What Was GSM?

GSM — Global System for Mobile Communications — was the digital mobile standard that replaced the analogue 1G networks of the 1980s. Launched commercially in Finland in 1991, GSM went on to become the world's dominant mobile standard, used by billions of subscribers across more than 200 countries at its peak.

Understanding how GSM worked isn't just historical curiosity — it illuminates how every subsequent mobile generation was built, and why 2G was such a radical leap forward from what came before.

The Core Problem: Sharing the Airwaves

Radio spectrum is a finite resource. You can't have thousands of people in a city all transmitting on the same frequency simultaneously — they'd drown each other out. GSM solved this with a combination of two techniques:

  • FDMA (Frequency Division Multiple Access): The available spectrum is divided into channels, each 200 kHz wide. Different conversations use different frequency channels.
  • TDMA (Time Division Multiple Access): Each 200 kHz channel is further divided into eight time slots. Up to eight calls share the same frequency channel, each taking turns in rapid succession — so fast that users experience a continuous, uninterrupted call.

The Cell: Why It's Called "Cellular"

A GSM network is divided into geographic areas called cells, each served by a Base Transceiver Station (BTS) — what most people call a mobile mast. Crucially, the same frequencies can be reused in non-adjacent cells without interference. This frequency reuse is what allows a network to serve an entire country with a limited amount of spectrum.

As you move between cells, your call is handed off seamlessly from one BTS to the next — a process called a handover. In GSM, this happened so smoothly that most users never noticed it.

The Network Architecture

Beyond the masts, a GSM network had several key components working together:

  1. Base Station Controller (BSC): Manages a group of BTS masts, handling handovers and radio resource management.
  2. Mobile Switching Centre (MSC): The core of the network — routes calls, connects to the public telephone network, and manages mobility.
  3. Home Location Register (HLR): A central database holding subscriber data — your phone number, services enabled, and current location.
  4. Visitor Location Register (VLR): A temporary database held by the MSC you're currently connected to, storing your details while you're in that area.
  5. Authentication Centre (AuC): Holds your SIM's secret key and handles the challenge-response authentication that proved your identity to the network.

How a Call Actually Connected

When you dialled a number on a 2G phone, the following happened in fractions of a second:

  1. Your handset broadcast a channel request to the nearest BTS.
  2. The network authenticated your SIM using a cryptographic challenge.
  3. The MSC looked up the dialled number's HLR to find where the destination handset was registered.
  4. A circuit was established end-to-end, allocating a time slot on the radio channel and a path through the switching network.
  5. The destination phone rang.

SMS: The Feature Nobody Expected to Matter

Short Message Service was designed as an engineering afterthought — a way to send brief text notifications using the signalling channel that was already active between your phone and the network. Nobody predicted it would become one of the most commercially important features in telecommunications history.

SMS messages in GSM were routed through a Short Message Service Centre (SMSC), which stored and forwarded messages. If your phone was off, the SMSC held the message until you came back online — an early form of asynchronous messaging that we now take for granted.

Why GSM Mattered

Before GSM, mobile calls were analogue and trivially easy to intercept with a radio scanner. GSM introduced encryption over the air interface — a fundamental security improvement. It also standardised roaming, meaning a single phone could work in multiple countries. These weren't technical footnotes; they were transformative changes that made mobile phones genuinely useful for everyday life.

GSM networks continued operating in many countries until the 2010s and beyond, with some 2G infrastructure still active today for IoT devices and legacy systems. Few technologies have had such a long and productive lifespan.