Reality Check: Diameter routing LTE and IMS use cases

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.
Diameter signaling routers are becoming the central point in 3G, IMS and LTE networks for connecting, translating and interoperating Diameter traffic. As outlined in our previous article, a Diameter signaling infrastructure at the network core facilitates signaling between network elements, endpoints such as online and offline charging systems (OCSs/OFCSs), mobility management entities (MMEs), policy control and charging rules functions (PCRFs) and home subscriber servers (HSSs).
Joe McGarvey, principal analyst at Current Analysis, wrote that, “Operators face a looming problem: the steady but significant build up of signaling traffic created by internal and external messaging associated with the delivery of IP-based services. While the exchange of this information, which is governed by the Diameter protocol, has yet to put a strain on existing fixed and 2G/3G network infrastructures, the adoption of all-IP based technologies – namely LTE and IMS – is anticipated to introduce significant scaling and complexity challenges.”
Operators have identified the first wave of these scaling and complexity challenges. In RFPs, they have most often cited the following use cases for efficient Diameter routing in their LTE and/or IMS networks.
Centralized routing
Adding new resources, such as MMEs, call session control functions (CSCFs), packet data gateways (PGWs) and front-end applications for the HSS, in an LTE or IMS network complicates network architecture in the evolved packet core (EPC). Each Diameter-based element requires a direct connection with one another, creating a mesh network requiring configuration and routing updates each and every time the operator adds a new network element.
Centralizing Diameter routing in a dedicated LTE or IMS core element, however, reduces the number of network connections, provides consistent congestions control and message throttling and offloads Diameter routing requirements from the nodes themselves. Further, when an operator deploys new network elements, routing updates are required only at the Diameter signaling router, lowering operational expense and simplifying provisioning.
LTE-to-LTE roaming
The 3GPP defines this use case as the Diameter Edge Agent. A Diameter signaling router is required at the edge of an operator’s network to provide security, interoperability support and to hide the operator’s network architecture from the rest of the world.
Implementing a Diameter signaling router at the edge of the network in concert with a centralized routing layer provides the operator with the ability to gracefully control the flow of Diameter traffic and to ensure that the impact from internal signaling issues or malicious assaults from outside can be minimized.
LTE-to -2G/3G roaming
Similar to the above scenario, a Diameter router at the edge of the network alleviates roaming issues from LTE networks to 2G/3G networks. As long as subscribers have multi-mode handsets and LTE coverage isn’t ubiquitous, subscribers will encounter mobility management and authentication problems when roaming in 2G/3G environments. A centralized Diameter routing element interworks Diameter with SS7 to enable seamless subscriber roaming and handoffs between LTE and 2G/3G networks.
HSS address resolution
Both IMS and LTE networks need a subscriber locator function (SLF) to map subscribers to HSSs. Many operators choose to reduce risk and improve performance by dividing their subscriber population among several HSS instances. A centralized subscriber locator function offloads that function from the HSS front end and provides for the deployment of HSSs from multiple vendors. This can be by choice or forced on operators when they perform acquisitions and thereby gain new subscriber data management infrastructure. A Diameter routing engine performs the subscriber locator function by mapping between a subscriber identity (e.g., MSISDN, IMSI or SIP URI) and an HSS. Operators gain flexibility to move subscribers from one HSS to another, to split subscriber number ranges over different HSSs, and assign individual subscriber numbers to any HSS.
PCRF binding
The 3GPP has defined this requirement as the policy diameter routing agent (DRA) proxy function. Operators often deploy multiple policy and charging rules function (PCRF) elements in their network for scalability reasons. This necessitates that operators bind subscribers’ sessions to the appropriate PCRFs to ensure proper billing and tracking of service and application usage. Operators are requesting that Diameter routers support static binding and dynamic load sharing across PCRFs when sessions are first established and require support for correlation between sessions across different Diameter interfaces.
Charging load-balancing and topology hiding
With the expected significant growth of Diameter signaling brought on by LTE and IMS networks, operators are evaluating how to bring new efficiencies to their charging deployments. Functions like topology hiding and load-balancing can be provided by a centralized Diameter router in the LTE and/or IMS signaling core.
Diameter troubleshooting
Operators’ legacy monitoring systems often lack the flexibility to provide real-time subscriber and service data or the end-to-end transaction visibility required to pinpoint network anomalies in an all-IP network. Integrating a Diameter signaling router with a performance management system gives operators a unique vantage point for collecting network traffic and subscriber data for troubleshooting, roaming, services and revenue leakage prevention. In addition, a tight coupling of the two elements streamlines configuration in the performance management system when new endpoints and elements join the network.
These are the first of what will surely become a long list of use cases for Diameter routing elements in LTE and IMS networks. Operators have already identified these as their most immediate needs, and as the number of subscribers, devices, applications and services over these networks grows, operators will find new uses requiring additional flexibility and scalability for their core network needs.
Jason Emery is Director of Product Management at Tekelec. He has more than 20 years of next-generation signaling experience, and has authored or co-authored more than 10 U.S. patents and invention disclosures.