The 16-port OC-3c/STM-1 Single-Mode Long-Reach ATM module is a high-density module for the Catalyst 8540 Multiservice Switch Router. It provides double the density of OC-3c ports per slot, compared to the Catalyst 8510 four-port OC-3c module. The Catalyst 8540 supports a maximum of 128 OC-3c ports. The 16-port OC-3c/STM-1 Single-Mode Long Reach module is identical to the 16-port OC-3c Multimode ATM module, except for the single-mode long-reach optical interface. Single-mode long-reach optics allows much greater distance between nodes than multimode optics.