to determine that the device is up and functioning. A second example is the use of multiple IP addresses as a way to simulate the multiple D- channels used for signaling in large-scale ISDN networks. The transition from TDM to IP trunking involved many steps, such as those shown in Figure 2-5. Finally the quality of IP trunking needs to meet and exceed the quality of TDM trunk- ing. This is most evident in the area of FAX, in which long duration and complex fax sce- narios have a higher success rate on traditional TDM-based interconnect services than IP trunking services. Table 2-3 summarizes trends in SIP trunk offerings. The long-term trend to IP trunking as a replacement to TDM trunking is strong and is expected to replace TDM as a method for interconnecting enterprise customers to the PSTN within the next 5 to 10 years. Number of TDM Trunks Deployed Worldwide for PSTN Connections Enterprise Interconnect Method Number of IP Trunks Deployed Worldwide for PSTN Connections Figure 2-5 Transition from TDM to IP Trunking 26 SIP Trunking Table 2-3 Trends in SIP Trunk Offerings Trend Why Is This Trend What New Service Offering Occurring ? Results from This Trend? There is a trend toward This trend expands the cus- SIP trunking services required more diversity in endpoints tomer base as end users look to connect to a specific device, that can connect to SIP toward a variety of IP-PBX instead of allowing any device trunking service. vendors. to connect. Customers are becoming Customers have the same Options Ping and SLAs based more sensitive to the ability expectation from SIP trunks on SIP are available. to quickly respond to as from TDM trunks. outages. Better FAX connectivity Customers have an expecta- T.38 FAX and SLAs around options. tion of FAX quality that is FAX success rates will be not met. TDM trunks are offered to alleviate the expecta- perceived to have a higher tion that SIP trunking is not as quality. high quality as TDM trunking. Better redundancy in ISDN has a natural redun- Multiple signaling channels 