MAKING ETHERNET OVER SONET FIT A TRANSPORT NETWORK OPERATIONS MODEL

摘要

Many carriers deploy or evaluate deployment of Ethernet over SONET (EoS) as a private line offering to enhance existing service offerings. Service definitions vary between carriers, but all believe the service needs to interoperate between vendors in a manner similar to todays T1, El or T3 services. Several technology and operations issues, including Ethernet transparency, PAUSE frames, Performance Management (PM), and alarm and failure propagation, are all important issues in the deployment of EoS. Most carriers plan to deploy EoS using Generic Framing Procedure (GFP), an ITU recommendation (G.7041). GFP provides the ability to map various traffic types―in this case, Ethernet―over a transport facility. It has an optional client control function that allows for Loss of Signal (LOS) to be propagated across the network. GFP allows two mappings of Ethernet over SONET―GigE transparent mapping and frame mapping. The benefit of frame-based GFP (GFP-F) for Ethernet is that it handles various bit rates and network topologies. Unfortunately, the existence of variable length frames creates a problem that must be accounted for. GFP-F and virtual concatenation offer carriers service definition flexibility, enabling carriers to offer fractional Ethernet service based on STS-1 or VT 1.5 granularity. For fractional service to work, carriers will use PAUSE frames as defined by IEEE 802.3x to retard the incoming pipe if a customer bursts too much traffic onto a SONET network. For example, if a customer has a 100 Mbps Ethernet service but the available bandwidth is limited to an STS-1 at 51 Mbps, the carrier will use a PAUSE frame to slow down incoming traffic. Implementing this PAUSE frame and fractional Ethernet functionality is quite different from how a Tl, T3 or OC-3 private line service is offered. When the PAUSE frame is used by an optical or SONET network, it is not possible for customer switches to use PAUSE frames to manage their connectivity transparently. EoS, like other private line services, has PM requirements so the carrier can develop Service Level Agreements (SLAs) and do trouble-shooting. One key difference between Ethernet and other transport services is that Ethernet does not have a fixed frame size, which impacts the PM data that is created. Another network operation requirement is that SONET networks, and therefore private line services, have the ability to propagate alarm and failure information through the network. It is critical for some Layer 2 Ethernet switched networks to learn about physical faults so they can provide protection.