What are the Basics Of IP Telephony, Basic Principles, Terms And Protocols Recovery Mode

What are the Basics Of IP Telephony, Basic Principles, Terms And Protocols Recovery Mode

Good afternoon, dear friends. In this article we will try to consider the basic principles of IP-telephony, describe the most frequently used protocols, specify the ways of encoding and decode the voice, to disassemble some characteristic problems.

IP-telephony refers to voice communication, which is carried out over data networks, in particular over IP-networks (IP-Internet Protocol). To date, IP telephony is increasingly replacing traditional telephone networks due to the ease of deployment, low cost of calls, ease of configuration, high quality of communication and comparative connection security. In this article, we will adhere to the principles of the OSI (Open Systems Interconnection basic reference model) and talk about the subject “from below upwards”, starting from the physical and link levels to the data levels.

OSI model and data encapsulation

Principles of IP telephony

When making a call, the voice signal is converted into a compressed data packet. Then, the packets are transferred over packet-switched networks, in particular IP networks. When they reach the recipient’s packets, they are decoded into original voice signals. These processes are possible due to a large number of auxiliary protocols, some of which will be discussed later.
In this context, the data transfer protocol is a language that allows two subscribers to understand each other and ensure a high-quality transfer of data between two points.
The difference from the traditional telephony
In traditional telephony, the connection is established using a telephone exchange and is intended solely for the purpose of the conversation. Here, voice signals are transmitted over telephone lines via a dedicated connection. In the case of IP telephony, the compressed data packets are sent to a global or local network with a specific address and are transmitted based on this address. In this case, IP addressing is already used, with all the inherent features (such as routing).

In this case, IP-telephony is a cheaper solution for both the operator and the subscriber.

This is due to the fact that:

• Traditional telephone networks have redundant performance, while the IP telephony uses voice compression technology and allows full utilization of the capacity of the telephone line.
• As a rule, to date, access to the global network is available to all comers, which allows you to reduce the cost of connecting or completely exclude them.
• Calls to the local network can use the internal server and take place without the participation of an external PBX.
Together with the above, IP-telephony allows improving the quality of communication. This is achieved, again, thanks to three main factors:
• Telephone servers are constantly improving and their work algorithms become more resistant to delays or other problems of IP networks.
• In private networks, their owners have full control over the situation and can change parameters such as bandwidth, the number of subscribers on one line, and, as a consequence, the amount of delay.
• Packet switched networks are evolving, and new protocols and technologies are introduced every year to improve the quality of communication (for example, the RSVP bandwidth reservation protocol).
Thanks to IP-telephony, the problem of a busy line is very elegantly solved, since the redirection or a transfer to the standby mode can be carried out by several commands in the configuration file on the PBX.

Physical Layer

At the physical level, the bit stream is transmitted over the physical medium through the appropriate interface. IP-telephony is almost completely based on the already existing network infrastructure. As a medium of information transmission, a typically twisted pair of Category 5 (UTP5), single mode or multimode optical fiber, or a coaxial cable is used. Thus, the principle of convergence of telecommunication networks is fully realized.


It is interesting to consider the technology PoE (Power Over Ethernet) – standards IEEE 802.3 af-2003 and IEEE 802.3at-2009. Its essence lies in the ability to provide power to devices through a standard twisted pair. Most modern IP phones, in particular the Cisco Unified IP Phones 7900 Series, are equipped with PoE. According to the 2009 standard, the devices can receive the current up to 25.5 watts.
When the power is applied, only two twisted pairs of 100BASE-TX cables are used, but some manufacturers use all four pairs, reaching a power of 51 watts. It should be noted that the technology does not require the modification of existing cable systems, including Cat 5 cables.
To determine whether the connected device is powered (PD-powered device), a voltage of 2.8 – 10 V is applied to the cable, thereby calculating the resistance of the connected device. If this resistance is in the range 19 – 26.5 kOhm, then the process goes to the next stage. If not, the test is repeated at intervals of ≥2 ms.
After that, the power range of the device being fed is searched for by applying a higher voltage and measuring the current in the line. After this, the line is supplied with 48 V – supply voltage. Constant control of overloads is also carried out.

Data Link Layer

According to the IEEE 802 specification, the channel layer is divided into two sublevels:

• MAC (Media Access Control) – provides interaction with the physical layer;
• LLC (Logical Link Control) – Serves the network layer.

At the channel level, the switches are operating. These are the devices that enable the connection of several nodes of the computer network and the distribution of frames between hosts based on physical (MAC) addressing.

It is necessary to mention the mechanism of virtual local area networks (Virtual Local Area Network). This technology allows you to create a logical topology of the network without regard for its physical properties. This is achieved by tagging traffic, which is described in detail in the IEEE 802.1Q standard.

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