And, although
SONET originally was designed to carry DS1 and DS3 signals, its tremendous base
of installed equipment (and engineering know-how), as well as its strong operational
and survivability characteristics, remain attractive to service providers who provide
Ethernet services.
EoS Overview
The initial drive to carry Ethernet over SONET dates back to the mid-1990s and the
first wave of telco Ethernet services.2 Many of these services used dedicated, proprietary
networks. At the same time, SONET deployment was in full swing. Service providers
and equipment vendors saw a simple opportunity to lower costs by integrating the
2 For example, Bell Atlantic??™s FDDI Network Service (FNS) and Ameritech LAN Interconnect Service (ALIS)
SONET/MSPP 303
delivery of Ethernet services with the delivery of circuit services over the burgeoning
SONET infrastructure. This drove the need for network elements that could carry
Ethernet over SONET.
The fundamental technical issue with EoS technology is the mapping of Ethernet
frames, which ride on asynchronous interfaces, within synchronous SONET payloads.
While there is nothing technically foreboding about this (recall that SONET was invented
to carry plesiochronous signals within synchronous payloads), the industry first needed
to define a standard set of protocols to map Ethernet frames into the SONET SPE.
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