Ethernet transport over SONET & ROADMs

Ethernet is transported over bidirectional fiber rings located between carriers' COs or POPs and customers. For the most part, these rings are based on SONET technology. However, SONET was not designed for Ethernet service. Ethernet on SONET wastes capacity because of high overhead (nonuser data for addressing, signaling, and maintenance) and mismatches in frame sizes between SONET and Ethernet. In addition to using bandwidth inefficiently, it is more complex to separate out Ethernet traffic for individual customers on SONET than on newer dense wavelength division multiplexers.

As they carry more IP and Ethernet traffic, some telephone companies are testing dense wavelength division multiplexers (DWDM) equipped with reconfigurable optimal add and drop multiplexers (ROADMs) cards. DWDM equipment combines up to 768 channels (called wavelengths or colors) of traffic onto a single pair of fiber cabling. ROADMs enable carriers to more easily add, separate out, and drop off traffic carried on optical rings to and from customers.
ROADM-equipped multiplexers will carry CO-to-CO traffic in addition to Ethernet and IP-based customer traffic. It will encapsulate individual customers' Ethernet traffic on single wavelengths (colors) derived from DWDMs. Many carriers currently plan to use ROADM equipment on shorter fiber rings with high amounts of IP, Ethernet, and storage area network protocols such as fibre channel traffic. The cost per bit for carrying this type of traffic is lower on DWDM than on SONET.

These DWDM devices will be built as carrier class, fully redundant configurations. Each fiber ring, multiplexer, and power supply will be duplicated. The multiplexers will sense failures and fiber cuts and automatically use the backup ring and multiplexers. This is referred to as automatic failover or resilient packet ring. Resilient packet ring (RPR) is an IEEE standard that is used in SONET and GigE rings. Luminous Networks and Cisco offer GigE-based resilient ring products.

SONET will continue to be used on fiber rings that carry less traffic and that aggregate streams of slower speed voice and T-1/E-1 data. Newer providers, such as OnFiber Communications, already carry metro Ethernet traffic on DWDM. Some phone companies are upgrading SONET to make it more suitable for data rather than planning to transition to DWDM with add and drop multiplexers.