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Diameter Signaling

Sonus Special Edition

by Lawrence C. Miller


Diameter Signaling For Dummies®, Sonus Special EditionPublished byJohn Wiley & Sons, Inc.111 River St.Hoboken, NJ 07030‐5774www.wiley.com

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Table of ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

About This Book ........................................................................ 1Foolish Assumptions ................................................................. 2Icons Used in This Book ............................................................ 2Beyond the Book ........................................................................ 3Where to Go from Here ............................................................. 3

Chapter 1: The Evolution of Signaling . . . . . . . . . . . . . . . .5Signaling System 7 (SS7) ........................................................... 5SS7 SIGTRAN Links ..................................................................... 8LTE/EPC/Diameter Network ................................................... 10

Chapter 2: Defining the Role of the STP in SS7 Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Gateway Screening and Advanced Routing .......................... 14Point Code Emulation .............................................................. 14Security ..................................................................................... 14Interworking ............................................................................. 15Gateway Accounting ................................................................ 15Global Title Translation .......................................................... 16SS7 and Diameter ..................................................................... 16

Chapter 3: Exploring Diameter Routing Use Cases . . . .17Interconnect for Roaming ....................................................... 17

Topology hiding ............................................................. 18Routing of messages in the DEA .................................. 19Diameter‐level screening in a DEA environment ....... 20

Centralized Routing ................................................................. 20Deployment of Multiple HSSs ................................................. 21PCRF Binding ............................................................................ 22Roaming to Non‐LTE Networks .............................................. 22

Chapter 4: Defining the Role of the DSC in LTE and VoLTE Networks . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Interconnect and Security ...................................................... 25Traffic Management ................................................................. 27Diameter Agent Interoperability ............................................ 27

Diameter Signaling For Dummies, Sonus Special Edition iv


Session Binding and Subscribers ........................................... 28Multi‐Protocol Interworking ................................................... 28Virtualization and Multiple Instances for

Routing Efficiency ................................................................ 29

Chapter 5: Number Portability and Subscriber Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

Number Portability (NP) ......................................................... 31Subscriber Location Function (SLF) ...................................... 32E.164 Number Mapping (ENUM) ............................................ 32

Chapter 6: Recognizing the Role of NFV in Signaling Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Defining NFV ............................................................................. 35STPs and NFV ........................................................................... 36Exploring Diameter NFV Use Cases ....................................... 37

Virtual Diameter Edge Agents ...................................... 37Virtual Core Diameter Signaling Controllers .............. 39

Chapter 7: Ten Things to Consider in a Diameter and SS7 Signaling Solution . . . . . . . . . . . . . . . . . . . . .41


Introduction

D iameter is the signaling protocol used in LTE networks around the world. However, telecommunications net-

works still use Signaling System 7 (SS7) networks extensively, and SS7 has become the most reliable, secure, and feature rich signaling methodology in telecommunications history. SS7 networks will exist for many more decades as signaling networks slowly evolve from SS7 to Diameter and Session Initiation Protocol (SIP).

The evolution from SS7 to Diameter and SIP is being driven by advances in technology and service providers’ desires to host voice and data services on a common, all‐IP infrastruc-ture and transport network. Service providers also need to balance this move to new network infrastructure with the need to monetize their networks, and the subscribers’ insa-tiable demand for applications and their associated band-width requirements. Since the initial inception of SS7 there have been significant advances in telecommunication network technology, including the introduction of Internet Protocol (IP) into service providers’ networks, thus driving the con-vergence between voice and data. This convergence has opened telecommunications networks allowing them to take advantage of protocol advances including Signaling Transport (SIGTRAN), Stream Control Transmission Protocol (SCTP), Diameter, and SIP.

About This BookThis book covers how Diameter Signaling networks are evolving from SS7 networks (Chapter 1), explores the many functions of signaling transfer points (STPs) in SS7 networks (Chapter 2), describes Diameter use cases (Chapter 3), looks at the role of Diameter Signaling Controllers (DSCs) in LTE


and VoLTE networks (Chapter 4), reviews number portability and subscriber database requirements (Chapter 5), intro-duces Network Functions Virtualization (NFV, Chapter 6), and identifies some important requirements to consider in an SS7/Diameter solution (Chapter 7).

Foolish AssumptionsIt’s been said that most assumptions have outlived their uselessness, but we’ll assume a few things nonetheless! We assume that you work in the telecommunications industry and have at least a basic understanding of telecommunica-tions terms and concepts. As such, we assume you are a somewhat technical reader. If these assumptions describe you, then this book is for you!

Icons Used in This BookThroughout this book, we occasionally use special icons to call attention to important information. Here’s what to expect:

This icon points out information that you should commit to your non‐volatile memory or your noggin’ — along with anniversaries and birthdays!

You won’t find a map of the human genome here, but if you seek to attain the seventh level of NERD‐vana, perk up! This icon explains the jargon beneath the jargon!

Thank you for reading, hope you enjoy the book, please take care of your writers! Seriously, this icon points out helpful suggestions and useful nuggets of information.

This icon points out the stuff your mother warned you about. Okay, probably not. But you should take heed nonetheless — you might just save yourself some time and frustration!

Diameter Signaling For Dummies, Sonus Special Edition 2


Beyond the BookThere’s only so much we can cover in 48 short pages, so if you find yourself at the end of this book, thinking “gosh, this was an amazing book, where can I learn more?” just go to www.sonus.net.

Where to Go from HereIf you don’t know where you’re going, any chapter will get you there — but Chapter 1 might be a good place to start!

Introduction 3

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The Evolution of SignalingIn This Chapter

▶▶ Recognizing different SS7 implementations

▶▶ Addressing bandwidth and facility constraints with SIGTRAN

▶▶ Getting acquainted with Diameter

I n this chapter, you learn how telecommunications signaling networks are evolving from mature Signaling System 7 (SS7)

network architectures to Long Term Evolution (LTE)/Evolved Packet Core (EPC)/Diameter networks that support today’s high‐speed, high-bandwidth requirements.

Signaling System 7 (SS7)SS7 is an international telecommunications standard defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU‐T) in 1980. SS7 is used to set up, manage, and tear down telephone calls over public switched telephone networks (PSTN) throughout the world. It sounds simple enough, but there’s actually quite a bit that goes into setting up, managing, and tearing down calls. For example, when you call someone, SS7 has to figure out where the person you’re calling actually is — perhaps it’s a friend roaming in a different country. Then, SS7 has to figure out if that person’s phone is busy. Next, it has to securely attach your phone to the local network and the foreign net-work, then determine whether your carrier and the foreign carrier have an agreement that allows the two networks to communicate. These examples are just a few of the many things that SS7 must do so your friend can “hear you now!”

Chapter 1



SS7 performs the following functions and services (among others):

✓ Call forwarding

✓ Caller ID (name and number)

✓ Local number portability

✓ Mobile telephone subscriber authentication

✓ Number translation

✓ Personal communication service (PCS)

✓ Prepaid billing

✓ Roaming

✓ Short Message Service (SMS)

✓ Three‐way calling

✓ Tollfree (800 and 888) and toll (900) calls

Nodes in an SS7 network are referred to as signaling points and consist of the following:

✓ Service Switching Points (SSPs): SSPs set up or tear down a call and communicate with SCPs to determine how to route a call, or to set up and manage a call fea-ture. SSPs are generally found within a voice switch.

✓ Signal Transfer Points (STPs): Routers or switches that relay SS7 messages to SSPs, SCPs, and other STPs on the SS7 network.

✓ Service Control Points (SCPs): SCPs connect with STPs and SSPs (less common) and are used to control the service. There are many different types of SCPs. For example, a database that converts toll‐free 800 numbers into normal phone numbers is a type of SCP. Another example is a number portability (NP) database used to determine whether a phone number has been transferred to another service provider.

The initial deployment of SS7 in North America more than 30 years ago included STPs in the network topology, deployed in a distributed core‐edge topology or a centralized core rout-ing topology:

Chapter 1: The Evolution of Signaling 7


✓ SS7 Core‐Edge Network Architecture: Early deploy-ments used a distributed architecture that included network (core) and local (edge) STP pairs. These early deployments continue to be used today. The core STP pairs provided access to companywide database ser-vices, aggregated connectivity to local STPs, and served as access points to other service providers. The edge STPs provided SS7 services and connectivity to all end offices and tandems within a geographical region. All requests for services that required database intervention were routed from the edge STPs to the core STPs and then to the appropriate database.

✓ SS7 Core Network Architecture: Later deployments are totally centralized and comprised of large core STPs providing all SS7 connectivity and database services. The evolution to this network configuration was influenced by government mandates to implement NP in both the wire-line and wireless telecommunications market segments. The NP service required extremely large and fast data-bases that could be accessed from every end office in the network. To accomplish these requirements, a solution was developed that integrated the database within STP functionality. The operating companies determined that a large core STP with an included database was the most cost‐effective use of this expensive technology.

The evolution of SS7 deployments outside North America was quite different due, in part, to the size of the networks, the starting point of the network, and the design of network elements.

Typically, the size of the individual international telecom-munications networks was much smaller than those in North America. The international switching equipment vendors incorporated some STP functionality into each of the network elements. The network size, coupled with the differences in switching equipment, facilitated the implementation of an associated or mesh network.

During the initial international implementation of SS7, net-work elements were interconnected directly with each other to create a fully meshed network. As the network continued to grow with more traffic and more interconnected elements,



network operators found that the management and adminis-tration of this meshed network became untenable and fraught with human related errors affecting network routing, address assignment, and security when interconnected to foreign net-works. This is where STPs got introduced into international networks to solve the complexity in operating a fully meshed network.

SS7 SIGTRAN LinksThe explosive growth in the number of users and the amount of traffic stretched the SS7 network architectures to the breaking point during the 1990s. The solution was simple: implement high‐speed signaling links. However, because the SS7 protocol defines the entire message delivery mechanism from the physical layer to the application layers, modifica-tions to the protocol had to be made at both the physical and transport layers.

During this time period, IP networks had grown extensively in use, and improvements in quality enabled them to provide higher reliability. Additionally, IP networks held a transport cost advantage over time‐division multiplexing (TDM)-based SS7 networks. This led to new standards being developed in order to enable the SS7 signaling protocol to run on IP‐based networks.

The SIGTRAN working group of the Internet Engineering Task Force (IETF) was formed in 1999 to define the architecture for transporting real‐time signaling information over an Internet Protocol (IP) network. The group’s effort yielded three key results:

✓ New network architecture: The segmenting of legacy switch functionality creates a more distributed switching architecture and enables a total separation of signaling from the media and the media control plane. The switching functionality split is defined in IETF Request For Comments (RFC) 2719.

✓ New transport protocol: Stream Control Transmission Protocol (SCTP) was defined to carry SS7‐related pro-tocol levels over an IP backbone network. SCTP meets



the rigid constraints of a real‐time protocol such as SS7 including guaranteed delivery, sequence delivery, and multi‐homing for reliability. SCTP provides the following functions and capabilities:

• Reliable data transfer

• Multiple streams to help avoid head‐of‐line blocking issues

• Ordered and unordered data delivery on a per‐stream basis

• Bundling and fragmentation of user data

• Congestion and flow control

• Support for continuous monitoring of reachability

• Graceful termination of association

• Support for multi‐homing

• Protection against blind denial‐of‐service and blind masquerade attacks

✓ Numerous adaptation layers: Adaptation layers encapsu-late upper levels of the SS7 protocol and transport them over IP utilizing the services of SCTP. Because each adap-tation layer is based on the SS7 level being transported or replaced, there are common capabilities across all adaptation layers. Each adaptation layer must provide the following:

• A seamless operation of SS7 level peers over an IP network

• A primitive interface boundary that the correspond-ing SS7 level had with its underlying SS7 level

• Management of SCTP transport associations and traffic between SGs and IP Signaling Endpoints (ISEPs) or two ISEPs

• Asynchronous reporting of status changes to man-agement functions

The SS7 network, including its transport capabilities and protocol technologies, are mature, well established, and well understood by telecommunications service providers world-wide. SS7 remains the preeminent signaling standard for many



operators providing network‐based, revenue‐generating ser-vices. With the large installed base, any change from legacy SS7 network architectures to next‐generation networks, such as LTE/EPC/Diameter and IP Multimedia Subsystem (IMS), will be evolutionary rather than revolutionary. As a result, hybrid networks combining parts of both SS7 and LTE/EPC/Diameter will be the standard for the near term.

LTE/EPC/Diameter NetworkThe mobile subscriber’s ever increasing demand for large volumes of bandwidth is driving the deployment of LTE/EPC/Diameter networks globally. Subscriber devices, such as smartphones and tablets with the always‐on applications they support, are having a huge impact on a mobile operator’s abil-ity to keep up with bandwidth demands and the associated signaling requirements.

The Remote Authentication Dial‐In User Service (RADIUS) protocol is Diameter’s direct predecessor. RADIUS provides centralized authentication, authorization, and accounting (known as “AAA” or “Triple A” services) management for users who connect and use a network service. It’s commonly used to manage access to networks, virtual private networks (VPNs), and email.

Diameter was created to replace the RADIUS protocol, and has better reliability, scalability, security, and flexibility than RADIUS (you might even say Diameter is “twice” as good as RADIUS!). Diameter is the required protocol in 3G and LTE mobile networks. SCTP (discussed earlier in this chapter) is the transport protocol for Diameter, due to the reliability and survivability required in EPC.

The architecture of the LTE/EPC/Diameter network (see Figure 1‐1) defines a large quantity of network elements, each with its own functionality. Each network element can have multiple interfaces to other elements based on the proce-dures and information exchanged.



Additionally, the peer concepts of the Diameter protocol and the connection‐oriented methodologies of SCTP, significantly increase the complexity and quantity of routing rules within the network. As the network continues to evolve and grow, additional routing rules have to be provisioned in every net-work element. The routing complexity inherent within this mesh‐type network presents a twofold problem. First, the maintenance and administration of the routing rules on indi-vidual nodes directly affects the consistency and scalability of the network. Second, placing the routing responsibility on the individual network elements can degrade the network element’s ability to perform its primary function.

A network deployment including Diameter Routing Agents at both the core and edge of the network provides a more effi-cient and scalable architecture (see Figure 1‐2). By placing the Diameter router in the core of the network, routing is central-ized to reduce the quantity and complexity of internetwork

Figure 1-1: Diameter mesh network.



and intra‐network routing. Also, because the routing responsi-bility is removed from individual network elements, expensive resources are freed to perform their primary function — thus reducing network-wide capital expenditures.

Figure 1-2: Diameter router network.


Defining the Role of the STP in SS7 Networks

In This Chapter▶▶ Understanding gateway screening and advanced routing features

▶▶ Simplifying switch migration with Point Code Emulation (PCE)

▶▶ Securing SS7 networks with Gateway Screening

▶▶ Supporting SS7 Interworking

▶▶ Keeping track of SS7 messages with Gateway Accounting (GWA)

▶▶ Performing Global Title Translation (GTT)

▶▶ Supporting SS7 and Diameter on a single platform

S ignaling Transfer Points (STPs) are packet switches that provide SS7 message routing between network elements

of different types. STPs are used to create a hub‐and‐spoke architecture in the SS7 signaling network. With this architec-ture, a central place for network monitoring and management, value‐added processing, and alternate routing and screening is created.

STPs are also used to interconnect different service provider networks and provide security and screening capabilities to enable secure interconnect between providers. Monitoring, screening, and security are particularly important when ser-vices cross multiple providers’ networks.

In this chapter, you learn about the role of STPs in SS7 networks.

Chapter 2



Gateway Screening and Advanced Routing

Gateway Screening and Advanced Routing allows operators greater control over the network routing of messages and is used in situations where a core network of STPs has a dual role of routing and monitoring/billing for the network.

Gateway Screening and Advanced Routing provides a very flexible and powerful mechanism for inspection and examina-tion of SS7 messages transiting the network. This mechanism is utilized to provide a rich set of features, giving operators full control over which messages are allowed to transit their networks, how these messages should be routed, and the accounting rules associated with them.

Point Code EmulationPoint Code Emulation (PCE) is a feature that can be used to significantly simplify and mitigate risk in switch migration activities. PCE allows service providers to grow their signaling networks transparently. This task is accomplished by giving the carrier a method to translate between public and internal point codes. Internal point codes are those defined by the car-rier and are only visible within the carrier’s network. Internal point codes are associated with a public point code that’s vis-ible by the SS7 network.

PCE is similar to network address translation (NAT) on an IP network. Similarly to NAT, PCE allows STPs to use a single point code to appear to the network as one or more shared point codes (PCs). PCE is a cost-effective solution for expand-ing networks where new point codes (PCs) are rare and expensive, and by minimizing the number of direct SS7 links to an STP pair.

SecuritySTPs placed on the boundaries between two networks are con-figured with a feature referred to as Gateway Screening (see Figure 2‐1). These STPs provide firewall security functions and

Chapter 2: Defining the Role of the STP in SS7 Networks 15


admission control over messages that are allowed into and out of the network.

InterworkingThere are many different SS7 protocol standards written by the International Telecommunication Union (ITU) and American National Standards Institute (ANSI), as well as local country variants. For this reason, the SS7 STP must support numerous SS7 variants and provide protocol conversion between these variants. The STP must convert between these variants and becomes a critical element when interconnecting SS7 networks between countries.

SS7 STPs also need to interwork between different layers within the SS7 stack; for example, Integrated Services Digital Network User Part (ISUP) and Transaction Capabilities Application Part (TCAP).

Gateway AccountingUsing Gateway Accounting (GWA), network administrators can define rule sets based on SS7 message parameters to col-lect statistical information which may be used for auditing purposes.

In order to provide auditing, the STP performing GWA needs to be able to generate statistics in a flexible manner that is configurable by the operator. It’s very common for these statistics to come in the form of ASCII text files.

Figure 2-1: STPs at the boundaries between two networks with Gateway Screening.



Global Title TranslationGlobal Title Translation (GTT) frees the originating signal-ing points from the burden of having to know every potential destination to which they might have to route a message. A switch can originate an SS7 Signaling Connection Control Part (SCCP) query and address it to an STP, along with a request to perform GTT.

The GTT application searches for matching Global Title com-ponents or fields of the Called Party and/or the Calling Party address. The most common GTT configuration is to translate based on the Called Party Address.

The receiving STP makes a determination of the intended address (Destination Point Code) based on the digits and other Called Party Address parameters of the SCCP message. The digit type varies depending on the service that’s trying to be reached, dialed digits, International Mobile Subscriber Identity (IMSI), and so on.

SS7 and DiameterDiameter can replace many of the functions previously per-formed by SS7/SIGTRAN protocols in Long Term Evolution (LTE) and IP Multimedia Subsystems (IMS) networks. For this reason, mobile service providers are deploying Diameter Signaling Controllers (DSC) to provide functions very similar to those of STPs in traditional SS7 networks.

Yet for most service providers, the deployment of Diameter is being done as part of a “cap and grow” rather than a “rip and replace” strategy. This is due to the extended, decades‐long migration of traffic and subscribers away from SS7‐based ser-vices. In fact, subscriber and usage growth in SS7‐based 2G and 3G networks is still expected well into 2020.

Vendor solutions that can concurrently support both STP (SS7) and DSC (Diameter) functions on a single platform and interwork between the two protocols are ideal to accommo-date the changing dynamics of signaling during this multi‐decade transition.


Exploring Diameter Routing Use Cases

In This Chapter▶▶ Interconnecting networks to support roaming

▶▶ Eliminating complexity with centralized routing

▶▶ Supporting multiple Home Subscriber Servers (HSS)

▶▶ Allocating bandwidth with Policy Charging Rules Function (PCRF)

▶▶ Facilitating roaming between LTE and non‐LTE networks

T oday, mobile network operators are faced with the challenges of building 4G Long Term Evolution/Evolved

Packet Core (LTE/EPC) networks to meet the demands of subscriber devices such as smartphones and tablets, and the “always on” applications they support. As IP Multimedia Subsystem (IMS) and LTE infrastructures are deployed, new elements such as policy servers, gateways, session controllers, and charging systems are interconnected. These new elements interwork via the Diameter protocol to exchange critical network and services information.

In this chapter, you learn about several common Diameter routing use cases and how to solve them with Diameter Signaling Controllers (DSCs).

Interconnect for RoamingRather than connecting sensitive end nodes directly to net-work elements from other administrative realms, a Diameter Edge Agent (DEA) can be used on the boundaries between

Chapter 3



two interconnected roaming partners, thereby simplifying net-work architecture and improving robustness (see Figure 3‐1).

Specifically, a DEA provides the following capabilities:

✓ Creates a single point of connection into a mobile service provider’s network

✓ Hides the topology of the local network so that unneces-sary outages don’t occur if the internal topology changes

✓ Simplifies monitoring, facilitates interworking, and pro-vides message normalization

✓ Protects the local Diameter network from denial‐of‐service (DoS) attacks

✓ Enables a powerful set of routing and screening functions in order to protect the network on any message or any Attribute‐Value Pair (AVP)

Topology hiding, message routing in the DEA, and Diameter‐level screening are further explained in the following sections.

Topology hidingThere are two key aspects of topology hiding:

✓ Limiting the information that an originator must have about a destination network in order to send a message, which is accomplished by basic DEA functionality (much like Global Title Translation and Network Interfaces in an SS7 network)

Figure 3-1: A DEA simplifies network architecture and improves robustness.

Chapter 3: Exploring Diameter Routing Use Cases 19


✓ Modifying messages so the receiver can’t discern the structure of the originator’s network

The DSC allows modification of message parameters that con-tain topology information.

A DEA can be used to modify messages — via routing tables or a User Agent application programming interface (API) — to facilitate interoperability, possibly including changes to Origin‐Host or other fundamental Diameter parameters.

A DEA can modify messages to obfuscate information, poten-tially removing information about the identity of local serv-ers and relays. The following are some examples of internal identities that should be prevented from being “leaked” in Diameter message exchanges:

✓ Origin‐Host AVP holds the identity of the originating client or server.

✓ Route‐Record AVPs hold the identities of the message originator and any intermediate relays.

✓ Session‐Id AVP holds the Diameter identity of the client that originated the session.

Routing of messages in the DEADSCs contain powerful routing tables that can be used to make DEA message routing more efficient. For example, assume a network operator has a direct connection to another network operator’s DEA, along with connectivity to two different IP exchange (IPX) providers. Based on this configuration, the operator will direct traffic based on the destination of the message. A message destined for the directly connected operator’s network will be routed directly to that operator’s DEA because the realm is known. However, for other destinations the operator makes routing decisions for one IPX or the other. These routing decisions need to be easy to configure and maintain in the DSC’s routing tables.



Diameter‐level screening in a DEA environmentMost network operators deploy a firewall at their network perimeter, thereby preventing the outside IP network from accessing the DSC. This process enables the DSC to focus on examining Diameter traffic in order to implement routing and screening at the Diameter level, rather than dealing with lower‐level DoS attacks and other security issues. In this con-figuration, the DEA will allow messages from known adjacent networks. However, even if traffic from an untrusted network is sent through a trusted source, such as the IPX provider, the DSC could reject this traffic. No direct connections from untrusted networks are allowed, and must instead be made through the DEA. This forces all external Diameter signal-ing traffic through the DEA function, and limits the types of attacks that have to be handled.

Centralized RoutingThe architecture of the LTE/EPC network defines a large quan-tity of network elements, each with its own functionality. Each network element can have multiple interfaces to other ele-ments, based on the procedures and information exchanged.

Additionally, Diameter is a peer‐to‐peer protocol and the connection‐oriented methodologies of the Stream Control Transmission Protocol (SCTP) significantly increase the com-plexity, monitoring, and control of this signaling network.

As the network continues to evolve and grow, additional Diameter peers have to be provisioned in every network ele-ment. This is often referred to as the N‐squared problem. The Diameter peer configuration complexity inherent within this mesh‐type network presents a twofold problem:

✓ First, the large task of maintenance and administration of peer configurations on individual nodes directly affects the consistency and scalability of the network.

✓ Second, placing the routing responsibility on the individ-ual network elements can degrade the network element’s ability to perform its primary functions.



The N‐squared problem, or Metcalfe’s Law, states that the value of a telecommunications network is proportional to the square of the number of connected users (or nodes) of the system. As more network nodes get added to the network, the complexity keeps rising exponentially.

To solve this problem, a DSC can be deployed to provide a more efficient and scalable network architecture. By placing the DSC in the core of the network, routing is centralized, reducing the quantity and complexity of inter‐network and intra‐network routing. Also, since the routing responsibility is removed from individual network elements, their expensive resources are freed up to perform their primary function, thus reducing network‐wide capital expenditures.

Deployment of Multiple HSSsHome Subscriber Servers (HSSs) are deployed in LTE/EPC and IMS networks. In IMS networks, HSSs are responsible for subscriber‐specific authorizations, service profiles, and preferences. In LTE/EPC networks, the HSS is responsible for Mobile Authentication and other Home Location Register (HLR) functionalities. Either an increase in numbers of subscribers or the need for network diversification can drive network operators to deploy multiple HSSs. In networks that contain multiple HSSs, the subscriber identity is used to route to the appropriate HSS containing a particular subscriber’s information. Each node requesting the subscriber information would have to be provisioned with routing information, including subscriber identity and the HSS’s address containing the subscriber’s profile. Unfortunately, this leads to complex and redundant routing tables.

By deploying a DSC with Subscription Locator Functionality (SLF), a more efficient routing methodology can be used. The DSC with SLF would be provisioned with subscriber identity and mapping to the appropriate HSS. This provides a centralized routing mechanism that is much more efficient to configure and maintain.



PCRF BindingThe Policy Charging Rules Function (PCRF) is becoming increasingly more critical in managing network operators’ resources while balancing the subscriber’s network data uti-lization experience. As network operators push to increase their Annual Revenue per User (ARPU), by offering tiered data plans and the introduction of Voice over LTE (VoLTE), the role of the PCRF becomes one of the most critical within the network.

The PCRF is used for the authorization of a subscriber’s bandwidth allocation based on multiple factors, including the subscriber’s past usage, the level of service a subscriber has purchased and the amount of resources currently available in the network.

When a subscriber establishes an IP/data session (IP‐CAN) within the network, a PCRF is assigned to authorize the ses-sion and maintain a Quality of Service (QoS) for the session. IP‐CAN/PCRF binding ensures the initial PCRF assigned for the session is responsible for maintaining the rules and QoS during the life of the session. When multiple PCRFs are deployed in the network based on either network scalability requirements or PCRF vendor product architectures, this type of network topology requires an IP‐CAN/PCRF binding capabil-ity, within the network.

By deploying a DSC including IP‐CAN/PCRF binding capabili-ties, multiple PCRFs can be provisioned in the network with-out having to replicate the binding information in every PCRF. The flexibility provided by the DSC enables efficient network design to meet the needs of both operators and subscribers.

Roaming to Non‐LTE NetworksWhen mobile network operators start their deployment of LTE/EPC, it is important to provide subscribers with the widest breadth of coverage possible. In order to provide the cover-age required by the subscribers, the home network (LTE/EPC) provider might have roaming agreements with non‐LTE net-works such as 2G or 3G operators. The home network provider would have provided its customers with multi‐mode handsets



to facilitate this roaming. Since the 2G/3G networks are SS7 signaling–based, and the LTE/EPC networks are Diameter signaling–based, an interworking function is required to trans-late from the Diameter signaling protocol in the LTE network to the SS7/MAP signaling protocol used in the 2G/3G networks. Without this translation/interworking function, it is impossible for a subscriber to roam from an LTE/EPC/Diameter network to a 2G/3G/SS7/MAP network (see Figure 3‐2).

The deployment of a DSC with Interworking Function (IWF), solves the problem of subscribers roaming from an LTE/EPC Diameter signaling protocol network to a non‐LTE network, by providing the translation and mapping capability required for communication between Diameter‐based networks and SS7‐based networks (see Figure 3‐3).

Figure 3-2: Roaming to non‐LTE network.

Figure 3-3: Roaming to non‐LTE network with DSC.


Defining the Role of the DSC in LTE and VoLTE Networks

In This Chapter▶▶ Addressing interconnected network and security challenges

▶▶ Managing congestion and flow control

▶▶ Ensuring interoperability between agents

▶▶ Binding multiple sessions

▶▶ Supporting multi‐protocol interworking between networks

▶▶ Using virtualization for routing efficiency

D iameter Signaling Controllers (DSCs) are key elements in Long Term Evolution/Evolved Packet Core (LTE/EPC)

networks, and are used for routing and securing Diameter messages. Diameter Signaling Controllers provide routing, traffic management, load balancing, and session binding. In this chapter, you learn about the role of DSCs in LTE and VoLTE networks.

Interconnect and SecurityInternetwork connections — whether bilateral or through an IP Exchange/GPRS Roaming Exchange (IPX/GRX) provider — pose a unique set of problems to mobile service providers. These internetwork connections are used when subscribers

Chapter 4



are roaming beyond their service provider’s coverage area. The combination of a complex LTE/EPC network, numerous interconnected networks, and the vendors’ wide diversity of equipment and software releases present significant chal-lenges for setting up routing rules and security policies on who can access which networks.

To simplify the roaming interface between peer networks, a Diameter Edge Agent (DEA) provides an entry point to pro-vide efficient connection methodologies and network secu-rity. The DEA hides the topology of the network behind it and advertises itself to roaming partners as a Diameter relay, serving all Diameter applications in the network.

The DEA is essentially a signaling firewall that protects the internal network from malformed messages, unauthorized senders, and exposure of internal information to external networks. Figure 4‐1 depicts this architecture.

The DEA must address the following types of security:

✓ Transport security: Transport security guarantees the integrity of transmitted and received Diameter messages by implementing secure protocols (Transport Layer Security or Datagram Transport Layer Security).

✓ Application security (topology hiding): Topology hiding prevents disclosure of certain network configuration information, by changing or removing internal informa-tion about a Public Mobile Network (PMN), which isn’t required outside the PMN.

Figure 4-1: GSMA PRD IR.88 Diameter roaming implementation architecture.

Chapter 4: Role of the DSC in LTE and VoLTE Networks 27


✓ Application security (admission control): Admission control ensures message validity. The DEA is expected to filter Diameter messages to accept only supported Application IDs, Command Codes, and Attribute‐Value Pairs (AVPs).

Traffic ManagementCongestion can be divided into incoming and outgoing con-gestion. Incoming congestion is reflected by growth of the incoming queue and outgoing congestion by growth of the outgoing queue.

Flow control can be thought of as a rate‐limiter while process-ing the incoming or outgoing queues. Incoming flow control limits how quickly messages may be accepted from the peer, and outgoing flow control limits how quickly messages may be sent to a peer. If messages are received, processed, or sent faster than the allowed rate, the corresponding queue grows and congestion eventually results.

Congestion Management is accomplished by determining the maximum age for a given type of message along with the amount of space left on the queue. Congestion is related to the arrival rate of messages on the queue and the Flow Control rate of taking messages off the queue.

Diameter Agent InteroperabilityDiameter agent interoperability, also known as protocol mediation, refers to scenarios where routing is required between two Diameter nodes using different versions of the protocol, or different implementations of the same version of the protocol. In these scenarios, the DSC’s job is to modify messages as they pass through the DSC to ensure disparate Diameter nodes can talk to each other.

The DSC provides advanced capabilities that allow opera-tors to overcome Diameter protocol incompatibility issues, encountered during network commissioning and turn‐up, in real‐time. Capabilities built into the advanced routing infrastructure allow operators to modify the AVP content of Diameter messages that traverse the system.



The DSC advanced routing mechanisms provide operators the ability to identify specific Diameter messages that require modification. Messages may be selected by any combination of the following:

✓ Header contents

✓ Originating or receiving Diameter node

✓ AVP contents

After a message is selected, it can then be directed to internal functions that allow real‐time modification, such as

✓ Add or delete AVPs

✓ Modify contents of AVPs

✓ Count subsets of AVPs

✓ Dump the contents of a subset of messages to log files

✓ Send the message in Extensible Markup Language (XML) format for further processing by a script or external system

Session Binding and SubscribersDiameter Session Binding is used in networks with multiple Policy and Charging Rules Function (PCRF) instances perform-ing the same function, and where the network operator wants messages with the same Diameter Session‐ID AVP to go to the same PCRF. This is useful when application‐level requests need to be routed to the correct PCRF that’s hosting the subscriber session, or when a single subscriber has multiple sessions in a multi‐PCRF network and each session from that subscriber needs to be terminated on the same PCRF. When deploying VoLTE, it’s essential to have Diameter Session Binding capabil-ity to connect IMS layer with PCRF.

Multi‐Protocol InterworkingAs service providers deploy LTE/EPC networks and begin to offer the enhanced capabilities of LTE/EPC to their subscrib-ers, they must address a new problem that arises when their

Chapter 4: Role of the DSC in LTE and VoLTE Networks 29


subscribers roam to non‐LTE/EPC‐based networks, such as 2G/3G networks. The network architectures and underlying protocols of LTE/EPC and non‐LTE/EPC networks differ in the following ways:

✓ 2G/3G uses SS7/Transaction Capabilities Application Part (TCAP)/Mobile Application Part (MAP) to manage mobility

✓ LTE/EPC uses Diameter to manage mobility

Multi‐protocol interworking is the capability to convert between these different Diameter and SS7 protocols.

The DSC solves the issue of subscriber roaming to dispa-rate networks by fully supporting the Third‐Generation Partnership Project (3GPP) specification for interworking between Diameter and MAP (see Figure 4‐2), thus allowing LTE/EPC subscribers to roam seamlessly between LTE/EPC and 2G/3G networks.

Virtualization and Multiple Instances for Routing Efficiency

Another aspect of routing to consider is the ability to consoli-date routing rules for both intra‐network and internetwork traffic. Having a single massive routing configuration inher-ently leads to complexity and increases the chances of errors

Figure 4-2: A DSC provides multi‐protocol interworking between LTE/EPC and 2G/3G networks.



when making routing/traffic rules changes. Therefore, the abil-ity to deploy multiple virtual DEAs, configured within a single network entity, enables routing segmentation (routing seg-mented on a per‐interconnected network basis) and provides an efficient routing mechanism. Each of these virtual DEAs has its own separate routing and screening rules that include the ability to shape traffic on a per‐peer basis. This shaping includes traffic flow control, throttling, and congestion on a per‐peer basis. This flexible routing concept provides increased control as well as ease of implementation.

Using virtual DEA instances, the DSC is uniquely enabled to provide for the needs of hub providers and wholesale inter-connect operators. Individual clients of these providers can be managed with individual routing table and configuration databases.

In a hub provider environment, one instance will typically be dedicated to each carrier customer to act as a Diameter Edge for that customer. One central instance for the hub provider will join these instances together. In its routing and configu-ration tables, the central instance will then be able to very efficiently

✓ Set rules defining which carriers may roam and with whom.

✓ Create statistics and peg counters on thousands of dif-ferent combinations of messages (for example, counting messages between two carriers).

✓ Throttle messages coming from or going to specific carriers.

✓ Copy messages to external systems for downstream billing creation.


Number Portability and Subscriber Databases

In This Chapter▶▶ Supporting Number Portability (NP)

▶▶ Understanding Subscriber Location Function (SLF)

▶▶ Mapping numbers internationally with E.164 (ENUM)

I n this chapter, you learn about some of the additional functions of signaling networks.

Number Portability (NP)Number Portability (NP) was introduced by regulators country‐by‐country in the late 1990s, to lower the barriers for subscribers to change service providers and increase competition for subscribers between service providers. With NP, individual subscribers and businesses can move to a new service provider without changing their existing telephone number. Depending on local regulations, telephone numbers can be ported between different fixed line providers, between mobile network providers, or between fixed and mobile providers.

NP affects the routing mechanisms for terminating voice and data calls. The fundamental nature of the dialed number changes from a physical routing address to a virtual address. Transparency to subscribers is achieved by incorporating a translation function to map a dialed potentially ported number into a network routing address (either a number prefix or another number) which can be routed.

Chapter 5



Two approaches to NP include an Intelligent Network (IN)-based NP solution and a Mobile Number Portability‐Signaling Relay Function (MNP‐SRF)-based solution. Both solutions may be implemented as a standalone solution or integrated with STP functionality.

Subscriber Location Function (SLF)

When a mobile operator grows very large, it may consider dividing up its subscriber database to be hosted on different Home Subscriber Servers (HSS) complexes. This allows the operator to scale up its HSS infrastructure and have increased resiliency and redundancy.

If an operator divides up its subscriber database, it needs a supporting Subscriber Location Function (SLF) to assist other Diameter elements in figuring out which HSS complex to contact in order to find a particular subscriber profile.

In its simplest configuration, the SLF will be a set of operator‐ defined rules in which there is a logical breakdown of how the subscribers are distributed between HSS complexes, for example, by ranges of subscriber identifiers or odd/even phone numbers. In more complex cases, the SLF could have a database of its own and would perform lookups of subscriber identifiers and retrieve the proper HSS addresses.

E.164 Number Mapping (ENUM)

The ability to dial a telephone number is critical to allow subscribers on classic SS7 telephone networks and Internet telephony (Voice over IP, or VoIP) networks to call each other. The Electronic Number Mapping System (ENUM) was developed by the Internet Engineering Task Force (IETF) to allow a single, universal personal identifier for different communication services. ENUM uses E.164 telephone num-bers and enables VoIP calls to be connected to traditional SS7 telephone networks.

Chapter 5: Number Portability and Subscriber Databases 33


E.164 defines a numbering plan for international telephone numbers. The format consists of a 1‐ to 3‐digit Country Code and a 12‐ to 14‐digit Subscriber Number for a maximum of 15 digits.

ENUM database is a critical service that brings together SS7 telephone networks and VoIP services, such as Session Initiation Protocol (SIP) and Voice over Long Term Evolution (VoLTE).


Recognizing the Role of NFV in Signaling Networks

In This Chapter▶▶ Learning about Network Function Virtualization

▶▶ Understanding how NFV fits with SS7 STPs

▶▶ Solving Diameter routing challenges with NFV

T he telecommunications industry has traditionally been characterized by a very methodical and sometimes slow

approach to major network and technology changes. However, this model runs contrary to the new realities of our modern business world in which agility, change, and innovation drive competitive advantage.

In this chapter, you learn how Network Function Virtualization (NFV) is enabling a paradigm shift in the telecommunications industry and how this will affect SS7 and Diameter networks in the future.

Defining NFVNFV focuses on new methods for the deployment and delivery of telecommunication services over a software‐based network infrastructure. This is very similar to the way the information technology (IT) industry uses virtualization in the data center and in the cloud. The benefits and objectives of NFV are

✓ Increased network design flexibility

✓ Rapid service innovation

Chapter 6



✓ Reduced capital expenditures and operational costs

✓ Reduced power consumption

✓ Standardized and open interfaces

The main concepts in the NFV methodology are to

✓ Decouple network functions from proprietary hardware and allow them to be instantiated on industry standard, commercial off‐the‐shelf hardware

✓ Shift control of network functions from hardware to software by using a hypervisor layer that abstracts the underlying hardware from the software functions

✓ Provide flexibility across locations — data centers and other network nodes — to maximize efficiencies and performance

✓ Create a more applications‐aware network to facilitate faster time‐to‐market for new services

STPs and NFVFor many service providers, evolution to an IP‐only network is an ongoing process. However, in parallel with this evolution most service providers still have a need to support traditional SS7/SIGTRAN protocols for the foreseeable future. In turn, this means STPs are still needed in fixed and mobile networks.

For service providers, this continued need for STPs is pressed up against the reality of fewer STP options in the market. Many STP vendors have announced End‐of‐Sale of their STP offerings, and those products are now reaching End‐of‐Service life. Other vendors have been acquired by non‐service provider‐focused companies. In aggregate, this leaves service providers with fewer choices, leading to higher risks and concerns about vendor commitments to STP product longev-ity. Service providers need to enter into relationships with vendors who can help evolve their STP architectures to a future‐proof design.

For many vendors, STPs have already evolved from SS7/ Time‐Division Multiplexing (TDM) proprietary hardware to an SS7/IP proprietary hardware solution. So what is the next step? The answer is moving to a virtualized solution.

Chapter 6: Recognizing the Role of NFV in Signaling Networks 37


NFV is the separation of what was previously tightly coupled and often proprietary hardware and software, in order to enable software to operate on industry‐standard commercial off‐the‐shelf (COTS) servers. For service providers who need to expand deployment of IP‐based STPs or are considering replacement of traditional TDM‐based STPs, virtualization is the way to go.

Exploring Diameter NFV Use Cases

The use cases discussed in the following sections provide some examples of NFV in Diameter routing. Additional NFV/Diameter use cases will continue to be defined as more ven-dors and service providers move forward with NFV in the LTE environment.

Virtual Diameter Edge AgentsAs more and more network operators sign bilateral roaming agreements, the management of these interconnected net-works is becoming problematic in terms of security, topology hiding, traffic handling, and the costs associated with using purpose‐built Diameter Signaling Controllers (DSCs).

Figure 6‐1 depicts a network operator who has multiple bilat-eral roaming agreements with other network operators. The home network operator is using NFV to implement virtual Diameter Edge Agents (DEA).

Similar to the challenges described in the service provider use case, the IP Exchange/GRPS Roaming Exchange (IPX/GRX) market is growing, driven by the increases in roaming agree-ments between network operators. This growth can have an adverse effect on IPX/GRX providers’ ability to effectively manage the interconnections in terms of operations, security, and capital expenditures for DSCs. Additionally, not all DSCs have the segmentation ability to address multiple tenants.

Figure 6‐2 depicts an IPX/GRX network operator who has mul-tiple network operator clients and is using NFV to implement virtual DEAs.



Figure 6-1: Service provider with bilateral roaming agreements.

Figure 6-2: IPX/GRX provider with multiple interconnected networks.

Chapter 6: Recognizing the Role of NFV in Signaling Networks 39


Each instantiation of the virtual DEA (vDEA) in the service provider and IPX/GRX provider’s network can provide

✓ Routing on an interconnected network basis.

✓ Security mechanisms tailored to interconnected networks.

✓ Screening of incoming messages by interconnected networks.

✓ Traffic shaping based on individual service level agree-ments (SLAs).

✓ CapEX savings over individual purpose-built platforms.

✓ OpEX savings with simpler implementation required for scaling.

✓ Reduction in risk; configuration changes for an intercon-nected network do not affect configurations for any other interconnected networks.

Virtual Core Diameter Signaling ControllersThe industry‐coined phrase “Diameter Signaling Storm” describes an exponential increase in Diameter traffic as LTE subscriber subscription rates increase. Addressing the increase in Diameter traffic using conventional DSCs based on purpose‐built hardware platforms requires that the DSC be engineered for the worst‐case traffic scenario. This concept of over‐engineering reduces the need for in‐service upgrades, but adds significantly to the capital investment costs.

Using NFV to instantiate virtual DSCs (vDSCs) based on real‐time traffic requirements is a far more efficient solution. This is shown in Figure 6‐3. Because most network signaling traffic, including Diameter, isn’t constant, a given vDSC can be instan-tiated or de‐instantiated in real‐time, based on variability in traffic. This concept saves operations costs in commissioning new processing capabilities and capital costs in purpose‐built computing power.



A vDSC can also be used when network operators would rather design, implement, and manage their networks on a regional basis. A vDSC is a far more effective solution than a purpose‐built DSC that could be cost prohibitive and logisti-cally challenging.

Look for the following capabilities and features in a virtual Diameter solution:

✓ Routing segmentation: The solution should allow for virtualization based on segmentation of routing rules on a per‐interconnected network basis. This segmentation would provide the ability to administer routing rules, traffic shaping, Diameter‐to‐Diameter interworking, and Diameter‐to‐SS7 interworking on a roaming partner or interconnected network basis. This capability allows increased control, reduces administrative risks, and pro-vides the flexibility required in network design.

✓ Standards‐based: In order for telecommunications ven-dors to be in the forefront of new and evolving concepts such as NFV, it’s important that they be actively involved in the standardization process. Their involvement ensures that they are committed to the concept and knowledgeable about upcoming changes in standards. This commitment will be reflected in the vendor’s imple-mentation of standards within their products.

Figure 6-3: Core DSC (traffic‐based instantiation).


Ten Things to Consider in a Diameter and SS7

Signaling SolutionIn This Chapter

▶▶ Evaluating Diameter and SS7 solutions

Y ou know ’em, you love ’em, and so without further ado . . . the Diameter Signaling For Dummies Part of Tens:

✓ Experience in telecommunications, SS7, and Diameter: In order to provide solutions that span the evolution-ary stages of telecommunications signaling (SS7 to Diameter), the solutions vendor has to have experience in the concepts of both SS7 and Diameter signaling. The experience in the legacy SS7 protocol and its associ-ated network provides the Signal Transfer Point (STP)/Diameter Signaling Controller (DSC) vendor with the unique knowledge of issues and concerns that occurred within legacy networks. This knowledge allows the vendor to provide solutions that mitigate these issues in new networks and protocols such as Long Term Evolution (LTE)/Evolved Packet Core (EPC)/Diameter.

✓ BSS/OSS integration: Native integration with business support systems (BSS) and operations support systems (OSS) is critical to ensure that evolving network archi-tectures can be properly managed. This is critical for managing the complex hybrid infrastructure of legacy SS7 components and newer Diameter components that are likely to exist in most networks for the near term.

Chapter 7



This can be achieved directly within the signaling solu-tion or via an Element Management System (EMS) that supports both SS7 and Diameter signaling.

✓ Combined Signal Transfer Points (STPs) and Diameter Signaling Controllers (DSCs): Having a combined STP and DSC allows the operator to seamlessly evolve their network as subscribers migrate, and it provides invest-ment protection and asset longevity.

✓ Consistent routing engine: An important architectural issue to be considered in the selection of a Diameter Signaling Controller (DSC) with Diameter Edge Agent (DEA) capabilities is whether the internal software design is based on universal protocol switching and rout-ing concepts.

✓ Diameter function requirements: Check whether the solution supports all of your network requirements such as Interworking Function (IWF), Policy Charging Rules Function (PCRF), and Subscription Locator Function (SLF).

✓ Routing segmentation: The DEA should include the ability to segment the routing rules on a per intercon-nected network basis. This segmentation would provide the ability to administer routing rules, shape traffic, implement Diameter to Diameter protocol mediation, and provide Diameter to SS7 interworking on a roaming partner or interconnected network basis. This capability allows increased control, reduces administrative risks, and provides the flexibility required in network design.

✓ Robustness at scale: Future proof your network archi-tecture with a scalable design and components that will support demand variability and long‐term growth. Scale without robustness isn’t carrier‐grade. STP and DSC solutions are critical to keep end user services up and running, and any service impacting failure will negatively impact the service provider’s financial bottom‐line.

✓ Security at scale: Diameter signaling is the lynchpin for successful 4G/LTE interconnection and roaming. Mobile operators must have the utmost confidence in their deployment decisions for DEA functionality in order to absolutely know their Diameter message exchange is secure at both the transport and application level. Diameter message use is increasing exponentially, but many Diameter architectures can’t scale to perform securely at high message rates.

Chapter 7: Considerations for a Diameter and SS7 Signaling Solution 43


✓ Specializing on network signaling and routing: There will always be differences in the implementation and interpretation of specifications when any network or protocol is deployed. These differences can cause catastrophic problems within networks and across the boundaries between different networks. A vendor spe-cializing in protocols and routing can provide mediation capabilities that solve the protocol inconsistencies and thus eliminate their network impact.

✓ Virtualization capabilities: The STP and DSC should include the ability to be deployed virtually in a Network Function Virtualization (NFV) environment. The virtual solution should be based on software that is common to appliance‐based solutions, thus providing the flexibility to be deployed virtually yet fit seamlessly within the net-work operators’ existing infrastructure. This provides a way to reduce implementation costs during the migration from SS7 to Diameter.

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