The D4 channel bank is the most recent Western Electric product in an evolution of time‐division multiplex terminals for digital transmission facilities. D4 incorporates many technical innovations relative to the earlier developed D1D, D2, and D3 channel banks but is fully compatible with these earlier banks. D4 banks may be operated in any of five different modes as required to connect to a particular digital carrier facility (T1, T1C, T2, or FT2). The D4 channel bank is partitioned into common equipment and channel units. The common equipment provides digital line interface, pulse code modulation coding and multiplexing, alarms, trunk processing power, power fusing, and distribution. Individual channel units provide the circuits that interface to external circuits. There are many different versions of channel units including standard interoffice voice‐frequency trunk types and those supplying data and special services.
A new system has been designed to provide protection switching capability for the L5 repeatered line. This line‐protection switching system, called lpss‐3, provides one standby line for protection of as many as 10 service carrying lines. Switching equipment is located in terminal, terminal main, and switching power‐feed stations. Economic and reliability considerations allow switching spans of up to 150 miles. Either temperature pilot deviations or excessive total signal energy initiates automatic switching. Manual control of all switching functions is provided locally and via telemetry. A dedicated pcm signaling system maintains inter station switching system coordination. Identical information is transmitted in complementary form over two adjacent L5 channels. Parity and complementary checking as well as automatic retransmission of failed codes enhances error immunity.
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