Learn 5 Basics Of IP Telephony – #1 – Transport Layer

Learn 5 Basics Of IP Telephony – #1 – Transport Layer

For the transport level, the following features are typical:

  • Segmentation of data from top-level applications;
  • Providing end-to-end connection;
  • Data reliability guarantee.

The main protocols that are used by VoIP services.

UDP (User Datagram Protocol)

UDP is a protocol based on the IP network protocol used by VoIP services and provides transportation services to the processes performed. Its main difference from TCP is the provision of non-guaranteed delivery, that is, when sending and receiving data, there is no acknowledgment requested. Also, when sending information, it is not necessary to establish a logical connection between the UDP modules (source and receiver).

RTP (Real-time Transport Protocol)

Despite the fact that RTP is considered to be a transport layer protocol, it generally works on top of UDP. With the help of RTP, the type of traffic is recognized, work with timestamps and control the transmission and number the sequence of packets are performed.

The main purpose of RTP is that it assigns to each outgoing packet the time tags, processed on the receiving side. This allows you to receive data in the proper order, reduces the impact of uneven packet transit time across the network, and restores the synchronization between audio and video data.

Data Layers

The last three levels of the OSI model are discussed together. Such a combination is permissible, since the processes occurring at these levels are closely related, and to describe them without the division into sublevels will be more logical.


The first step is to describe the H.323 protocol stack, developed in 1996. This standard describes equipment, network services and terminal devices intended for audio and video communication in packet-switched networks (Internet). For any H.323 device, the voice exchange is required.

H.323 recommendations suggest:

  • Platform independence
  • Standards for encoding analog data
  • Bandwidth management
  • Flexibility and compatibility

Note a very important fact: the recommendations do not define the physical medium of transmission, the transport protocol, and the network interface. This means that devices that support the H.323 standard can work in any existing packet-switched networks.

According to H.323, the four main components of a VoIP connection are:

  • The terminal;
  • Gateway;
  • Zone controller;

Controller for Multipoint Control Unit (MCU).

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