Information Technology Reference
In-Depth Information
Line [DSL]), and more options exist to run voice service over data networks. Fol-
lowing is an overview of voice-over-data with a bit more detail on IP telephony.
Voice-over-data extends the telephony (telecommunications) transport fabric
beyond the PSTN and private links to include Ethernet, Frame Relay, and ATM.
Internet Protocol (IP) telephony is the extension of traditional telecommunica-
tions/telephony services, capabilities, and applications across TCP/IP. Any topol-
ogy that supports TCP/IP or encapsulates TCP/IP may facilitate VoIP traic. IP
telephony is the broader term encompassing applications, topology, and transport;
VoIP is merely one of many voice-over-data transport methods.
Telephony operations include bearer traffic (voice) and data traffic (signaling).
Signaling enables the voice system to operate. Signals are sent from PBX to station,
PBX to central office, PBX to PBX, PBX to voice mail, and much more. These sig-
nals traditionally traverse dedicated voice links like house wire, PSTN T1s, and tie
lines. Convergence enables the bearer and data traffic to traverse the data network.
Bearer traffic on a data network is generally referred to as voice-over-data; signaling
over the data network is generally thought of as just another data application—in
this case the application resides on a voice server (e.g., old term: PBX) and voice
clients (e.g., station). Transmitting telephony bearer channels and signaling over a
data network has special meaning and exciting implications to voice experts, but is
generally unknown and unappreciated by most others, including those proficient
in the data world.
As an example of the complexities involved with IP telephony, consider Fig-
u
re 7.3, which depicts various IP telephony endpoints and connectivity of the voice
server and the PSTN. These connectivity options present a variety of operating
conditions using network convergence.
Points 1 and 3 show voice server connectivity to the local (LAN) data network.
Point 2 shows private tie lines (IP trunks) between voice servers. Points 4 to 6 show
various IP station connectivity options, including IP softphone (phone software on
a PC) or IP phone (voice hardware connecting directly to the LAN). An extension
call between a traditional analog phone and IP phone happens exactly as a tradi-
tional extension call would; the same goes for ten-digit external calls from an IP
phone, long-distance calls, etc. There is complete user transparency.
Rules may be set up within the voice server to direct calls over the IP trunks or
over traditional PBX tie lines. This latter presents many options to build in redun-
dant trunk paths and provide user transparency if either the data service or PSTN
service goes down. All of these together offer many options for IA planning with
respect to voice services.
Applied IP Telephony course by Keith D. Willett, July 2000.
Voice-over-IP.
