GSM Basics: Overview

GSM (Global System for Mobile Communications) is the most widely used cellular standard in the world, originally designed for 2G voice and SMS, and later extended to data (GPRS/EDGE).

What is GSM?

  • Digital cellular standard for mobile voice and data

  • Launched in Europe in 1991; adopted globally

  • 900/1800 MHz (EU/Asia), 850/1900 MHz (Americas)

Core Concepts

  • Cellular network: Divides coverage into “cells,” each served by a Base Station

  • SIM card: Holds subscriber identity and keys

  • Voice, SMS, low-rate data: Core services (with later upgrades for Internet access)

GSM Architecture

  • MS (Mobile Station): Your phone (hardware + SIM)

  • BTS (Base Transceiver Station): “Cell tower” – handles radio link with phones

  • BSC (Base Station Controller): Manages multiple BTS, handles handover and resources

  • MSC (Mobile Switching Center): Switches calls/SMS, manages mobility and authentication

  • HLR/VLR (Home/Visitor Location Register): Databases for subscriber info and roaming

  • AUC: Authentication Center (manages SIM/IMSI/Ki)

  • EIR: Equipment Identity Register (blocks stolen phones)


\[MS] ⇄ \[BTS] ⇄ \[BSC] ⇄ \[MSC] ⇄ \[HLR/VLR]

Frequency & Modulation

  • TDMA: Time Division Multiple Access (multiple users per frequency, via timeslots)

  • GMSK: Gaussian Minimum Shift Keying (modulation for efficient, robust RF)

Security & SIM

  • Each SIM has a unique IMSI and a secret Ki key

  • Authentication: Uses A3/A8 algorithms, with session keys (Kc) for each call

Radio Channels & Timeslots

  • Frequency bands: GSM900, 1800, 850, 1900 MHz

  • Channel = carrier frequency (200 kHz wide) divided into 8 timeslots (TDMA)

  • One user = one timeslot per frame (except for control channels or “packet” data)

Channel Types

  • Traffic Channels (TCH): Carry voice or user data

  • Control Channels (CCH):

    • BCCH: Broadcast Control (network info, cell parameters)

    • CCCH: Common Control (paging, access grant)

    • DCCH: Dedicated Control (setup, authentication, SMS signaling)

Protocol Stack

  • Layer 1 (Physical): Handles modulation, RF transmission (GMSK)

  • Layer 2 (Data Link): LAPDm – error correction, framing

  • Layer 3 (Network): RR/MM/CC – radio resource management, mobility, call control

Call Flow (Simplified)

  1. Power on → Phone scans BCCH, selects best cell

  2. Registration → Sends IMSI, network authenticates SIM

  3. Call/SMS → Signaling on DCCH, voice/data on TCH

  4. Handover → If moving, network switches BTS/BSC

  5. Encryption → Stream cipher (A5/1 etc.) enabled after authentication

Packet Data

  • GPRS/EDGE: Uses extra timeslots for data (“packet channels”), handled by SGSN/GGSN in the core

  • Data rates: From ~9.6 kbps (GSM) to 236 kbps (EDGE)

What is the role of SS7 in GSM?

SS7 (Signaling System No. 7) is the universal signaling protocol used in the core of telecom networks (both fixed and mobile).

In GSM, SS7 is fundamental for all network management, subscriber handling, roaming, and service operations.

###Key functions of SS7 in GSM:**

  1. Mobility Management / Roaming

    • SS7 allows the network to track the location of each subscriber—even when roaming internationally.

    • SS7 messages update the HLR/VLR (Home/Visitor Location Registers) whenever a user moves to a new area.

  2. Authentication and Security

    • SS7 is used to exchange authentication messages between MSC (Mobile Switching Center), HLR, and AUC (Authentication Center).

    • It handles key transfer, challenge/response during mobile registration.

  3. Call Setup and Routing

    • All call setup, routing, and release (voice, SMS) is managed through SS7 signaling.

    • Smart routing: SS7 ensures the call/SMS reaches the right destination even if the subscriber moves or uses services like call forwarding.

  4. SMS Delivery

    • SMS messages are delivered over SS7 signaling, passing between MSC, SMSC (Short Message Service Center), HLR, etc.

  5. Advanced Services

    • Handles short codes, value-added services (e.g., call forwarding, voicemail, USSD, etc.).

Examples of SS7 messages in GSM

  • MAP (Mobile Application Part): handles subscriber location, authentication, status transfer.

  • ISUP (ISDN User Part): manages voice call setup.

  • SCCP (Signaling Connection Control Part): advanced routing for signaling messages.

Why is SS7 critical in GSM?

  • It’s the “language” used between all major core network components (MSC, HLR, VLR, SMSC, AUC, EIR, etc.).

  • Without SS7, there would be no roaming, no subscriber management, and no automatic routing.

  • It enables global mobility and real-time management of millions of subscribers for operators.

In summary

SS7 in GSM = the “nervous system” of the mobile network, coordinating all behind-the-scenes operations—from simple calls and SMS to international roaming and security.