Getting Started with OpenBTS

Appendix A. Quick Reference

Here’s a quick reference for many facts scattered throughout the book. I find myself looking these things up constantly and wanted to present them in a unified section.

GSM Hierarchy

This book cannot attempt to teach you GSM in a few short chapters. However, a very helpful thing to have a grasp on is a hierarchy of GSM jargon from macro to micro.

A tower is the mast with antennas that we’re all familiar with, sometimes disguised as a cactus or tree. Each tower can be divided into multiple sectors. These sectors cover a portion of the possible 360º (e.g., four 90º sectors on a tower). A single sector tower covers all 360º. Each sector will be assigned one or more ARFCNs, sometimes also called carriers, because they are the actual pair of frequencies where data is physically exchanged.

Each ARFCN has eight time slots. Each time slot has a combination assigned to it. Each combination is composed of multiple logical channels. These logical channels are divided into two main categories of signaling and media. SDCCHs (standalone dedicated control channels) carry signaling such as handset registration traffic or SMS traffic. TCHs (traffic channels) carry media such as GPRS data or voice traffic. Logical channels are made up of multiple frames and frames are made up of bursts.

To see this system in action, open up the GSM Timeslot and Channel Visualizer.

Decibels and Decibel Milliwatts

Decibels can be used to express ratios between two values or, when paired with a ...