A local area network/wide area network (LAN/WAN) switch is presented which aggregates LAN traffic onto SONET/SDH transport streams in a programmable manner. The system can concentrate Ethernet packets and map the data onto SONET streams at data rates varying from STS-1 to STS-12C. Depending on the bandwidth available in the WAN interface, different numbers and combinations of STS-n streams can be formed. The system allows for efficient and flexible mapping of LAN traffic onto a WAN for transport over fiber optic networks.