rk Core network Last mile (serves cells) IPsec overhead Transport protocol overhead OAM, sync, etc. X2 user-plane and controlplane Control plane S1 User-plane traffic Transport network Figure 1.39 Illustration of backhaul components in a typical cellular network [66]. Mobile backhaul is an example application area where packet optical technology48 enables the paradigm shift from traditional TDM-based backhaul to high performance, low cost, and scalable solutions that are currently required. Modern packet optical solutions enable the optimization of IP traffic between cell site gateways and core routers, avoiding unnecessary router hops. New generation of mobile backhaul supports Ethernet transport for all cell types and all locations regardless of last mile technology, whether CPRI-based fronthaul, DASs, fiber-connected small-cells or macro-cells, or fiber aggregation points supporting microwave backhaul in non-fiber environments are utilized. The use of frequency-division duplex scheme over the air-interface requires only simple frequency synchronization using SyncE or IEEE 1588v2 packet-based synchronization schemes. However, time-division duplex scheme requires phase synchronization, in which the network needs to track the phase of the synchronization signal and to receive accurate time-of-day timestamps. The more complex radio-access network features such as CoMP transmission and reception and eICIC further require phase synchronization. Frequency synchronization is provided through a number of methods, where the most com- mon solution is SyncE. It can also be provided in some regions using the global navigation Packet optical transport covers technologies and architectures that enable the transport of Internet protocol packets on both fixed and mobile optical networks. Converged products include the functional switching capability of wavelength division multiplexing schemes, Ethernet switching via various protocols, as well as time division multiplexing and optical transport network switching. The technologi