Voice telephony, internet telephony, voice and voice over IP are technically complex topics.

IDB informs pro actively and in depth on the technology and its application. Enclosed technical information is for free and is understood as a transfer of knowledge.

Public tele-
phony bases worldwide
on binding technical standards
and regula-
tory require-


Telephone operators render telecommunication services such as fix net and/or mobile telephony and data transmission for their subscribers. Increasingly value added services are being offered such as IP-TV. Most of the large telephone operators were or are governmental or governmentally regulated monopolists. Predominantly in Europe such monopolists were named PTT’s (Post, Telegraph and Telephone Administration). These telephony companies are also known in the jargon as Telco, Carrier, Local Exchange Carrier or Telecom Service Providers.

The classic telephone operators, the “wire-line carriers” operate on the base of historic standards in analogous and digital telephony. Not only their networks but also their active components (e.g. switches) base on the historic standards. A switch in telecommunication is a system of integrated electronic components that switches calls. The central office (the switching branch) is historically the physical location that houses all equipment including the telephony switches. They enable to route calls to the destination in the sense that a link is established between the parties and voice information may be exchanged. In times of analogue technology the switches required a lot of space.

With the digitalisation of telephony electronic switches emerged the introduction of the TDM-Technology (Time Division Multiplex). In the current third phase of technological history the switches for IP-telephony consists mainly of software. Besides the switching the billing or rating is an area of major significance. Contrary to other telecommunication services the accurate billing of services to subscribers and other carriers is one of the key disciplines.Telecom systems may be distinguished in two categories: Systems of Class 4 and Class 5. A Class 4 system (Transit Switch) is used for long distance communication in the PSTN (Public Switches Telephone Network) and links Telco’s together. Class 5 systems (Local Switch) are deployed to connect subscribers. Class 5 Systems enable the voice communication of subscribers. Each phone connection of a subscriber is allocated with one or more telephone numbers, which may be dialled and accessed in the world-wide telephony network (PSTN). The numbering system is a strictly hierarchical structured convention, consisting of international area code, the regional or network code and the actual subscriber code.

The techno-
logy of
Voice over
IP enables telephony
on data links based on the Internet Protocol (IP)


Voice over IP (VoIP) is telephony transmitted using the Internet Protocol (IP). The Internet Protocol is package based. The traditional telephony (Plain Old Telephony, POT) either analogue or digital (ISDN) is circuit-switched on dedicated connections. Circuits-switched transmissions require higher network capacities, since the links may not be utilized by several services concurrently. IP based Multimedia Systems (IMS) utilize the same transmission technology to transmit multimedia content (voice, video, data, etc) in networks simultaneously. VoIP technology is deployed quite differently. In-house communication in enterprises or organisational entities LAN or IP-PBX’s (Private Branch Exchange) support VoIP for telephony over data networks efficiently. Most of today’s deployed PBX’s support VoIP. Next to all of the new PBX are IP based systems with full IP capabilities. The deployment of VoIP at a large scale in public telephone networks is in its infancy despite the bulk of long distance calls are handled by VoIP. One of the reasons: The switching in public telephony is by far a larger challenge than in a closed user group or in a point-to-point connection. Second reason: VoIP is based on a shared transmission medium (Shared Media) which makes the operation from a service point of view more complex. Third reason: The penetration of broadband connections in homes and houses is in many countries at the starting point. The expansion grows with the roll-out of the base infrastructure by the deployment of FTTH/FTTB (Fibre to the Home / Fibre to the Building). The proficiency of VoIP switching technology and broadband access are the two key factors in the transformation of traditional telephony to VoIP. In the long term the benefits will outweigh. VoIP utilises the network capacity by factors better than conventional technology.

The quality of service in Voice over IP depends on several parameters


To assure a high quality several measures need to be in place. Most critical is the last mile to the subscriber, where usually only limited bandwidth is available. To ensure sufficient quality of the voice communication the allocation of bandwidth for voice communication is required on connections transmitting shared media such as IP-TV. This will be realised with VPN connections or other means allowing bandwidth allocation.
The deciding factors on the quality of the voice communication transmission delays, packet losses and voice compression.

Transmission delays are actually no major issue in up-to-date IP networks. Difficulties originate by nomadic subscribers who try to place voice calls on bad or limited IP infrastructures.
Packet losses in IP networks may not be avoided. They occur usually in situations with limited bandwidth, generally on the last mile (e.g. ADSL-Connections). To prevent packet loss IP packages with voice need to be prioritised aver other packets. IDB uses components to prevent packet loss on the last mile. An additional impact on the quality of voice service derives from the choice of voice compression. The applied voice compressions depend on the available bandwidths. The quality of voice services declines with increased voice compression. Voice compressions should be avoided whenever possible. Not only the voice quality is at its best, but at the same time a reliable fax transmission will be assured.