t frame DA - Destination address S-Tag - Service VLAN tag (service VLAN ID) SA - Source address I-Tag - service instance tag UNI - User-network interface Figure 4.7 Provider bridges and provider backbone bridges for carrier ethernet network architecture. 4 QoS for Fixed Ultra-Broadband In some network environments synchronization is highly important for certain events, such as handovers in mobile networks. So, when Carrier Ethernet is used for mobile backhaul networks, there is a need for synchronization for mobile applica- tions and services in a similar manner as with TDM-based transport technologies. That is particularly required to minimize interference in the radio access network, as well as to support handovers in the cellular radio access network (handing over calls/sessions between neighboring cells), as well as to satisfy certain regulatory requirements on QoS. In general, synchronization refers to distribution of common reference for time and/or frequency to all networks (or network segments) to align their frequency and/or time scales. For example, different generations of mobile technologies have different demands regarding synchronization. However, most synchronization requirements are related to distribution of a common frequency reference within the mobile network, expressed in unit ppb (parts per billion). The ppb for global systems for mobile communication (GSM) (2G), universal mobile telecommunication system (UMTS) (3G), long-term evolution (LTE)/LTE-Advanced (4G), should be less than +50 ppb for wide-area base station/eNodeB with cells sizes in range of kilometers, +100 ppb for local area base station/eNodeB, and + 250 ppb for home base station/eNodeB [30]. 4.4.1 Class of Service for the Carrier Ethernet The main approach of QoS provisioning in the Carrier Ethernet network is related to CoS. Bandwidth profiles per EVC and per CoS are governed by six parameters: CIR (committed information rate). This defines assured bandwidth, which is assured via bandwidth reservation based on traffic engineeri