sion Multiplexing) systems. The metro transport functionality is performed by WDM which multiples the basic functionality of optical fibers and resides above it. In the most traditional sense and 1 Transmission hub location is a site (normally a cell site also) that aggregates traffic from various cell sites via leased lines or microwaves and using the same transport methods or fiber forward this traffic to the controllers (BSC/RNC). Next Generation Mobile Communications Ecosystem Layers 4-7 APPLICATION Layers 2/3 ATM/Ethernet Layer 1 SONET/SDH Physical Transport OPTICAL FIBER Figure 7.9 Traditional metro transport layered architecture perhaps till the mid 2000s, the SONET/SDH layer sits above the DWDM layer and IP/ATM, and SO on, lies above the SDH layer as shown in Figure 7.9. The light-wave systems were introduced in the 1970s to address the growing capacity needs of the telecommunication systems. Historically [14]: In the mid to late 1970s, telecommunication services were primarily voice and fax services at 64 kb/s. The trunks that interconnected service switches used PDH (Plesiochronous Digital Hierarchy) at bit rates from 1.5 to 140 Mb/s over copper or fiber, which usually carried numerous 64 kb/s circuits combined with bit multiplexing. In the mid to late 1980s, advances in optical transmission technologies and development of more accurate clocks led to the development of SDH. The SDH hierarchy began with bit rates of 155 Mb/s, increasing every few years by a factor of four to 622 Mbps, 2.5 Gbps, 10 Gbps, to the currently deployed (per wavelength) transmission rate of 40 Gbps. The client signals of SDH are mainly PDH signals. Moving into the 1990s, data services became increasingly important. Initial data connectivity included dialup modem services (still at 64 kbps) and then using private line service to bridge together Ethernet 10/100 Mbps LANs. To address the needs of high capacity demands of the metro networks, Next Generation SDH and later MSTP (Multi Service Transport Platform) were introduced in the