Reality Check: Agile core necessary for successful EPC deployment

Editor’s Note: Welcome to our weekly Reality Check column. We’ve gathered a group of visionaries and veterans in the mobile industry to give their insights into the marketplace.
In the mobile market, flexibility is critical. Subscribers want flexibility in their voice and data plans and operators want flexibility to architect their networks. In my last article, we explored the intelligence needed in successful evolved packet core (EPC) deployments. This article continues on that theme and takes a look at additional – and flexible – elements as you build out your EPC network.
Open sesame
The industry tends to couple the LTE radio (the E-UTRAN) and the EPC. However, the EPC is being standardized as the core network for all access mechanisms, including: LTE, 2G, 3G, non-3GPP and even wireline networks.
The “open” EPC allows the operator to realize a truly converged packet core supporting all access technologies. The planning around the EPC must consider how all of these access networks enter the core, interwork with legacy systems, maintain seamless mobility and provide consistent and optimized services.
For example, it is possible to migrate the 2G/3G core network to EPC after EPC is deployed to support LTE radios. It is also possible to migrate the 2G/3G core network to EPC before or at the same time the LTE radios are deployed using an SGSN S4 or HRPD Serving Gateway (HSGW) for eHRPD networks. The preparation and planning for the EPC network is ideally done together with the 3G network to provide a seamless migration and to account for the integrated networks.
One of the key deployment considerations is the location of each of the EPC functions both initially and over time. Each operator has unique requirements; therefore, no one deployment model will suit all operators. A 3G UMTS operator will differ significantly from a CDMA operator, small operators from large, wireline from wireless; and there will be a clear dependency on what equipment has already been deployed.
Integration of functions
A key optimization and deployment consideration is integration (or co-location) of multiple core functions on a single platform. Options to consider are the integration of discreet 4G functions, and integration of 2G/3G, 4G, and/or non-3GPP core network functions to achieve capital and operational efficiencies along the upgrade path. For example, a single node acting as a collocated SGSN + MME and a node acting as a co-located GGSN+SGW + PGW can serve both the 2G/3G network and 4G network.
The integration of functions simplifies the network topology and management, while providing service uniformity. The reduction in “box” count could lower CapEx and OpEx, but also eliminates external servers, load-balancers, interfaces and related management equipment.
Break out
Deployment flexibility is more than just specific functions in specific locations. Consideration must include flexibility for individual applications, subscribers, services, and call flows. Local breakout of media is important in the management of bearer traffic. For example, it may make economic sense in some networks to offload Internet traffic locally at the wireless edge. This reduces the backhaul overload and cost between the edge and the core for traffic that is a significant revenue contributor.
Other traffic that adds to higher profitability, such as operator branded Video on Demand (VoD) or gaming, can be sent to a centralized location that supports and enables the higher revenue-generating services.
Similarly, since the majority of voice traffic remains local to a regional aggregation point, voice can be localized by hair pinning within the same regional aggregation point where the SGW and the distributed PGWs are deployed.
To enable these types of services requires the flexibility to support the PGW functionality in two different physical locations – distributed and centralized. Each of which may process different applications, potentially from the same subscriber, based on the destination.
The sheer volume of the aggregated throughput to backhaul all user data from a local or regional aggregation cluster with SGWs to centralized data centers with PGWs can be expensive. Support for call localization, internet offload, and local breakout provides an opportunity to significantly reduce the core network backhaul.
The building blocks
One of the key EPC considerations is the deployment architecture. The majority of 3G core deployments use a centralized architecture where a centralized GGSN serves multiple SGSNs at distributed locations. EPC, with many of the considerations already described, opens the door to revisit deployment architectures, including:
–Centralized Bearer/Distributed Control – The traditional 3G architecture expanded to 4G where the PGW is located at a centralized location and the MME and SGW is distributed;
–Distributed Bearer/Centralized Control – A scenario where the PGW/SGW is distributed and the MME is located at a centralized location;
–Completely Centralized – An architecture where all the EPC functions are centralized;
–Completely Distributed – An architecture where all the EPC functions are distributed and generally deployed together.
It is also important to consider that the deployment architecture may vary over time. The first steps toward evolving the existing 2G/3G mobile packet core will be to provide initial EPC functional capabilities. Scaling and densification of the EPC will be required at a later stage (generally three to four years after initial deployment), as the rollout of LTE coverage progresses and subscriber numbers increase. Depending on the operator requirements, the architecture may vary between initial deployment and densification.
The road to LTE
In the evolution to LTE/EPC, a critical decision is whether EPC is initially an overlay network just for LTE access or whether the EPC integrates LTE plus 3G networks. The integrated approach is possible from day one, if desired. An example of the integrated approach would be the deployment of MME and/or SGW capabilities in regionally distributed nodes co-located with the existing SGSN. The PGW capabilities could be in a more centralized location with the GGSN or an integrated SGW and PGW could be provided in a distributed node.
Another potential near term deployment model is a complete EPC overlay with separate functional elements handling LTE connections. This approach may mitigate some risk and allow slow migration to EPC.
Scalability
LTE brings multiple challenges to signaling. With the flattening of the radio network, the MME and SGW are bound to have a massive load of transactions per second. The elimination of a node equivalent to the RNC in the LTE radio network hierarchy increases the signaling requirements as the eNodeB’s are connected directly to the MME. This means the MME will be handling significantly greater signaling loads than a typical SGSN, including; paging requests to all eNodeBs, exposure to all inter eNodeB mobility events, in addition to NAS signaling ciphering and integrity protection.
The signaling challenges will further increase as circuit services, such as SMS and mobile terminated voice calls, migrate to the packet domain, requiring more frequent paging than today’s 3G networks. A second significant issue is the complexity of 2G-3G-4G mobility management between SGSNs and MMEs. The attach, detach, and mobility management (TAU, RAU, I-RAT handover) procedures will generate a significant amount of signaling load between the control nodes with or without Idle-Mode Signalin
g Reduction.
Finally, the MME/SGW has to handle an extremely large nu
mber of idle/active transitions. Since radio resources are expensive, the eNodeBs attempt to transition calls to idle ASAP. This is not a major issue in current 3G network as many 3G applications are typically not “always on.” This will also impact the SGSNs that are providing mobility with 4G elements.
Roam around the world
In the standardization of LTE and the EPC, 3GPP specifies mobility protocols from both traditional 3GPP networks and non-3GPP networks – GTP and PMIP6, DSMIP6, MIP4. The Mobile IP based protocols will typically be used for connectivity to non-3GPP networks, such as, CDMA, Wi-Fi and femtocell. The design of the EPC core must consider subscriber roaming both on to other LTE networks, as well as non-3GPP accesses. The selection of the core vendor must have expertise in both GTP and these mobile IP based protocols, but also consideration must be made toward supporting both technologies within a single platform to minimize complexity and cost.
Summary
It is clear that a tidal wave of data traffic is coming and the network must be adequately prepared. These considerations will help as you build out network requirements to successfully evolve from 3G to 4G.
Jonathan Morgan is senior director of product marketing for Starent Networks, a leading provider of infrastructure solutions that enable mobile operators to deliver multimedia services.