. Pseudowire (PW) is an emulation of a native service (ATM, Frame Relay, Ethernet, SONET/SDH, etc.) over a packet switched network (IP, MPLS, etc.), commonly it is used for Ethernet over PSN (packet switched network). Figure 7.15 illustrates a PWE3 stack reference model. It illustrates a layered architecture for service emulation PSN based on PWs. The emulated service payload is encapsulated within the PW. In order to transport the PW end-to-end, the PW is carried within a PSN tunnel. Pseudowire solutions began quietly as a way to transport legacy services over operators' MPLS core and have been implemented by several vendors. However, it should be noted that there is a limited time-frame in which PW technology is a relevant option. As more and more mobile node equipments support Ethernet interfaces and native IP transport, PWE3 may have to find other drivers [23]. Emulated Service Emulated Service (e.g. TDM, ATM) Emulated Service (e.g. TDM, ATM) Payload Payload Encapsulation Pseudo Wire Encapsulation PW Demultiplexer PW Demultiplexer PSN Tunnel, PSN & PSN Tunnel PSN Tunnel, PSN & Physical Layers Physical Layers Packet Switched Network (PSN) Figure 7.15 PWE3 stack reference model. Reproduced from C 2008 Metro Ethernet Forum [24] Transmission Networks 7.4.2.3 Carrier Ethernet The Ethernet based LAN networks have been around since the 1970s. It is the most widely deployed LAN technology due to its simplicity, cost efficiency and ease of deployment. The Ethernet which was originally designed for simple data sharing over LAN has seen significant development in the last ten years. From the simple 10 Mbps shared operation in 1980s to today's 10 Gbps switched operation and to tomorrow's 100 Gbps, Ethernet is expected to be a key transport technology in MAN (Metropolitan Area Network) and WAN (Wide Area Network) networks. In recent years, service providers have started to deploy Ethernet as a transport technology in the MAN or WAN environments. The key disadvantages of traditional transport technologies like Frame Relay,