TekelecWP1557 - Diameter

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The Rise of Diameter Signaling

The Rise of DiameTeR signalingThe Rise of DiameTeR signaling

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The Rise of DiameTeR signaling

Table of Contentsintroduction.................................................................................................................4

siP as the succesor to ss7............................................................................................5

Diameter Routing Challanges.......................................................................................5

Comparing ss7 with Diameter......................................................................................6

Diameter signaling Router overview.............................................................................7

signaling architecture for Diameter-based networks.........................................................7

Diameter mediation and Protocol interworking.............................................................8

additional Value of a Centralized Diameter Router........................................................9

in summary...................................................................................................................9

some Relevant acronyms.............................................................................................10

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Introductionmobile data traffic is skyrocketing, fueled by the introduction of smartphones, laptop

dongles, flat-rate plans, social networking and compelling applications like mobile video.

and, there is no slowdown in sight.

aBi Research (9 august 2009) predicts that by 2014 the mobile data traffic registered

per month will equal the total data traffic logged during all of 2008. operators have

realized that their 3g networks are not equipped to sustain this high level of traffic

growth. They are looking to all-internet protocol (iP) networks such as long term

evolution (lTe) and iP multimedia subsystem (ims) to provide the bandwidth required to

support data-hungry devices and applications and to cost effectively address the growing

gap between traffic and revenue growth.

The Diameter protocol, using stream control transmission protocol (sCTP) for transport,

is used widely in the all-iP, service-oriented ims and lTe architectures. Within the

ims control and service planes, Diameter plays a central role in policy, charging,

authentication and mobility management. for years operators have employed

signaling system 7 (ss7) as the international, standardized protocol to communicate

globally between operator networks. isDn user part (isUP) is used for voice-oriented

communication, Camel application Part (CaP) is used for service control and mobile

application part (maP) for non-call-related signaling, including mobility management.

The digital era is on the horizon, yet the majority of operators around the globe still

connect their networks through the exchange of well-defined ss7 messages. all of that

is about to change.

This paper focuses on the signaling role that Diameter is going to play in end-to-end

iP networks like lTe. Diameter’s part in service-related functions such as charging is

not addressed in detail. however, Tekelec expects common protocol functions such

as routing, screening and Diameter-normalization to be implemented in common but

centralized Diameter-specific network elements.

79%

10%

11%CAP/INAP

ISUP/BICC

MAP

Figure 1. Signaling Volumes in GSM Core Network

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SIP as the Successor to SS7While voice-related isUP traffic represents only 10% of the overall signaling traffic, the

functions it performs will be required in all-iP networks such as ims. as the networks

evolve, operators will move away from ss7-based isUP to session initiation protocol (siP).

a relevant development in this area is the iP eXchange (iPX), which the gsm association

(gsma) developed to create a technical and commercial infrastructure that supports

packet voice interconnects. it is also used for other purposes such as Diameter-based

roaming for lTe and ims interconnect. The iPX, carrying siP traffic, can be seen as an

alternative to isUP-based voice interconnects. in practice, the gPRs Roaming exchange

infrastructure (gRX) is evolving to iPX, which combines the data and voice worlds into

one infrastructure.

in lTe networks, the functions performed by ss7-based maP signaling also will be

replaced by equivalent operations based on the Diameter protocol. This is detailed in

relevant 3gPP specifications. over time, the number of ss7 networks will stabilize and

more and more Diameter-based interfaces will be used.

Diameter Routing ChallengesWithout 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. initially, implementing an ims or lTe network without a signaling core

may be sufficient, but as traffic levels grow, the lack of a capable signaling infrastructure

poses a number of challenges, including:

• Scalability: each endpoint must maintain a separate sCTP association with each of its Diameter peers as well as the status of each, placing a heavy burden on the endpoints as the number of nodes grows.

• Congestion control: Diameter lacks the 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.

• 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 testing (IOT): Protocol interworking becomes unmanageable as the number of devices supplied by multiple vendors increases. With no separate signaling or session framework, ioTs must be performed at every existing node when a new node or software load is placed in service. ioT activities consume a

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considerable amount of operator time and resources, with costs increasing in proportion to the number of tests that must be performed.

• Support for both SCTP- and transmission control protocol (TCP)-based implementations: sCTP-based elements cannot communicate with TCP-based elements unless they are upgraded or all of the elements support both protocol stacks.

• Subscriber to HSS mapping: When there are multiple hsss in the network, subscribers may be homed on different hsss. 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.

Comparing SS7 with DiameterThere are key differences between the ss7 and Diameter protocols that also create

challenges. operators used to managing ss7 networks need to be aware that certain

“common rules” no longer apply in lTe, and they will need to take other measures to get

the network behavior they have become accustomed to with ss7. one example is network

failures. in ss7, the network is designed and configured to route around a failed path,

and, therefore, the signaling is optimized upon detection of a failure. for Diameter, the

failed path must be (re)discovered every time a request is sent over it. in this way, Diameter

Figure 2. A Comparison of SS7 and Diameter Characteristics

Characteristic

Routing Each message independently routed

Answer message does not contain

routing info and follows same path

as associated request

Network-wide

Relies on reliable transport

Specialized, app-specific proxies

required

Early days; lots of prove-in

still required

DRA for PCRF

HSS address resolution for

EPC and IMS

No dedicated SNM messages

Hop-by-hop

No defined traffic priorities

Failures ‘discovered’ with each new

request

Dedicated SNM messages

GTT can be used by upper layers

Wide-scale deployment; years of

experience

Routed around failed paths

Congestion levels and traffic

priorities defined

Signaling network management

(SNM) and Congestion control

Subscriber number based routing

Robustness

SS7 Diameter

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signaling is less efficient. a central Diameter routing functionality could provide some

improvements.

Tekelec believes that as with ss7, a centralized node to manage signaling routing

and related functionalities will continue to play an important role in future signaling

networks. The introduction of Diameter-based signaling does not change this. industry

organizations have recognized this need, including the 3gPP. in fact, the Diameter

standard defines Diameter agents to support the efficient and correct handling of

Diameter signaling. These agents include: Diameter relay, Diameter redirect, Diameter

proxy and Diameter translation.

Diameter Signaling Router Overview Tekelec’s Diameter signaling Router (DsR) creates a centralized core Diameter signaling

layer that relieves lTe and ims 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/sCTP (or

TCP) mesh that is created by having direct signaling connections between each network

element. having one or more sCTP hubs that centralize the sCTP connections to all

end nodes simplifies interoperability between different network elements and enhances

network scalability.

Signaling Architecture for Diameter-based NetworksCentralizing 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.

a centralized signaling architecture:

• improves signaling performance and scalability by alleviating issues related to the limited signaling capacity of mmes, hsss, CsCfs and other Diameter endpoints;

• simplifies network expansion because routing configuration changes for new endpoints are performed only on the DsR;

• increases reliability by providing geographic redundancy;

• Provides mediation of 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;

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Diameter Mediation and Protocol InterworkingThe DsR’s ability to centralize the mediation of different Diameter variants deserves special

focus because it brings immediate benefit to the network. as with other protocols, vendors

are likely to use their own variants of the Diameter protocol based on how they believe a

specific interface should be implemented. This implementation can be slightly different from

that of another vendor, although both claim to work to specifications. interworking issues

arise when multi-vendor equipment is combined in one network, a common approach

for operators that are building a best-of-breed solution. The issue can also emerge when

operators connect their network to other parties, like mobile virtual network operators

(mVnos).

interworking Diameter and non-Diameter nodes that are built on legacy protocols

presents a special challenge. operators that want to introduce Diameter-based protocols

into a network where legacy protocols must also be supported – either for costs or

practical reasons – must take Diameter mediation into account. That mediation can be

handled by having a decentralized functionality where one of the two platforms implements

both Diameter and the non-Diameter, legacy protocol. or, a “purpose built” protocol

mediation solution can be deployed between the nodes.

• enables hss routing flexibility with integrated hss address resolution function;

• Creates a centralized monitoring and network intelligence data collection point to isolate problems and track key performance indicators (KPis); and

• 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.

Diameter Mediation and Protocol Interworking

Figure 3. Typical Network Architecture Using the DSR as a Central Diameter Routing Engine

DSR

DiameterAgent

EIR

DiameterAgent

Foreign LTE Domain Foreign GPRS Domain

Home LTE/IMS Domain

I/S-CSCFP-CSCFMMEPCRF

SLF

!

vMME vSGSN

vS4-SGSN

SS7

vPCRF

PGW OFCF

OCF

ABMF RF

AF

IP-SM-GW

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Tekelec’s view is that a centralized protocol mediation function is a more effective approach.

it allows operators to maintain “legacy” nodes without investing in temporary, new

functionality and capacity for interworking purposes. The centralized protocol mediation

function can provide immediate value – even without solving complex routing

tasks – by enabling:

• Diameter-to-Diameter interworking for two different Diameter implementations;

• Diameter-to-legacy protocol interworking where each operator has different needs based on the particular legacy protocols that need to be supported; and,

• iPv4-iPv6 and sCTP-TCP interworking.

Additional Value of a Centralized Diameter RouterThe Diameter-based router is the perfect vantage point at the network core to cost effectively

and flexibly enable a variety of other functions and services. it is a much more efficient way

to implement routing tables from a central point in the network and deploy capabilities that

need access to other network resources, either inside or outside the operator’s own domain.

The centralized node can then proxy information for decentralized elements like hsss, mmes

or PCRfs.

When connecting to a Diameter-based network element in another operator’s network,

there is always the risk that the Diameter implementations will not match perfectly, even if

the elements are provided by the same vendor. Therefore, a Diameter agent is required to

adapt Diameter messaging. By implementing the functionality at the central network core

level, operators can perform ioTs faster and more cost effectively.

in the ss7 world, operators steer roaming traffic to enable them to control the network

on which their subscribers roam. Typically, the solution is connected to a centralized node

that handles the roaming signaling traffic, usually next to the signal transfer point (sTP).

The use of Diameter-based roaming combined with the always-on nature of lTe data

sessions requires a new solution to steer roaming traffic. This is yet another application

of a centralized Diameter router. in the near future, traffic steering can be enriched with

personalized steering defined by policy information. for example, an operator could steer

traffic based on the requested services and the availability of the best network, either in

terms of cost or quality for that service at that time of day. a centralized signaling router

with access to policy information and real-time network status can provide this next level of

traffic steering.

In Summarystep by step, ss7 signaling is being replaced by Diameter and siP-based equivalents. While lTe defines

Diameter-based rather than ss7 interfaces, the majority of the functionalities performed by ss7 will have

to be carried into the Diameter realm. as Diameter networks become larger, the number of interfaces

and complexity will increase, creating the need to optimize networks for Diameter-related tasks.

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The Diameter signaling Router creates a Diameter signaling core that relieves lTe and

ims endpoints of routing, traffic management and load balancing tasks and provides

a single interconnect point to other networks. 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 address resolution, Diameter interworking and traffic steering.

Some Relevant Acronyms BiCC Bearer independent Call Control

Camel Customized applications for mobile network enhanced logic

CaP Camel application Part

CsCf Call session Control function

DRa Diameter Routing agent

DsR Diameter signaling Router

enUm electronic number mapping

ePC evolved Packet Core

gRX gPRs Roaming eXchange

hss home subscriber server

inaP intelligent network application Part

ims iP multimedia subsystem

ioT interoperability Test

iP internet Protocol

iP-Can iP Connectivity access network

iPX iP Packet eXchange

isUP isDn User Part

iWf interworking function

KPi Key Performance indicator

lTe long Term evolution

maP mobile application Part

mme mobility management entity

mVno mobile Virtual network operator

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PCRf Policy and Charging Rules function

PDn Packet Data network

sB service Broker

sCCP signaling Connection Control Part

sCTP stream Control Transmission Protocol

siP session initiation Protocol

sms short message service

ss7 signaling system 7

sTP signal Transfer Point

TCP Transmission Control Protocol

ss7 signaling system 7

sTP signal Transfer Point

TCP Transmission Control Protocol

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