er 2 transport technologies in this section. Transmission Networks Fixed DWDM DWDM ROADM Node Site A Site A Site B Site C Site D Site E Site C Site D Site B Figure 7.14 Fixed DWDM VS. Dynamic ROADM 7.4.2.1 ATM 3GPP Rel-99 prescribed ATM (Asynchronous Transfer Mode) as the de facto interface for bearing UMTS traffic while GSM flows can be carried over AAL1 (ATM Adaptation Layer 1). The protocol can cope with the multiple technologies of the various mobile generations, and can handle different traffic types with great efficiency. This is due to ATMs built-in QoS capabilities, offering traffic prioritization and service differentiation as well as traffic engineering capabilities. For example, ATM gives a mobile operator the ability to transport voice and video streaming with fixed resources using CBR (Constant Bit Rate, i.e., AAL1) while adaptive traffic flows like Internet surfing or messaging can be handled with VBR (Variable Bit Rate). The details on ATM can be found in [17]. 7.4.2.2 Native Ethernet Ethernet was standardized as IEEE 802.3 in the 1980s for local area networks. The combination of the twisted pair versions of Ethernet for connecting end systems to the network is the most widespread wired LAN technology. Native Ethernet can be carried directly over DWDM or fiber (SONET) or on copper (PDH). The two key challenges for using Ethernet in the mobile world are accurate synchronization and service emulation as discussed next. 7.4.2.2.1 Synchronization Mobile base stations need a reliable source of highly accurate synchronization, complying with ITU recommendations for several reasons, including smooth handover between cells, legal interception, and SO on. Circuit transport networks have been built using circuit technology such as E1, T1, and SDH/SONET, which are inherently synchronous, as bit rate and line utilization, always have the same value (e.g., E1 has 2048-b periods as well as 2048 clock cycles) [14,19]. End-to-end synchronization (aligning the timing of network elements for the purpose of transmitt