Abstract: Protection for service lines linking offices in a communications system is realized by linking the offices in a sequence with a serial connection of protection lines. Protection for service lines which bypass one or more of the offices in the serial sequence is achieved by automatically controllably cascading appropriate ones of the protection lines. The cascading of protection lines is realized on a nonhierarchical basis by assigning each service line in the system a unique address and by supplying a protection switch request message including the address of the requesting service line sequentially to each office in the serial sequence until the transmit-end of the requesting line is located. A switch is automatically effected by equipment located at the offices only if all the protection lines to be cascaded are available.