In some ways, wireless networks are completely misnamed, since the vast majority of wireless traffic rides traditional wireline copper and fiber facilities for most of its trip from originator to destination. But for the portion of the network that is wireless, data services growth on 3G networks has created a mobile backhaul problem in terms of transporting this traffic to or from the wireline network to the user.
For 4G Long Term Evolution (LTE) services, the new Evolved Packet Core (EPC) gives operators an architectural advantage for transporting wireless traffic. Yet for operators deploying LTE from a 3G service-base, evolving 3G mobile backhaul to EPC may be as big a planning issue as evolving 3G radio networks and handsets to 4G.
Transport infrastructure for 3G and 4G services can be divided into two categories:
- Mobile service elements with components that are aware of registration, mobility and service control aspects of mobile services;
- Transport/backhaul elements that provide connectivity between tower locations and service points.
Making sure mobile backhaul accommodates both 3G and 4G services
Both mobile service and transport/backhaul elements must be evolved in harmony so they can transition between 3G and 4G services
Mobile backhaul and 3G transport strategies vary depending on exactly what kind of 3G services are used (e.g., EDGE, HSPA, CDMA). In practice, however, most operators have deployed either time-division multiplexing (TDM) or asynchronous transfer mode (ATM) (AAL2) backhaul facilities to take advantage of their metro infrastructure and core technology.
This backhaul structure must be migrated to EPC during the evolution to 4G, and since fiber-feeding the tower sites is the preferred approach to 4G deployment, either parallel TDM/Ethernet or TDM over Ethernet Pseudowire Emulation Edge-to-Edge) is likely to be deployed..
The challenge of integrating TDM backhaul for 3G with HSPA and LTE/4G backhaul draws into question the Evolved Packet Core's basic topology. EPC architects know that the logical architecture of an EPC connection in the data plane is tower (eNodeB)-to-gateway-to-service. The gateway in this context is the place where mobility registration and service control meet address assignment and data network connectivity.
Evolved Packet Core specifications, however, provide for the separation of the gateway into a serving gateway (SGW) and a packet data network (PDN) gateway, or PGW. This separation creates an EPC sub-network of tunnels where QoS) and traffic management are more directly under the control of the service control logic (such as IMS). This capability can be used to provide low-latency transport for TDM being transported over LTE facilities.