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Một cấu hình của PRI được ghi nhận như là 30B + D. PRI này cung cấp một tỷ lệ bit 2,048 Mbps và thường được cung cấp ở châu Âu và Australia.Các điểm tham chiếu cho dòng PRI đơn giản hơn so với dòng BRI. Các chức năng của các điểm tham chiếu là tương tự như trong dòng BRI. | Using ISDN and DDR to Enhance Remote Access Connectivity Chapter 5 157 figurations for the PRI. In North America and Japan the configuration is noted as 23B D or 23 B-channels and one D-channel operating at 64 Kbps. The bit rate of this type of PRI is 24 X 64 Kpbs 1.544 Mbps. Another configuration of the PRI is noted as 30B D. This PRI offers a bit rate of 2.048 Mbps and is commonly offered in Europe and Australia. PRI Reference Points and Functional Groups The reference points for PRI lines are simpler than for BRI lines. The functions of the reference points are the same as in the BRI line. The major difference is that PRI does not support multiple ISDN devices on the same line whereas a BRI network supports connecting multiple devices to the same line. As shown in Figure 5.3 in PRI lines the Terminal Equipment TE connects directly to the Data Service Unit Channel Service Unit DSU CSU which then connects to the Local Exchange LE . The DSU CSU is similar to a modem but does not convert digital signals into analog signals. Since there is no support for non-ISDN multiple devices the reference points and functional groups for the PRI line can be kept simple. Figure 5.3 ISDN PRI reference points and functional groups. S T U TE DSU CSU LE ISDN Protocol Layers ISDN uses several different protocols for both control signaling and user data. The protocols can be correlated to the Open System Interconnection OSI reference model. The OSI reference model regulates all communication between systems to ensure interoperability between vendors. The OSI reference model consists of seven functional layers including Physical Data Link Network Transport Session Presentation and Application. Since signaling protocols and user data protocols are different yet still operate in the same OSI layers it further divides the OSI model into protocol planes. The user plane U-plane contains the protocols required for sending user data such as voice video and data. The control plane C-plane .
