 and 10 GbE speeds. Fiber deployment in major metro areas and advances in last-mile access technologies, such as wireless broadband and free-space optics (FSO), aim at removing the access network bottleneck. Unfortunately, today only a very small fraction (a few percentage points) of buildings are Advances and Opportunities in Next-Generation SONET actually on fiber links to the metro core network, and alternative access technologies are not proven or widely deployed. 2. Metro core networks interface with both the local metro access network and the WAN network. Historically, SONET-based transmission systems have provided MAN connectivity. These systems were originally designed and built to carry voice traffic. As such, they operate in a time- division multiplexing (TDM) mode at the physical (PHY) layer and rely on an added data link layer (DLL) protocol such as the Point-to-Point Protocol (PPP), Asynchronous Transfer Mode (ATM), or Frame Relay. Most of the traffic growth in these networks is now due to data applications. The critical question is whether the embedded SONET transport infrastructure (which represents billions of dollars of partially depreciated assets in an estimated 100,000 rings in the United States)¹3 is adequate and/or optimized for data traffic. For a reference point, a typical GbE-based metro backbone deployed by the ELECs uses a trivialized statistical TDM (STDM) mode at the PHY layer14 and a Media Access Control (MAC) protocol at the DLL level. Metro service providers are looking for cost-efficient, data-optimized solutions. The question then becomes whether a TDM PHY plus a PPP (or ATM) DLL is better than a trivialized STDM PHY plus a MAC DLL, or vice versa. The key to the answer is simple: Follow the money. In rendering an end- to-end reliable secure monitored service, where are the cost components that comprise the total cost? Is the cost in the protocol engine or is it in the physical fiber link from the building to the metro core network? 3. WANs have been implemented using a variety of