Ethernet and SONET/SDH are the two primary link technologies used in the network communications while Ethernet in the LAN and SONET/SDH in the Telco/PTT WAN. Ethernet over SONET/SDH (EoS) collectively represents a group of industry standard specifications for optimal transport of Ethernet through SONET/SDH, because Ethernet is not directly supported over the SONET/SDH network traditionally. Basically, EoS capability turns the Sonet/SDH MAN/WAN infrastructure backbone into a transparent Ethernet segment for attached servers and clients.

The core technologies in the EoS architectures are the encapsulation schemes. Currently, there are four encapsulation techniques available-virtual concatenation (VC) and the link capacity adjustment scheme (LCAS) techniques, and the generic framing procedure (GFP) and link access procedure for SDH (LAPS) techniques.

The Link Access Procedure for SDH (LAPS), the traditional way of carry Ethernet traffic over SONET/SDH, is defined as a type of high-level data link controller (HDLC) that includes data link service and protocol specification used in transporting IP packets over SDH networks. LAPS enables the encapsulation of IPv6, IPv4, PPP, and other higher-layer protocols. LAPS provides a point-to-point unacknowledged connectionless service over SDH.

Virtual Concatenation allows for non-standard SONET/SDH multiplexing in order to address the bandwidth mismatch problem. Using virtual concatenation, the SONET/SDH transport pipes may be right-sized for Ethernet transport. Virtual Concatenation is a technique that allows SONET channels to be multiplexed together in arbitrary arrangements. This permits custom-sized SONET pipes to be created that are any multiple of the basic rates. Virtual concatenation is valid for STS-1 rates as well as for Virtual Tributary (VT) rates. All the intelligence to handle virtual concatenation is located at the endpoints of the connections, so each SONET channel may be routed independently through the network without it requiring any knowledge of the virtual concatenation. In this manner, virtually concatenated channels may be deployed on the existing SONET/SDH network with a simple endpoint upgrade. All the equipment currently in the center of the network need not be aware of the virtual concatenation.

Link Capacity Adjustment Scheme (LCAS) which dynamically changes the amount a bandwidth used for a virtual concatenated channel. Using Link Capacity Adjustment Scheme (LCAS), signaling messages are exchanged within the SONET overhead in order to change the number of tributaries being used by a Virtually Concatenated Group (VCG). The number of tributaries may be either reduced or increased, and the resulting bandwidth change may be applied without loss of data in the absence of network errors.

Generic Framing Procedure (GFP) is the protocol for mapping packet data into an octet-synchronous transport such as SONET. GFP has adapted the cell delineation protocol used by ATM to encapsulate variable length packets. A fixed amount of overhead is required by the GFP encapsulation that is independent of the contents of the packets. In contrast to HDLC whose overhead is data dependent, the fixed amount of overhead per packet allows deterministic matching of bandwidth between the Ethernet stream and the virtually concatenated SONET stream. Within GFP, there are two different mapping modes defined: frame based mapping and

transparent mapping. Each mode is optimized for providing different services.