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652 VOICE OVERINTERNETPROTOCOL(IP)This occurs because the electronics of a typical telephone
network cause the far end of almost every call connection
to echo back a weakened version of the transmitted sig-
nal to the originating end. Although this echo is always
present, it is not noticeable until the round trip delay be-
comes long enough.
All of these deficiencies of a transmission channel have
the effect of reducing the channel capacity, or, throughput,
of a transmission link. Channel capacity refers to the the-
oretical upper limit on how much information, in terms
of bits per second, can be transmitted through a channel
(Wozencraft & Jacobs, 1965).Circuit-Switched Connections of a Call
in a Conventional Telephony Network
VOIP networks must interoperate with conventional
circuit-switched networks, in particular the ubiquitous
public switched telephone network or PSTN. A VOIP
network that cannot interoperate with, and therefore ex-
change calls with, the PSTN would be of little value to
most telephone users. A network’s value increases as it
can be used to reach more users.
Like other networks, the PSTN uses a hub and spoke
architecture of transmission links and switches (Bell Lab-
oratories, 1977, 1983). Each user’s telephone is typically
connected by an individual circuit (usually referred to as
local loop) to a central office switching hub. In a small
town, there may be only one central office, whereas in
a large major metropolitan area, there could be several
dozen central offices.
In each central office hub, there is usually at least one
switch. The switches used in traditional telephony are
called circuit switches. A circuit switch will route a call
over a dedicated path from one transmission link to an-
other for at least the duration of the call. See Figure 4.
As will be described more fully below, this contrasts with
the packet switching used in the Internet, which does
not maintain a dedicated path connecting network links,
but instead passes information along shared paths on a
packet-by-packet demand basis.Circuit
Switching
NodeLink LinkDedicated Path Created for Call Duration
Figure 4: Circuit switched connection between
two links.Originating the Call
As Figure 5 shows, when a caller picks up a telephone
to make a call, the local switch in the caller’s local cen-
tral office (Node A) provides a dial tone that the caller
hears through the telephone. That dial tone indicates to
the caller that the local telephone switch has seized his or
her local telephone line and is ready to receive the digits
of the telephone number that he or she wishes to dial. The
processes of seizing the telephone line and returning dial
tone are types of supervisory signaling.
The caller then enters, or dials, the telephone num-
ber of the called telephone. If the calling telephone is
enabled with touch-tone signaling, the calling telephone
signals to the local switch the digits dialed by using dif-
ferent combination of two touch-tones for each digit di-
aled. Such transmitting of routing information is called
address signaling. The caller’s local switch Node A detects
these tone combinations and, in turn, determines the tele-
phone number the caller is trying to call.Circuit Switching to the Call’s Destination
Each circuit switch has a routing look-up table for deter-
mining how to handle a call based on some or all of the
calling and called numbers. If the local switch does not
have adequate information in its routing tables to make a
routing decision, most modern switches will hold the callLink Link
Local Loop Local LoopInteroffice
Transmission
LinkInteroffice
Transmission
LinkInteroffice
Transmission
LinkCalling
TelephoneCalled
TelephoneOffice""CentralNode ANode B
"Toll Office"SS7
Packet
SwitchSMS
DatabaseNode C"Central
Office"Figure 5: Conventional circuit switched telephony network.