Evolving the LTE packet core

“The 3G core network and the 4G core network are, in essence, the same,” Lynch said, meaning its CDMA EV-DO packet data network and the LTE EPC network will eventually meld into a single entity. The new evolved high-rate packet data (eHRPD) standard will serve as a bridge between the two, incorporating elements from LTE’s service architecture evolution into the EV-DO core. The key element will be the HRPD serving gateway, which replaces the packet data serving node (PDSN) in the CDMA network and essentially acts as dual-standard S-gateway connecting both LTE and EV-DO networks.

Both Alcatel-Lucent and Ericsson will supply their EPC solutions to Verizon Wireless as part of their radio access network contract, but Verizon also named Starent Networks as a key core equipment supplier. Along with Alcatel-Lucent, Starent supplies most of the PDSNs in Verizon’s 3G network. Starent director of marketing Andy Capener would not reveal what specific core elements Starent would deploy for Verizon, saying only that the gateway has an entire SAE portfolio, including MME, S-gateway and P-gateway, at Verizon’s disposal.

It’s probably no coincidence, though, that at Mobile World Congress, Starent announced both its HRPD-serving gateway. “It’s the access gateway between the EV-DO network and the EPC core,” Capener said. “It would be a key step in linking the 3G and LTE networks.”

If Verizon were to task Starent and Alcatel-Lucent to merge its packet cores, the vendors would either replace or upgrade their current PDSNs in the EV-DO core with the HRPD S-gateway, which could perform both the functions of the PDSN and EPC S-gateway. The gateway would not only route data and signal traffic but provide the critical link between the mobility management functions of the EV-DO and LTE networks, thus allowing a device to pass between the two networks with no interruption in service.

For UMTS, the migration process follows a similar path, except the key bridging element will be the high-speed packet access (HSPA) network’s  serving GPRS support node (SGSN), which will be upgraded to link the packet cores of the GPRS core of the 3G network to the SAE core of the evolved 4G network.

Bringing Voice to LTE
While upgrades to the packet core will ensure interoperability between 3G and LTE data networks, it doesn’t do the same for voice. The end-to-end IP architecture of the LTE network means any voice service must be VoIP, which can’t easily be integrated with circuit-switched 2G and 3G voice networks.