hown in Figure 7.26. There is also considerable effort to use Carrier Ethernet technologies in the aggregation and metro networks to replace the SONET. Transmission Networks PBB-TE Gigabit Ethernet Microwave Radio 802.1D/802.1Q Microwave Radio TDM/IP Hybrid Microwave Radio MPLS-TP TDM-based (63E1s) NG-SDH/MSPP Microwave Radio TDM-based Ethernet (LAN) SDH/SONET 1950s 1960s 1970s 1980s 1990s 2001-2005 2006-2010 2010s Figure 7.26 Transport network evolution Currently, the transition to Hybrid (TDM/IP) Transport is under progress where the microwave radios have both TDM and Ethernet interfaces. The traffic from the radios to be carried all the way to core network where IP/MPLS is used which can carry various types of traffic. Many new base stations already support IP interfaces, but the intermediate backhaul network is rarely IP routed. The backhaul infrastructure legacy nodes, based on ATM and TDM remain abundant. And thus the transition to an all-IP transport network is a long way to go. Microwave Radio has been evolving to cater for all forms of traffic, namely TDM, TDM/IP hybrid and Ethernet from the cell sites. It is common practice for operators to rely on TDM-based leased lines and microwave radio links. In the last two years, a considerable effort has been put into investigating the use of Gigabit Ethernet Radios for 3G, LTE WiMAX and HSPA+ networks. Optical Technology had undergone a major evolution since its birth in the 1980s. A revolution started in the second half of the 1990s with the development of optical amplification for increasing the repeater spacing and WDM for increasing the aggregate bit rate. Erbium-doped fiber amplifiers compensating for fiber losses made it possible to deploy optical amplifiers periodically spaced 70-80 km apart with a regeneration of the signal on distances of more than 1000 km. The advent of WDM has made it possible to double the system capacity every few months. The present evolution of optical technology is based from one side on further developments of optical amplifica