Institute of Electronic SystemsAalborg University

Future Mobile Networks:

Quality of Service Provisioning for Macro-Mobility in IMS-based Networks

Master Thesis / Group 05gr995-06gr112 / June 1, 2006

2

Aalborg University Institute of Electronic Systems

TITLE QoS provisioning for Macro-mobility in IMS-based networks PROJECT PERIOD September, 2005 - June, 2006 PROJECT GROUP Mobile Communications 05gr995/06gr1112 GROUP MEMBER German E. Castro D. SUPERVISORS Hans-Peter Schwefel Kim Lynggaard Larsen

ABSTRACTMultimedia content is becoming a popular service for actual and next generations of mobile wireless networks; the IP multimedia subsystem (IMS) is a platform intended to provide such content in an efcient way. Additional to the IMS, the provision of the necessary Quality of Service (QoS) and performing handover (HO) are used in order to improve customer experience while using multimedia services. The perceived quality can be affected by the time for session setup and the QoS negotiation procedures, but this is not always an issue when it has to be done just once and at the establishment of the communication. Nevertheless, certain handover scenarios imply that the connection to the multimedia session has to be re-established and the QoS parameters re-negotiated; this can seriously affect the QoS perceived by the user. The aim of this project is to study and to propose strategies in order to provide QoS efcient reestablishment of an ongoing IMS real time multimedia session in case of macro mobility from the IMS perspective. The study is conducted by two approaches, rst by a theoretical analysis of the involved concepts and technologies and second by simulation using NS2 in order to validate concepts and to compare different solutions.

Number of reports 8 Total number of pages 130

PrefaceThis report has been written by German Castro on the 9th and 10th semester of the International Master of Science (M.Sc.) in Mobile Communications. It constitutes the documentation of the project done about Quality of Service Provisioning for Macro-Mobility in IMS-based Networks. It is primarily addressed to the supervisors, examiners and students at Aalborg University. The structure of the report is as follows, rst an introductory description of the problem is given and then the report is divided in three parts. The rst one, gives the theoretical background required in order to understand the components and the technologies involved. Once the background knowledge is given a second part is presented, it contains a redenition of the problem, the description of existing solutions and nally a conceptual proposal about how to solve the problem. In the third part the simulations and their results will be shown, followed by an analysis of the results and a comparison between the theoretical and simulation approach. The literature references are numbers in square brackets, e.g. [1], which means the rst material in the bibliography list. The equations are marked by numbers in brackets, e.g. (3.1), where the rst number indicates that the equation belongs to the third chapter, and the second number indicates that it is the rst equation in the chapter. The same method is also used for tables and gures. A copy of this report can be found at: http://kom.auc.dk/group/05gr995/05995/ Mobile Communications group 05gr995, Aalborg University. June 21, 2006.

German Castro

i

AcknowledgementsThis work was partially supported by Siemens Communications Mobile, Munich, Germany. I would like to thank in particular to Gerhard Kuhn and Enzo Scotto di Carlo for their input and helpful comments. I would like to thank my supervisors, Hans-Peter Schwefel and Kim Lynggaard Larsen for their guidance, support, motivation and knowledge during this project. I want to thank specially Dorthe Sparre and Muhammad Imadur Rahman for their kind help always and all the professors and PhD students who helped me during the Master studies. I would also like to thank my parents and my brother for their support during my education and my life. Finally, I would like to specially thank Anyela, my wife, for being always at my side, helping me, supporting me and giving me all her happiness and unconditional love. German Castro June, 2006

ii

ContentsPreface Acknowledgements Abbreviation List 1 Introduction - General Project Description 1.1 Motivation . . . . . . . . . . . . . . . . 1.2 Overview . . . . . . . . . . . . . . . . 1.3 Possible Sub-problems . . . . . . . . . 1.4 Work to be done . . . . . . . . . . . . . i ii x 1 1 2 3 4

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

I2

Theoretical BackgroundQoS and Handover 2.1 QoS Denition . . . . . . . . . . . . . . . . . 2.2 QoS Parameters . . . . . . . . . . . . . . . . . 2.2.1 Delay . . . . . . . . . . . . . . . . . . 2.2.2 Jitter . . . . . . . . . . . . . . . . . . 2.2.3 Loss Rate . . . . . . . . . . . . . . . . 2.2.4 Throughput . . . . . . . . . . . . . . . 2.3 Signaling for QoS . . . . . . . . . . . . . . . . 2.3.1 On-path / Off-path . . . . . . . . . . . 2.3.2 Soft-State / Hard-State . . . . . . . . . 2.3.3 Uni-directional / Bi-directional . . . . . 2.4 QoS Mechanisms . . . . . . . . . . . . . . . . 2.4.1 Differentiated Services (DiffServ) . . . 2.4.2 802.1P . . . . . . . . . . . . . . . . . 2.4.3 Other IP QoS provisioning mechanism 2.5 Handover Denition . . . . . . . . . . . . . . 2.6 Types of Handover . . . . . . . . . . . . . . . 2.7 Conclusions about handover . . . . . . . . . . iii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

56 6 6 8 8 8 8 9 9 10 11 11 11 12 12 15 15 15

3 Access technologies 3.1 WLAN . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Architecture of WLAN and modes of operation 3.1.2 QoS in WLAN . . . . . . . . . . . . . . . . . 3.1.3 Mobility in WLAN . . . . . . . . . . . . . . . 3.1.4 Authentication and association . . . . . . . . . 3.1.5 Authentication Methods . . . . . . . . . . . . 3.2 WiMAX . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Architecture of WiMAX . . . . . . . . . . . . 3.2.2 QoS in WiMAX . . . . . . . . . . . . . . . . 3.2.3 Mobility in WiMAX . . . . . . . . . . . . . . 3.3 UMTS . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 UMTS Services . . . . . . . . . . . . . . . . . 3.3.2 Architecture of UMTS . . . . . . . . . . . . . 3.3.3 Packet Data Protocol (PDP) Context . . . . . . 3.3.4 QoS in UMTS . . . . . . . . . . . . . . . . . 3.3.5 Mobility in UMTS . . . . . . . . . . . . . . . 3.4 Other technologies . . . . . . . . . . . . . . . . . . . 4 IMS 4.1 Denition . . . . . . . . . . . . . . . . . . . . . . . . 4.2 IMS and SIP Architecture . . . . . . . . . . . . . . . . 4.3 QoS in IMS . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Description of functionalities . . . . . . . . . . 4.3.2 Requirements for IP Multimedia Core Network 4.3.3 Logical components for QoS management . . . 4.4 Mobility support in IMS . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

17 17 17 19 21 22 22 24 25 25 28 29 29 29 31 32 34 36 38 38 38 40 40 41 41 43

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

II Specic problem denition and possible solutions5 Problem Statement 5.1 Access Network Architecture . . . . . . . . . . . . . . . . . . 5.1.1 Architecture description and important considerations 5.2 Layer 1/2/3 attachment . . . . . . . . . . . . . . . . . . . . . 5.3 PDP Context establishment . . . . . . . . . . . . . . . . . . . 5.4 QoS Management . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Call admission Control (CAC) . . . . . . . . . . . . . . . . . 5.6 Handover analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4445 45 46 48 48 49 51 52 53 53 54 56

6 Proposed Solutions 6.1 Optimized SIP handover . . . . . . . . . . . . . . . . . . . . . . 6.2 QoS Context transfer . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Dropping calls in the CAC . . . . . . . . . . . . . . . . . iv

6.3 6.4

6.5

6.2.2 Service Downgrade in the CAC . . . . . . . . . . . . . . 6.2.3 Call queueing in the CAC . . . . . . . . . . . . . . . . . Bandwidth Broker . . . . . . . . . . . . . . . . . . . . . . . . . . Delayed QoS negotiation . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Start with BE, update to maximum available BW and nally update to the requested BW when possible . . . . . 6.4.2 Accept maximum available BW and update to the requested BW when possible . . . . . . . . . . . . . . . . . . . . . 6.4.3 Mantain BE until requested BW is available . . . . . . . . Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 58 58 59 61 61 63 64

III7

Setup, results and conclusionsSimulation 7.1 Simulation tool . . . . . . . . . . 7.2 Important Considerations . . . . . 7.3 IMS simulator . . . . . . . . . . . 7.3.1 Description and Topology 7.3.2 Session setup simulation . 7.4 QoS Simulation Structure . . . . . 7.4.1 Description and Topology 7.4.2 Trafc . . . . . . . . . . . 7.4.3 CAC . . . . . . . . . . . 7.4.4 Differentiated Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6566 66 66 67 68 69 71 72 73 75 76 81 81 81 84 87 91 91 94 94 94 94 95 95 97

8

Results comparison and analysis 8.1 Improvements simulation . . . . . . . . . . . . . . . . . . . . . . 8.1.1 Optimized SIP handover . . . . . . . . . . . . . . . . . . 8.1.2 QoS Context Transfer . . . . . . . . . . . . . . . . . . . 8.1.3 Bandwidht Broker . . . . . . . . . . . . . . . . . . . . . 8.1.4 Delayed QoS negotiation - Mantain BE until requested QoS is available . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Combined simulation of improvements - Performance evaluation . Conclusion 9.1 Access Network . . 9.2 IMS/SIP signaling . 9.3 QoS . . . . . . . . 9.4 Future Work . . . .

9

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

References Appendixes v

A QoS A.1 QoS and network performance . . . . . . . . . A.1.1 Reliability . . . . . . . . . . . . . . . . A.1.2 Availability/Accessibility . . . . . . . . A.1.3 Dependability . . . . . . . . . . . . . . A.1.4 Signaling Plane . . . . . . . . . . . . . A.1.5 Handover performance . . . . . . . . . A.2 QoS Mechanisms . . . . . . . . . . . . . . . . A.2.1 Multiprotocol Label Switching(MPLS) A.2.2 Resource Reservation Protocol (RSVP) A.2.3 Integrated Services (IntServ) . . . . . . A.3 Applications and QoS . . . . . . . . . . . . . . A.3.1 Inelastic . . . . . . . . . . . . . . . . . A.3.2 Elastic . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

. . . . . . . . . . . . .

98 98 98 98 99 99 99 99 99 100 100 100 100 101

B WLAN 102 B.1 PHY Layer in 802.11 . . . . . . . . . . . . . . . . . . . . . . . . 102 B.2 Data Link Layer in 802.11 . . . . . . . . . . . . . . . . . . . . . 104 C WIMAX C.1 MAC generalities . . . . . . . C.1.1 MAC packet format . C.1.2 MAC Data transport . C.1.3 Bandwidth allocation . C.1.4 MAC support of PHY D SIP 106 106 107 107 108 108 110

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

E Programming tools 112 E.1 Overview of NS-2 . . . . . . . . . . . . . . . . . . . . . . . . . . 112 E.1.1 DiffServ module . . . . . . . . . . . . . . . . . . . . . . 113

vi

List of Figures1.1 2.1 3.1 3.2 3.3 3.4 3.5 3.6 3.7 4.1 4.2 5.1 5.2 5.3 5.4 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 General scenario . . . . . . . . . . . . . . . . . . . . . . . . . . Relation between user-perceived QoS and network layered model . ISO and WLAN . . . . . . . . . Infrastructure mode for WLAN . QoS in WLAN - EDCF . . . . . Message exchange for 801.1x . . UMTS architecture . . . . . . . PDP context . . . . . . . . . . . Bearer service in UMTS [www3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 7 18 18 20 24 30 32 33 39 40 46 47 49 49 54 54 55 56 57 58 59 60 60 62 62 63

IMS architecture . . . . . . . . . . . . . . . . . . . . . . . . . . SIP signaling for session intiation and QoS setup . . . . . . . . . Access Network Architecture . . . . . . IP addresses within WLAN or WiMAX . Procedure for attaching to the network . Procedure for establishing PDP contexts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Re-authorization message ow for optimization of handover . . . Message ow of session Re-establishment . . . . . . . . . . . . . PDP context generation . . . . . . . . . . . . . . . . . . . . . . . QoS context transfer and activation . . . . . . . . . . . . . . . . QoS context transfer with CAC dropping calls . . . . . . . . . . . QoS context transfer with negotiated downgrade . . . . . . . . . . QoS context transfer with Call queueing in the CAC . . . . . . . . Core Network Architecture including Bandwidth Broker . . . . . . Session re-establishment for a Bandwidth Broker transfering the context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.10 Message ow for multiple updating of QoS . . . . . . . . . . . . . 6.11 Message ow for updating of QoS starting with the maximum available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12 Message ow for updating of QoS starting with Best Effort . . . . vii

7.1 7.2 7.3 7.4 7.5 7.6 7.7 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13

IMS simulation Input/Output block diagram . . . . . . . . . . . IMS architecture . . . . . . . . . . . . . . . . . . . . . . . . . Message ow for session establishment . . . . . . . . . . . . . Total time Vs BS detection time . . . . . . . . . . . . . . . . . . QoS simulation Input/Output block diagram . . . . . . . . . . . Simulation topology . . . . . . . . . . . . . . . . . . . . . . . . Edge and Core routers implementation in the simulated scenario

. . . . . . .

68 68 69 70 72 72 80 82 83 83 84 85 85 88 88 89 90 90 92 93

Simulation architecture for Optimized SIP handover . . . . . . . . Register/Reauthotization comparing Optimized and Unoptimized solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Register/Reauthotization and Invite/Re-establishment comparing Optimized and Unoptimized solutions . . . . . . . . . . . . . . . . . Setup for simulating Context Tranfer between PDGs and ASNGs . Simulated architecture for signaling in QoS Context Transfer . . . Simulated architecture for trafc in QoS Context Transfer . . . . . Time in queue for a HO video call according to percentage of VoIP sessions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time in queue for a HO video call according to percentage of Video sessions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time in queue for a HO video call according to percentage of HTTP sessions . . . . . . . . . . . . . . . . . . . . . . . . . . . Setup for simulating Bandwidht Brokers and Context Transfer . . Simulated architecture for signaling for the Bandwidth Broker approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Available BW during the time of simulation . . . . . . . . . . . . Used BW Vs time . . . . . . . . . . . . . . . . . . . . . . . . . .

D.1 SIP architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 E.1 User view of NS . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

viii

List of Tables3.1 3.2 3.3 6.1 7.1 7.2 7.3 7.4 7.5 7.6 7.7 8.1 8.2 8.3 8.4 8.5 8.6 8.7 Variants of the 802.16 standard . . . . . . . . . . . . . . . . . . QoS classes in UMTS. [www3] . . . . . . . . . . . . . . . . . . . UMTS bearer attributes dened for each trafc class. [2] . . . . . Combination of improvements to be tested . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 34 35 64 70 71 74 74 76 77 78 82 86 86 87 89 91 92

Input parameters for the IMS/SIP simulation model Approximated times for IMS session setup . . . . Simulation Parameters . . . . . . . . . . . . . . . Trafc Parameters in Simulation . . . . . . . . . . Assumed EB for the different trafc sources . . . . DSCPs for the different trafcs and priorities . . . RED queue parameters . . . . . . . . . . . . . . .

Time comparison for Optimized SIP handover . . . . . . . . . Handover delay for QoS Context Transfer . . . . . . . . . . . . Handover delay for QoS Context Transfer in Service downgrade Session characteristics for call queueing . . . . . . . . . . . . . Handover delay for QoS Context Transfer . . . . . . . . . . . . Comparison between AF and BE in a 100% load scenario . . . . QoS requirements for the different trafcs . . . . . . . . . . . .

B.1 802.11 WLAN Speeds. . . . . . . . . . . . . . . . . . . . . . . . 103 B.2 Modulation and Coding for WLAN OFDM Data Rates. . . . . . 104

ix

Abbreviation List3G 3GPP AAS AF AK AN AP APC APN APR ARQ AS ASN ASNG ASP ATM BB BBM BE BER BPSK BR BS BW BWA C/I C/N CID CIR CN CRC CS DHCP Third Generation Third Generation Parthership Program Adaptative Antenna System Assured Forwarding Authorization Key Access Network Access Point AP Controller Access Point Name Ap Router Automatic Repeat Request Application Server Access Serving Network Access Serving Network Gateway Application Server Provider Asynchronous Transfer Mode Bandwidht Broker Break-Before-Make Best Effort Bit Error Rate Binary Phase Shift Keying Bandwidth Request Base Station Bandwidth Broadband Wireless Access Carrier-to-Interference Ratio Carrier-to-Noise Ratio Connection Identier Commited Information Rate Core Network Cyclic Redundancy Check Convergence Sublayer Dynamic Host Conguration Protocol

x

DL DRM DSCP E2E EB EDCF EF FA FDD FER FFT GGSN GPRS GSM HO HUMAN HSS IMS IP IP-CAN LAN LER LOS LSB LSP MA MAC MAN MBB MIP MN MPLS MSB MSH MSS NAR NLOS NRT NS NS2 OFDM OFDMA OSI

Downlink Domain Resource Manager DiffServ Code Point End-to-End Equivalent Bandwidth Enhanced Distribution Coordination Function Expedite Forwarding Foreing Agent Frequency Division Duplex or Dluplexing Frame Error Rate Fast Fourier Transform Gateway GPRS Support Node General Packet Radio Service Global System for Mobile Communications Handover High-Speed Unlicensed Metropolitan Area Network Home Subscriber Server IP- Based Multimedia Subsystem Internet Protocol IP Connectivity Access Network Local Area Network LAbel Edge Routers Line-of-Sight Least Signicant bit Label Switched Paths Mobility Agent Medium Access Control Layer Metropolitan Area Network Make-Before-Break Mobile IP Mobile Node Multiprotocol Label Switching Most Signicant Bit Mesh Mobile/Fixed Subscriber Station New Access Router Non-Line-of-Sight Non Real Time Networks Simulation Networks Simulation 2 Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiple Access Open System Interconnection

xi

P-CSCF PAR PDF PDG PDP PDSN PDU PHB PHS PHSF PHY PLMN PMP PSTN PTT QAM QoS QPSK RAN RED RNC RNG RNSN RRM RT SAP SBLP SDP SDU SGSN SIP SLA SNR SPI SS TDD TDM TDMA TE TFT ToS

Proxy Call State Control Function Previous Access Router Policy Decision Function Packet Data Gateway Packet Data Protocol Packet Data Serving Node Protocol Data Unit Per Hope Behaviour Payload Header Suppression Payload Header Supression Field Physical Layer Public Land Mobile Network Point-to-Multipoint Public Switched Telephony Network Push to Talk Quadrature Amplitude modulation Quality of Service Quadrature Phase-Shift Keying Router Access Network Random Early Detection Radio Network Controller Ranging Radio Network Serving Node Radio Resource Management Real Time Service Access Point Service Based Local Policy Session Description Protocol Service Data Unit Serving GPRS Support Node Session Initiation Protocol Service Level Agreement Signal-to-Noise Ratio IPsec Security Parameter Index Subscriber Station Time Division Duplex or Duplexing Time Division Multiplexing Time Division Multiple Access Terminal Equipment Tracc Flow Template Type of Service

xii

UDP UE UL UMTS UTRAN VLAN VLR VoIP VoWIP WiMAX WirelessMAN WirelessHUMAN WLAN WRR

User Datagram Protocol User Equipment Uplink Universal Mobile Telecommunications System UMTS Terrestrial Radio Access Network Virtual Local Area Network Visited Location Register Voice Over IP Voice Over Wireless IP Worldwide Interoperability for Microwave Access Wireless Metropolitan Area Networks Wireless High-speed Unlicensed Metropolitan Area Networks Wireless Local Area Network Weighted Round Robin

xiii

Chapter 1

Introduction - General Project Description1.1 Motivation

Actual and future applications in mobile communications are becoming more and more complex in terms of the amount of data to be transfered and the technological requirements of the new contents. One of the challenges for the service providers is to attract their users to use these new applications and at the same time provide them in a efcient way and billing them accurately. Real time multimedia content is an example of this type of content, it can be very heavy in terms of amount of data and being real time represents extra complexity from the technological and quality point of view. To deal with this multimedia content, 3GPP (Third Generation Partnership Project) created the IP-Based Multimedia Subsystem (IMS). IMS is a platform that allow mobile operators to offer services based on Internet applications independently from the actual conectivity (UMTS, WLAN, among others). The IMS provides session and connection control as well as an application service framework. All entities in an IMS network use the Session Initiation Protocol (SIP) to communicate one with another. SIP provides session to a user equipment (UE) connecting to the IMS. Using SIP, the IMS offers services such as user registration, instant messaging or presence services. SIP manages multimedia sessions, mobility and QoS. The quality of the offered services, can be seen or meassured by many different parameters or categorized in many different ways, in the case of multimedia services one of the most important parameters (if not the most important one) is the human perception. The way the user perceives the quality of the multimedia content being presented, is what can make a difference in terms of what the user is aiming to pay for.

2

Introduction - General Project Description

Different access technologies provide different quality of service capabilities due to diverse transmission conditions like coverage, data rate capabilities, among others; each one of them at certain cost/price. Switching between access technologies or the simultaneous use of them can be useful in order to increase capabilities or decreasing costs. The aim of this project is to analize possible solutions for providing real time multimedia services under certain quality conditions in the cases where a user is switching from one access network to another, it is important to notice that the use of IMS as a core is implied. In other words, what mainly motivates this project is to study the implications that the inter-technology handover brings to the QoS in a connection in the IMS platform during real time multimedia transfer and to propose possible solutions to the related issues.

1.2 Overview

Figure 1.1: General scenario A general scheme of the actual scenario is presented in Figure 1.1, it shows a communication between an application server (AS) and a user equipment (UE). Additional to these components two additional block are present, the rst one is the IMS platform and transport and the second one are the access networks. The UE is moving from the rst access network (AN) to the second one. There are several options about the access networks, in the present case the idea is to work with access technologies that allow to manage the QoS and that are new or at least that are expected to be in use or in the market for a considerable period of time, for examples WiMAX, WLAN, UMTS. When the UE is moving from one AN to another, it is for example from WLAN to WiMAX and viceversa

1.3 Possible Sub-problems

3

or from WLAN to UMTS and viceversa. When changing the access network, it is important to notice that each AN has its own schemes or strategies for providing the QoS. Therefore, the rst part of the theoretical study presents the reader a common denition of quality of service, the parameters that can affect it and show the elements that dene the QoS. After the denition of QoS for the project, the issues related to the handover are presented, as shown in Figure 1.1 the UE is performing a HO from one access network to another, so, dening handover and studying different categories of it is important in order to delimit the problem denition to the type (or types) of handover that can be more relevant for the present case of study. After the presentation of these two concepts, it is important to understand how different access networks work. Aspects like the components of the network, their functionality and the way each one of them manages the mobility and the QoS are going to be studied in order to determine the most relevant aspects to be considered while changing from one access network to another. Finally, when a UE is moving from one access network to another, it is not only relevant to the session within the access networks, as shown in Figure 1.1 the IMS & transport are also part of the communication between the AS and the UE so it is worth to know how the IMS works, its components, how the QoS is managed within the IMS and how the handover impacts current sessions and their related QoS.

1.3

Possible Sub-problems

From this rst analysis some issues can be expected for the project, some of them are shown below: Determine the entity in charge of triggering the handover. Select the most appropiated components of the network to negotiate the QoS. Choosing between different handover scenarios: Breaking the rst connection before having a second one. Simultaneous connectivity to both access networks: Improvement via temporary multicast. Flow splitting. Dene the components that are more relevant for the whole problem in order to be simulated.

4

Introduction - General Project Description

After the theoretical study has been done and during the simulation more subproblems are expected, because of that in chapter 5 a redenition of the problem statement will be presented.

1.4 Work to be doneThe research work in this project will be conducted rst by a theoretical analysis of the problem, where a study of the technical elements will be performed in order to determine the specic problem(s) to be studied, as well as the most relevant components for the case of study. Then, a conceptual solution will be proposed based on the background knowledge, current existing solutions, previous studies about similar problems and of course personal contribution. Finally, simulation work will be performed. In order to do this, a simulation tool has to be chosen and as mentioned before, the most relevant parameters and/or components have to be included in the simulation test bed in order to have enough tools to analyze the problem and compare the solutions. The main purpose of having theoretical and simulation approaches is to prove the concepts and also propose and/or develope new solutions from the results obtained by testing.

Part I

Theoretical Background

Chapter 2

QoS and Handover2.1 QoS DenitionQoS stands for Quality of Service and it can refer to the probability of a telecommunication network meeting a given trafc contract for a specic user and/or content. In many cases it is used informally to refer to the probability of a particular packet succeeding in passing between two points in the network under certain conditions [www1][www2]. Additionally, the idea of implementing QoS is strongly related with features as distinguishing trafc into different types, features, demands to the networks, and of course to charge the customers differently according to these. When talking about real time applications, the term QoS becomes more specic and can be understood as user-perceived QoS. This means that even if the basis for providing QoS are related to technical parameters, they are aimed to satisfy agreements mainly based on certain quality provided at the user interface. In order to study the user-perceived QoS in an IP-based network architecture with hetereogeneous wireless access, the factors that inuence it are presented in Figure 2.1. It shows the relation between the network layer model and the user-perceived QoS; It can be seen that the user-perceived QoS depends on application QoS which at the same time is depending on network QoS [5]. The present case of study focus on Network QoS, meaning layers below layer ve in the protocol stack.

2.2 QoS ParametersThere are some factors that have an impact on the QoS in a network, the most signicant ones are listed below [5]: Network architecture and transmission paths (routing) Processing delays (e.g., for encryption) Congestion/trafc loads

2.2 QoS Parameters

7

Figure 2.1: Relation between user-perceived QoS and network layered model

8

QoS and Handover

Link properties (in particular for wireless links) Failure events Mobility events Even if the main concern for real time aplications is the user-perceived QoS as mentioned before, there must exist clear, measurable metrics that specify quantiable QoS parameters of the system components, these are delay, jitter, loss rate and throughput [2].

2.2.1

Delay

It is the elapsed time for a packet to traverse the network from the instant a packet is generated at the source until it reaches the destination or the arrival of a correct aknowledgement of the successful reception of the sent packet is received. At the network layer, the end-to-end packet latency is the sum of processing, transmission, queuing and propagation delays.

2.2.2

Jitter

It is dened as the variation of the delay encountered by similar packets having the same source and destination through the network, the jitter requirement can mainly affect real time streaming applications. Jitter is generally included as a performance parameter, since it is very important at the transport layer in packet data systems, due to the inherent variability in arrival times of individual packets. Services intolerant of delay variation will usually try to reduce it by means of buffering. However, delayed data arrivals make data useless resulting in receiver buffer underow and early arrival can lead to receiver buffer overow.

2.2.3

Loss Rate

Loss rate refers to the percentage of data loss among all the delivered data in a given transmission time interval, it can be evaluated in bit lavel (Bit Error Rate BER), packet level (Packet Error Rate - PER) or frame level (Frame Error Rate FER), loss rate requirements apply to all classes of applications. Real time (RT) applications might tolerate a limited amount of data loss depending on the error resiliency of the decoder and the type of application; whereas non real time (NRT) applications, typically, have much more strict requirement on data loss.

2.2.4

Throughput

It can be dened as the amount of data from a source to a destination processed by the protocol for which throughput is to be measured during a specic time interval. The throughput, differs between protocol layers, for example, a loss of one IP packet that generates a loss of also one packet in the application layer, impacts the

2.3 Signaling for QoS

9

meassure of the throughput in a different way, because it is possible that there are less packets in the application layer, because, each one of the these packets can be composed and therefore affected by many packets in the IP layer. The throughput, can be expressed as a peak rate or an average rate depending, for example, if the rate at which packets are transmited is constant or not.If retransmissions are involved in protocols, the throughput is reduced due to gaps in the transmission while waiting for acknowledgements.

2.3

Signaling for QoS

QoS signaling refers to the exchange of information between network elements that is aimed to setup a specic QoS treatment for certain data packets. Examples of QoS signaling are [5]: Communicate and negotiate QoS requirements for a certain type of data packets. Discover the next relevant QoS-signaling aware entity in the network. Exchange information about current QoS state like available bandwidths or buffer-occupancy in terminals or network elements. Messages modicating the QoS state. It is important to notice that in some cases, QoS signaling can be part of the user-data packets, like for example, when a congestion notication bit in the header of the packets is being setted or cleared by the routers in order to generate notication. A categorization of the QoS signaling can be done by the following properties:

2.3.1

On-path / Off-path

If the packets containing the signaling information follow exactly the same path as the data packets, the approach implements on-path signaling also called pathcoupled signaling, it is characterized by: QoS resources are managed locally by each router. Signaling is triggered by the terminals and follows the data path. End-to-end reservations are set up hop-by-hop by a signaling protocol that installs states in routers.

10

QoS and Handover

The RSVP protocol is an example of such approach. The signaling messages normally install or modify QoS state of the network elements that it crosses. As a disadvantage, RSVP is not mobility-aware and because of that, it does not support a change of the IP address. By the other hand, an off-path (or path-decoupled) signaling, the signaling messages are directed by entities like bandwidth brokers (BB) or domain resource managers (DRM), the main functions of these entities are to: Handles the resources for one domain. Maintains an up-to-date image of the resources and reservations in its domain. Requests resources from DRMs in adjacent domains along the data path in order to provide end-to-end reservations. These central QoS entities in most of the cases have other interfaces like for example COPS based (RFC2748 and RFC 3084) in order to control entities like router in the data-paths. There are different QoS mechanisms on the data path that can be used in the off-path signaling, such as DiffServ or IntServ which are explained in subsections 2.4.1 on the facing page and A.2.3 on page 100. An advantage of the use of a DRM over an on-path signaling scheme is that if a router fails, the failure is not directly noticed by the terminals in the case of hop-by-hop signaling and re-establishment may require new signaling from the terminals. In comparison, a DRM can be notied of local changes and can adapt the reservation. On the other hand, the DRM itself can fail but its reliability can be improved by the use of redundant systems.

2.3.2

Soft-State / Hard-State

Signaling messages commonly install some kind of QoS state in the entities that they cross. If this state has to be modied or erased with the help of additional signaling messages, it is called a hard QoS state. The danger of this hard state is that improper use of signaling or loss of connectivity could result in permanent, un-used QoS state and will consume resource unnecessarily. This problem can accumulate over the time and nally block any network activity. In the soft QoS state, the state is automatically removed, for example, after certain timeout intervals. If state needs to be maintained over a longer period of time, refresh messages have to be sent by the initiating component. This adds additional signaling overhead but it has the advantage that no unneeded QoS state blocks resources.

2.4 QoS Mechanisms

11

2.3.3

Uni-directional / Bi-directional

In many applications, like video streaming, the QoS requirements are non-symmetrical with respect to the direction of transmission. This mean that even if signaling messages are being exchanged in both directions, unidirectional QoS signaling establishes only state with respect to one transmission direction. For uni-directional signaling, it can be distinguished between receiver initiation and sender initiation. In case of on-path signaling problems can arise with receiver initiated signaling if routers through the network are not symmetric, for example if a message send from receiver to sender passes through different entities than in the reverse direction.

2.4

QoS Mechanisms

There are several mechanisms to provide the QoS over a network, the most relevants for the present project are:

2.4.1

Differentiated Services (DiffServ)

The DiffServ provides a mechanism for enabling different treatment of packets in an IP network, this is done by marking (using) the type-of-service (TOS) bits previously dened in the IP standard, so that differential levels of service can be given to different aggregate ows at the entry points to the network. According to this, DiffServ takes a very local view of the problems involved in providing a certain quality of service. DiffServ species the called Per Hop Behaviour (PHB), which determines how a router or switch should forward the packet to the next node in the network, the requested PHB is stored in the IP packet. There are three dened PHBs: Assured forwarding (AF) Denes four trafc classes, each of which can have three drop-precedence values for a total of twelve. Expedited forwarding (EF) Provided for low latency, low jitter, low loss, and assured bandwidth. Best-effort forwarding The Differentiated Services architecture does not have a well-dened concept of a service class. Instead it leaves the classication of trafc to the nodes at the edge of a DiffServ network. The classication used there is based on the SLA existing between the DiffServ network and its neighboring networks. DiffServ contains no signaling protocol so the edge nodes must in general be manually congured to match the Service Level Agreement, and therefore the resource reservation in the

12

QoS and Handover

nodes is more or less static. The main disadvantage of DiffServ is that it can not guarantee a specic QoS, especially on an end-to-end link. Because no signaling is involved, it has no prior knowledge of whether a specic ow will receive adequate QoS even if it is marked preferentially, if a route or router is heavily congested all packets will be rejected whether they are priority packets or not. Similarly, because there is no signaling, applications cannot adjust their requirements in advance in response to network conditions and many IP applications do adjust to network conditions in this way, given the chance.

2.4.2

802.1P

IEEE 802.1p specication enables layer two switches to prioritize trafc and perform dynamic multicast ltering. The prioritization specication works at the MAC framing layer. The standard also offers provisions to lter multicast trafc to ensure it does not proliferate over layer two switched networks. The IEEE 802.1p is an extension of the IEEE 802.1Q (VLANs tagging) standard. The 802.1Q standard species a tag that appends to an Ethernet MAC frame. The VLAN tag has two parts: The VLAN ID (12-bit only used by the 802.1Q) and Prioritization (3-bit) eld used by the 802.1P, the three-bit eld for prioritization, allows packets to be grouped into various trafc classes. It can also be dened as best-effort QoS at layer two and it is implemented in network adapters and switches without involving any reservation setup. 802.1p trafc is simply classied and sent to the destination; no bandwidth reservations are established. By the use of the prioritization eld, IEEE 802.1p establishes eight levels of priority. The highest priority is seven, which might go to network critical trafc such as Routing Information Protocol (RIP). Values ve and six might be for delay sensitive applications such as interactive video and voice. Data classes four through one range from controlled load applications such as streaming multimedia and business critical trafc down to loss eligible trafc. The zero value is used as a best-effort default, invoked automatically when no other value has been set.

2.4.3

Other IP QoS provisioning mechanism

Scheduling and buffering are common mechanisms for handling IP packets and important in order to implement QoS differentiation between User Equipments. Scheduling When multiple queues are sharing a common transmission media, a scheduler is needed in order to decide how to pick up packets from each queue to send out.

2.4 QoS Mechanisms

13

Generally a packet scheduler is expected to have the following properties: Low time complexity to select and forward a packet. Treat different ows fairly. Provide low worst-case delay and delay variation. Simple enough to be implemented efciently. When talking about schedulers, the simplicity and time-complexity properties collide with fairness and delay-bound properties. Schedulers with short term fairness and strict delay bound generally have high time complexity and are hard to implement. The simplest scheduler is called Round Robin (RR) and it works by selecting a packet from each queue in a circular way, so all the queues have equal chances to send. Weighted round robin (WRR) is an extension of RR, and allows the bandwidth between queues to be distributed according to weights. Each time a non-empty queue is examined, it may send as many packets as its weight indicates. The WRR carry several improvements like: To use bytes or chunks of bytes, instead of packets. This gives greater granularity on the resource to be distributed. Allow queues to be selected several times within short intervals, instead of being selected many times in series. There could be two implementation methods of WRR: a standard WRR and a Weighted Interleaved round point (WIRR) implementation. Another type of scheduler is Priority (PRI). With PRI the queue with highest priority will always be served rst until it is empty, then the second highest priority queue will be served so that the lowest priority queue can only be picked when all the other queues have been emptied. Queuing The simplest queue algorithm for routers is Drop Tail. The way it works is that when there is sufcient buffer space Drop Tail queue accept any incoming packet but when the buffer is full it simply drops any new arriving packet. RED (Random Early Detection) is a congestion avoidance algorithm that can be implemented in routers, RED queue computes a weighted average queue size to detect a congestion before the queue becomes really full, because a sustained long queue is a sign of network congestion. As any packet arrives, a RED gateway checks the weighted average queue size and compare with it is minimum and maximum thresholds, if there is congestion, it either drops a packet or sets a bit in a header eld of the packets according to certain probability. There are three phases for RED gateways which drops packets:

14

QoS and Handover

Normal operation: If the average queue size is less than the minimum threshold, no packets are dropped. Congestion avoidance: If the average queue size between the minimum and maximum thresholds, packets are dropped with a certain probability. This probability is a function of the average queue size, so that larger queues lead to higher drop probabilities. Congestion Control: If the average queue size is greater than the maximum threshold, all incoming packets are dropped. The effects of RED gateways in a network will be: Control the average queue size, and reduce the average queuing delay. Act as a low-pass lter, so the burst trafc gets easier to be dropped. Avoid synchronization of TCP connections. A TCP based trafc source will reduce its transmitting rate when some of its burst packets will be dropped which cause RTT time-out. But the Drop Tail queue drops all the packets in congestion which cause that all the TCP sources reduce their transmitting rate at the same time and the total throughput suddenly becomes very low after the congestions period. RED drops packets randomly before the buffer gets overowed, which prevents this synchronization problem, reduces trafc oscillation effect and hence increase the overall throughput.

2.5 Handover Denition

15

2.5

Handover Denition

Handover (HO) can be understood as the process by which an active Mobile Node (MN - MN is the same as a UE and this two terms are going to be used during the project), changes its point of attachment to the network, or when such change is attempted. The access network may provide features to minimize the interruption to sessions in progress. It is also called handoff. [6].

2.6

Types of Handover

As can be seen in [6], there are many different kinds of handovers, classied according to the aspects involved, like: The scope: Which refers about from where to where is moving the MN. The control: This classication refers to the node initiating the handover, the node having the primary control over the handover process, the node collecting the measurements for supporting the decision of when and where to handover to, if the handover is initiated by the new or the previous Access Router (AR) and nally if the handover is proactive or reactive (Planned or Unplanned respectively). Connectivity to Access Routers: Depending if a new connection is made before or after the old one is broken, the handover can be classied as Makebefore-break (MBB or Soft Handover) or Break-before-make (BBM or Hard Handover)

2.7

Conclusions about handover

As mentioned in the previous chapter, the QoS for real-time applications is related with the user perception of the quality, according to this, the present project aims to improve the quality of service re-establishment, so it gets reected in application and user- perceived QoS and therefore contributes to get closer to a seamless handover, understandig this concept as a handover in which even if in practice some degradation in service is to be expected, some protocols, applications or end users do not detect/perceive any change in the service capability, security or quality. It also means that the aim is to have a smooth and fast handover, implying respectively reducing the packet loss and the handover latency. Even if this reduction does not directly imply having a seamless handover, the idea is to study solutions that may help to decrease the impact of the handover compared with the case where no additional solution is implemented. From the scope of the handover point of view, there are some denitions of types of handover that can overlap with each other, like for example, the case of

16

QoS and Handover

Inter-technology handover and Vertical handover. The rst one is dened as a handover between equipment of different technology and the second one involve MNs moving between access point of different type. Even if one is talking about equipment and the other about AP, a clear distintion is not made. Also, for example the differences between a horizontal and vertical handover can be vague. For example, a handover from an AP with 802.11b WLAN link to an AP with 802.11g WLAN link may be considered as either vertical or horizontal handover, depending on an individuals point of view. Understandig the horizontal handover as the one that involves MNs moving between access points of the same type. The actual case can be seen as an Inter-technology / vertical handover which despite of the overlaping denitions, implies that the MN is moving between access points of different type and technology, for example, from WLAN to WIMAX and/or viceversa. The sub-classications of the handover from the control type talk by themselfs. Noticing that this ve handover types are independent, and the present handover case should be classiable according to each one of this types. Node initiating the handover: This can depend on the AN architecture and will be discussed later when studying estrategies for the HO that will be used in the present project. Node having the primary control: This is also related to the AN technology and can depend for example if the QoS is managed on-path or off-path, a BB can be the entity also controlling the HO procedure. Node collecting the measurements: It is similar to the previous one. AR initianting the HO: If the case of study is a sudden break up of the communication it can result that the new AR is the one initiating the HO and the previous one is not completely involved. Proactive or reactive: If the case of study is a sudden break up of the ongoing communication, this is a clear reactive handover, otherwise it can be proactive. In case of classifying according to the connectivity to the access routers, both cases result interesting for the present study, the BBM gives the challenge of reestablish in an efcient way the QoS of an ongoing session that was suddenly interrupted and the case of MBB has to set minimum times in order to have the full QoS provisioning pre-established before being able to move completely to the second connection, the second one also has the additional issue of managing the data content during the time the handover is being done, this in case a simultaneous connection to both access networks is possible.

Chapter 3

Access technologiesThere are many different access technologies in the market, each one of them intended to serve specic user requirements and dealing with situations as handover or QoS managing in a particular way. This chapter presents background knowledge about different access technologies, their architecture and the way they deal with situations as HO and QoS; the emphasis is done in those technologies which are strong candidates to be used for the present project.

3.1

WLAN

A wireless LAN (WLAN) is a data transmission system, designed to provide locationindependent network access between computing devices by using radio waves rather than a cable infrastructure. The IEEE ratied 802.11 specication as the standard for wireless LANs. Like all IEEE 802 standards, the 802.11 standard focus on the bottom two levels of the ISO model, the physical (PHY) layer and data link layer. Any LAN application, network operating system or protocol, like for example TCP/IP, will run on an 802.11-compliant WLAN as they run over Ethernet. See Figure 3.1

3.1.1

Architecture of WLAN and modes of operation

802.11 denes two pieces of equipment, a wireless station and an access point (AP), the last one acts as a bridge between the wireless and wired networks. An access point usually consists of a radio, a wired network interface like for example 802.3 and a bridging software. The access point acts as the base station for the wireless network, aggregating access for multiple wireless stations onto the wired network. The 802.11 standard denes two modes: infrastructure mode and ad hoc mode. In infrastructure mode the wireless network consists of at least one access point connected to the wired network infrastructure and a set of wireless end stations.

18

Access technologies

Figure 3.1: ISO and WLAN This conguration is called a Basic Service Set (BSS). An Extended Service Set (ESS) is a set of two or more BSSs forming a single subnetwork. This mode can be seen in Figure 3.2

Figure 3.2: Infrastructure mode for WLAN The Ad hoc mode (also called peer-to-peer mode or Independent Basic Service Set - IBSS) is simply a set of 802.11 wireless stations that communicate directly with one another without using an access point or any connection to a wired network. This mode is useful for quickly and easily setting up a wireless network

3.1 WLAN

19

anywhere that a wireless infrastructure does not exist or is not required for service but, is not relevant for the present case of study and therefore will no longer be considered. As the situation for the actual project implies that the WLAN has to be connected within the IMS and a full scheme of QoS provisioning has to be established, additional components to the infraestructure mode in an ESS have to be included, this elements for the interconnection are discussed further in the second part of this document.

3.1.2

QoS in WLAN

As users experience the convenience of wireless connectivity, they are beginning to demand support for the same applications that they run over wired networks. Because wireless bandwidth availability is restricted, quality of service is increasingly important in 802.11 networks. On WLANs based on this standard, all users share the networks capacity and no packet gets priority over any other. This usually is not a problem with typical data applications, such as exchanging e-mail and browsing the Web, but with voice calls and streaming video, packets have to get across the network at the right time. The original 802.11 MAC protocol was designed with two modes of communication for wireless stations. The rst, Distributed Coordination Function (DCF), is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes referred to as listen before talk. In this mode, a station waits for a quiet period on the network, begins to transmit data and detect collisions. DCF provides coordination, but it does not support any type of priority access of the wireless medium. The second mode is called Point Coordination Function (PCF) and it supports time-sensitive trafc ows. Wireless access points periodically send beacon frames to communicate network identication and management parameters specic to the wireless network, between the sending of beacon frames, PCF splits the time into a contention-free period and a contention period. With PCF enabled, a station can transmit data during contention-free polling periods, however, PCF has not been widely implemented because the technologys transmission times are unpredictable. Because DCF and PCF do not differentiate between trafc types or sources, the IEEE developed enhancements in 802.11e to both coordination modes in order to facilitate QoS. This standard denes QoS mechanisms that gives support to bandwidth-sensitive applications such as voice and video by prioritizing trafc and preventing packet collisions and delays, which should improve the experience of the users while maintaining backward-compatibility with current 802.11 standards.

20

Access technologies

The enhancement to DCF - Enhanced Distribution Coordination Function (EDCF) - introduces the concept of trafc categories in the MAC layer. Each station has eight trafc categories, or priority levels. Using EDCF, stations try to send data after detecting the medium is idle and after waiting a period of time. This time is dened by the corresponding trafc category called the Arbitration Interframe Space (AIFS). A higher-priority trafc category will have a shorter AIFS than a lower-priority trafc category. Thus stations with lower-priority trafc must wait longer than those with high-priority trafc before trying to access the medium. To avoid collisions within a trafc category, the station counts down an additional random number of time slots, known as a contention window, before attempting to transmit data. If another station transmits before the countdown has ended, the station waits for the next idle period, after which it continues the countdown where it left off. No guarantees of service are provided, but EDCF establishes a probabilistic priority mechanism to allocate bandwidth based on trafc categories. In Figure 3.3, an example of how EDCF works, is shown. Three main steps can be distinguished from there [www4]:

Figure 3.3: QoS in WLAN - EDCF 1. Devices with trafc categories of high, medium and low, have data to send on the WLAN. After the rst device nishes sending a packet, the Access Point acknowledge its receipt. 2. After the acknowledgement, comes the AIFS which is based on the trafc priority. The lower is the priority, the longer is this waiting time. 3. Once this AIFS ends, the stations has a random contention window and begin to countdown over it. Once this contention window is over the device begins to transmit and the other stations (normally the ones with lower priority)

3.1 WLAN

21

suspend the countdown once the device who has the access begins to transmit a packet. The way 802.11e aims to extend the polling mechanism of PCF is with the Hybrid Coordination Function (HCF). A hybrid controller polls stations during a contention-free period. The polling grants a station a specic start time and a maximum transmit duration. EDCF appears to be gaining more early acceptance than HCF.

3.1.3

Mobility in WLAN

The 802.11 MAC layer is responsible for how a client associates with an access point. When an 802.11 client enters the range of one or more APs, it chooses an access point to associate with, based mainly on signal strength. Once it is accepted by the access point, the client tunes to the radio channel to which the access point is set. Periodically it surveys all 802.11 channels in order to assess whether a different access point would provide it with better performance characteristics. If it determines that this is the case, it reassociates with the new access point, tuning to the radio channel to which that access point is set. Reassociation usually occurs because the wireless station has physically moved away from the original access point, causing the signal to be weak or due to considerable changes in the radio communication characteristics, or just because of high network trafc on the initial access point. The last case is known as load balancing, since its primary function is to distribute the total WLAN load in an efcient way across the available wireless infrastructure. The process of associating and reassociating users with APs in a dynamic way can be used in order to setup WLANs with very broad coverage, this coverage can be achieved by the use of overlapping cells through the areas that are inteded to be covered. The overlapping can employ techniques such as channel reuse. While the standard denes how a station associates with APs, it does not dene how APs track users as they roam about, either at layer 2 between two APs on the same subnet or at layer 3 when the user crosses a router boundary between subnets. The rst issue is handled by vendor-specic inter-AP protocols and therefore it may vary in performance. If the protocol is not efcient, there is a chance of packets being lost while the user moves from one access point to another. The second one can be handled by layer 3 roaming mechanisms as for example the implementation of a DHCP that just implies that in the new subnetwork, the user obtains a new IP address or by estrategies like mobile IP (MIP). The issue of the layer 3 mobility is highly related with the access network architecture and will be discussed in detail in the following part of the document about the WLAN being interconnected with the IMS platform.

22

Access technologies

3.1.4

Authentication and association

The rst step for connecting to a wireless LAN is authentication, it is the process through which a wireless node has its identity veried by the network to which the node is attempting to connect. Sometimes this process is null, meaning that, although both the client and access point have to proceed through this step in order to associate, there is really no special identity required for association. In infrastructure mode, the client begins the authentication process by sending an authentication request frame to the access point, then the access point will either accept or deny this request and notify the station of its decision with an authentication response frame. The authentication process can be accomplished at the access point or the access point might pass along this responsibility to an upstream authentication server. This server would perform the authentication based on a list of criteria and return the results to the access point so that it could return the results to the client station. Once the authentication is completed, the station sends an association request frame to the access point who replies to the client with an association response frame either allowing or disallowing association. The complete process of authentication and association has three distinct states: 1. Unauthenticated and Unassociated: In this initial state, the wireless node is completely disconnected from the network and unable to pass frames through the access point. Access points keep a table of client connection states known as the association table. 2. Authenticated and Unassociated: In this second state, the wireless client has passed the authentication process, but is not yet allowed to send or receive data through the access point. Normally, clients pass the authentication stage and immediately proceed into the association stage very quickly, (milliseconds) rarely at the access point the step of authenticated could be seen, it is far more likely that unauthenticated or associated are seen. 3. Authenticated and Associated: In this nal state, the wireless node is completely connected to the network and able to send and receive data through the access point to which the node is connected (associated). At the access points association table, it can be seen that this client is fully connected and authorized to pass trafc through the access point.

3.1.5

Authentication Methods

The authentication process for a client means a series of steps with the access point that vary according to the authentication procedure. The IEEE 802.11 standard species two methods of authentication: Open System Authentication (OSA) and Shared Key Authentication (SKA). This methods do not provide end-to-end or

3.1 WLAN

23

user-to-user authentication, therefore, some additional authentication mechanisms were added to the specication, one of them is the EAP/802.11x. The three of them are explained in the following subsections: OSA OSA is not really authentication, because the access point accepts the mobile station without verifying the identity of the station. The AP authenticates a client when the client simply responds with a MAC address during a two-message exchange, the process is as follows: 1. The client makes an active probe request. 2. The client receives response from the neighbours AP. 3. After a timeout, the client starts the authentication. 4. As OSA is null authentication, the AP allows the client to enter the network. 5. The association procedure starts. 6. The client is associated with the AP and can pass data through the network. SKA SKA uses criptography in order to have a secure communication between the client and the AP. It uses the principle of challenge-response, the client is assumed to know a shared key previous to the start of the authentication process, the process is as follows: 1. The client makes an active probe request. 2. The client receives response from the neighbours AP. 3. After a timeout, the client starts the authentication. 4. The client receives an unencrypted challenge from the AP. 5. The client encrypts it and send it back to the AP. 6. If the challenge is encrypted correctly, the AP allows the client to enter the network. 7. The association procedure starts. 8. The client is associated with the AP and can pass data through the network.

24

Access technologies

EAP/802.1x Extensible Authentication Protocol (EAP) uses legacy OSA authentication-association before 802.1x authentication. It has two main characteristics. First, it separates the message exchange from the process of authentication by providing an independent exchange layer and second, the authentication process can be extended by adopting a new mechanism without necessarily affecting the EAP layer. The message exchange can be seen in Figure 3.4.

Figure 3.4: Message exchange for 801.1x

3.2 WiMAXWorldwide Interoperability for Microwave Access (WiMAX) is the name given to the type of network that makes Broadband Wireless Access (BWA) available for the IEEE standard 802.16, therefore, in this section the standard 802.16 and the name WiMAX will be used without making any distinction. The 802.16, as many other IEEE standards mainly focuses on the PHY and MAC layers. There are three main variants of WiMAX (see Table 3.1). The most relevant for the actual case of study and the one that will be studied in this section is the 802.16e because is the only one supporting mobility. It is worth to mention that this standard is still on drafts and has not been completely approved, so, some of the characteristics mentioned in this document about it, are subject to be changed

3.2 WiMAX

25

in the nal approved standard documentation even if changes in the fundamentals are not expected.

3.2.1

Architecture of WiMAX

According to the standard, there are two types of equipment: Base Station - BS is an equipment providing connectivity, management and control Subscriber Stations (SS) Subscriber/Mobile Station - SS/MS is an equipment set providing connectivity between subscriber and BS There are two main network architectures mentioned in the 802.16 standard, the rst one is called mesh (MSH), in this architecture SSs are capable of forwarding trafc from and to multiple other SS. The second architecture and the one that can be used for the present project is called PMP (Point-to-Multipoint), it works with a centralized BS dealing with multiple SS The wireless links for PMP are managed by a central BS and a sectorized antenna that is capable of handling multiple independent sectors simultaneously. Within a given frequency channel and antenna sector, all stations receive the same transmission. The BS is the only transmitter operating in this direction, so it transmit without having to coordinate with other stations, except for the overall TDD that may divide time into uplink and downlink transmission periods. SS share the uplink to the BS on a demand basis. Depending on the class of service utilized, the SS may be issued continuing rights to transmit or the right to transmit may be granted by the BS after receipt of a request from the user. Within each sector, users adhere to a transmission protocol that controls contention betwen users and enables the service to be tailored to the delay and bandwidth requirement of each user application. This is accomplished through different types of uplink scheduling mechanisms. These are implemented using unsolicited bandwidth grants, polling, and contention procedures. For example, contention may be used to avoid the individual polling of SSs that have been inactive for a long period of time but, the use of polling simplies the access operation and guarantees that application receive service on a deterministic basis if it is required.

3.2.2

QoS in WiMAX

The principal mechanism for providing QoS is to asociate packets traversing the MAC interface into a service ow identied by a connection identier (CID). A service ow is a unidirectional ow of packets providing a particular QoS, the SS and BS provide this QoS according a set of QoS parameters dened for the service

26

Access technologies

Description

802.16a Initial version of 802.16, line-of-sight communication to xed devices

802.16-2004 Revised version of 802.16 standard, non-line-ofsight communication to xed or portable devices July 2004 From 1H06 10-66GHz and