Figure 6.4: Public Switched Telephone Network(PSTN Class 3 Switching Hierarchy Trunk Line (干线) Digital Class 4 Class 5 Class 5 End End Office Office 6-146-14 Figure 6.4: Public Switched Telephone Network (PSTN) Switching Hierarchy Trunk Line (干线) Digital Class 4 Class 3 Class 5 End Office Class 5 End Office