delay, the seven layers of OSI were truncated at layer 4. In fact, CCITT Rec. Q.709 specifies no more than 2.2 seconds of postdial delay for 95% of calls. To accomplish this, a limit is placed on the number of relay points, called STPs, that can be traversed by a signaling message and by the inherent design of SS No. 7 as a four-layer system. Figure 14.2 relates SS No. 7 protocol layers to the OSI reference model. Remember that reducing the number of OSI layers reduces processing, and thus processing time. As result, postdial delay is also reduced. We should note that SS No. 7 layer 3 signaling network functions include signaling message-handling functions and network management functions. Figure 14.3 shows the general structure of the SS No. 7 signaling system. There seem to have been various efforts to force-fit SS No. 7 into OSI layer 4 upwards. These efforts have resulted in the sublayering of layer 4 into user parts and the SCCP (signaling connection control part). In Section 14.4 we briefly describe the basic functions of the four SS No. 7 layers, which are covered in more detail in Section 14.5 through 14.7. Signaling System No. 7 Open system interconnections (SS No. 7) (OSI) Application layer Presentation layer Session layer User parts (SCCP) Transport layer Signaling network Network layer functions Signaling link control Data link layer Signaling data link Physical layer Figure 14.2 How SS No. 7 relates to OSI. CCITT SIGNALING SYSTEM NO. 7 Layer 4 Layer 3 Layer 2 Layer 1 parts Message transfer part Signalling network functions Signalling data link Signalling message handling Message Message Signalling discrimin- Switching Transmission distribution functions channels ation functions Message routing Signalling network management Signalling traffic management Signalling route management Signalling management Testing and maintenance Signalling message flow SCCP-Signalling connection control part Controls and indications Figure 14.3 General structure of signaling functions. [From ITU-T Rec. Q.701, Figure 6/Q.