VoIP is the process of sending sound or voice over the internet. In a typical call using the current technology, the voice starts as an analog signal. When the sound reaches the transmitter in the phone, the vibrations are turned into electrical impulses. The impulses are transmitted over a pair of wires to the central office switch. The switch processes the electrical impulses into digital information. This process is known as Analog to Digital or Digital to Analog (A to D / D to A). This digital signal is transmitted to a distant switch over a connection called a trunk circuit. The main function of a telephone switch is to connect the callers phone to the called party phone and convert analog signals to digital information. The network of switches used every day in the United States is known as the Public Switched Telephone Network (PSTN).

There are two sides to a switch, first is the line side connection which is a two-wire circuit that connects the phone to the switch. This connection is normally an analog link between the switch and the central office (the name of the building where the switch is located). Second is the trunk side of the switch. This connection is normally four-wire in nature and in most cases a digital link. The four-wire circuit is a full duplex circuit where there is a dedicated transmission path in both directions. Switches have many dedicated trunk groups that connect the PSTN together in a spider web fashion.

VoIP is the next generation of telephone technology. Major portions of the network are being replaced. The traditional switch has been replaced with a device known as a Gateway Controller. The GC provides an A to D function as well as placing voice packets into the IP network for delivery to a distant GC. Unlike the PSTN where switches use dedicated trunks, the VoIP network uses a shotgun approach to deliver packets to the far end.

The theory is much the same in the VoIP and traditional PSTN. The difference is in the method used to establish a call. The PSTN uses multiple switches to connect a caller in Chicago to someone in LA. In this example, let's say there are five switches in the path between Chicago and LA. Each switch in the path uses trunks that are dedicated between each switch. Switch one has trunks connecting to switch two and switch two is connected to switch three with dedicated trunks and so on and so on. The PSTN uses phone numbers as the routing key to determine the next step in the switching sequence. As a call arrives at the next switch in the path, that switch queries its database to determine the trunk group to use for routing the call to the distant end. A call making its way across the nation hops from switch to switch in what appears to be a helter-skelter fashion. The important factor in this scenario is that for each switch pair involved there is a dedicated trunk circuit locked up for the duration of the call. If there are two calls between two switches then there are two trunks in use for the length of each call.

In the VoIP network there are still switches in the network, however; the switches are packet switches moving small packets of data from point to point. The trunks have been replaced with data links that are used to deliver packets where circuit dedication is not required. Gateway Controllers break the voice into small packets of information much the same as a switch in the PSTN. The GC also starts the routing function by routing packets to a distant IP address rather than routing on a phone number. The call setup information, as well as the voice packets, hop through the IP network in a ping-pong fashion much like the PSTN without dedicating a circuit per call. Packets containing voice information are mixed with data packets containing a web page or an e-mail in route to the IP address assigned.

One thing that begins to come clear in the VoIP network is the cost of call delivery. In the PSTN each call requires a dedicated circuit and is not available for any other use during a call. The VoIP network uses circuits in small bits at a time, that is a phone call shares available circuits with anything else that needs to be transmitted without the costly locked up circuits.

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