Page 208 - Asterisk™: The Future of Telephony

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.
Even the ubiquitous analog circuit is sampled into a DS-0 as soon as possible. Some-
times this happens where your circuit terminates at the central office, and sometimes
well before. ‖

T-carrier circuits
The venerable T1 is one of the more recognized digital telephony terms. A T1 is a digital
#
circuit consisting of 24 DS-0s multiplexed together into a 1.544 Mbps bitstream. This
bit stream is properly defined as a DS-1. Voice is encoded on a T1 using the μlaw
companding algorithm.

§ 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.
‖ Digital telephone sets (including IP sets) do the analog-to-digital conversion right at the point where the
handset plugs into the phone, so the DS-0 is created right at the phone set.
# The 24 DS-0s use 1.536 Mbps, and the remaining .008 Mbps is used by framing bits.

180 | Chapter 7: Understanding Telephony