In computer networking and telecommunications, a pseudowire (PW) is an emulation of a native service over a Packet Switched Network (PSN). The native service may be ATM, Frame Relay, Ethernet, low-rate TDM, or SONET/SDH, while the PSN may be MPLS, IP (either IPv4 or IPv6), or L2TPv3. The PW emulates the operation of a "transparent wire" carrying the native service, but it is realized that this emulation will rarely be perfect. The first PW specifications were the Martini draft for ATM PWs, and the TDMoIP draft for transport of E1/T1 over IP. In 2001, the IETF set up the PWE3 working group, which was chartered to develop an architecture for service provider edge-to-edge PWs, and service-specific documents detailing the encapsulation techniques. Other standardization forums, including the ITU and the MFA Forum, are also active in producing standards and implementation agreements for PWs. There are now many PW standards, the most important of which are IETF RFCs 3985 (PWE3 architecture), 4447 (PW setup using LDP), 4448 (Ethernet PW), and 4553 (SAToP TDM PW), as well as ITU-T Y.1411 through Y.1415, Y.1452 and Y.1453 (ATM, TDM, voice services, and Ethernet PWs), and X.84 (Frame Relay PW).
