System information

Aliasing
If you’ve ever watched the wheels on a wagon turn backward in an old Western movie,
you’ve seen the effects of aliasing. The frame rate of the movie cannot keep up with the
rotational frequency of the spokes, and a false rotation is perceived.
In a digital audio system (which the modern PSTN arguably is), aliasing always occurs
if frequencies that are greater than one-half the sampling rate are presented to the an-
alog-to-digital (A/D) converter. In the PSTN, that includes any audio frequencies above
4,000 Hz (half the sampling rate of 8,000 Hz). This problem is easily corrected by
passing the audio through a low-pass filter
†
before presenting it to the A/D converter.
‡
The Digital Circuit-Switched Telephone Network
For over a hundred years, telephone networks were exclusively circuit-switched. What
this meant was that for every telephone call made, a dedicated connection was estab-
lished between the two endpoints, with a fixed amount of bandwidth allocated to that
circuit. Creating such a network was costly, and where distance was concerned, using
that network was costly as well. Although we are all predicting the end of the circuit-
switched network, many people still use it every day, and it really does work rather well.
Circuit Types
In the PSTN, there are many different sizes of circuits serving the various needs of the
network. Between the central office and a subscriber, one or more analog circuits, or a
few dozen channels delivered over a digital circuit, generally suffice. Between PSTN
offices (and with larger customers), fiber-optic circuits are generally used.
The humble DS-0―The foundation of it all
Since the standard method of digitizing a telephone call is to record an 8-bit sample
8,000 times per second, we can see that a PCM-encoded telephone circuit will need a
bandwidth of eight times 8,000 bits per second, or 64,000 bps. This 64-Kbps channel
is referred to as a DS-0 (that’s “Dee-Ess-Zero”). The DS-0 is the fundamental building
block of all digital telecommunications circuits.
† A low-pass filter, as its name implies, allows through only frequencies that are lower than its cut off frequency.
Other types of filters are high-pass filters (which remove low frequencies) and band-pass filters (which filter
out both high and low frequencies).
‡ If you ever have to do audio recordings for a system, you might want to take advantage of the band-pass filter
that is built into most telephone sets. Doing a recording using even high-end recording equipment can pick
up all kinds of background noise that you don’t even hear until you downsample, at which point the
background noise produces aliasing (which can sound like all kinds of weird things). Conversely, the phone
records in the correct format already, so the noise never enters the audio stream. Having said all that, no
matter what you use to do recordings, avoid environments that have a lot of background noise. Typical offices
can be a lot noisier than you’d think, as HVAC equipment can produce noise that we don’t even realize is there.
610 | Appendix A: Understanding Telephony