Evolved Packet Core migration and integration planning
For migration planning, it is convenient to view the mobile service elements of 3G and 4G services by pairing elements roughly according to function. 3G circuit-mode connections used for voice can be simply tunneled or carried as noted above using parallel TDM/ATM or integrated pseudowires over Ethernet or IP. 3G packet traffic handling is where the tightest integration between the mobile service elements and backhaul strategies must be considered.
A good starting point is to consider the ultimate 4G logical model of mobile service elements and how they integrate with the 3G model. The Serving GPRS Support Nodes (SGSN) and Gateway GPRS Support Nodes (GGSN) relationship in 3G corresponds to the "trio" of Mobility Management Entity (MME)/SGW/PGW in LTE's EPC. The question, then, is whether the 3G and 4G elements are interconnected or integrated, which will depend on each operator's 4G architecture.
Mainstream network equipment vendors support three basic models for providing mobile service elements in Evolved Packet Core:
Elements can be discrete nodes, with each logical EPC component representing a unique device.
Elements can be fixed cards or interfaces on a router/switch device, so that logical EPC components are mapped not to their own devices but to specific packet metro/core elements.
Elements can be "logical" and hosted by one of many switch/router or other service components in the network.
For any of these models, 3G and 4G functionality can be provided either independently or hosted in a single device. The latter solution is optimal where there will be considerable 4G deployment, where 3G elements are older (and thus represent less asset displacement cost), and where service evolution to 4G is expected to be rapid.
Where it's not feasible to replace SGSN/GGSN functionality with a dual 3G/4G/EPC node set (MME/SGW/PGW), the only option is to link the 3G elements with the 4G network to combine the traffic. Where integrated fiber backhaul connects tower sites with both 3G and 4G radio access networks, the use of integrated functionality for 3G/4G elements is much preferred because all traffic will emerge from backhaul at the same point.
Where operators are integrating 4G mobile elements into packet edge devices, it may be difficult or impossible to support both 3G and 4G missions on the same node because of constraints in 3G support by packet edge cards. Because of the flexibility that packet-edge hosting of EPC components offers, it is possible to host EPC components at the edge adjacent to the location of the 3G SGSN/GGSN that must be connected, and thus to reduce handling and latency.