystem, Network and Switching Subsystem, Operation Support Subsystem. (Key point: GSM subsystems)

2. What does the SIM hold?
   A. IMEI only
   B. The IMSI, secret key Ki, A3/A8 algorithms and PIN
   C. The HLR database
   D. Radio transceivers
   Answer: B. The SIM stores subscriber identity so any ME works with any SIM; the IMEI belongs to the equipment. (Key point: SIM role)

3. What distinguishes HLR from VLR?
   A. HLR is temporary; VLR is permanent
   B. HLR identifies stolen handsets
   C. VLR stores the secret key
   D. HLR is the permanent subscriber database; VLR is a temporary database for subscribers roaming in an MSC area
   Answer: D. VLR holds copied data for visitors, reducing HLR queries. (Key point: HLR vs VLR)

4. What do AuC and EIR do?
   A. AuC stores Ki and generates triplets for authentication; EIR lists white/grey/black IMEIs
   B. AuC lists IMEIs; EIR stores Ki
   C. Both route calls
   D. Both perform transcoding
   Answer: A. AuC handles authentication and ciphering keys; EIR controls equipment validity. (Key point: AuC and EIR)

5. What does the TRAU do?
   A. Pages the mobile
   B. Converts 13 kbps GSM speech to 64 kbps PCM for the MSC
   C. Stores subscriber data
   D. Connects to the PSTN
   Answer: B. Transcoding and rate adaptation between GSM codec and PCM. (Key point: TRAU function)

## Related in Wireless Communication

- [GSM and CDMA standards; GSM architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-cdma-standard-architecture) — Wireless Communication, chapter 8
- [GSM BCH and CCCH operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-bch-ccch) — Wireless Communication, chapter 8
- [CDMA vs LTE system architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-vs-lte-architecture) — Wireless Communication, chapter 8
- [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
- [GSM control channels and their purposes](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-control-channels) — Wireless Communication, chapter 8, asked 2×
- [Numerical: rms delay spread; flat or selective for AMPS/GSM](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Arms-delay-spread-amps-gsm) — Wireless Communication, chapter 3

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