All data travelling over the Internet is made up of packets that contain a payload as well as extra information that determine where and how that payload will be delivered. In VoIP the payload is the actual voice data. The packet also consists of several other ‘layers’ that aid in the speedy delivery of the data which allows real time conversations to take place over the Internet.
The Internet Protocol Suite (IPS) is composed of 5 layers which encapsulate the actual payload. The layers contain information about how the payload is to be delivered – for example, if all the data has to be delivered or not – and how it will be treated on delivery. There are three layers that make connections between computers and two physical layers that data must pass through en route to its destination. These physical layers are part of the computer.
The top layer of the IPS is the Application Layer. The VoIP soft-phone controls the Application Layer – in VoIP a common application layer is SIP (Session Initiation Protocol). It specifies the type of connection the caller wants to make (voice, video or instant messaging for example), and identifies the other party with a unique number similar to an IP address (the 4 part number that identifies every computer on the Internet).
The Transport Layer in any Internet connection determines the format for delivering data. Web pages usually use TCP (Transmission Control Protocol) which guarantees data delivery – sometimes at the expense of speed. VoIP depends more on speed than data integrity, so TCP is not usually used. Instead RTP (Real-time Transport Protocol) in conjunction with UDP (User Datagram Protocol) are used to control data flow. RTP identifies the payload and provides sequencing information so that the data can be reassembled correctly when it reaches its destination. UDP provides a fast method of delivery but by itself cannot determine data sequence or delivery information.
Next is the Network Layer, which for the Internet (and other networks) is the Internet Protocol (IP). IP is used to set up a path, but offers no guarantees for data delivery or integrity. For dependable data delivery the upper layers (transport and application) are needed.
The two physical layers of the Internet Protocol Suite are the Data Link layer and the Physical layer. Ethernet is used as the Data Link layer in VoIP. It provides a means to transmit data reliably by controlling and synchronizing the flow. The physical layer provides the pathway that transmits bits to the Data Link layer. In VoIP the physical layer is the twisted-pair cable that connects the network card, routers, modems, Analog Telephone Adaptors (ATAs) and IP phones.
If we follow the data path of a voice packet, it originates in the sound card which converts the voice into digital data. The audio stream is compressed by the Voice Over Internet Protocol software and divided up into packets which contain information about where the data is to be delivered. This data is transmitted from the computer to the Internet through the twisted-pair cable attached to the modem.
The data packets may take several different routes to their destination because of the ever-changing conditions of the Internet. On arrival, the voice data has to be reassembled in the correct order and converted to an analog signal which the receiving party can hear. All this should happen in less than half a second no matter where in the world the two parties are located.