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October 2007 | Volume 2 / Number 5
Feature Articles

Quality of Service in IMS – Essential for Success

By Keith Cobler
At the same time, the market for multimedia communication services has become increasingly competitive. Although brand loyalty plays a dominant role in some customer segments, it cannot be the only barrier to entry as new technologies and changing customer needs and demographics are reshaping the market with increasing speed.

So for new IMS multimedia services, what factors are important for customer retention? Today, cost is certainly one of the key factors as consumers become savvier in terms of what is available and how much they are willing to pay for it. But given the relative infancy of IMS services, quality of service (QoS) becomes an increasingly important factor - especially during the initial deployment and introduction of new services. Consumers rely on communications as a fundamental component of their everyday lives. They are unwilling to compromise when it comes to poor quality - and are likely to drop a service in favor of another service provider that promises better quality and an exceptional experience.

Why Network Quality Matters

In the past, there were only a few companies that delivered communication services. But over the years, the lines between fixed, mobile and wireless have blurred - a trend that has accelerated with the introduction of IMS. For the traditional fixed or mobile network operator, they now have to deal with new competitors whose strategies and tactics are less familiar, but are creating unrest with their conventional customer base. With convergence, it's almost anyone's market for the taking.

Within this new market environment, network operators have to become more responsive to changing market dynamics and must have the ability to roll out new services and applications as needs arise. With IMS, network operators can develop and leverage single applications for multiple markets, avoiding the silo approach that has plagued them in the past.

At the same time, network operators must streamline their costs by minimizing future network expenditures (CAPEX) and minimizing ongoing network maintenance costs (OPEX). In moving from their current service-specific architecture to a multi-service architecture, there is the potential for huge cost savings from both infrastructure and maintenance economies of scale.

As networks continue to converge, the concept of quality at a given cost for network operators will become more significant - from the customer perspective because they are willing to pay for additional services, and from the network operator perspective because they must leverage existing network infrastructure and resources to deliver quality at a given cost.

The Evolution of Quality of Service

Historically, networks were developed with specific services in mind and it was understood that all applications did not have the same QoS. This same mindset held true for the initial IP networks that were introduced. On the low end of the scale, the QoS involved with sending data over an IP network was most often a best-case effort. It wasn't until the advent of VoIP that the industry began to focus its attention on improving the quality of voice calls over the Internet. But through constant network monitoring and optimization, VoIP services today are beginning to approach the same quality of service as PSTN - the benchmark for voice services - but now with the CAPEX and OPEX advantages of a converged IP network.

Because of the VoIP experience, expectations for quality are high for video and for converged services using video, voice and data. But within the converged IP environment, it is important to keep in mind that video, voice and data are not all created equal - voice is very sensitive to latency and jitter, while video is very sensitive to dropped packets.

Driving the connection between the customer's need for service and what the network can deliver is the concept of quality of experience, or QoE, which is a measure of the customer's perception of quality. Although it may be easy to believe that all that matters is QoE since it focuses on the end user, but it's important to remember the very foundation of QoE rests on having a network that can deliver services and applications with an acceptable QoS.

From a network management and monitoring perspective, it's critical to have complete end-to-end visibility of an entire IMS session - from the core IMS network to the end-user's device - if you want to ensure a given QoS and take full advantage of available network resources.

Challenges in Ensuring QoS in an IMS Network

QoS is a key component in the original design and conception of IMS. Different from some network architectures of the past, IMS allows service providers the ability to differentiate the QoS they deliver to a given customer or group of customers - a key aspect of future business models.

However, there are challenges in delivering and ensuring QoS in an IMS network.

To begin, IMS is complex. With more than a dozen new network elements, 20+ new interfaces and dozens of protocols involved, there is a great deal of complexity that needs to managed and monitored. (See Figure 1.)

Some of the new elements in IMS include the Home Subscriber Server database (HSS); the Call/Session Control Function (CSCF) which processes SIP signaling and is divided into the Proxy-CSCF (P-CSCF); the Interrogating-CSCF (I-CSCF); the Serving-CSCF (S-CSCF); the Application Server (AS) that hosts and executes services and includes the SIP AS (Application Server); the OSA-SCS (Open Service Access-Service Capability Server) and the IM-SSF (IP Multimedia Service Switching Function); the Media Resource Function (MRF) which provides a source of media for the home network; the BGCF server which connects IMS originated calls to circuit switched terminations;

Figure 1. IMS introduces many new network elements in the IMS core, as well as many new interfaces and protocols.


the Signaling Gateway (SGW) that interfaces the signaling plane of the circuit switched network; Media Gateway Control Function (MFCF) that performs protocol conversion; and the Media Gateway (MGW) that interfaces with the media plane or the circuit switched domain.

Another point regarding IMS elements is that 3GPP does not call for the standardization of nodes - but functions instead. So, what does that mean? In simple terms, it means that IMS is a collection of functions linked together by a common set of interfaces. With this approach, network equipment manufacturers are free to split elements apart or combine them - but the underlying basis is for a given functionality. There are potential challenges raised by this approach, including possible interoperability issues since some network equipment manufacturers may choose to spread functionality across several nodes, while others may combine multiple functionality into a single node.

In addition, there is increased session complexity, and depending on the networks involved, it may increase ten-fold over an existing stand-alone network. For example, the signaling associated with a multimedia mobile phone session that accesses IMS resources might include 30-40 individual messages and span multiple network elements and protocols. To further complicate the issue, a given session may contain multiple streams of different data types, each with their own QoS characteristics. If a problem should arise in any step along the way, a service provider needs to have the ability to trace the signaling path and quickly isolate network elements, interfaces or protocols involved with the problem.

Ensuring Network QoS for Maximum Impact

It is difficult to challenge the importance of QoS in the delivery of next generation services and applications over IMS. However, as we've also seen, delivering QoS within an IMS environment is not without its challenges. Most importantly, network operators need to realize the decisions made today about what they deploy and how they manage and monitor it will have a lasting impact on their long-term business success or failure. And, different from network technologies of the past, IMS is all about delivering those individual services and applications to those individuals willing to pay for them.

From an operator's perspective, it's no longer just about managing the network, but providing the right information concerning services and applications to the marketing, business operations and customer-related departments that depend on it to be successful. By adapting a network monitoring platform to support a wide variety of network types and technologies and diving deep into IMS and other associated protocols, operators will be able to ensure a network's end-to-end QoS.

Keith Cobler is Marketing Manager, Network Management, for Tektronix. For more information on Tektronix' communications test, measurement and network monitoring solutions, visit

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