There are also several communications protocols used for VoIP technology
that address signaling and voice transmission. These addressing signaling pro-
tocols include the Session Initiation Protocol (SIP), the Media Gateway Con-
trol Protocol (MGCP) and H.323. The SIP initiates, modifies, and terminates
sessions with call participants and is integral to providing call services such as
call forwarding and caller identification.
The MGCP is primarily used for controlling telephony gateways—devices
that convert analog signals to digital signals such as analog telephone
service from a telephone service provider connected to a VoIP system. The
H.323 standard is an umbrella of standards focusing on audio, video, and
data communications across IP-based networks that do not provide a
guaranteed QoS.
For voice transmission the standard protocol is the Real-time Transport
Protocol (RTP). This protocol provides end-to-end network transport func-
tions, such as control and identification and is supplemented by a control
protocol (RTCP) to allow monitoring of data or digital voice delivery.
Another VoIP issue has been providing power to VoIP telephones. In a con-
ventional TDM system power is centralized and feeds each of the instruments
attached to the system. In early deployments of VoIP, VoIP telephones had to
be powered locally, at the location of the telephone instrument. This meant
that not only did the telephone need a network connection but it also required
a power outlet, thus adding cost to VoIP deployments and making the moving,
adding, and changing of instruments cumbersome.
The market response was to develop a method to provide power to the tele-
phone instruments through the network connection. It resulted in the IEEE
802.3af standard that set guidelines for providing power over a network cable
(commonly referred to as POE) and discussed previously.
Telephone users can use analog or digital telephones. On conventional
TDM telephone systems standard analog telephones or proprietary digital
instruments would be used. On VoIP systems, analog telephones can be used
with an adapter at individual instruments or a network gateway that converts
multiple analog telephones to the digital network. Other analog devices, such
as fax machines, are serviced in the same manner as analog telephones.
Digital telephones for a VoIP system can connect directly to a LAN net-
work switch, much like a desktop PC would be connected. Many digital tele-
phones come equipped with a built-in “mini-data switch,” allowing one
network connection to be connected to the instrument which serves both the
telephone instrument and a desktop PC connected to the mini-switch.
Softphones are another option for VoIP systems.
Softphones are software applications for a PC that essentially turn it into a
telephone. Users utilize a headset or a USB-connected telephone to
116 Smart Building Systems for Architects, Owners, and Builders