"This technical report provides transmission planning guidelines for voiceband services over connections that include Internet and PSTN segments@ i.e.@ hybrid Internet/PSTN connections. In particular@ ""voice over the Internet"" or ""Voice over Internet Protocol"" (VoIP) connections are considered@ where at least part of the connection is over an Internet Protocol (IP) network. PSTNonly connections (i.e.@ connections that begin@ end@ and are carried only on the PSTN) are considered as the reference against which the performance of other connections is compared. This report provides end-to-end transmission performance guidance for hybrid Internet/PSTN connections carrying voiceband services. When one of the terminals is attached to an IP network@ it is assumed that this terminal has functionality defined in ITU?CT Rec. H.323. (The terminal need not be an H.323 terminal@ but is assumed to have that functionality.) To support these end-to-end guidelines@ guidance is given for interworking functions between Internet and PSTN. The functionality provided by Internet/PSTN gateways presents special performance issues that must be addressed to ensure high quality service. Protocol details of IP and higher level protocols (e.g.@ TCP@ UDP@ RTP@ RSVP) are not considered in this report. Rather@ the impact on performance of voiceband services of the packetized aspects of IP networks@ such as delay associated with packet assembly@ lost or discarded packets@ is of primary interest here. This report also includes discussion of issues associated with the interworking functions provided by Internet/PSTN gateways. Purpose This report provides guidance on basic telephony issues involved in providing voiceband services over hybrid Internet/PSTN connections. This information will also be useful to equipment manufacturers@ VoIP service providers@ Internet Service Providers (ISPs)@ software developers@ and PSTN providers where service goes over a gateway to IP networks. Traditional PSTN engineers will be familiar with the telephony aspects of this report@ but will find useful the material on Internet-specific features of voiceband services over connections that incorporate an Internet segment. This technical report serves as preliminary material. It lays the groundwork for a future report on statistical tools for evaluating and comparing the performance of various networks. This report is complementary to other work in T1A1 on performance of voiceband services over ATM@ Frame Relay@ etc. Application The importance of user expectations for voiceband services on hybrid Internet/PSTN networks is recognized. However@ this report does not address this subject@ nor does it offer detailed suggestions of how to assess the impact of user expectations on the acceptability of these services. Additional guidance on many of these issues is available in ITU?CT P-series and G.100-series Recommendations. It is intended that the guidelines provided in this report be applied by equipment manufacturers@ VoIP service providers@ ISPs@ software developers@ and PSTN providers. Those who provide services where the PSTN interconnects with IP networks through a gateway should find this information useful as they design equipment@ software and services for voiceband services over hybrid Internet/PSTN connections. Application of the guidelines herein by equipment manufacturers@ VoIP service providers@ Internet Service Providers@ software developers@ and PSTN providers should enhance the quality of voiceband services delivered over hybrid Internet/PSTN connections. Furthermore@ if widespread customer acceptance of VoIP service is desired@ the resulting voiceband service quality should approach that of the PSTN. The guidance in this report applies to situations/connections illustrated in Figure 1@ with special attention focussed on the ""Gateway/Inter-Working Function"" (which provides the essential interworking capabilities of these hybrid connections). A major application of this report will be Internet Telephony where the terminal (e.g.@ a PC) enjoys H.323 stack functionality. As noted above@ this report will use PSTN-only connections as the reference against which the performance of other connections will be compared. This report applies to connections where the data transfer phase occurs over hybrid Internet/PSTN connections. Issues of call set-up@ number translation (between PSTN and IP numbering)@ etc.@ are not considered in this report."