Voice over IP – Transmission Channel

VoIP calls are not transmitted over dedicated channels as in circuit-switched telephone networks (PSTN, GSM). In general, packet-switched networks provide shared channels where the speech packets have to compete with packets from other users and applications, unless the network provides prioritized scheduling for real-time service classes. Therefore, the packets of a VoIP transmission experience different delays on their way through the network.

The resulting variation of inter-arrival times at the receiver, also called jitter, can be compensated to some extent by a (possibly adaptive) jitter buffer. The dimensioning of the receiver buffer has to find a trade-off between the resulting delay and the amount of packet losses due to late arrivals.

Besides these delay inflicted losses, packets may also get lost due to buffer overflows in routers in case of network congestion. On wireless transmission links, packets get lost due to residual bit errors.