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Ethernet Over SDH or Ethernet over SONET refers to a set of protocols which allow Ethernet traffic to be carried over synchronous digital hierarchy networks in an efficient and flexible way. The same functions are available using SONET.
Ethernet frames which are to be sent on the SDH link are sent through an "encapsulation" block to create a synchronous stream of data from the asynchronous Ethernet packets. The synchronous stream of encapsulated data is then passed through a mapping block which typically uses virtual concatenation to route the stream of bits over one or more SDH paths. As this is byte interleaved, it provides a better level of security compared to other mechanisms for Ethernet transport.
After traversing SDH paths, the traffic is processed in the reverse fashion: virtual concatenation path processing to recreate the original synchronous byte stream, followed by decapsulation to converting the synchronous data stream to an asynchronous stream of Ethernet frames.
The SDH paths may be VC-4, VC-3, VC-12 or VC-11 paths. Up to 64 VC-11 or VC-12 paths can be concatenated together to form a single larger virtually concatenated group. Up to 256 VC-3 or VC-4 paths can be concatenated together to form a single larger virtually concatenated group. The paths within a group are referred to as "members". A virtually concatenated group is typically referred to by the notation pathType-Xv, where pathType is VC-4, VC-3, VC-12 or VC-11 and X is the number of members in the group.