In this arrangement, there is a one-to-one correspondence between the sparing switch cards 230 and the primary line cards 140A. Thatis, each sparing switch card 230 receives a group of secondary TX and RX lines corresponding to a group of primary TX and RX lines attached to one of the primary line cards 140A. The backplane 200 also carries a switch selection control card 180 that isadapted to cont