Information Technology Reference
In-Depth Information
The jagged line over the wires in the analog phone in Figure 1-3 represents a broken cir-
cuit. Anytime the phone is on hook, the phone separates the two wires, preventing electric
signal from flowing through the phone. When the phone is lifted off hook, the phone con-
nects the two wires, causing an electrical signal (48V DC voltage) to flow from the phone
company central office (CO) into the phone. This is known as
loop start signaling
.
Loop start signaling is the typical signaling type used in home environments. Loop start
signaling is susceptible to a problem known as glare. Glare occurs when you pick up the
phone to make an outgoing call at the same time as a call comes in on the phone line be-
fore the phone has a chance to ring. This gives you the awkward moment of, “Uhhh...Oh!
Hello, Bob! I'm sorry, I didn't know you were on the phone.” In home environments, this is
not usually a problem for a couple reasons. First, the chances of having a simultaneous
outgoing and incoming call are slim. Second, if you do happen to have an incoming call,
it's always meant for your house (unless the caller dialed the wrong number).
In business environments, glare can become a significant problem because of the large
number of employees and high call volume. For example, a corporation may have a key
system (which allows it to run its own, internal phone system) with five analog trunks to
the PSTN, as shown in Figure 1-4.
x5000
Analog Loop Start Connections
x5001
Key System
Central Office
x5002
Figure 1-4
Illustration of Glare
If a call comes in for x5002 at the same time as x5000 picks up the phone, the key sys-
tem will connect the two signals, causing x5000 to receive the call for x5002. This hap-
pens because the loop start signal from x5000 seizes the outgoing PSTN line at the same
time as the key system receives the incoming call on the same PSTN line. This is an in-
stance of glare.
Because of glare, most modern PBX systems designed for larger, corporate environments
use ground start signaling. Ground start signaling originated from its implementation in
pay phone systems. Many years ago, when a person lifted the handset of a pay phone, he
did not receive a dial tone until he dropped in a coin. The coin would brush past the tip
and ring wires and temporarily ground them. The grounding of the wires signaled the
phone company to send a dial tone on the line. Using this type of signaling in PBX sys-
tems allows the PBX to separate an answering phone from an incoming phone line, reduc-
ing the problem of glare. To receive a dial tone from the CO, the PBX must send a ground
signal on the wires. This intentionally signals to the telephone CO that an outgoing call is
