Figure 6.4: Public Switched Telephone Network(PSTN Switching office Access Line(接入线) Central office Local Loop(本地回路) End office Single Twisted Pair Of Low Quality lr Customer Premises Customer Premises 客户驻地 (Residential) (Business) 6-136-13 Figure 6.4: Public Switched Telephone Network (PSTN) Customer Premises 客户驻地 (Residential) Customer Premises (Business) Switching Office Central Office End Office Access Line (接入线) Local Loop (本地回路) Single Twisted Pair Of Low Quality