performance in ims

Monitoring and improving performance in IMS networks

Innovative Communications

Technologies & Entrepreneurship

8 t h S e m e s t e r

5 / 2 4 / 2 0 1 2

Student: Alexandros Fragkopoulos

Group Number: 12GR890

Supervisors:Rasmus Hjorth Nielsen

Neeli R. Prasad

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Abstract

During the last decades the development and improvement of telecommunications is massive. If one considers how long ago the telephone was a privilege that only a few possessed, then one can see the big onward steps technology has made. In this rapid rhythm of technological advances, people demanded to be connected more and more time until they reached the point that they want anywhere and anytime full access into the digital world. It either serves their needs for work or entertainment. Looking in this direction, research has been made the last years on the development of efficient usage of the internet services via smartphones or any other portable device and through any type of network. An important issue that has to be solved and clarified is how does someone utilize same services through all these different technologies (old and new, complex and simple, wired and wireless oriented)? And if one manages to do that, then who is benefited from that? All these questions bring more and more issues on the surface. In an effort to address all these issues the IMS framework has been created. Although it is relatively still new as a framework, numerous benefits have been discovered through research for this network. IMS networks help to interconnect devices through different technologies and still provide the same quality and number of services. In this way users are satisfied by the operators and operators find the opportunity to setup a unified charging policy (through the IMS network) that will help them increase their profits. This project though, is being focused on the performance of these networks from the point of improving them but also defining what are the most important KPI’s that depict weaknesses in performance issues. Concerning the part of improvement, some ideas from two papers are being presented and an approach of them is being implemented. In the end of this project some results from simulations are derived and discussed. Last but not least is a small reference to the future technologies such LTE and LTE-A.

Alexandros Fragkopoulos ICTE 8

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Table of Contents

Abstract.........................................................................................................1List of Figures..................................................................................................3List of Tables...................................................................................................3List of Abbreviations..........................................................................................3Chapter 1 – Introduction.....................................................................................5Chapter 2 - IMS................................................................................................7

2.1 Why IMS?...............................................................................................72.2 3GPP: Requirements and Standards...............................................................72.3 IMS Architecture.......................................................................................92.4 Entities inside an IMS...............................................................................10

Chapter 3 – Analysis........................................................................................153.1 State of the art.......................................................................................153.2 Performance in IMS services......................................................................163.3 KPIs for the IMS network..........................................................................17

3.3.1 Accessibility KPIs...............................................................................173.3.2 Retainability and Utilization KPIs...........................................................19

Chapter 4 – Design..........................................................................................204.1 First use case.........................................................................................204.2 Second use case.....................................................................................214.3 Improving performance in IMS networks.......................................................22

Chapter 5 – Implementation..............................................................................235.1 Description of the algorithm......................................................................235.2 Figures and Results..................................................................................255.3 Results’ Analysis.....................................................................................28

Chapter 6 – Conclusions...................................................................................306.1 Future Vision in Communication Technologies................................................30

Bibliography..................................................................................................31


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List of Figures

Figure 1 - Layered Architecture of the IMS [2]..........................................................9Figure 2 – Successful IMS Voice Session Establishment [1].........................................13Figure 3 – Plots from First Case Scenario..................................................25Figure 4 - Plots from Second Case Scenario..............................................26Figure 5 - Plots from Third Case Scenario..................................................27Figure 6 - Plots from Fourth Case Scenario...............................................28Figure 7 - Ranking of the Best Case Scenario............................................29

List of Tables

Table 1 – Acquisition of data from the simulations....................................28

List of Abbreviations

PSTN Public Switched Telephone NetworkIP Internet ProtocolDSL Digital Subscriber Line3G 3rd Generation4G 4th GenerationIMS IP Multimedia Subsystem3GPP 3rd Generation Partnership ProjectVoIP Voice over IPQoS Quality of ServiceQoE Quality of ExperienceGPRS General Packet Radio ServiceLAN Local Area NetworkWCDMA Wideband Code Division Multiple AccessWLAN Wireless Local Area NetworkWiMAX Worldwide Interoperability for Microwave AccessOFDM Orthogonal Frequency Division MultiplexFlash-OFDM Fast Low-latency Access with Seamless Handoff OFDMLTE Long Term EvolutionCSCF Call Session Control FunctionHSS Home Subscriber ServerP-CSCF Proxy- Call Session Control FunctionI-CSCF Interrogating- Call Session Control FunctionS-CSCF Serving- Call Session Control Function


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E-CSCF Emergency-Call Session Control FunctionAS Application ServerSIP Session Initiation ProtocolSLF Subscriber Location FunctionMGCF Media Gateway Control FunctionBGCF Breakout Gateway Control FunctionDL DownloadUL UploadSISO Single Input Single OutputMIMO Multiple Input, Multiple OutputCS Circuit SwitchedCSN Circuit Switched NetworkNACF Network Attachment Control FunctionIP-CAN IP Connectivity Access NetworkUE User EquipmentUA User AgentCLF Connectivity Session Location and Repository FunctionCN Core NetworkRAN Radio Access Network


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Chapter 1 – Introduction

Telecommunications is a very valuable and indispensable part of people’s lives. In every aspect of everyday life there is an increasing need to communicate with people that are far from someone. This need started in the ancient times for different reasons and purposes than nowadays. Since then it developed and today we’ve reached to a point that telecommunications is a very important part of our work, lives and also free time activities. Since telecommunications is so important in various different levels, there has being great effort to combine different types of telecommunications. In reality combining different types of telecommunications is much more complicated than it looks like. There are old (PSTN) and new (IP networks) technologies that have to be combined. Another factor should also be met and that is the combination of wired (DSL) and wireless networks (3g/4g). An effort in combining all these networks, technologies, mediums etc. under one architecture is called IMS (IP Multimedia Subsystem) networks. In the evolving world of telecommunications the need for creating converged networks has prevailed and the IMS networks pointing to this direction. Although they already operate in many places of the world many things still remain unsolved.

This project will give an insight on how IMS networks function, which are their features but also which are their advantages and disadvantages in combining different technologies. Furthermore, there will be an analysis on how does someone monitor performance on these networks and which are the challenges. Significant key factor in performance of the networks is security but this project will not get into security issues.

Motivation

IMS networks seem to be the next big thing in the market of telecommunications from the point of combining different technologies and exchange content between them, making our communication experience richer and the technologies interoperable. Since IMS are in the center of attention, industry is pushing through R&D (research and development) for improvement of these networks and defining some aspects of them that are not yet so well defined, such as performance or security. By allowing this act as an incentive, this project focuses on matters of performance. Performance is quite important in general but specifically in IMS networks, performance is really important since it needs to:

1. Be well defined. Which are the performance characteristics that affect an IMS network and why.

2. How could one improve the performance on a framework that uses different technologies?


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By monitoring and better understanding of the ideas behind performance and control, one would be able to set the best parameters or make the most optimum alterations in order to have the best possible results, and thus efficiency in a network that has IMS functionality. Thus, this project is trying to address some issues in two entities of the IMS networks, the Proxy-CSCF and the Server-CSCF. There is still space for improvement there and by investigating some alternatives on how the user could get connected in these two entities; one could derive useful information concerning the general improvement of the IMS.

Problem Formulation

Performance in IMS networks is of vital importance as the wireless networks and services becoming more demanding and QoS needs to be increased. The idea is that in this project there will be a description of the main KPIs (Key Performance Indicators) and then there will be an effort to determine how some changes in the parameters of the IMS or some extra attention in the aforementioned entities could increase their efficiency and performance. This will be done by the introduction of different cases for the same problem and then there will be a discussion on which approach fitted better and why.


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Chapter 2 - IMS

In this chapter there will be an analytical description of what is an IMS – through investigation of why do we need it –, which are its requirements and how does its entities function. There will also be examples of how the services are provided to the end users in certain cases.

2.1 Why IMS?

Cellular systems have been around for a couple of decades. Circuit switched networks have made a big impact – and still do – in cellular systems with the use of voice service. It is, until today, the most popular and reliable deployment for cellular networks. In the cellular networks there is an effort to bring all services together under one roof. In order to accomplish that, cellular systems have been using for some years now, IP in fixed parts of the network, whereas as air interface not that much. Since two to three years there has been a great effort to try and include real time services over the air and use IP also there until the end user and not only in the radio access and core network. Introduction of real-time services in cellular systems is linked with VoIP (Voice over IP) technology. So, the near-future vision is to have packet switched networks and end to end IP usage regardless if it is fixed or mobile systems. Migration from the current network is quite difficult but it can be phased and cut down into parts.

Packet switched networks are deployed to cover the need for a more interactive form of communication via any type of device and between any technology. It is possible through them to exchange voice, data, video and messaging and in that way communication is also enriched. In order to enable simultaneous usage of circuit switched networks together with packet switched networks and to have an equivalent quality of packet switched networks as with circuit switched an architecture is being specified by 3GPP and is called IMS (IP Multimedia Services). IMS includes this functionality to enable these different types of networks to co-exist.

In IP multimedia services there are some parts of the protocol stack that can be reused such as the addressing and routing, whereas other, like parts from the application logic, differ in order for the service to be unique. Thus, there is a common base in IMS networks and services are interoperable with others [1].

2.2 3GPP: Requirements and Standards

In order for the IMS to be able to cover a wide spectrum of technologies it should contain some requirements that an IP multimedia application should fulfill. By being standardized it


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functions as a starting point for a common basis for manufactures, operators and vendors. These requirements are mentioned below:

Potential for QoS negotiation during a session but also at the beginning of it. End-to-end voice quality the same or greater than circuit switched telephony

services. Support of roaming and negotiation for QoS and service capabilities between service

operators. Reassurance of interoperability with default media types between all the services.

But also support for other media types. Possibility to have several IP multimedia applications within each IP multimedia

session. Same level of privacy, security and authentication protection with GPRS and circuit

switched services. Access independence support. So the users could be able to connect through

different technologies (such as: GPRS, fixed lines, LAN, WCDMA (3G)) Potential to support applications that have been developed outside 3GPP

community.

All of the above requirements bring a unified structure for all different applications and benefit users, vendors and operators in different ways each of them.

Users benefit from rich multimedia applications and rich services provided in within. They are also benefit from the high level security and integrity while inserting their personal data. And last but not least, users benefit also from the possibility of using one device and passing from one technology to the other and still being able to use their services seamlessly but also being able to use their services through different terminals.

Operators benefits from common authentication and authorization mechanisms, service control and fraud management, charging mechanisms but also legacy telephony interworking. One last thing that an operator benefits from is that IMS helps to improve efficiency by providing information concerning radio bearer establishment process and services. And indeed this improves the efficiency – for the operator – as it aids to wisely select a radio bearer and make use of header compression as a performance booster.

Vendors benefit from the introduction of one technology platform for various services because it works as an incentive in improving infrastructure and terminal devices [1].


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2.3 IMS Architecture

IMS basically consists of three layers and in these layers there is communication for two different reasons: signaling and data. Some components exchange only signaling but others data and signaling.

Figure 1 - Layered Architecture of the IMS [2]

This happens because only at the “lowest” level/layer of bit exchange, there is the need to exchange data between users. The aforementioned layers are the following (from bottom to top):

1. Media/transport layer2. Control/Signaling layer3. Service/Application layer

Each layer aids in a specific way so that a communication process between different technologies and architectures could be achieved successfully.


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The media/transport layer includes different types of access technologies such as WLAN, 3G, xDSL, GPRS, WiMax, Flash-OFDM and LTE, that users can access in order to be interconnected to the network. In this layer except signaling there is also data exchange, as mentioned above.

The Control/Signaling layer includes the most important components of IMS which are all the **CF (Control Functions) and the HSS (Home Subscriber Server). The CSCF (Control Session Control Function) are the following: Proxy-CSCF, Interrogating-CSCF , Serving-CSCF and Emergency-CSCF. The Control functions will be explained later. In this layer the SIP signals are being processed and routed to the desired destination.

The Service/Application layer includes various AS (Application Servers) that host and execute a wide range of services through the IMS architecture [2].

2.4 Entities inside an IMS

In this section there will be an insight to the entities of the IMS and a description of their usability. There will also be a reference to SIP and DIAMETER protocols, as they are the most important protocols in this architectural framework. And since SIP is the “messenger” inside most entities of the IMS, it will be described before any entity.

SIP in IMS

SIP (Session Initiation Protocol) is the most important signaling protocol of the IMS. SIP is used for arranging sessions and establishes them but also manages and controls them. Basically, in the Control and Signaling layers are the places where this protocol is mostly used. The SIP protocol is based in requests and acknowledgements or rejections. These messages are sent in the form of a number and a word summarizes the function of the message. These numbers are three digits long and the first digit gives information about the type of the message. Below are presented the six different types of messages:

1xx: Provisional Messages

2xx: Successful answers

3xx: Redirection Answers

4xx: Method Failures

5xx: Server Failures

6xx: Global Failures [3]

All these messages are being exchanged between the entities in an IMS. The complete process on how does SIP exactly functions will be explained in detail below.


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CSCF

Call/Session Control Functions are components that make use of SIP signaling and help in various cases – such as establishment, monitoring and controlling sessions – and are consisted of P-CSCF, I-CSCF, S-CSCF, E-CSCF. Generally, CSCF is responsible for the following matters:

Keep track of session status Querying HSS for authentication information concerning users Establishment and allocation of the resources and routing of the information

through the correct nodes for a successful connection

A more detailed description of the components of the CSCF is presented below [4].

P-CSCF

The Proxy-CSCF is the component that makes the direct communication with the user. Any IMS requests from the user are routed to a P-CSCF node, through a RAN (Radio Access Network) or a WLAN or in general any other type of network and from the P-CSCF node to an S-CSCF node. P-CSCF features are SIP compression (SigComp (Signaling Compression)), interaction with PCRF and establishment a mutual authenticated communication with the user with the help of IPsec [5]. SigComp is used because if the available bandwidth is low, then the establishment of the connection will take a long time to be completed. In order to avoid that, a compression method is used [1].

I-CSCF

The Interrogating-CSCF is responsible for querying the HSS to assign an S-CSCF node to the user that have communicated with the P-CSCF, and by querying the HSS, then the HSS assigns an S-CSCF to the user/subscriber. The I-CSCF together with P-CSCF were also enrolled to hide the rest of the IMS network from the users and making a hiding topology but that happened until the 7th release of 3GPP. Thereafter, these two elements stopped having this function and became part of the IBCF (Interconnection Border Control Function) [5], [6].

S-CSCF

The responsibilities of the Serving-CSCF are about: maintaining sessions, decisions on how to route data, storage of service profiles and handling for the SIP registrations coming from the subscriber. The S-CSCF checks in the HSS whether the user is authenticated to perform a certain registration for a certain service. After the HSS’s approval, the S-CSCF continues to


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monitor the registration [5]. Another important process that is used by the S-CSCF in order to provide routing services and support the connectivity with older technologies such as PSTN or ISDN is ENUM (E.164 Number Mapping). ENUM is used to interconnect the telephone number based systems (such as PSTN) to the URI’s based system (such as SIP protocol). This function is managed by ENUM with the translation of telephone numbers into URI’s. ‘E’ in ENUM stands for E.164 ITU-T standard for international numbering where all globally-reachable telephone numbers are registered and organized [7].

E-CSCF

The Emergency-CSCF is a newer component in the control functions and is responsible for controlling the request of an emergency call. The subscriber is calling an emergency number and the SIP request is being directed directly to the E-CSCF through the SGSN (Serving GPRS Support Node). Information concerning the user’s location is also important and that is the responsibility of HSS and GMLC (Gateway Mobile Location Center) collaborating to locate the SGSN and send the information to the E-CSCF and then to determine which is the closest PSAP (Public Safety Answering Point) [8].

MGCF & BGCF

In order to connect to a circuit switched network through the IMS platform, all the SIP messages from the Control Functions should be altered to comply with the signaling protocols in this type of network. Because of that, there are two functions that implement the translation, BGCF (Breakout Gateway Control Function) and MGCF (Media Gateway Control Function).

In order for the messages to be translated MGCF uses a H.248-based Mn interface. With the aid of this interface, the RTP (Real Time Protocol)-based data/media, are translated in the media format which is acceptable by the circuit switched network. The interwork with the circuit switched network is accomplished by the use, from both sides, of a circuit switched signaling over IP.

Finally, the control function of the BGCF is actually choosing which MGCF will handle the session, whether it is in the same domain or in a different one, where, in the latter case, it directs the SIP request to the desired BGCF [1].

HSS & SLF databases

HSS and SLF are the databases used in IMS. HSS is the database that includes all the subscribers’ data such as: identities, access parameters, service-enabling information and registration information. The SLF (Subscriber Location Function) on the other hand is the responsible database that gives information to the I-CSCF and S-CSCF for which HSS has the information of a specific user, even in case there is more than one public user identities. All


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these information that are being sent from and to, HSS and SLF, make use of the DIAMETER protocol, which is a networking protocol for AAA (Authentication Authorization Accounting)[9].


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SIP’s Functionality

In Figure 2 it is presented how SIP signaling will function in order to establish a session.

Figure 2 – Successful IMS Voice Session Establishment [1]


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Initially, there is a SIP Invite request to the P-CSCF entity. This entity sends the request to the S-CSCF that the HSS assigned the user to. Then, the request is being forwarded to the I-CSCF where this entity queries HSS for information about the other S-CSCF. The query is sent to the HSS with the aid of DIAMETER protocol. After a successful reply from the HSS, the I-CSCF connects with the other S-CSCF that makes an evaluation of the user’s filters and controls whether it needs an AS invocation or not. Afterwards, the request is being forwarded to the user through the P-CSCF that is assigned for it. After the process is completed and before the establishment of the session, a provisional response is sent and at that time both terminals try to reserve resource from their access network. In this way there is an effort to reassure QoS during the initiation of the session. After the QoS is achieved the terminal starts ringing and the session is established. The example shown above describes a successful establishment of a Voice session scenario between two terminals through the IMS network[1].


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Chapter 3 – Analysis

In this chapter there will be a reference to state of the art work which is happening in the field of IMS. There will also be a detailed explanation of all important KPIs that play an important role in improving IMS networks and how to estimate the improvement accurately.

3.1 State of the art

In the area of the IMS, and specifically their performance, various papers have been published. These papers focus on different aspects and from different angles of the performance indicators of an IMS network. Some papers focus specifically in the internal performance of the IMS (e.g. the cooperation between the CSCF entities) and some others to the collaboration of an IMS network with RANs that utilize it.

In [10] the case was that they observed several RANs and compared them under the same basis. By plotting some important time parameters for sessions, they gave an idea about faster and slower combinations of different RANs. According to their paper, they investigated four different times in sessions: signaling request time, signaling reply time, signaling release time and total session setup and release time. By doing these computations, they came to the conclusion that when the end users use GSM network to connect to each other, they have the highest delays in all four measurements. On the other hand, users that use WLAN from both sides have the least delays in all four categories. So, according to this paper, the best performance in heterogeneous access networks is the WLAN-IMS-WLAN connection. As a conclusion it is stated that various factors affect these times such as: which technology ones uses, different access nodes, capacity of the IMS core network and available bandwidth of RANs.

Another paper [5] goes more into detail by comparing the performance of the IMS networks to the performance of the SIP based networks. Under the same workload both networks have been tested on messaging delays and session initiation delays. As messaging procedure, it is stated the delivery of the message and the acknowledgement for its successful delivery. As the session initiation procedure, it is determined the time from the first INVITE message until the acknowledgement from the called party. There were three workloads: 200,400 and 600scenarios/sec. The aim was to observe in which network the time needed to either initiate a session or send a message, was higher. Some results from this paper project that both networks performed quite well but the SIP based network was faster in both cases and under all workloads. The users that were exchanging these messages or initiating these calls had different statuses and they could be registered or not - available or executing some other scenario. The selection of their statuses was based on a Poisson distribution. One last thing that it is included in these results is the addition of a probability that 50%, 90%, 95% or 99% of the service was handled smoothly. So one could see where the time was increased and under which circumstances (what amount of workload).

The last work that is presented here is entirely based on internal performance of the IMS core network. According to [11] there have been several tests regarding different issues in


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an IMS core network. The first test determined some performance factors by increasing the number of simultaneous calls/sec. It was observed that as long the server could handle the routing of more calls everything were steadily increasing. The number of calls/sec was increasing and the processor load was increasing too; until the point that the number of simultaneous calls/sec reached to the value of 150 and the server rebooted due to overloading. Throughout the test, the memory utilization didn’t exceed 65%. The second test was more focused into security and robustness matters by sending malformed INVITE requests. The messages were sent either to the UA (User Agent) or the SIP proxy server. These messages were containing exceptional elements; that is, data that could arise undesired behavior to the receiving point as the device or process could crash, or needed to be rebooted manually or consume a considerable amount of memory and/or CPU for a fair or infinite amount of time. If one of the above actions takes place, the test fails. The third and last test that was introduced in this paper evaluated the behavior and the response of the system when sending SIP requests. The test cases that were used are introduced in the ETSI TS 102 027-2 V4.1.1. The test cases included messaging, call control and querying capabilities series. The conclusions that were drawn showed that all three tests passed successfully. The first one determined the maximum amount of simultaneous routable calls and it could be used to benchmark the capacity of different vendors. The second one passed the test, as there was no failure to the devices or the services due to malformed messages. The third test finished also successfully and the responses were correct according to the RFC 3161 standard.

3.2 Performance in IMS services

Performance is an important factor concerning the efficiency of an entity. When there were only circuit switched networks, measuring performance was easy. For voice services the performance was measured as the highest amount of satisfied users that the system could support. Nowadays, cellular systems support also VoIP technology via IMS connectivity. In VoIP, things are not that clear, as to how one evaluates a performance of a network that is based not only in voice services but also in video and other components. A point of such an inquiry is the interaction between different forms of multimedia, such as audio and video[1].

Service Performance Requirements

Services have some characteristics in order to be evaluated concerning their performance. Some of them are: service availability, retainability and quality. A circuit switched network has a minimum downtime of 0.0001% and some similar standards for speech quality and mouth-to-ear delay. VoIP standards are not as high as in the circuit switched networks.

Users perceive two characteristics as far as concerning performance through VoIP telephony. The first characteristic that affects users’ experience is – individually – every form of multimedia. For example, for video sessions, the codec that is used has a large impact on the quality and performance of the service and for voice sessions the speech quality has more aspects, such as: sampling rate, frame loss etc. On the other hand, the quality of the


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services, individually, is not enough because of the potential end-to-end delay of the media. So the second characteristic is the combination of them, for a high quality end-to-end experience [1].

3.3 KPIs for the IMS network

According to 3GPP and its latest update TS 32.454 V11.0.0 (2011-12) on Key Performance Indicators (KPIs) for the IMS networks, they are divided to three major categories and these include, the Accessibility KPIs from network and user perspective, the Retainability KPIs from network perspective as well as the Utilization KPIs from network perspective [12].

3.3.1 Accessibility KPIsThe accessibility KPIs are divided into various subcategories for a better understanding and defining of the performance of the IMS networks, both from user and network perspective. The KPIs for the accessibility of the IMS network are listed and presented below.

Initial Registration Success rate of the Serving-CSCF Session Setup – mean – Time Session Establishment Success Rate Third Party Registration Success Rate Re-registration Success Rate for the S-CSCF Session Setup – mean – Time (regarding the messages originate from an IMS) Session Setup – mean – Time (regarding the messages originate from a CSN) Immediate Messaging Success Rate Session Establishment Network Success Rate [12]

Initial Registration Success rate of the Serving-CSCF:

This KPI evaluates the success rate concerning the number of successful registrations to the S-CSCF over the number of attempted registrations. This evaluation aids to control the accessibility of the network [12].

Session Setup – mean – Time

This KPI acquires the mean setup time for a session. It is very important to know if the setup times are low or high, both from users’ perspective and satisfaction but also from network transaction performance [12].


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Session Establishment Success Rate

This KPI evaluates the session establishment success rate by giving two results from two fractions representing two different perspectives. One from the originating side and one from the terminating side of the session establishment. These two fractions are the successful session establishments over the attempted session establishments. This KPI differentiates between session successes from the originating and the terminating side, in order to have the real number of success rate. It also includes users’ behavior as no attribute is excluded [12].

Third Party Registration Success Rate

This KPI calculates a very vital feature of the IMS networks and that is the success rate regarding registration to third parties. That is the information provided by the S-CSCF to the AS (Application Servers) as to whether a user is registered or not. The KPI is calculated by the successful 3rd party registrations over the attempted ones [12].

Re-registration Success Rate for the S-CSCF

In this KPI it is investigated whether the success rate of re-registration is high or not. This is calculated by the success re-registrations over the attempted ones. The re-registration is useful for some reasons; it can either aid to inform the network of a change into the registration status or just refresh the existing status or to inform the network about a change in the capabilities of the UE or possibly to inform for a change in the IP-CAN (IP- Connectivity Access Network) [12].

Session Setup – mean – Time (regarding the messages originate from an IMS and CS)

Another KPI is the mean time regarding Sessions’ Setup from the IMS CN (Core Network) and the mean time regarding the Sessions’ Setup from the CS point of view. In both cases the results are helpful to IMS and to other network operators as they can control whether the performance issues originating from the IMS endpoint or the CS side [12].

Immediate Messaging Success Rate

In immediate messaging service the content could vary as there could be any type of multimedia. The success rate affects users’ experience and is expected to be high. It is calculated by the number of successful immediate messaging procedures over the attempted immediate messaging procedures [12].


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Session Establishment Network Success Rate

This KPI gives the performance of the IMS based on the successful session establishments together with the number of failed session establishments due to users’ behavior over the number of attempted session establishments. This KPI gives an insight to the actual success rate of the sessions that have been established and therefore a more accurate evaluation of the performance of the network, concerning this aspect. In this KPI there is again a different calculation for the originating and the terminating side of the session establishments [12].

3.3.2 Retainability and Utilization KPIsThe KPIs above are responsible for addressing problems or malfunctions in the network or just evaluation of the normal behavior by controlling accessibility to the network. But there are also other forms of performance matters that can come up. Below the last two KPIs will be discussed, concerning retainability of a session and utilization of an IMS network. So these aspects are:

Call Drop Rate of IMS Sessions Mean Session Utilization [12]

Call Drop Rate of IMS Sessions

This KPI helps for the evaluation of the retainability of the sessions. In order to calculate this rate, the fraction would be the amount of dropped sessions over the number of successful ones. This is also an important key indicator but doesn’t give a lot of insight as to where could be the problem. It indicates performance matters that could be caused by the IMS network or the user side [12].

Mean Session Utilization

This KPI tries to address the utilization of the network by calculating the mean number of simultaneously online and answered sessions over the capacity of the network. This indicator reflects the relation between the size of the network and the utilization of it. So, in case this number is low, this gives an indicator that the network is utilized up to a good level[12].


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Chapter 4 – Design

All the KPI’s in the chapter above test the performance of the IMS networks from different aspects and from different technologies that cooperate with the IMS networks. In this chapter the project will move one step further and provide an insight to what exactly is happening when the network reaches its limitations. From that point and after, its entities become unstable and the whole system can’t provide services until it reboots or until it overcomes the overload of the processor. There will also be an analysis on how the sessions are directed into the IMS when balancing a load of calls and what is happening while trying to establish the nearest P-CSCF in a network. Both cases mentioned will be represented by two scenarios.

4.1 First use case

In the case that there is an increased amount of sessions that needed to be setup, then the entire load is being directed from the P-CSCFs to the S-CSCFs by having the I-CSCFs choose – via HSS information – which S-CSCF will make this setup for its session.

Root cause of the Problem

The large number of sessions is not a problem for an adequate number of S-CSCFs. Though, if the amount of traffic is not well balanced then problems might occur as one of the Serving nodes will be overloaded with requests or established sessions and will cause a restart to it. As a result all the sessions of data, video or audio will be lost and the performance of the network will drop significantly. So, S-CSCF is playing an important role in the good performance of the whole IMS architecture.

Solution

As stated in [13] I-CSCF is responsible for assigning the users to an S-CSCF but the control for an overloaded S-CSCF comes afterwards. That means that a I-CSCF directs a user to the S-CSCF and if the S-CSCF node become overloaded then the I-CSCF starts de-register users from there and registers them to another S-CSCF node. The problem that derives from this action is low performance and a possibility that the user will be assigned again at the same S-CSCF node as it will not seem to be overloaded after a number of de-registrations. So, in [13] there is a proposal that supports the use of a load balancing method and that this function should be handled by I-CSCF before even registering the user to a S-CSCF. This method does not add any new entities or any significant message signaling overhead. The change is that the selection of the S-CSCF is done basically in the I-CSCF by knowing beforehand which S-CSCF has available capacity to support more load.


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This is done by SIP specific event notifications which will be sent from the S-CSCF to the I-CSCF and inform every time about the load level of the entity. The criteria that should be fulfilled are that the S-CSCF has free capacity to serve the user and that it has the required capabilities to do it. They propose that the notification messages are sent only every time there is a change in the load of the S-CSCF and the content of these messages will be only the identifier of the S-CSCF and the value that indicates the amount of load in it. That message has the size of 512bytes. In this way the I-CSCF knows every moment which S-CSCF has available slots to accept a new user for registration. In the case that all available S-CSCFs are full, a 5xx SIP message is sent, informs that the server has failed.

Having an improved load balancing system provides a positive effect in nearly all KPIs which are stated in the TS 32.454 V11.0.0 (2011-12). These KPIs concern the Accessibility, Retainability and Utilization of the IMS network. Generally, if one improves how load is balanced in the S-CSCF entity then all the mean times are decreased and reliability and durability of the whole architecture is improved.

4.2 Second use case

In the current case there is the need for a UE to register, the first step is the connection of the UE to a P-CSCF from which the UE will enter the IMS network.

Root cause of the Problem

Although this scenario does not imply any fail of the system or some entity individually, it presents the need of a new approach to get connected more intelligently and smoothly to a P-CSCF.

Solution

In [14] they propose a model based on two individual things. Initially, the idea is that every P-CSCF will be characterized by two factors: firstly a geographical location (coordinates) and secondly four pointers stating where the P-CSCF is located by having NE, SE, SW, and NW as signs. Then, this idea will be used by a near neighbor range search in two dimensional quad tree, algorithm and the UE will get connected to the correct P-CSCF regarding its geographical location. This paper is more focused on the condition that the UE is moving along visited networks.

The quad tree algorithm is functioning by dividing the two dimensional space into four quadrants with the initial point and then subdividing into four sub-quadrants every time there is a new point noted. In every quadrant there is a characterization with NE, SE, SW and NW and it is helping in keeping a good architecture over the created network. This algorithm proved to be useful because by stating which node (with the aid of coordinates) and which quadrant, it actually gives the right information about the location of the UE.


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CLF hold the quad tree information with all the P-CSCF nodes and the creation of four quadrants for each P-CSCF node. P-CSCF will provide the CLF, through e2 interface, with local information. Executing the algorithm determine which nodes are actually nearest to the UE and then check which one of these has an available capacity. The information regarding the active sessions are stored in CLF together with the information on the relay agent that is used for each session. The information on the relay agent is sent via the a2 interface from NACF. When the right P-CSCF is found, then CLF will inform NACF with a Bind Acknowledgment. This message also contains the identity of the P-CSCF that serves the session.

4.3 Improving performance in IMS networks

Above there was a detailed reference in two improvements about two different entities of the IMS network. Both ideas contribute in two things and these are performance and availability.

The first improvement is targeted in the part where all the decisions are taken and a big part of the intelligence of the IMS network is located and that is the S-CSCF. Whereas, the second improvement is located in an “entry” entity that connects users with the rest of the IMS architecture and this is the P-CSCF. The combination of them can be applied since there is no conflict between them. The improvement in the P-CSCF is using a modified algorithm for discovery of the P-CSCF and some changes in the way DHCP messages are utilized. The improvement though in the other scenario adds a small overhead to the signaling between the I-CSCF and S-CSCF but the result is a completely balanced load on all S-CSCFs of the network. So, the cost of change for the network is only some minor software changes. On the other hand, the benefits one receives by the introduction of both changes are much greater, and these are speed, reliability and higher utilization of the network.


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Chapter 5 – Implementation

After having analyzed many different aspects of the IMS networks and presented two papers that actually propose improvements in two important entities of the IMS framework, the project will go further in presenting an implementation inspired by these two proposals mentioned in the two different use cases in chapter 4. In this chapter the implementation will be shown and important aspects of it will be discussed. Results will also be given and compared with different cases.

5.1 Description of the algorithm

For the needs of the implementation an algorithm was made to simulate the procedure of a user that gets connected to a P-CSCF and then to the S-CSCF where a significant part of the intelligence of an IMS network is located. In order to study some differences between different scenarios, some parameters of the algorithm were changing so it could be possible to create four different scenarios and then compare them.

These four scenarios are the following:

1. Random search of an available slot in a P-CSCF node for a user and then random search of an available slot in an S-CSCF node.

2. Search of an available slot in a P-CSCF based on the minimum possible delay of all the available P-CSCF nodes and then random search of an S-CSCF.

3. Search of an available slot in a P-CSCF based on the minimum possible delay of all the available P-CSCF nodes and then search of an available slot in an S-CSCF based on the minimum possible delay of all the available S-CSCF nodes.

4. Random search of an available slot in a P-CSCF for a user and then search of an available slot in an S-CSCF based on the minimum possible delay of all the available S-CSCF nodes.

In order to simulate these scenarios and get a realistic sense in our results, some things should be taken into consideration in making the algorithm. So, before this project go any further, a clarification should be given on what exactly the delay takes into consideration in this work for both the user and the node side. This delay is a factor that represents:

1. For users: signal quality, processing time or other factors that the user’s position or device is responsible for the delay.


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2. For nodes: the node’s distance from the user, the load of the node or other parameters that affect the node’s processing time and acknowledgement of the user’s request.

So, given the aforementioned clarification, the algorithm should have a list of users that request a connection to IMS services. These users have a delay from zero to one and it is based on a uniform distribution, meaning that in the given interval the probability, of any value is the same. The algorithm is making use also of a delay for the nodes of the P-CSCF and the S-CSCF based also on a uniform distribution with a value range of zero to one. In order to give a more realistic sense to the whole program, the distributions for the delays of the P-CSCF and the S-CSCF nodes are changing after every iteration, meaning that the nodes have longer or shorter distance from each user – since the users are moving – and that their processing time is varying in time. We also assume that the S-CSCF’s capabilities are adequate for the service that the user wishes to use.

The algorithm functions as introduced in the following steps:

1. The number of users is 500 and the number of available slots is 200 in ten nodes for each entity of twenty free slots each.

2. The user requests a connection with a P-CSCF.3. Search for a Proxy based on the node with the minimum delay or a random search

(it depends which scenario it is studied)4. If all the P-CSCF nodes are full then the program retries a few times in case a node

had a free slot but it wasn’t found yet (that applies only to the random search schemes) and then exits.

5. If a P-CSCF node is found for the user then the algorithm proceeds in finding an S-CSCF.

6. Search for a Server is done based also on the node with the minimum delay or a random search (it depends which scenario it is studied)

7. If all the S-CSCF nodes are full then the program retries a few times in case something was not done correctly and then exits but before exiting, it erases the user from the P-CSCF connected list.

8. When the maximum number of users that can be connected is reached the program retries a few times in case a node had a free slot but it wasn’t found yet (that applies only to the random search schemes) and then exits.

9. As long as this procedure takes place, five plots are showing in every iteration the following measurements:

a. The amount of users that are connected to a P-CSCF node and in which node they are connected to (top left position)

b. The total delay of a user to get connected to a P-CSCF and to an S-CSCF and this user’s own delay (top middle position). Above this plot one can also see the current total delay of each user as he gets connected after each iteration.

c. The amount of users that are connected to a S-CSCF node and in which node they are connected to (top right position)

d. A normal distribution which depicts the delay that most users have in order to get connected until an S-CSCF (bottom left position). Above this plot one can also see the average total delay after every iteration.


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e. How many users are connected to an S-CSCF and in which iteration this happens (bottom right position). Above this plot one can also see the current number of total connected users after each iteration.


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5.2 Figures and Results

The algorithm produced some plots that will be illustrated in the following pages. Above each plot there will be a description where one can see which the scenario is and the measurements that were taken.

First Case

In this scenario (Figure 3) the user is connected with a random selection of a P-CSCF and a random selection of a S-CSCF.

Figure 3 – Plots from First Case Scenario

This algorithm needed 111.3922 seconds to finish its users’ registration. Note that the ‘User Delay: 0’ shows that there was no user registered in the last iteration of the program so the variable remained without a value.


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Second Case

In this scenario (Figure 4) the user is connected with a selection based on the minimum possible delay to a P-CSCF and a random selection of an S-CSCF.

Figure 4 - Plots from Second Case Scenario



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Third Case

In this scenario (Figure 5) the user is connected with a selection based on the minimum possible delay to a P-CSCF and a selection based on the minimum possible delay to a S-CSCF.

Figure 5 - Plots from Third Case Scenario



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Fourth Case

In this scenario (Figure 6) the user is connected with a random selection of a P-CSCF and a selection based on the minimum possible delay to a S-CSCF.

Figure 6 - Plots from Fourth Case Scenario


5.3 Results’ Analysis

The following table (Table 1) shows some important measurements derived from the scenarios presented above.

Table 1 – Acquisition of data from the simulations

First Case Second Case Third Case Fourth CaseUsers Connected 195 183 193 200Elapsed Time 111.3922 75.7921 70.5785 110.5606Average Connection Time 1.4659 1.0588 0.6169 1.1107


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In most of the cases (except the total number of connected users), the third case (selection of node with minimum delay in both P-CSCF & S-CSCF entities) is better. The performance of this approach of an IMS network is improved by applying the selection of a node based on its minimum delay in both P-CSCF and S-CSCF.

The following figure (Figure 7) derived from the sum of the normalized values of the table above and shows clearly which case achieves the best results.

First Case Second Case Third Case Fourth Case0

0.5

1

1.5

2

2.5

2.025

1.487

1.089

1.750

Total Number for each case

Figure 7 - Ranking of the Best Case Scenario

As it was stated above, the algorithm in the third case decreased the delay by 46.22% from the first case where every selection in each entity was random. The only parameter that the first case had exceeded by the third was the number of users that assigned during the process but the difference was only 1.01% more users compared to the third one. The third case had also an exceptional average time of connection (until an S-CSCF) for each user (57.91% lower delay from the first case) since the algorithm was searching in both entities the connection path with the lowest available delay factor. Even the elapsed time for the algorithm to finish directing all the users to the available slots, was smaller than all the other cases. With respect to the first case the third case was faster by 36.64%. The overall behavior of the customization in the third case is the optimal one in respect to the other three cases.


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Chapter 6 – Conclusions

Along this report many topics were discussed concerning the IMS networks. Moreover, two papers were presented and a suggestion that the combination of them could improve even more the IMS framework, in terms of performance and reliability, was given. Finally, an approach inspired from these two papers was introduced and some results derived, showing that in a system where randomness is narrowed down and information is exchanged, the system becomes more reliable and faster so the final user experience and the QoS are increased. The only disadvantage in order to exchange information that will make the system more intelligent, flexible and more reliable, is that one has to add overhead in messages. In the next subchapter there will be a reference to the LTE and LTE-A networks and what users should expect in the near future.

6.1 Future Vision in Communication Technologies

IMS is an emerging architectural framework, as the 4G networks will push things for better QoS and QoE and more demanding services but still there is way to go. Currently, circuit switched networks are still in use – maybe phasing out but still used by a big part of the market.

LTE Networks

Since the LTE (Long Term Evolution) Networks will be the next big thing in the area of mobile communications, it is only right to present some of the key aspects of them.LTE networks were first started to be developed as an evolution of 3G mobile system in 2004. Later, in the 8th release of 3GPP the form was standardized and in January 2008 it was approved and finalized. Since then minor changes are made to this. Some of the requirements for LTE are that for high mobility scenarios, the speeds would vary around 100Mbps in DL and 50Mbps in UL with a 20MHz bandwidth. One other requirement is that the QoS should be enhanced for end-to-end services and also that there should be flexibility concerning the spectrum, varying from 1.25 to 20MHz [15]. The LTE use the OFDM (Orthogonal Frequency Division Multiplex) modulation for the physical layer and for the transmission MIMO (Multiple Input Multiple Output) antennas system, as it performs a lot better than SISO (Single Input Single Output). Although for most people LTE is the specification that complies with the name of 4G, in fact this is not correct. With the improvement of LTE – LTE advanced –, the connection is now correct. LTE advanced among other things will support higher speeds for scenarios of low but also high mobility [9].


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