Analiza metod współpracy urządzeń Voice over IP z siecią telekomunikacyjną

W artykule przedstawiono możliwości współpracy systemu Voice over IP z siecią telekomunikacyjną (siecią telefoniczną). Na wstępie nakreślono zakres pojęcia Voice over IP, a następnie zdefiniowano pojęcia: współpracy oraz systemu Voice over IP, a także porównano pojęcie centrali Voice over IP z klasyczną centralą. Omówiono najbardziej typowe metody przyłączenia systemu VoIP do sieci PSTN, takie jak: analogowe łącze telefoniczne (reprezentowane przez styki FXS i FXO), łącze ISDN (w dostępie podstawowym) i łącze E1 (dostęp pierwotnogrupowy). Poszczególne rodzaje dostępów zostały zilustrowane przykładami wykorzystania. Oceniono ich przydatność, funkcjonalność i złożoność realizacji. Wskazano także usługi dodatkowe, warunkujące zapewnienie dobrej współpracy systemów VoIP i PSTN, w szczególności poprawną transmisję informacji wybierczych po zestawieniu połączenia. Zaproponowano kryteria oceny jakości współpracy, zwracając jednocześnie uwagę na brak formalnych wymagań w kwestii badania testowania protokołów Voice over IP i urządzeń realizujących współpracę.

This paper provides overview and analysis of a typical cooperation method for the Voice over IP system and public switched telephone network (PSTN). At the beginning, general aspects of the Voice over IP were presented. The Voice over IP is a technique that offers the users a telephone service in the IP network. Therefore there is a necessity for VoIP system to make possible connection from the VoIP system subscriber to PSTN subscriber and in reverse direction. There is a lack of uniform onomastics in Voice over IP and therefore basic definitions have been prepared. The first discussed and defined term was "the VoIP exchange". Differences between a typical telephone exchange and the VoIP exchange were presented. Next the terms "VoIP system" and "cooperation" were defined. It is assumed that "the VoIP system" is a set of stations in IP network using the same VoIP protocol for communication between them. And "the cooperation" was defined as possibility of making telephone connection (with specific quality) between the users of different telephone systems. In the main part of the paper, three types of VoIP-PSTN connections were presented and discussed. First one was an analogue telephone line (typical subscriber line in PSTN), that can act as FXO or FXS interface. This kind of access is available in almost all situations, but it has some limitations such as: lack of signalling information after making connection, necessity of using IVR system for incoming call control, etc. Second discussed interface was basic rate interface (BRI) ISDN. It offers two digital B-channels and additional signalling channel (D). Access via BRI ISDN interface enable us to use additional service like CLIP, CLIR, MSN, DDI, MWI. The last presented interface was PCM-30 interface (called E1). PCM-30 can act on various types of signalization, typically it works on DSS1 (digital signalization using in ISDN). Application of the discussed interface has been illustrated by the examples of configuration of two VoIP exchanges (Alcatel OmniPCX and Asterisk PBX). Other additional aspects of interworking, like signals transmission after connection establishing were presented too. Especially, transmission of DTMF signals in audio channel was discussed. The test results of on tones in-band transmission were presented. In the last part of the paper, appraisal of the cooperation performance was proposed. Based on the cooperation definition, such factors as time of connection establishing, speech quality, latency, and echo, presence of the additional services, were introduced. Conclusions end the paper.