Management of Low and Variable Bit Rate. ATM Adaptation Layer Type 2 Traffic

Transcription

1 Western Australian Telecommunications Research Institute and The University of Western Australia Management of Low and Variable Bit Rate ATM Adaptation Layer Type 2 Traffic Charles Voo This thesis is presented for the Degree of Doctor of Philosophy of The University of Western Australia School of Electrical, Electronic and Computer Engineering October 2003

2 Acknowledgments I wish to express my sincere thanks to my supervisors, Associate Professor John Siliquini and Professor Zigmantas Budrikis for their guidance and assistance throughout my studies towards the Ph.D. degree. I would also like to acknowledge the support of the Western Australia Telecommunications Research Institute (WATRI) throughout my Ph.D. In addition, I would like to acknowledge the financial support provided to me for my Ph.D. studies by an Australian Postgraduate Award and an Australian Telecommunications Cooperative Research Centre Award. More thanks are due to my parents for their encouragement and support throughout my studies. Special thanks to Tarith Devadason for his valuable comments. Finally, I would like to thank Jin for her continual love and support. i

3 ABSTRACT Abstract Asynchronous Transfer Mode (ATM) Adaptation Layer Type 2 (AAL2) has been developed to carry low and variable bit rate traffic. It provides high bandwidth efficiency with low packing delay by allowing voice traffic from different AAL2 channels to be multiplexed onto a single ATM virtual channel connection. Examples of where AAL2 are used include the Code Division Multiple Access and the Third Generation mobile telephony networks. The main objective of this thesis is to study traditional and novel AAL2 multiplexing methods and to characterise their performance when carrying low and variable bit rate (VBR) voice traffic. This work develops a comprehensive QoS framework which is used as a basis to study the performance of the AAL2 multiplexer system. In this QoS framework the effects of packet delay, delay variation, subjective voice quality and bandwidth utilisation are all used to determine the overall performance of the end-to-end system for the support of real time voice communications. Extensions to existing AAL2 voice multiplexers are proposed and characterised. In the case where different types of voice applications are presented to the AAL2 multiplexer, it was observed that increased efficiency gains are possible when a priority queuing scheme is introduced into the traditional AAL2 multiplexer system. Studies of the voice traffic characteristics and their effects on the performance of the AAL2 multiplexer are also investigated. It is shown that particular source behaviours can have deleterious effect on the performance of the AAL2 multiplexer. Methods of isolating these voice sources are examined and the performance of the AAL2 multiplexer re-evaluated to show the beneficial effects of a particular source isolation technique. The extent to which statistical multiplexing is possible for real time variable VBR sources is theoretically examined. These calculations highlight the difficulties in multiplexing VBR real time traffic while maintaining guaranteed delay bounds for these sources. Based on these calculations, multiplexing schemes that incorporate data transfers within the real time traffic transfer are proposed as alternatives for utilising unused bandwidth caused by the VBR nature of the voice traffic. ii

4 CONTENTS Table of Contents Chapter 1 Introduction Broadband Integrated Services Digital Network (B-ISDN) Asynchronous Transfer Mode (ATM) ATM Cell Structure ATM Switching Principles ATM Transfer Capabilities Deterministic Bit Rate (DBR) Using the DBR ATC to Transport CBR Traffic Using the DBR ATC to Transport VBR Traffic Statistical Bit Rate (SBR) ATM Adaptation Layer (AAL) AAL Original AAL AAL3/ AAL5 19 iii

5 CONTENTS AAL Summary The New AAL Comparisons AAL1 and AAL AAL5 and AAL AAL2 Work Objectives Thesis Contents 34 Chapter 2 Establishing Real Time Connections in ATM Networks Using AAL AAL2 Network Structure Real Time Communications Delay Constancy Source Network Destination Continuity of Data Flow Spacer buffer Play-out buffer overflow 50 iv

6 CONTENTS Sink starvation Establishing the real time connection Quality of Service (QoS) Framework 57 Chapter 3 AAL2 Multiplexer Model AAL2 Multiplexer System Model Voice Sources AAL2 Multiplexer Model Simulation in OPNET 69 Chapter 4 Priority Queuing Delay Budget Scenario Example General AAL2 Multiplexer Performance Performance of Prioritised AAL2 Multiplexer 83 Chapter 5 Source Sensitivity Simulation Example Performance Sensitivity of AAL2 Multiplexer Usage Parameter Control (UPC) Token Bucket Policer 94 v

7 CONTENTS Selection of Token Bucket Parameters Conclusion 103 Chapter 6 Alternative Multiplexing Method Simulation Example Worst Case Behaviour of the Token Bucket Parameters Future Work - Integrated Multiplexing Scheme Conclusion 114 Chapter 7 Conclusion 115 References 118 Appendix A Implementation of the DBR and SBR Cell Dispatch Processes 127 Appendix B An Analysis Establishing the Equivalence between DBR and SBR ATCs 139 Appendix C UPC Software Implementation 148 vi

8 ACRONYMS Acronyms 3G Third Generation AAL1 ATM Adaptation Layer Type 1 AAL2 ATM Adaptation Layer Type 2 AAL5 ATM Adaptation Layer Type 5 ATM CDMA CDV CID CPS DBR E-ADPCM ETSI FCFS HEC IP ITU-T Asynchronous Transfer Mode Code Division Multiple Access Cell Delay Variation Channel Identifier Common Part Sublayer Deterministic Bit Rate Embedded-Adaptive Differential Pulse Code Modulation European Telecommunications Standards Institute First Come First Serve Header Error Control Internet Protocol International Telecommunications Union Telecommunications vii

9 ACRONYMS LI MBS MOS OSF P PCM PDU PPR PSTN QoS SAP SBR SMG SN SNR SPR SSCS UPC UTRAN Length Indicator Maximum Burst Size Mean Opinion Score Offset Field Parity Pulse Code Modulation Protocol Data Unit Peak Packet Rate Public Switched Telephone Network Quality of Service Service Access Point Sustainable Bit Rate Statistical Multiplexing Gain Sequence Number Signal to Noise Ratio Sustainable Packet Rate Service Specific Convergence Sublayer Usage Parameter Control Universal Mobile Telecommunications System Terrestrial Radio viii

10 ACRONYMS Access Network UUI VAD VBR User to User Indication Voice Activity Factor Variable Bit Rate ix

11 AUTHOR S PUBLICATIONS LIST Author s Publications List [1]. C. Voo, J. F. Siliquini, and G. Mercankosk, Service differentiation of variable bit rate voice in AAL2 multiplexers, in Proceedings of IEEE Region 10 International Conference on Electrical and Electronic Technology (TENCON 01), vol. 2, pp , [2]. C. Voo, J. F. Siliquini and G. Mercankosk, Performance of AAL Type 2 Voice Multiplexers, Proceedings of the 9th IEEE International Conference on Telecommunications (ICT 02), vol. 1, pp , June, [3]. C. Voo and J. F. Siliquini, Performance Comparison of Multiplexing Methods for Voice over ATM using AAL2, in Proceedings of the 9th IEEE International Conference on Telecommunications (ICT 02), vol. 1, pp , June, [4]. C. Voo, A Review of the New Adaptation Layer Type 2, Inter-University Postgraduate Electrical Engineering Symposium (IUPEES 99), pp , July [5]. C. Voo, Performance of Statistical Multiplexed Voice over ATM using AAL2 and Deterministic Bit Dropping, Inter-University Postgraduate Electrical Engineering Symposium (IUPEES 00), pp , July [6]. J. F. Siliquini, G. Mercankosk, S. Ivandich, C. Voo, Z. L. Budrikis, and A. Cantoni, On Statistical Multiplexing Gain for Variable Bit Rate Voice Sources, in Proceedings of the 8th IEEE International Conference on Telecommunications (ICT 01), vol. 2, pp , June, x

12 CHAPTER 1 INTRODUCTION Chapter 1 Introduction The Broadband Integrated Services Digital Network (B-ISDN) has been defined for integrating the transport of different traffic types onto a single network infrastructure. The underlying technology chosen for the B-ISDN is the Asynchronous Transfer Mode (ATM). In this chapter, some important characteristics of the ATM such as cell structure, switching principles and transfer capabilities are described. Also, descriptions for the roles of ATM adaptation layers (AAL) are given. Comparisons between existing AAL protocols will highlight the need for the recently defined AAL type 2. Finally, the AAL2 is described and the thesis aims listed. 1.1 Broadband Integrated Services Digital Network (B-ISDN) Historically, land based telecommunications systems started with the integration of data communications equipment into the existing Public Switched Telephone Network (PSTN). The connection of remote computer equipment across different countries was financially economical as the network infrastructure was already installed internationally. However, the PSTN was designed for low bandwidth voice traffic and therefore not suitable for transmitting data, especially when speed requirements increased. As a result, these networks were later modified with the addition of high speed cabling but still requiring Modem (Modulator Demodulators using analogue signalling) connections. These networks had become known as Public Switched Data Networks (PSDN). 1

13 CHAPTER 1 INTRODUCTION As the demands on bandwidth increased, the need for a true digital media became apparent and this led to the development of the Integrated Service Digital Network (ISDN). The ISDN supports telephony and a wide range of data applications such as teletext and facsimile in the same network at connection speeds of 64kbit/s. This rate was chosen because, at that time, it was the standard rate for digitised voice. With the development of ISDN, the possibility of new services such as video conferencing, video telephony and other multimedia type applications were being investigated. However the introduction of these new services into the ISDN was hampered by the limitation that ISDN can only support applications compatible with the 64kbit/s switched digital connections. Therefore, the Broadband Integrated Service Digital Network (B-ISDN) was developed and standardised by the International Telecommunications Union Telecommunications (ITU-T) to support multimedia services with different bandwidths and delay requirements. At the time B-ISDN was developed, there were two existing technologies that could be used to support the B-ISDN. These were the Synchronous Digital Hierarchy (SDH) and the Asynchronous Transfer Mode (ATM) technologies. ATM was chosen as a candidate to be the transport mechanism for B-ISDN due to its simplicity and its capability to support a variety of both delay and loss sensitive traffic types. 1.2 Asynchronous Transfer Mode (ATM) ATM is a cell based networking and switching technology which can support a variety of both delay and loss sensitive traffic. As a cell based transmission technology, ATM packs data from various sources attached to the B-ISDN into a standard ATM cell format and the network transports these cells across the network. This uniform cell structure standardises the processing of cells and simplifies the integration of network components. ATM has been standardised by both the International Telecommunications Union (ITU) [1][2] and the ATM Forum for use in the planned public network of the future. 2

14 CHAPTER 1 INTRODUCTION ITU was established on May 17, It was formed to allow the interconnection of telegraph networks between countries. Now there are three general ITU sectors: Telecommunications (ITU-T), Radiocommunications (ITU-R) and Development (ITU- D). The main objective of the ITU is to define international standards that are to be adopted by all countries. Because the standardisation body is large, much time is required before a standard is adopted. Therefore the ATM Forum was established in October 1991 and was meant to accelerate the development of ATM products and services through a rapid convergence of interoperability specifications. In addition, it was supposed to promote industry cooperation and market awareness. For the work presented in this thesis, descriptions of any ATM terms will be based on the ITU-T standards as ITU is internationally recognised ATM Cell Structure With ATM, information for all services is conveyed and switched in fixed sized segments called cells. Each cell is 53 octets in length, consisting of a 5 octet header and a 48 octet payload field. There are two different types of ATM cells as shown in Figure 1. ATM cells transferred between a terminal and the local ATM switch follow the User- Network Interface (UNI) cell structure, which includes a Generic Flow Control (GFC) field. ATM cells transferred within the network between ATM switches follow the Network-Network Interface (NNI) cell structure, which has an expanded Virtual Path field in place of the GFC field. 3

15 CHAPTER 1 INTRODUCTION UNI Octet NNI GFC VPI VPI VCI VCI VCI PTI CLP HEC 1 ST octet of payload 2 nd octet of payload VPI VPI VCI VCI VCI PTI CLP HEC 1 ST octet of payload 2 nd octet of payload 48 octet of payload octet of payload 7 bit 0 7 bit 0 Figure 1: UNI and NNI ATM cell structure. The following gives a description of the various ATM fields: Generic Flow Control (GFC): Consists of 4 bits and is optionally used to regulate the entry of cells into the ATM network. Virtual Path Identifier (VPI): Consists of 12 bits in the NNI and 8 bits in the UNI, and is used for the identification and routing of cells. Virtual Channel Identifier (VCI): Consists of 16 bits and is also used for the identification and routing of cells. Payload Type Identifier (PTI): Consists of 3 bits and is used to identify the type of information contained in the ATM cell. Cell Loss Priority (CLP): Consists of 1 bit and is used to identify the priority of the cell with regards to its discard potential. Cells with CLP = 1 are considered 4

16 CHAPTER 1 INTRODUCTION low priority and discarded first when a network element experiences congestion. Header Error Control (HEC): Consists of 8 bits and is used for error checking on the first 4 octets of the header ATM Switching Principles The VPI and VCI fields within each ATM cell are used to identify and switch cells across the ATM network. The size of the fields are minimised by only providing switching information between the current and next switching elements and not an endto-end global address as in the Internet Protocol (IP) address. The VPI/VCI fields within the cells are updated as they are passed from switch to switch. VPI_1 VPI_2 VCI_1 VCI_1 VCI_2 VCI_1 ATM Switch VPI_3 Figure 2: VPI/VCI Translation within an ATM switch. Figure 2 shows the relationship between VPIs and VCIs and how they are translated when processed in an ATM switch. From Figure 2, there are two connections both having a VPI of 1. Within a single virtual path, there can be theoretically up to 2 16 virtual connections. In the above example, there are only 2 VCI labelled 1 and 2. 5

17 CHAPTER 1 INTRODUCTION ATM Transfer Capabilities Applications with different service requirements are supported in ATM by different transfer schemes [2][3][4]. Delay-sensitive applications, such as telephony and video require a timing relation between source and destination and there are transfer capabilities which exist for these. These real-time applications require a limit on the variation in the end-to-end delay to allow the communications to be real-time in nature. It also allows for the relevant transmit and receive buffers to be dimensioned. On the other hand, for applications such as data transfers, which are non-delay sensitive but loss-sensitive, no timing relation is required between source and destination. There are also transfer capabilities to support this type of communications. ATM supports four different transfer capabilities, these being the Deterministic Bit Rate (DBR), Statistical Bit Rate (SBR), Available Bit Rate (ABR) and Unspecified Bit Rate (UBR) transfer capabilities. In Table 1 we summarise the characteristics of each of the ATM transfer capabilities in terms of their suitability for the transport of real time and non-real time traffic. Since this thesis is primarily concerned with the transfer of real time traffic, only DBR and SBR ATM transfer capabilities will be described in more detail. Service DBR SBR ABR UBR Characteristics Type 1 Type 2 and 3 Bandwidth Guarantee Yes Yes Yes Optional No Real time traffic Yes Yes No No No Bursty data traffic No No Yes Yes Yes Table 1: Summary of the ATM service characteristics. 6

18 CHAPTER 1 INTRODUCTION The difference in SBR types (excluding SBR Type 1 (SBR1)) is in the handling of cells based on the value of the CLP field in the ATM header (described in Section 1.2.3). For cells with CLP=1, selective cell discard applies to both SBR Type 2 (SBR2) and SBR3 Type 3 (SBR3). However SBR3 also allow cells with CLP=1 to be tagged. For voice and other real time applications, only DBR and SBR Type 1 (SBR1) transfer capabilities are suitable Deterministic Bit Rate (DBR) Deterministic Bit Rate (DBR) ATM transfer capability (ATC) is used for the transport of real time traffic with guaranteed bandwidth for delay sensitive applications. Quality of Service commitments provided by DBR ATC are guaranteed for each connection. The traffic characteristic of the DBR ATC is modelled by a single traffic descriptor, namely the Peak Cell Rate (PCR DBR ). Under ITU standardisation, the DBR ATC can be used to support both constant bit rate (CBR) traffic and variable bit rate (VBR) traffic. The general case of transporting ATM cells using DBR ATC is shown in Figure 3. time t k+1 T k,k+1 Source Network DBR Spacer t k t k-1 ζ k+1 ζ k ζ k-1 ATM τ κ time Incoming cells Destination d spacer,k ζ k = tk + dspacer, k + Dp + τ k ' Figure 3: General model showing distribution of cells through an ATM network using the DBR ATM transfer capability. Source Referring to Figure 3, t k denotes the time at which the last bit of the k th cell is presented to the spacer. The interarrival time between the k th and the k+1 th cell is defined as T k,k+1. 7

19 CHAPTER 1 INTRODUCTION Spacer The function of the spacer is to limit the presentation of cells into the ATM network to a rate less than or equal to the peak cell rate of the DBR connection i.e. PCR DBR. Let d spacer,k denote the waiting time experienced by the k th cell in the spacer buffer. This value is assumed to be statistically bounded by τ spacer (i.e. 0 d spacer,k < τ spacer ). Network There are two components of delay associated with the transport of cells through the ATM network. The first component delay is the fixed propagation delay and is the time it would take a cell to traverse the ATM network if the cell experienced no queuing delay along its path. It also includes the packet transmission and processing delay within the switches in the network. This component of delay is denoted as D p. The second component of delay is the queuing delay. Let τ k denote the total queuing delay experience by the k th cell along its path. The value of τ k is dependent on the amount of jitter or queuing delay experienced by the cells travelling through the ATM network. According to DBR traffic contract, the value of τ k is statistically bounded by the cell delay variation tolerance, τ CDV [5]. Destination At the destination, the last bit of the k th cell arrives at time ζ k defined as ζ = (1.1) k tk dspacer, k Dp τ k ' Using the DBR ATC to Transport CBR Traffic Constant bit rate (CBR) sources are sources that produce traffic at fixed rates and are characterised by a single traffic parameter, their peak cell rate. Figure 4 shows the transport of these CBR cells through the ATM network using the DBR ATC. 8

20 CHAPTER 1 INTRODUCTION T Source 1 PCR kk, + 1 DBR t k+1 t k t k-1 time Incoming cells Spacer d spacer,k PCR DBR Network DBR ATM τ κ ζ k+1 ζ k ζ k-1 time ζ = t + D + τ ' k k p k Destination Figure 4: Transport of CBR cells through an ATM network using the DBR ATM transfer capability. Referring to Figure 4, to conform to the DBR traffic contract the CBR input traffic rate must be less than or equal to the peak cell rate of the DBR connection (i.e. T k,k+1 1/PCR DBR ). In this case, the waiting time for each cell in the spacer, d spacer,k is 0. Note that the CBR input traffic rates cannot be greater than the PCR of the DBR connection because incoming cells will be discarded when the spacer inevitably overflows. Using (1.1) and τ CDV as the bound for τ k, we can write for ζ k : tk Dp ζ k tk Dp τ CDV (1.2) Using the DBR ATC to Transport VBR Traffic The DBR ATC can also be used to transport Variable Bit Rate (VBR) sources. VBR sources are characterised by variable inter-cell arrival times. We assume that the VBR sources generate packets that conform to the Generic Cell Rate Algorithm (GCRA) [6] that has three parameters; Peak Cell Rate (PCR source ), Sustainable Cell Rate (SCR source ) and Intrinsic Burst Tolerance (τ IBTsource ). In this case, the GCRA is a policer that discards non-conforming cells before sending them to the spacer buffer to prevent the possibility of spacer buffer overflow. Therefore the sustainable cell rate of the source (i.e. SCR source ) must be smaller or equal to the peak cell rate of the DBR ATC (i.e. PCR DBR ). The characteristics of the cell transport for a VBR source using the DBR ATC is shown in Figure 5. 9

21 CHAPTER 1 INTRODUCTION Cells conform to GCRA (PCR source, SCR source, τ IBTsource ) time Source t k+1 t k t k-1 Incoming cells Spacer d spacer,k PCR DBR Network DBR ATM τ κ ζ Destination ζ k+1 ζ k ζ k-1 time k = tk + dspacer, k + Dp + τ k ' Figure 5: Distribution of VBR packets through an ATM network with DBR transfer capabilities. Referring to Figure 5, the VBR input traffic rate can be greater than the PCR of the DBR connection (i.e. T k,k+1 1/PCR DBR ) when the burst of cells conform to GCRA (PCR source, SCR source and τ IBTsource ). The maximum burst size of the traffic source is given by τ IBTsource MBS = SCRsource PCR source (1.3) For VBR traffic, d spacer,k is statistically bounded by the intrinsic burst tolerance, τ IBTsource (i.e. 0 d spacer,k < τ IBTsource ). The maximum network delay, τ k experienced by each cell is again statistically bounded by τ CDV. At the destination, each cell arrives at time ζ k that has bounds in the range given by (1.4). t + D ζ t + d + D + τ (1.4) k p k k spacer, k p CDV 10

22 CHAPTER 1 INTRODUCTION Statistical Bit Rate (SBR) Statistical Bit Rate (SBR Type 1) ATM transfer capability (ATC) has 3 traffic parameters associated with it, namely the PCR SBR, SCR SBR and τ IBT SBR. The input rate into the network is limited to the PCR SBR, which with the specified τ IBT SBR provides a limit on the volume of traffic that may be input above the SCR SBR. Therefore, VBR sources must first be shaped according to the GCRA with its PCR SBR, SCR SBR and τ IBT SBR parameters before they are carried by the SBR ATC. The SBR service guarantees that the actual transfer will be a rate at least equal to the SCR SBR. However, at times the VBR traffic can be serviced at a higher rate than the SCR SBR. But that is not guaranteed. The model for VBR cell transport through an ATM network with SBR ATM transfer capability is shown in Figure 6. Note that no spacer is required in this case. Cells conform to GCRA (PCR source, SCR source, τ IBTsource ) ζ k+1 ζ k ζ k-1 t ATM k+1 t k t k-1 Policer τ κ time time Source Network Destination Incoming cells PCR SBR, SCR SBR, τ IBT SBR SBR ζ = t + D + τ ' k k p k Figure 6: Distribution of VBR packets through an ATM network with SBR transfer capabilities. Referring to Figure 6, incoming cells pass through a policer according to the GCRA (PCR SBR, SCR SBR and τ IBT SBR ). Conforming cells are passed to the ATM network with no delay incurred by the policer. Note that any cells found non-conforming by the GCRA (PCR SBR, SCR SBR and τ IBT SBR ) are unconditionally discarded. Within the ATM network, each cell will be subject to a variable queuing delay. The composition of the queuing delay includes not only the constant delay D p and the delay due to phase coincidences with other traffic (i.e. τ CDV ) but also a possible smoothing delay whenever the service rate within the network is less than the rate at which the cells enter the ATM network (i.e bounded by the intrinsic burst tolerance, τ IBT SBR ). The bounds on these 11

23 CHAPTER 1 INTRODUCTION various delay components for a number of scheduling disciplines used in the ATM switches are summarised in Table 2. Scheduling disciplines Parekh-Gallager [7] D Variable delay bounds < τ + τ K + SCR SBR PGPS i MUX IBT SBR Golestani [8] D < τ + τ + K N1 δ... N K δ δ SCR SBR SCFQ i MUX IBT SBR K ( ) Stiliadis-Varma [9] LR () 1 ( K ) D < τ + τ + θ θ i MUX IBT SBR i i Goyal-Vin-Cheng [10] SFQ D < τ + τ + ( Nδ + + N δ Kδ ) 1... i MUX IBT SBR K Table 2: Variable transfer delay bounds for various scheduling disciplines. Referring to Table 2, the bracketed term in each of the bounds corresponds to the delay due to phase coincidences (i.e. τ CDV ) with other traffic across the network. The number of hops a cell goes through is denoted by K and δ denotes one cell transmission time in seconds. N j denotes the number of connections competing for access at hop j. It can be seen in Table 2 that for any of the scheduling disciplines, the smoothing delay is bounded by τ IBT SBR. Therefore, when using the SBR ATC for transporting VBR traffic, cells arrive at the destination at time ζ k that has bounds in the range given by t D t D k ' k + p ζ k < k + p + τ (1.5) Where the term τ k is given by τ = τ + τ (1.6) R ' k CDV IBT SB 12

24 CHAPTER 1 INTRODUCTION 1.3 ATM Adaptation Layer (AAL) Transfer capabilities are supported by ATM Adaptation Layers (AAL). AALs provide a mapping protocol between higher layers and the ATM layer. In terms of hierarchy within the B-ISDN Protocol Reference Model, it rests on top of the ATM layer as shown in Figure 7. Higher Layers (4+) ATM Adaptation Layer (AAL) (3) Convergence Sublayer (CS) Segmentation & Reassembly Sublayer (SAR) ATM Layer (2) Physical Layer (1) Figure 7: B-ISDN Protocol Reference Model. Referring to Figure 7, the lowest layer is the Physical layer. This layer is concerned with the transmission of data as well as other low level functions such as bit timing, transmission of frames and error checking. The second layer is the ATM layer, which provides VPI/VCI translation as described in Section 1.2, cell header creation/retrieval and Generic Flow Control. Currently, there are four AAL protocols specified to cover the transfer capabilities described in Section The AAL chosen for use is one that best suits the characteristics of the higher layer applications. The characteristics of each AAL is summarised in Table 3. 13

25 CHAPTER 1 INTRODUCTION Characteristics ATM Adaptation Layer Type AAL1 AAL2 (old) AAL3/4 AAL5 Timing Relationship Yes Not Required Bit Rate Constant Variable Mode Connection Oriented Connectionless / Connection Oriented Connection Oriented Table 3: ATM Adaptation Layer Characteristics. The AAL layer consists of two sublayers known as the Convergence Sublayer (CS) and the Segmentation and Reassembly sublayer (SAR). The functionality of the CS is to receive/send packets from/to higher layers. For each AAL, the packet structure is different and as such, each AAL supports a different type of CS packet. The functionality of the SAR sublayer is to segment packets into sizes equivalent to the length of the ATM cell payload before sending these onto the ATM layer AAL1 AAL1 has been designed to provide a DBR, connection oriented service wherein the timing relationship between the source and the destination is required. This timing relationship is obtained by the use of the Source Clock Frequency Recovery [11][12]. The use of AAL1 is suitable for delay sensitive applications such as telephony. The Segmentation and Reassembly Protocol Data Unit (SAR-PDU) packet structure of AAL1 is shown in Figure 8. The size of the packet structure is exactly 48 octets in length (i.e. corresponding to the length of an ATM cell payload). A SAR-PDU is formed by prepending a Segmentation and Reassembly Service Data Unit (SAR-SDU) with an octet header when it leaves for the ATM layer. Note that the header is extended to 2 octets when the CSI field is set to 1 and the SCF field is of even count (i.e. 0, 2, 4 14

26 CHAPTER 1 INTRODUCTION or 6). The offset field in the additional octet is used as a pointer to indicate the end of the payload and the parity bit is used to provide protection over the offset field. For this SAR-PDU, the maximum payload is 46 octets Bits CSI SCF CRC PAR PAR Offset Field SAR-SDU 1 Octet Present when CSI=1 and SCF=0, 2, 4, or 6 SAR-SDU=47 Octets when CSI=0 Figure 8: AAL1 SAR-PDU Packet Structure. The following gives a description of the various AAL1 fields: Convergence Sublayer Indication (CSI): Consists of 1 bit. This indicates the presence of the convergence sublayer function. Some examples of CS functions include the handling of SAR-PDU for partially filled SAR-PDU payloads, the handling of cell delay variation for delivery of AAL-SDUs to an AAL user at a constant bit rate, and timing information transfers. Sequence Count Field (SCF): Consists of 3 bits. This is provided by the CS layer and is used for the detection of lost or mis-inserted SAR-SDUs at the receiver. Cyclic Redundancy Checksum (CRC): Consists of 3 bits. This is used for bit error detection and correction over the SAR-PDU header. Parity (PAR): Consists of 1 bit. This is set such that the 1 octet SAR-PDU header has even parity and is used to protect the CRC. The one octet header is checked and removed by the AAL1 SAR sublayer on reception and the payload sent to the higher layer from the CS layer. 15

27 CHAPTER 1 INTRODUCTION As previously mentioned, partially filled SAR-PDU payloads can be handled by the CS sublayer using the Structure Data Transfer (SDT) method where an additional octet in the SAR-PDU header (i.e. extended to 2 octets) is used as a pointer to indicate the end of the payload [13]. This cell format can only be used when the sequence count value (i.e. SCF) in the SAR-PDU header is 0, 2, 4, or 6. For delay-sensitive applications where the SAR-SDUs are always less than 47 octets, [13] defines a CS procedure for partially filling the payload of a SAR-PDU. This method (known as the partial fill procedure) requires the receiving AAL CS to know when the payload contains overhead, the number of overhead octets and the position of these octets in the payload. Using this method, the number and position of AAL user information octets and CS generated dummy value octets in the remaining payload octets can be determined. However, [13] does not specify how the receiver will be able to distinguish AAL user information from padding (i.e. dummy octets) using information obtained from the AAL header. This has yet to be implemented for the AAL Original AAL2 Referring to Table 3, the original AAL2 was intended to provide real time services that have variable bit rates. However, due to the standard having many undefined properties, it is no longer under development. Note that this AAL bears no structural relation to the new AAL2 that will be described later in Section AAL3/4 AAL3 and AAL4 merged to become AAL3/4 and provide both connection and connectionless data service for variable bit rate (VBR) traffic. However, the AAL itself does not perform all functions required by a connectionless service, since functions such as routing and network addressing are performed at the network layer. There are two modes in which the AAL can operate; Stream Mode and Message Mode. If the preservation of message boundaries is required, then Message Mode must be used. 16

28 CHAPTER 1 INTRODUCTION The size of an AAL3/4 packet can be as large as 65,535 octets. The large packet size introduces transmission latencies, thus making it unsuitable for real time traffic. Figure 9 shows both the AAL3/4 CS and SAR PDU packet structures. Note that the SAR-SDU can be smaller than 44 octets Octets =< CPI BETag BA AAL-SDU Pad AL BETag Length CS-PDU bits 6 10 ST SN MID 44 Octets of CS-PDU LI CRC SAR-PDU Figure 9: AAL3/4 CS and SAR packet structures. The following gives a description of the various AAL3/4 CS-PDU fields: Common Part Indicator (CPI): Consist of 1 octet. This is used to interpret subsequent fields for the CS functions in the CS-PDU header. Examples include identifying related AAL layer management messages such as performance and fault monitoring, and the transfer of Operation and Management (OAM) messages. Begin End Tag (BETag): Consist of 1 octet. This is a sequence number used for checking packet synchronisation. Made redundant for connectionless services. Note that it is also repeated in the tail. Buffer Allocation (BA): Consists of 2 octets. This allows the receiving CS to allocate the appropriate amount of memory resources for incoming data. When AAL3/4 operates in message mode, the BA value is encoded equal to the CS- 17

29 CHAPTER 1 INTRODUCTION PDU payload length. In streaming mode, the BA value is encoded equal to or greater than the CS-PDU payload length. AAL-Service Data Unit (AAL-SDU): Allows up to 65,535 octets of data to pass between the higher layer application and the CS. Pad: Consists of up to 3 octets. Size of padding required must be such that the total packet length is a multiple of 4. Alignment (AL): Consists of 1 octet. This is similar to Pad but ensuring that the trailer is 4 octets long. Length: Consists of 2 octets. This indicates the length of the CS-PDU payload field in octets and is also used by the receiver to detect loss or gain of information. For transmission, the CS-PDUs are segmented by the SAR sublayer into 44 octet blocks and prepended 2 octet headers and appended 2 octet trailers. Note that the SAR-SDUs can be smaller than 44 octets. The resultant 48 octet SAR-PDUs are sent to the ATM layer where they are encapsulated into ATM cells through the prepending of 5 octet headers. The following gives a description of the various AAL3/4 SAR-PDU fields: Segment Type (ST): This identifies a SAR-PDU as containing a Beginning of Message (BOM), a Continuation of Message (COM), an End of Message (EOM) or a Single Segment Message (SSM). Sequence Number (SN): Consists of 4 bits and can be used for the detection of missing SAR-SDUs. Multiplexing Identifier (MID): Consists of 10 bits used to identify which CS- PDU the received SAR-PDU relates to. This allows AAL3/4 to multiplex data 18

30 CHAPTER 1 INTRODUCTION from different AAL3/4 connections. Length Indicator (LI): Consists of 6 bits to indicate the length of SAR-SDU information in the SAR-PDU payload. Cyclic Redundancy Checksum (CRC): Consists of 10 bits used for bit error detection on transfer of SAR-PDU AAL5 Due to the merging of AAL3 and AAL4, AAL3/4 has large overheads. AAL5 was then developed to replace AAL3/4. It provides similar connectionless services support as AAL3/4 but with less transmission overheads and better error detection. However, unlike AAL3/4, AAL5 does not support multiplexing of different AAL5 connections onto a single VCC. The CS for AAL5 is further divided into two parts, the Common Part Convergence Sublayer (CPCS) and the Service Specific Convergence Sublayer (SSCS) [11]. The SSCS provides signalling functions as required by the higher layers, and may sometimes be null. Similar to AAL3/4, AAL5 has two modes of operations; Streaming Mode and Message Mode. This has been discussed in Section The AAL5 CS- PDU packet structure is shown in Figure 10. octets =< octets payload Pad UU CPI Length CRC CS-PDU Figure 10: AAL5 CS-PDU packet structure. 19

31 CHAPTER 1 INTRODUCTION The following gives a description of the various AAL5 CS-PDU fields: Pad: Padding of 0-47 octets added to ensure that the CS-PDU length is an integral number of 48 octet segments. This helps to simplify SAR segmentation process and receiver packet field decoding. User to User identifier (UU): Consists of 1 octet. It enables the AAL user layers to identify the associated Service Access Point (SAP). Common Part Indicator (CPI): Consist of 1 octet. This is not used but is included to ensure trailer without padding is 8 octets in length. Length: Consists of 2 octets. It indicates the length of the CS-PDU payload field and is also used by the receiver to detect loss or gain of information. Cyclic Redundancy Checksum (CRC): Consists of 4 octets used for error detection on transfer of CS-PDU. The SAR-PDUs are created by segmenting CS-PDUs into 48 octet blocks. There are no prepended header or appended trailer fields. AAL5 uses the ATM User to User (AUU) parameter in the ATM cell PTI field to indicate the existence of the end of a CS-PDU in a SAR-PDU payload. Note a SAR-PDU where the value of AUU is 1 indicates the end of a CS-PDU; the value of 0 indicates the beginning or continuation of a CS-PDU AAL Summary From the above descriptions of AALs, it is found that each different AAL is used for a different service class. AAL1 supports DBR real time, connection-oriented services and is therefore suitable for the transportation of voice band signals (e.g. One 64kbit/s A-law or µ-law coded G711 signal), video and high quality audio traffic. 20

32 CHAPTER 1 INTRODUCTION AAL3/4 is suitable for services that utilise long packets. This is because the longer the packet, the smaller the percentage of the transmission overheads. However, having a long packet may result in a received packet being discarded when only a bit is in error. AAL3/4 supports multiplexing of different AAL3/4 data streams unto a virtual channel through the use of the MID field. AAL5 is the improved version of AAL3/4. It provides the same functions but with less transmission overheads and better error detection than the AAL3/4. Hence, AAL5 has been adopted as the standard for the ATM signalling protocol. It does not support multiplexing of packets on a virtual channel. In summary, any real time services such as multimedia require the use of AAL1 and non real time services require the use of either AAL3/4 or AAL5. Since the mid 1990s, there has been an increasing need for the support of low bit rate time sensitive traffic. Although AAL1 is able to support this type of traffic, it is at the expense of inefficient use of bandwidth since the cell rate must be high to maintain real performance but for low bit rate traffic, only a small portion of the ATM cell payload is utilised. Here it has been assumed that AAL1 has the capability to support partially filled payloads using the partial fill method described in Section An example of such an application is the transfer of voice data using AAL1. Audio sources are usually sampled at 8kHz and quantified into an 8-bit word, therefore requiring a 64kbits/s channel. Using an ATM cell to transfer just one voice data word is very inefficient use of available bandwidth (i.e. 1 octet out of a possible 47-octet payload space). Packing more voice data words into the ATM cell payload can increase the bandwidth efficiency, but can only be done at the expense of delaying transmission of some data until the cell is partly or completely full. If ATM cells can only be transmitted when the payload is completely filled, then the first cell in the payload would have to wait for a total of 47 octets. Byte arrivals occur once every 125µs. Once the first byte in an ATM cell payload has been received, it 21

33 CHAPTER 1 INTRODUCTION would have to wait for another 46 octets. In terms of time to accumulate 47 octets, this corresponds to a total time of µs=5. 875ms at an effective rate of 64kbits/s. At this point, it should be noted that most voice sources compress their voice data prior to transmission. An operational characteristic of compressors is to buffer the voice data until sufficient data is obtained before transmitting them. This introduces additional delay especially when the system is already waiting for enough data to fill an entire ATM cell payload. So in the above example, if voice was compressed to an effective data rate of 8kbits/s, the total packetisation delay would increase to ms=48ms. This is unacceptable because for a voice connection, the delay budget that consists of delay components such as queuing delay, propagation delay, network delay and equalisation delay is tight (i.e. around 100ms one way). Hence with such a large packetisation delay, it is difficult to meet this delay budget, given the existence of the other delay components (refer to Section 4.1 for more details). The solution then is to provide an AAL that can multiplex data packets from multiple sources into a single ATM cell payload. This requires a small but variable length packet. It should also allow for processing of concurrent packet arrivals from different higher layer applications since a number of packets of different sizes can be multiplexed into the same ATM cell. This can greatly increase bandwidth efficiency and reduce transmission latencies due to the reduced time in filling the ATM payloads. The recently defined AAL2 supports such requirements The New AAL2 The new AAL2 standardised by the ITU-T in November 2000 [14][15][16] and the ATM Forum in [17] was developed specifically to support the transfer of low and variable bit rate traffic across the ATM network. It does this efficiently by supporting the multiplexing of AAL2 packets from different higher layer applications into the same ATM cell for transmission. The AAL2 SAR-PDU packet structure is shown in Figure

34 CHAPTER 1 INTRODUCTION bits CID LI UUI HEC Payload 3 octets =< 44 octets Figure 11: AAL2 SAR-PDU packet structure. Descriptions of the AAL2 packet fields are as follows: Channel Identifier (CID): Denotes the corresponding Convergence Part Sublayer (CPS) connection using 8 bits. Note that there are 8 reserved values. Length Indicator (LI): Indicates the number of valid octets in the payload within the range of 1 to 45 using 6 bits in the header. User to User Indication (UUI): Consists of 5 bits and is used by the Service Specific Convergence Sublayer (SSCS) for traffic management such as Operation Administration and Maintenance (OAM), long packet segmentation and carrying audio encoding format profiles for code points 0 to 15. Code points between 16 and 22 are reserved. Header Error Control (HEC): Consists of 5 bits and is used for error detection in the packet header. The AAL2 packet is variable in size and can be much smaller than an ATM cell payload of 48 octets. Due to its variable size, many AAL2 packets from different connections in the application layers can be packed into the same ATM cell for transfer. As mentioned previously, this reduces the network latency and improves bandwidth efficiency at high rates. The AAL2 Common Part Sublayer (CPS) provides the multiplexing capability for AAL2 packets when multiple packets are packed into a single ATM cell payload. To do this, the first byte in the ATM cell payload is used to carry the Start Field byte, shown in Figure 12. The first six bits of the Start Field define the location of the first new 23

35 CHAPTER 1 INTRODUCTION AAL2 packet. From there, the AAL2 packet lengths are used to maintain packet alignment within the 47-octet payload. The remaining bits in the Start Field are used for cell sequence checking and bad parity detection within the Start Field bits Hdr Offset SEQ PAR Payload = AAL2 SAR-PDUs Start Field 47 octets ATM Cell Payload Figure 12: AAL2 CPS-PDU packet structure. The multiplexing process for AAL2 is illustrated in Figure Sources Source Packets Service Specific Convergence Sublayer (SSCS) Common Part Sublayer (CPS) Start Field header ATM header 1 a b 3 47 Octet 48 Octet 3-byte header AAL2 SAR-PDU AAL2 CPS-PDU ATM Cells Figure 13: Voice transportation using AAL2. 24

36 CHAPTER 1 INTRODUCTION Referring to Figure 13, source packets (labelled 1 to 3) are collected by the SSCS and segmented into 44 octet segments. Note that source packets can be of variable sizes. Segmentation of packets is illustrated by Packet 2 whereby it is segmented into 2 smaller segments, labelled a (full 44 octets) and b (remaining octets). Each segment is AAL2 encapsulated (i.e. prepended a 3 byte header) to form AAL2 SAR-PDU packets. These AAL2 SAR-PDUs are then segmented by the CPS to form AAL2 CPS PDUs each with a Start Field header. Note that the CPS-PDU payload is fully utilised Comparisons The performance of the AAL2 is now compared with the performance of AAL1 and AAL5 in terms of packing efficiency and the transmission delay AAL1 and AAL2 As mentioned previously, even though AAL1 is suitable for real time traffic, it is not able to efficiently carry low and variable bit rate real time traffic such as voice due to bandwidth inefficiency. The example of Figure 13 used to show the transportation of source packets via AAL2 will be used to illustrate the transportation of the same packets using AAL1. This is shown in Figure

37 CHAPTER 1 INTRODUCTION Sources (Applications) Source Packets Padding AAL1 Layer AAL1 packets 1 octet header ATM Layer ATM packets 5 octet header VCC1 VCC2 VCC2 VCC3 Figure 14: Voice transportation using AAL1. Referring to Figure 14, each packet (whole or segmented) is encapsulated to form AAL1 packets. Note that partial fill procedure described in [13] is used. Due to the fixed length AAL1 payload, padding is required to fill the remaining spaces, thus resulting in poor bandwidth utilisation. The problem of low bandwidth utilisation is solved for the case of the AAL2, in which packets from different connections are multiplexed onto the same ATM payload, thereby utilizing the whole payload space. A case scenario shown in Figure 15 is used to compare the performance of AAL1 and AAL2. 26

38 CHAPTER 1 INTRODUCTION 8 kb/s Codec 1 n 1 n AAL1 Packetisation Delay AAL2 Source Packet Source Packet 3 octet header AAL1 Packet Padding 5 octet header 1 octet header 1 octet header AAL2 Packet AAL2 CPS PDU VCC 1 VCC n VCC 1 VCC 1 ATM Packet ATM Packet Figure 15: Case scenario for comparisons between AAL1 and AAL2. Referring to Figure 15, a number of 8kbits/s codecs are used. The size of the packets is dependent on the allowed packetisation delay. For AAL1, source packets are encapsulated into a fixed size payload of 47 octets. The remaining unused payload not filled by the SAR-SDU is padded and an AAL1 header is prepended. Each AAL1 packet is then further encapsulated into an ATM cell. Also packets from each source require a different Virtual Channel Connection (VCC). For the AAL2, source packets are encapsulated and prepended 3 octets header to form an AAL2 packet. These are then further segmented into fixed size AAL2 CPS PDU packets before being ATM encapsulated. AAL2 packets from different sources are multiplexed onto an AAL2 CPS-PDU packet. The remaining AAL2 packet that cannot 27

39 CHAPTER 1 INTRODUCTION be filled into the first CPS-PDU payload is filled into the second CPS-PDU payload, thus resulting in little to no bandwidth wastage (see Section 1.3.6). Note that packets from different sources share a common VCC. The bandwidth efficiency for the AAL1 can be calculated by dividing the number of useful octets over the total size (i.e. the total number of ATM cells used) as given by (1.7). The size of an ATM cell is 53 octets (i.e. 5 octet header and 48 octet payload). The size of an AAL1 PDU is 48 octets, which includes the one octet header. Efficiency = No. of useful octets total No. of ATM packets 53 (1.7) The maximum efficiency is obtained when number of useful octets equal 47 resulting in an efficiency of 88.7%. The bandwidth efficiency for the AAL2 requires obtaining the total number of AAL2 packets that result from the source packet, and then dividing the useful octets over the total size (i.e. the total number of ATM cells used). AAL2 packets are variable in length with a maximum of 47 octets (i.e. 3-octet header and 44-octet payload). The AAL2 CPS-PDU is 48 octets in length including the one octet header. Using (1.8), the total number of AAL2 packets that is required by the source packet can be determined. The bandwidth efficiency is calculated using the resultant value through the application of (1.7). No. of AAL2 packets ( ) Size of source packet in bits = 44 8 bits (1.8) Where x Rn, n= x x n< x+ 1 28

40 CHAPTER 1 INTRODUCTION The performance comparison between AAL1 and AAL2 is shown in Figure 16. Values in this figure is obtained using (1.7) and (1.8). The horizontal scale is obtained with reference to an 8kbit/sec sampling rate. 100 Bandwidth Efficiency (%) kb/sec voice codec AAL Type 1 AAL Type 2 Typical codec generation times Packetisation Delay (ms) Figure 16: Bandwidth efficiency vs codec delay. Referring to Figure 16, it can be observed that AAL2 performs better than AAL1 over a wide packetisation range. The performance of AAL1 is comparable to AAL2 only when the packet length reaches a certain size. The bandwidth efficiency for the AAL1 is directly proportional to the size of the packet whereas for the AAL2, it reaches seveneighths of its maximum bandwidth efficiency (i.e. 70%) for a packetisation delay of 10ms. 29