Voice over IP (VoIP) Fundamentals114“Minimise Delay” and “Maximise Throughput” for IP packetsmarked with this value.10.2.5 Call set-upVarious IP-based protocols are used for system telephony via the Internet protocol(“IP”) (see also Start Procedure starting on page 125). Multiple TCP connections aremade between an IP telephone and OpenCom 510 for the telephone’s start pro-cedure, registration and signalling.Call data is directly exchanged between IP telephones using the RTP (“RealtimeTransport Protocol”) protocol.Channels on a Media Gateway (MGW) are allocated for making a telephone con-nection with an ordinary terminal or for dial tones. The MGW converts IP voicedata into PCM data streams used with conventional telephony and vice versa. Forthis, IP voice data are exchanged between the IP telephone and the gateway.Tip: Switching between voice data channels may cause a slightdelay in some circumstances. For example: when accepting acall on an IP telephone, headset users should wait about onesecond before answering.10.2.6 Useful servicesThe type of data compression used for VoIP prevents these types of connectionsfrom using certain services. Take these notes into account especially if you want touse 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 uncompressedG.711 codec is used■ Analogue modems can not be usedTip: Configure the actual usage for the a/b ports, e.g. set them toFax or Data (analogue). Connections from and to these a/b
