Second generation SONET

Second generation SONET also referred to as multiservice platforms, achieved higher speeds (up to OC-192 [Optical Carrier-192], 10 gigabits), took up less space by supporting more ports on each card, and gave carriers the capability to increase and decrease speeds remotely without taking the ring out of service. They also enabled carriers to drop off lower optical carrier streams to customers for enterprise SONET (Synchronous Optical Networking) services such as Ethernet and storage area network services. However, next-generation multiplexers do not interface directly to MPLS (Multiprotocol Label Switching) networks. In addition, although they carry Ethernet and storage area network services, they do so inefficiently, in SONET frames. Next-generation SONET devices can have internal add and drop multiplexers and digital cross-connect systems.

ADD and Drop Multiplexers (ADM)
Add and drop multiplexers add and drop channels from fiber rings at the edge of the network. They drop off and pick up channels to a particular central office or to a small metropolitan area from rings that connect the core to the access network. Add and drop multiplexers are less complex and handle fewer streams of traffic than digital cross connects.

Digital Cross Connects
Digital cross connects rearrange channels of traffic between multiple routes. A digital cross connect system has the same functionality as a switch. For example, multiple rings may connect at a carrier's point of presence (POP) in the core network in the northeast. The digital cross connect sends some of the traffic to, for example, New York, some to Pennsylvania, and the rest to New Jersey. It also accepts traffic from these states and connects it to other routes. The newest digital cross connects are all-optical. They switch colors (channels) of traffic without converting light signals carried on fiber to electrical signals and electrical signals back to light. This eliminates the need for conversation equipment in these devices, which leads to lower prices and higher-speed switching.