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Reasons for using the echo-cancellation (EC) technique for full-duplex two-wire transmission of ISDN signals are given. Analysis of the performances achievable with different codes, such as 3B/2T, MMS43, partial response class 4 (PR4), and diphase, justifies the final selection of MMS43. A full-digital integrated U-interface circuit with MMS43 code and with Barker codeword synchronization is described. First laboratory test results are presented.