PTT

PTT is one of the earliest communication standards used for mobile telephony, and, in fact, was a mobile analogue mediated through a wireline telephone service provider. To call the operator, the user of the mobile phone pressed and held for several seconds the PTT button (tangent) on his device. After the subscriber released the button, the operator answered and requested the phone number to which the subscriber wanted to make a call.

The subscriber again switched to the transmission mode by pressing the button and called the desired number, after which the operator dialed the given number via a wired telephone line and established a connection. After the conversation has ended, if there was no activity in the line for some time or a hang-up signal was heard from the side of the telephone line, the operator could release the channel and use it for other connections. This standard was used in the 1980s in Canada, where it was later replaced by MTS , and is still used by some Indian operators.

Recently, the use of PTT in IP networks has become widespread. The main reason for this was the ease of organizing point-to-multipoint audio transmission for the PTT system. Due to this, separate groups are easily organized when all users participating in this group hear the speaker.

PTT is based on two main technologies:

• Server PTT

All PTT clients access the server (for example, the IMS server) to obtain permission to transmit the signal and to obtain information about the available groups and users. The exchange of service information between the client and the server is usually carried out using the SIP protocol .

The client selects a group (or single user) for a conversation and, when you click on the "talk" button, he contacts the server for permission to start a session. The server will not give permission to start the session if the group (user) is busy, for example, another user is already transmitting voice data to the same group. If permission is obtained, the client starts transmitting voice data to another user (like unicast ) or to a group of users (like multicast ).

After releasing the talk button, the server sends a signal about the end of the session.

Depending on the organization of the system, the server can independently terminate the session by sending a “end of session” signal to the client.

Advantages: centralized management of sessions, including between remote users.

Disadvantages: a central element (server) is required, which is very expensive.

• Serverless PTT

The client in serverless PTT, unlike the server PTT, independently decides on the start of the session, analyzing whether messages arrive for the group for which he wants to start the session. The client determines the end of the session by a specific signal from another client or by timeout. Lists of users / groups must be organized in a separate protocol (for example, based on a separate multicast group).

Advantages: low cost and ease of building a system.

Disadvantages: lack of centralized session management, transmission overlays are possible from two or more users at the same time, especially in the case of remote users (with a long signal delay).

• Ims
• PTV
• Zello

• E. Ahonen, Implementing a Multi-access Push-to-Talk System and Analysing User Experience, 2007.
• A. Woodruff, PM Aoki “Push-to-Talk Social Talk, 2005
• R. Koivisto "Push-to-Talk over Cellular: Still searching for the flow of success", 2005