Ethernet over SONET
Ethernet over SONET is a set of protocols and technologies for carrying Ethernet traffic over a SONET infrastructure. Historically, when SONET is being used to interconnect two Ethernet-based networks, a router is needed to forge the link between the disparate networks. This is primarily because, with the exception of 10GigE, SONET is not a supported Physical Layer option for Ethernet. So the more conventional approach to making the interconnection is to use two routers, each equipped with an Ethernet port for the local Ethernet, and a SONET port for the interconnection. Typically, the SONET circuit is between buildings or premises, and it could be metropolitan, regional, national, or global in scope. Assuming an IP router, the Point-to-Point Protocol (PPP) is typically implemented over the SONET link.
The use of this arrangement is rooted in history. The common carriers initially offered T-carrier services, and then later SONET services to their customers. SONET was engineered to be backwards compatible with the earlier T-carrier system, and SONET multiplexers were designed to handle these interfaces and these interfaces only.
Demand for Ethernet services, however, caused SONET multiplexer manufacturers to enhance their platforms and make it possible to insert Ethernet line cards into the multiplexer. The multiplexer then hands off an Ethernet circuit at either end, making it possible for the end user to use less-expensive routers with only Ethernet interfaces, or to eliminate one or both of the routers from the network completely.
Putting an Ethernet interface directly into a SONET multiplexer, however, poses some significant challenges for the SONET infrastructure. One issue concerns transmission rates. The Ethernet transmission rates of 10 Mbps, 100 Mbps, and 1 Gbps have no direct correlation to the traditional SONET payloads. In conventional SONET, this would force the service provider to either under-provision the circuit (e.g., and STS-12c for a GigE port), or over provision it (e.g., an STS-3c for a Fast Ethernet connection). Another issue concerns framing. The Ethernet practice of letting the transmission line return to an idle state when there is no frame to be transmitted would not be well received over a SONET network. SONET tends to interpret darkness as a failure. So the multiplexers have to provide a form of frame synchronization that would make it possible for the SONET multiplexer at the egress point of the ring correctly locate and extract the Ethernet frame from the SONET stream.
Although all of these issues can be resolved in a proprietary fashion, the use of industry standards makes it possible to build highly interoperable equipment, and to provision services that cross the networks of multiple carriers. Such standards now exist and fall under the umbrella of next generation SONET (NG-SONET).
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