The basis of IP telephony

At the network level, routing takes place, respectively, the main devices of the network level are routers. This is where it is determined how the data will reach the recipient with a specific IP address.

The main routable protocol for the vitrual PBX is IP (Internet Protocol), on the basis of which IP telephony is built, as well as the world wide Internet. There are also many dynamic routing protocols, the most popular of which is OSPF (Open Shortest Path First) - an internal protocol based on the current state of communication channels;

Today there are special VoIP-gateways (Voice Over IP Gateway) that provide connection of conventional analog phones to an IP-network. As a rule, they also have a built-in router that allows them to keep track of traffic, authorize users, automatically distribute IP addresses, and control bandwidth.

To combat potential transmission delays, IP needs to be supplemented with additional tools, such as queuing protocols (so that voice data does not compete with conventional data). As a rule, for this purpose, routers use low-latency queuing (LLQ - Low-Latency queuing) or weighted queuing based on classes (CBWFQ - Class-Based Weighted Fair Queuing). In addition, prioritized coding schemes are needed to consider voice data as critical to transmission.

The main transport layer protocols are TCP (Transmission Control Protocol), UDP (User Datagram Protocol), RTP (Real-time Transport Protocol). Directly in IP telephony, UDP and RTP protocols are used, and their main difference from TCP is that they do not provide reliable data delivery. This is a better option than delivery control (TCP) because telephony is highly dependent on transmission delays but less sensitive to packet loss.

UDP

UDP is based on the IP network protocol and provides transport services to application processes. Its main difference from TCP is the provision of non-guaranteed delivery, that is, no confirmation is requested when sending and receiving data. Also, when sending information, it is not necessary to establish a logical connection between the UDP modules (source and destination).

RTP

Although RTP is generally considered a transport layer protocol, it generally runs on top of UDP. With the help of RTP, recognition of the type of traffic, work with time stamps, transmission control and numbering of the sequence of packets are realized.

The main purpose of RTP is that it assigns timestamps to each outgoing packet, which are processed on the receiving side. This allows data to be received in the correct order, reduces the effect of uneven packet transit times, and restores synchronization between audio and video data.