irs that are able to carry data rates up to 2 Mbps. Ordinary subscriber pairs are used for analog telephone subscribers, analog and digital PBX/PABX con- nections, ISDN basic rate connections and ADSL. ADSL and ordinary ana- log telephone circuits use the same 2W subscriber loop. Data and speech connections may be used simultaneously and they are separated in the Introduction to Telecommunications Network Engineering Customer Local access network premises 64 Kbps PCM coded voice Analog 2W circuits: Analog Analog Analog Analog Analog Modem Digital Analog transmission basic Digital circuits: Digital Digital to trunk local exchanges 160 Kbps Digital 2M exchange Analog Digital functionality 1.5 or 2 Mbps Splitter 256 Kbps distribution Internet frame (MDF) Figure 2.15 Local-access network and digital local exchange site. exchange where speech is connected to an ordinary analog exchange interface and data are routed to the Internet, as shown in Figure 2.15. A digital exchange may include both analog and digital subscriber interfaces. For digital private (automatic) branch exchange (PBX/PABX) applications, 1.5- or 2-Mbps digital interfaces are available. If the local switch has ISDN capability, basic rate and primary rate interfaces are avail- able. Ordinary subscriber pairs are used for ISDN basic rate connections (160-Kbps bidirectional) and a network terminal (NT) is required on cus- tomer premises. The primary rate interface of ISDN (1.5 or 2 Mbps) is used for PABX connections. It requires two pairs, one for each transmission direc- tion, and supports many simultaneous external calls. In addition to MDF, network operators may use other distribution frames for transmission network management and maintenance. An optical distribution frame (ODF) contains two fields of optical fiber connectors. The optical cables of the network are connected to one connector field and the other one is connected to optical line terminal equipment. Cross- connections between two connector fields are created with optical fibers. This allow