Aastra OpenCom 130 User Guide - Page 114
Call Set-up, 9.2.6 Useful Services, Telephony, Extended, profile, Data analogue
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Voice over IP (VoIP) Fundamentals 9.2.5 Call Set-up Various IP-based protocols are used for system telephony via the Internet protocol ("IP") (see also Start Procedure starting on page 129). Multiple TCP connections are made between an IP telephone and OpenCom 100 for the telephone's start procedure, registration and signalling. All voice data are directly exchanged between IP telephones using the RTP ("Realtime Transport Protocol") protocol. Channels on a Media Gateway (MGW) are allocated for making a telephone connection with an ordinary terminal or for dial tones. The MGW converts IP voice data into PCM data streams used with conventional telephony and vice versa. For this, IP voice data are exchanged between the IP telephone and the gateway. Tip: Switching between voice data channels may cause a slight delay in some circumstances. For example: when accepting a call on an IP telephone, headset users should wait about one second before answering. 9.2.6 Useful Services The type of data compression used for VoIP prevents these types of connections from using certain services. Take these notes into account especially if you want to use connections made via Q.SIG-IP or SIP: ■ ISDN data services can not be used ■ Faxes can only be sent using the uncompressed G.711 codec ■ DTMF dial tones are only received by the other party if the uncompressed G.711 codec is used. Alternatively, DTMF dial tones can be transferred using the Internet standard RFC 2833/4733. For this, the "DTMF (RFC4733)" codec needs to be activated for the codec configuration on the Telephony: Extended: VoIP profile page. ■ Analogue modems can not be used Tip: Configure the actual usage for the a/b ports, e.g. set them to Fax or Data (analogue). Connections from and to these a/b ports will then be made using uncompressed or ISDN connec- tions where possible. 112