r to operator and country to country. At a high level, backhaul segment connects BTSs/NodeBs to one or more traffic aggregation sites (transmission hubs) which are then connected to BSCs/RNCs. The metro transport network connects multiple controller sites to MSCs and to Data Gateways and also connects multiple MSCs and Data Gateways to each other within a city. The metro segment further provides connectivity to the core network. The core transport network provides intercity connectivity and to the outside world (PSTN, Internet, etc.). The selection of transport technology for any of the segments is dependent on many factors, including but not limited to, distance between backhaul, regional and core transport networks. The transport distances between them vary greatly, depending on many factors which include cell site density, spectrum availability, terrain characteristics, point of presence, and SO on. Long-term network capacity growth, cost optimization, lease versus build and resilience are some of the other fundamental factors that further influence the selection of the transport technology [1]. The chapter starts with listing the challenges and drivers that necessitate continuous investments in the transport networks. In the next three sections various technologies for the backhaul, metro and backbone segments are discussed. These transport technologies belong either to the physical, data link or network layer of the OSI model. In the remaining sections we will present the evolution of transport network, some updates from the industry and summarize the potential solutions for the challenges documented earlier in the chapter. 7.2 Market Drivers and Challenges Transmission infrastructure represents a significant part of the overall mobile network Capex and Opex. In some cases backhaul represents 75% of the transport costs, which themselves make up 25-35% of Next Generation Mobile Communications Ecosystem: Technology Management for Mobile Communications Saad Z. Asif © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470