VoIP

VoIP (Voice over Internet Protocol) is a technology that converts speech into digital data and transmits it in packets over IP networks, such as the internet or internal networks. This differs from traditional circuit-switched telephony (PSTN), which maintains a dedicated circuit for the duration of a call.

The system operates in two parts: signalling to set up and end calls, and the voice transmission itself. A codec compresses the voice to reduce bandwidth consumption. Voice packets are typically sent over time-sensitive protocols, since latency and packet loss directly affect call quality.

Protocols

SIP (Session Initiation Protocol) is the most widely used signalling protocol; it manages how two parties connect and communicate. Voice traffic is commonly carried over RTP (Real-time Transport Protocol), which typically runs on UDP. Other protocols, older or specialized, include H.323 and Asterisk's IAX. Some commercial services use proprietary protocols that are not fully open.

Quality and Constraints

Call quality (QoS) depends on three key factors: latency, jitter (variation in packet arrival timing), and packet loss. On congested networks, policies can be set to prioritize voice packets. Another constraint is that NAT and firewalls can block direct connections; this is addressed using methods such as STUN, TURN, and ICE. VoIP also requires reliable power and internet access, unlike some older phones that drew power from the telephone line itself.

Uses

VoIP is used in corporate telephone systems (IP PBX), internet calling services, and mobile apps offering voice and video. Large telecommunications companies also use VoIP in their core networks, even when users perceive a normal call—for example, VoLTE carries voice over LTE. The lower cost of long-distance calls is one reason for widespread adoption of this technology.