wever both optical and electrical interfaces are recommended. Implemen- Cell by cell tations are to allow for terminal interchangeability. Maximum func- ALPHA tional commonality between the UNI physical layer and that of the consecutive incorrect HEC SYNCH NNI is aimed at. DELTA consecutive correct HEC 41.4.12.1 Physical medium characteristics of the UNI Physical characteristics of the UNI at both 155.520Mbit/s and Figure 41.17 Cell delineation state diagaram (Fig. 5/1.432) 622.080Mbit/s are considered, listing transmission media, interface range, electrical and optical parameters and connectors. 41/18 Asynchronous Transfer Mode randomise the cell data for possible improvement of the trans- transmission system: SDH based, Cell based and PDH based: use of mission performance. The self-synchronising scrambler poly- SDH path and section overheads, maintenance cells for the physical nomial x43 + 1 has been chosen for the SDH-based physical layer and bit messages in PDH are described. For ATM layer flows, layer. A Distributed Sample Scrambler (DSS) using the polyno- special cells dedicated to OAM functions are provided. mial x31 28 + 1 appears the probable choice for the cell-based A tabulated approach is used to specify the OAM functions of the physical; layer. (ETSI, 1990) UNI and their related OAM flows. Tables are provided for the SDH-based physical layer, the cell-based physical layer and the 41.4.13 1.610 OAM principles of the B-ISDN ATM layer. access This covers the maintenance of the B-UNI and the B-ISDN sub- scriber access. It follows the maintenance principles defined in Rec. 41.5 Quality of service performance M.20 and describes the minimum functions needed to maintain the requirements Physical layer and ATM layer of the customer access. Five phases have been assumed in specifying OAM functions for B-ISDN: Quality of Service (QoS) is a set of measures of service performance performance monitoring, defect and failure detection, system pro- which determine the degree of satisfaction of a user of the specific 