Telephony Signalling Basics

Although almost all high-end telecommunications equipment produced today uses digital transmission in some form or another, the most common element that home users come into contact with still usually functions in analog – namely their telephone handset. With analog communication, a user’s voice is represented as an analog waveform that is transmitted across the wires connecting their phone to the telecommunication carrier’s local central office (CO). Many years ago, the telecommunication carrier’s entire network was comprised of analog equipment, and that waveform was ultimately sent across a dedicated circuit all the way to the receiver’s phone.

Although analog transmission works, it is not particularly efficiently, and is susceptible to noise interference during the transmission process. As such, almost all carrier networks are now entirely digital, with the exception of the last portion, from the user’s home to the central office (the local loop). In the local loop, analog still reigns supreme, and likely will for many years to come.

Because the local loop is largely still analog, and the rest of the carrier network digital, a conversion needs to take place. Specifically, the analog signal that travels from the user telephone to the CO needs to be converted to a digital signal to cross the carrier network, and then back to analog from the recipient’s CO to their phone (and ultimately their ear, which is able to comprehend the analog waveform – your voice). The process of converting the originating analog signal to digital requires the local CO telephone switch to “sample” the analog waveform, and then produce a digital representation of it. At the receiving end, this process is reversed, with the switch taking the digital signal and converting it back into a waveform. In the world of telephony, this is usually accomplished using a technique known as pulse code modulation (PCM).

While you aren’t required to be familiar with all of the details of how PCM works for the CCDA exam, the steps below are provided to give you a better sense of the conversion process for your own understanding.

1. The original analog signal is filtered by a coder-decoder (codec) so that only the range of frequencies that can be heard by the human ear (approximately 300hz to 3400hz) are sampled.

2. The filtered signal is sampled approximately 8000 times per second using a technique known as pulse amplitude modulation (PAM).

3. Each sample is converted into binary using PCM, and then transferred across the digital network. At receiving CO switch, this process is reversed to convert the signal from digital back into an analog waveform.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.