Electronic Surveillance and Countermeasures on Analogue Telephone Lines

Researched, compiled and tested by Fone Ranger, 1999-2004

Foreword

This brief guide to phone surveillance This somewhat dated subject has been covered numerous times and will no doubt be considered old news by , but it's still relevant to anyone wanting to know how to construct and detect amateur taps on analogue lines.

Physically connecting taps to phone lines is limited to ASLs

• Switch
• PBXs
• ISDN, ADSL

Electrical Properties of ASLs

Phone Interfacing Basics

As pointed out by TAP magazine², devices which pass audio from phone lines essentially fall into one of four catagories:

Capacitor

The simplest method, a capacitor, typically between 0.01uF and 1uF, is used to block DC, thus passing only AC signals (e.g. audio)

Inductor

This can take two forms: o connected to the earpiece o wrapped around one wire in the line pair

Transformer

Direct

This is a slight misnomer in the context of tapping since it involves connecting to the phone handset itself

Series Transmitters

Parallel Transmitters

The simple passive monitor circuit can be connected to the input

Deployment

Countermesures

References

1. Understanding Telephones, Julian Macassey
2. TAP No. 37 pp 2-3,

Last updated on November 03, 2004