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VOICE OVERINTERNETPROTOCOL 653and launch a packet data message to an external database
(called a service management system or SMS). That mes-
sage is routed from the switch to the SMS over a par-
allel data network called a Signaling System 7 (or SS7)
packet network. The SMS returns routing instructions,
via the SS7 packet network, to the switch that originated
the routing query. The SS7 signaling standard came out
of the conventional telephony industry.
If the local switch determines that the call needs to be
connected to a local loop attached directly to that local
switch, the switch will ring the local loop of the called
telephone. At the same time, the switch returns a ringing
sound signal to the calling party. On the other hand, if the
local switch determines that the called party’s telephone
is busy, the local switch returns a slow busy signal to the
calling party.
As soon as the called party picks up its phone, the local
switch creates a circuit-switched dedicated audio commu-
nications path between the calling party’s local loop and
the called party’s local loop, thus completing a dedicated
audio transmission path between the calling telephone
and the called telephone. For the call’s duration, that ded-
icated audio path is available for the transmission of voice
signals, in both directions, between the calling and called
party’s telephones. If one or more of the links used to co-
nstruct that path are digital, a CODEC in the connecting
switch or other connecting device will make the neces-
sary analog-to-digital conversion so that the voice signal
may seamlessly traverse the boundaries between the ana-
log and digital links.
If the called telephone’s local loop is not directly con-
nected to the calling telephone’s local switch, the local
switch must establish a connection to the called telephone
through another switch or switches. For most modern
PSTN switches, this requires that the originating switch
launch an SS7 message, over the SS7 network, to reserve
a path through other switches in the network for complet-
ing the desired communications path to the call’s destina-
tion. If the far end switch where the called party is located
determines that the called party’s line is busy, no circuit
connections will be made, the path reservation for the con-
necting links is dropped, and the originating switch will
return a slow busy signal to the calling party. However, if
the reason the call cannot be completed is network con-
gestion, the originating switch will return a fast busy to
the calling party.
If the called party’s line is not busy and a complete path
to the called party’s line can be established, the called
party’s local switch rings the called party’s telephone.
While the called party’s phone is ringing, the calling party’s
local switch is returning an audible ringing signal to the
calling party. When and if the called party answers, the
switches instantly circuit-switch together the links along
the reserved path in order to complete a dedicated path
between the calling and called telephones.
Finally, it should be noted that some switches commu-
nicate interoffice signaling information using non-SS7 in-
teroffice signaling arrangements, such as in-band signal-
ing. Regardless of the supervisory and address signaling
arrangement used, the net result of establishing a commu-
nications path between the calling and called telephone
will be the same.VOICE OVER INTERNET PROTOCOL
VOIP literally refers to the transmission of a digitized
voice signal using digital packets, routed using the In-
ternet protocol or IP. The driving forces for using VOIP
are beliefs in its cost savings, flexibility, and the growing
desire to combine voice and data transmission on one net-
work. See, e.g., Morris (1998), Cisco VOIP Primer (2002),
Matthew (2002).How VOIP Transmission Works
Because a VOIP call is transmitted digitally, it begins with
a digitization process similar to that used in conventional
telephony. First, the voice signal is sampled at a rate
greater than the Nyquist sampling rate, and those sam-
ples are digitized. Whereas conventional circuit switched
telephony transmits the digitized samples in a constant
stream of synchronized (i.e., equally spaced in time) digi-
tal samples, VOIP transmits the digitized voice communi-
cations samples in asynchronous (i.e., unequally spaced
in time), sequentially numbered packets of data. Each
packet (which may contain many voice samples) contains
its own IP formatted address information, which allows it
to be routed over an IP network. Unlike conventional tele-
phony, where each sample follows the path of the sample
before it, each of the IP packets containing several sam-
ples of voice data may take an independent path (which
is shared with other data packets) to its destination. With
each packet potentially taking a different route, the pack-
ets often can arrive at their destination out of order (or
sometimes not at all). At the far end, the IP and other rout-
ing information is stripped from the packet, the voice sam-
ples are temporarily collected in a buffer and reordered
as required, and then, if all goes well, the original voice
signal is reconstructed, albeit, slightly delayed. For VOIP
technology to have the functionality and flexibility of con-
ventional telecommunications, some form of call control,
in the form of supervisory and address signaling, is re-
quired.
Figure 6 shows the basic VOIP transmission scheme.VOIP Signaling
VOIP signaling refers to signaling that can be used over
an IP network to establish a VOIP call. It provides the
needed functionality of supervisory and address signaling.
VOIP call signaling comes in two predominant compet-
ing schemes: H.323 and Session Initiation Protocol (SIP).
Each must satisfy the generic needs of telephonic calling.
Specifically, each must provide for the following:The calling phone to address and signal to the called tele-
phone,
The called telephone signaling its availability for receiving
calls,
Establishing a transmission path between the calling and
called telephone,
The called or calling telephone signaling to the other
phone it has hung up, and
The tearing down the transmission path once the call is
over.