unk for PSTN Access SIP Trunks for PSTN access is one of the fastest growing services currently offered by SPs. The basic service is purely as an ISDN replacement that offers no service beyond the same functionality of a TDM ISDN connection. The essential features provided in this basic offering are shown in Table 2-2. These are the least common denominators of SIP trunking features. Just getting all the SPs throughout the world to determine how those basic services are offered has taken years and, as of 2009, is still not consistently offered. The macro trends in the area of advanced SIP trunks for PSTN access are based on the other trends described in this chapter. The long-term direction of SIP trunking services is to be enhanced to deal with the increased number of endpoints, and the diversity to this trend is that the SP needs to deal with a larger number of problems as the intercon- nect methods for different types of premise equipment can result in additional mainte- nance costs. Table 2-2 Essential Features of a Basic SIP Trunk Offering Signaling Method Signaling Transport Mechanism Physical Layer 3 Over the top, DSL, T1/Frame Relay, cable modem, Metro Connectivity Method Ethernet, 3G, other Compression Method G.711, G.729 Features Call setup and teardown, Hold, Resume; ability to send DTMF tones; support for setting a CALLERID and CALLING NAME Chapter 2: Trends in IP Telephony 25 A second major developing trend is the transition to offer a more complete replacement to an ISDN circuit. ISDN circuits have built-in redundancy and alarming mechanisms not native to SIP trunks. SPs find that customers demand these features with this alternative connectivity method. They also are discovering that lower prices are no longer a motiva- tor to replace traditional TDM circuits. Specific examples of this are SPs offering Layer 7 Keepalive ping messages to simulate what was previously done in Q.921. In SIP, the method to do this is using an Options Ping sent from the SP to the customer and from the customer to the SP 