Page 198 - Asterisk™: The Future of Telephony

Ring Tip

Figure 7-1. Tip and Ring
Handset. The handset is composed of the transmitter and receiver. It performs the con-
version between the sound energy humans use and the electrical energy the telephone
network uses.

Tip and Ring

In an analog telephone circuit, there are two wires. In North America, these wires are
‡
referred to as Tip and Ring. This terminology comes from the days when telephone
calls were connected by live operators sitting at cord boards. The plugs that they used
had two contacts―one located at the tip of the plug and the other connected to the ring
around the middle (Figure 7-1).
The Tip lead is the positive polarity wire. In North America, this wire is typically green
and provides the return path. The Ring wire is the negative polarity wire. In North
America, this wire is normally red. For modern Cat 5 and 6 cables, the Tip is usually
the white wire, and Ring is the coloured wire. When your telephone is on-hook, this
wire will have a potential of –48V DC with respect to Tip. Off-hook, this voltage drops
to roughly –7V DC.

Digital Telephony

Analog telephony is almost dead.

In the PSTN, the famous Last Mile is the final remaining piece of the telephone network
still using technology pioneered well over a hundred years ago. §
One of the primary challenges when transmitting analog signals is that all sorts of things
can interfere with those signals, causing low volume, static, and all manner of other
undesired effects. Instead of trying to preserve an analog waveform over distances that
may span thousands of miles, why not simply measure the characteristics of the original

‡ They may have other names elsewhere in the world (such as “A” and “B”).
§ “The Last Mile” is a term that was originally used to describe the only portion of the PSTN that had not been
converted to fiber optics: the connection between the central office and the customer. The Last Mile is more
than that, however, as it also has significance as a valuable asset of the traditional phone companies; they
own a connection into your home. The Last Mile is becoming more and more difficult to describe in technical
terms, as there are now so many ways to connect the network to the customer. As a thing of strategic value
to telecom, cable, and other utilities, its importance is obvious.

170 | Chapter 7: Understanding Telephony