1 Introduction to Diameter Signaling Router

Mobile data traffic is growing exponentially, fueled by the introduction of smart phones, laptop dongles, flat-rate plans, social networking, and applications like mobile video. Operators are looking at Internet protocol (IP) networks such as Long Term Evolution (LTE) and IP multimedia subsystem (IMS) to provide the bandwidth required to support high data traffic requirements and applications. The CSPs need cost-effective solutions to address the growing gap between traffic and revenue growth.

The 3GPP Evolved Packet core (EPC) and IP Multimedia Subsystem (IMS) network architectures have specified the use of Diameter over Stream Control Transmission Protocol (SCTP) or Transmission Control Protocol (TCP) for many network interfaces such as for policy, charging, authentication, and mobility management. Many of interfaces are illustrated in the figure below. 3GPP and ETSI defines the Diameter protocol, it is the foundation Authentication, Authorization, and Accounting (AAA) functions in the Next Generation Network (NGN).

Figure 1-1 Selected Diameter Interfaces in LTE and IMS


Selected Diameter Interfaces in LTE and IMS

1.1 Reference

For more information, see Diameter Signaling Router Operations, Administration and Maintenance Guide (OAM) Guide.

1.2 Diameter Routing Challenges

For years operators have employed signaling system 7 (SS7) as the international, standardized protocol to communicate globally between operator networks. In LTE and IMS networks, many of the functions performed bySS7-based signaling in current networks are replaced by equivalent functions based on the Diameter protocol. Operators expect the same network behavior and robustness currently provided by SS7 networks.

Without a separate Diameter signaling infrastructure at the network core to facilitate signaling between network elements, endpoints such as Mobility Management Entities (MMEs) and Home Subscriber Servers (HSSs) must utilize direct signaling connections to each other, forming a mesh-like network architecture. Network endpoints must handle all session related tasks such as routing, traffic management, redundancy and service implementation. Implementing an IMS or LTE network without a signaling framework may be sufficient initially, butas traffic levels grow, the lack of a capable signaling infrastructure poses a number of challenges:
  • Scalability and load balancing: Each endpoint must maintain a separate SCTP association or TCP connection with each of its Diameter peers and keep track of the status of each association. This network arrangement increases the overheads on the endpoints as the number of nodes increases, and the endpoints have the additional responsibility of load balancing. This burden is made more complex with the responsibility of load balancing placed on each end point.
    • Congestion control: Diameter lacks well-defined congestion control mechanisms found in other protocols such as SS7. For example, if an HSS has multiple Diameter front ends, the lack of sufficient congestion control increases the risk of a cascading HSS failure.
    • Secure Network interconnect: A fully meshed network is completely unworkable when dealing with connections to other networks because there is no central interconnect point, which also exposes the operator’s network topology to other operators and can lead to security breaches.
    • Interoperability: Protocol inter working becomes unmanageable as the number of devices supplied by multiple vendors increases. With no separate signaling or session framework, interoperability testing (IOT) must be performed at every existing node when a new node or software load is placed in service. IOT activities consume a considerable amount of operator time and resources, with costs increasing in proportion to the number of tests that must be performed.
    • Support for legacy EIR: A need for MAP to Diameter inter working is required as transitions are made and LTEis quickly introduced into a network while still needing to support legacy HLRs.
    • Support for both SCTP and TCP implementations: SCTP elements cannot communicate with TCP elements. Without a central conversion element, operators will either have to upgrade TCP elements or require all elements in the network to support both stacks.
    • Subscriber to HSS mapping: When there are multiple HSS in the network, subscribers may be homed on different HSS. Therefore, there must be some function in the network that maps subscriber identities to HSSs. With no separate Diameter signaling infrastructure, that task must be handled by a standalone Subscription Locator Function (SLF), or by the HSS itself. Either approach wastes MME (or call session control function[CSCF]) processing and can add unnecessary delays. The HSS approach wastes HSS resources and may even result in the need for more HSSs than would otherwise be necessary.
    • Policy and charging rules function (PCRF) binding: When multiple PCRFs are required in the network, there must be a way to ensure that all messages associated with a user’s particular IP connectivity access network(IP-CAN) session are processed by the same PCRF. This requires an element in the network that maintains session binding dynamically.

In recognition of Diameter routing issues, 3GPP has defined the need for a Diameter signaling infrastructure and a Diameter border infrastructure as shown below which is taken from TR 29.909. In addition, the GSMA has specified the need for a Diameter Proxy Agent as shown below which is taken from PRD IR.88.

Figure 1-2 3GPP Inter/Intra-operator Diameter infrastructure


3GPP Inter/Intra-operator Diameter infrastructure

Figure 1-3 GSMA roaming implementation architecture


GSMA roaming implementation architecture

1.3 Diameter Signaling Router Solution

Oracle Communication’s Diameter Signaling Router (DSR) creates a centralized core Diameter signaling layer that relieves LTE, IMS and 3G Diameter endpoints of routing, traffic management and load balancing tasks and provides a single interconnect point to other networks. Each endpoint only needs one connection to a DSR to gain access to all other Diameter destinations reachable by the DSR. This approach eliminates the Diameter or SCTP(or TCP) mesh that is created by having direct signaling connections between each network element. Having one or more connection hubs that centralize the Diameter traffic to all end nodes simplifies interoperability between different network elements and enhances network scalability.

Centralizing Diameter routing with a DSR creates a signaling architecture that reduces the cost and complexity of the core network and enables core networks to grow incrementally to support increasing service and traffic demands. It also facilitates network monitoring by providing a centralized vantage point in the signaling network.

Advantages of a centralized signaling architecture as listed below:
  • Improves signaling performance and scalability by alleviating issues related to the limited signaling capacity of MMEs, HSSs, CSCFs and other Diameter endpoints.
  • Provides a centralized point from which to implement load balancing.
  • Simplifies network expansion because routing configuration changes for new endpoints are performed only on the DSR.
  • Increases reliability by providing geographic redundancy.
  • Provides mediation point for Diameter variants to support interoperability between multi-vendor endpoints.
  • Creates a gateway to other networks to support roaming, security and topology hiding.
  • Reduces provisioning, maintenance and IOT costs associated with adding new network nodes.
  • Enables HSS routing flexibility by providing a central point to perform HSS address resolution.
  • Creates a centralized monitoring and network intelligence data collection point to isolate problems and track Key Performance Indicators (KPIs).
  • Provides network wide PCRF binding to ensure that all messages associated with a user’s particular IP-CAN session are processed by the same PCRF.

    The DSR can be deployed as a core router routing traffic between Diameter elements in the home network and as a gateway router routing traffic between Diameter elements in the visited network and the home network. Refer to the figure below for a representation of an operator’s EPC or IMS core network with DSR.

Figure 1-4 Example of Operator’s EPC/IMS Core network with DSR


Example of Operator’s EPC/IMS Core network with DSR

The resulting architecture enables IP networks to grow incrementally and systematically to support increasing service and traffic demands. A centralized Diameter router is the ideal place to add other advanced network functionalities like network performance intelligence via centralized monitoring, address resolution, Diameter interworking and traffic steering.