ass Ethernet switches are deployed directly on an optical layer. In this case, in the quite frequent situation in which the former network architec- ture was based on SDH/SONET, some solution is required to manage legacy TDM equipment. The solution is the so-called pseudowiring. In general terms a pseudowire is an emula- tion of a native service over a packet switched network (PSN). The native service may be low-rate TDM, SDH/SONET, ATM, frame relay, or Ethernet while the PSN may be Ethernet, MPLS, or IP (either IPv4 or IPv6). A pseudowire emulates the operation of a "transparent wire" carrying the native service. There are now many pseudowire standards, the most important of which are IETF RFCs as well as ITU-T recommendations [85]. Switching Systems: Architecture and Performances TDM pseudowire design to carry a real-time bit stream like SONET/SDH has, how- ever, many specific characteristics. As a matter of fact, conventional TDM networks have numerous special features, in particular, those required to carry voice-grade telephony channels. These features imply signaling systems that support a wide range of telephony features, a rich standardization literature, and well-developed operations and manage- ment (OAM) mechanisms. All of these factors must be taken into account when emulating SDH/SONET over Ethernet. Another critical issue in implementing SDH/SONET pseudowires is clock recovery. In native TDM networks the physical layer carries highly accurate timing information along with the TDM data, but when emulating TDM over Ethernet this physical layer clock is absent. TDM timing standards dictate precise and demanding requirements, and confor- mance with these requires careful design of Ethernet equipment. A final issue that must be also addressed is SDH/SONET pseudowire packet loss con- cealment. Since TDM data is delivered at a constant rate over a dedicated channel, the native service may have bit errors, but data is never lost in transit. As all packet networks, also Ethernet suffers to some degree from p