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Abstract

The AAL2 protocol was chosen as a transport protocol within the UMTSTerrestrial RadioAccess Network (UTRAN). This protocol handles the radio channels between the mobile ter-minal and the Radio Network controller (RNC). On radio channels, the time constraints arestringent because of the synchronisation mechanisms at the MAC layer. The AAL2 protocolshould guarantee a certain Quality of Service (QoS) for the traffic handled on the AAL2connections.

In this paper, we introduce a general description of the AAL2 protocol and its position inUTRANand we study some performance issues of this protocol. We propose two schemes totransport the AAL2 traffic on the Iub and Iur interfaces and we study different schedulingalgorithms (FCFS, RR, WRR, EDF, Priority) at AAL2 level in order to evaluate the performanceof each algorithm. The optimal Timer-CU value is studied in this paper in order to choose atrade-off between time constraints and bandwidth efficiency. The equivalent bandwidth iscalculated in order to evaluate the capacity of a VC supporting AAL2 connections. Finally, theAAL2 switching technology is compared with the ATM switching technology in the case of atraffic concentrator in order to evaluate the advantages of the AAL2 switching.

Keys words:

ÉVALUATION DES PERFORMANCES DU PROTOCOLE AAL2 DANS L’UTRAN

Résumé

Le protocole AAL2 a été choisi comme protocole de transport dans le réseau terrestred’accès radio de l’UMTS(UTRAN). Ce protocole transporte les canaux radio entre le terminalmobile et le RNC(Radio Resource Controller). Sur les canaux radio, les contraintes de tempssont strictes à cause des mécanismes de synchronisation de la couche MAC. Le protocoleAAL2 doit garantir une certaine qualité de service pour le trafic transporté sur les connexionsAAL2.

Performance Evaluation of the AAL2 protocol within the UTRAN

Rani MAKKÉ*, Samir TOHMÉ*, Jean-Yves COCHENNEC**, Sophie PAUTONNIER***

* GET/Télécom Paris, 46 rue Barrault – 75634 Paris CEDEX 13 – France E-mail: [email protected]** France Télécom R&D, 2 avenue Pierre Marzin 22300 Lannion, FranceE-mail:[email protected]***Sophie Pautonnier, Mitsubishi Electric ITE-TCL, 1 allée de Beaulieu,CS 10806, 35708 Rennes CEDEX 7, FranceE-mail:[email protected]

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Dans cet article, nous décrivons le protocole AAL2 et sa position au sein de l’UTRAN etnous étudions quelque aspects de performance de ce protocole. Nous proposons deux sché-mas pour transporter le trafic AAL2 sur les interfaces Iub et Iur et nous étudions différentsalgorithmes d’ordonnancement (FIFO, RR, WRR, EDF, Priorité) au niveau AAL2 pour évaluer lesperformances de chaque algorithme. La valeur optimale du Timer-CU est aussi étudiée pourchoisir un compromis entre les contraintes de délai et l’utilisation de la bande passante. Labande passante équivalente est calculée pour évaluer la capacité d’un VC transportant descanaux AAL2. Enfin, La commutation AAL2 est comparée avec la commutation ATM dans lecas de concentration de trafic pour évaluer les avantages de la commutation AAL2.

Mots clés :

Contents

I. INTRODUCTION

Asynchronous Transfer Mode (ATM) is now widely used in many core networks to trans-port high data rate with a highly reliable Quality of Service (QoS) in particular for multime-dia applications. In 1997,ITU-T has standardised ATM Adaptation Layer 2 (AAL 2) whichallows multiplexing of several small packets called “mini-cells” in one ATM cell [1,2]. This ismore and more useful with the introduction of new low bit-rate codec. AAL 2 is therefore wellsuited for transport in the Core Network, in particular for real-time applications. Anotheradvantage of the AAL 2 protocol is its switching capability as each AAL 2 mini-cell contains aheader including an AAL 2 channel identifier.

In IMT2000/UMTS, AAL 2 was chosen as a transport technology for the radio access net-work UTRAN (UMTS Terrestrial Radio Access Network) in release 99 of the 3GPP standard[3]. The transmission delays in UTRAN have to be minimised as much as possible because ofthe real time services carried over UTRAN. In fact, even non real-time traffic coming from thecore network is becoming more or less “real-time” inside UTRAN because of the air interfacesynchronisation. As the delay requirements are very important in the access network, a pre-cise evaluation of the performance of the AAL 2 protocol is needed.

This paper presents the main theoretical results of a collaborative project (RNRT MINICEL

Project) between the ENST (Télécom-Paris), France Télécom R&D and Mitsubishi ElectricITE in which a prototype including a UTRAN environment simulator and an AAL 2 switch hasbeen developed [4]. This work was supported by the French Ministry of Industry.

2 R. MAKKÉ – PERFORMANCE EVALUATION OF THE AAL2 PROTOCOL WITHIN THE UTRAN

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I. IntroductionII. The AAL2 protocol

III. The UTRANarchitectureIV. AAL2 in UTRAN

V. Performance issues

VI. Traffic modelVII. Simulation modelVIII. Simulation resultsIC. ConclusionReferences (18 réf.)

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II. THE AAL2 PROTOCOL

The AAL 2 protocol has been standardised to transport very low bit-rate applications withreal-time constraint and variable bit-rate (e.g. the compressed voice).

AAL 2 was defined to get around the problem of the ATM cell packetization delay thatbecomes critical for the low bit-rates (at 16kbps, its value is 24 ms). The solution is simple :when multiplexing several communication flows in the same ATM channel, the delay becomesreasonable for a given communication.

The AAL 2 protocol consists of variable length data units called mini-cells with a maxi-mum payload length of 45 bytes (optionally 64 bytes).

The AAL 2 layer is divided into two sub-layers : the SSCS(Service Specific ConvergenceSub-layer) [5] and the CPS (Common Part Sub-layer). The SSCSis divided into three sub-layers : the SS-ADT (Service Specific – Assured Data Transfer), the SS-TED (Service Specific –Transmission Error Detection) and the SS-SAR (Service Specific – Segmentation And Reas-sembly) (Fig. 1).

Only SS-SAR is used in UTRAN. It segments higher-level data units exceeding 45 bytes(optionally 64 bytes) into packets with a maximum length of 45 bytes (optionally 64 bytes).The CPS layer overloads each mini-cell by a 3-byte header, whose format is described in

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FIG. 1 – AAL 2 sub-layers.

Sous-couches AAL2.

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Figure 2. The CID (Channel IDentifier) field identifies the AAL 2 connection. There are 256possible CID values, 8 of them are reserved for signalling purpose, and the rest may be used toidentify 248 different AAL 2 connections. The LI (Length Indicator) field determines the mini-cell payload length. By default, the maximum length is 45 bytes, but it may be 64 bytes ifthere is an indication at the connection establishment procedure. The UUI field is assigned tothe SSCS. Its 5 bits (32 code-points) are not interpreted by the CPSsub-layer and they are pas-sed transparently from the SSCStransmitting entity to the SSCSreceiving entity. The HEC (Hea-der Error Control) is used for error detection in the mini-cell header.

The ATM header allows two levels of addressing (Virtual Path Identifier VPI and VirtualCircuit Identifier VCI). Thus, it is possible to set up ATM VPCs between AAL2 end-points and toallow them to use VCI and CID to create multiple native connections. With a 16-bit VCI field,an ATM VPC will be able to support up to 248 × 216 AAL 2 connections.

The mini-cells are inserted in the ATM cells and 1-byte field (STF : STart Field) is added atthe beginning of the ATM payload. The STFcontains a pointer to the first byte of the first mini-cell header in the ATM payload. Overlapping is used : one minicell inserted in the ATM cell canoverlap onto the next cell (Fig. 3). Padding may be added at the end of the ATM cell payload ifthere are no additional minicells to be inserted.


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FIG. 2 – AAL 2 minicell header format.

Format de l’en-tête d’une mini-cellule AAL2.

FIG. 3 – Overlapping

Chevauchement.

CID LI UUI HEC

8 bits 6 bits 5 bits 5 bits

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The Timer-CU is an important parameter of the AAL 2 protocol. When a CPS-SDU (CPSSer-vice Data Unit) arrives in the CPSsub-layer, the CPSprotocol adds the 3-byte header to formthe CPS-packet. Then the Timer-CU is armed and STF is calculated. CPS-PDU is then transmittedto the ATM layer either when it is full, i.e. the 47-byte payload field is filled, or when theTimer-CU has expired. If the Timer-CU expires before the CPS-PDU becomes full, padding isadded to fillCPS-PDU that is transmitted to the ATM layer.

III. THE UTRAN ARCHITECTURE

The UTRAN is the UMTS Terrestrial Radio Access Network [6]. Its architecture is very simi-lar to the architecture of the GSM radio access network. Figure 4 represents the general archi-tecture of UTRAN.

The different elements of UTRAN are:– RNC - Radio Network Controller : it controls the radio resources.– Node B – the equivalent of the BTS (Base Transceiver Station) in GSM. The principal role

of this node is to transmit data and signalling on the radio interface.– RNS- Radio Network Subsystem : it is the access part of the UMTS network that manages

the allocation and the release of the radio resources for a set of cells. There is only one RNC ineach RNS.

The different UTRAN interfaces are:– Iu : interface between the RNS and the Core Network [7].– Iub : interface between one Node B and one RNC [8,9].– Iur : interface between two RNCs [9,10].When the mobile terminal is in soft handover state (when the terminal has several radio

links with different cells corresponding to different RNS), one RNS has an inter-connectionpoint with the Core Network. This RNS is called S-RNS (Serving RNS). The other RNS is calledD-RNS (Drift RNS) and can transmit all the user data flows to the S-RNC which establishes therecombination. The recombination principle consists of recombining all the data flows recei-ved from one user into one data flow to the core network.

IV. AAL2 IN UTRAN

The AAL 2 protocol is deployed on the Iub and Iur interfaces within UTRAN for real-timeand non real-time flows. On these interfaces, the AAL 2 protocol supports radio channels [11]extended between the UE (User Equipment) and the RNC as shown in Figure 5.

The RLC (Radio Link Control) layer [12] is transparent for voice flows. For data flows, theRLC layer segments data packets received from the core network into several RLC-PDUs (RLC -Packet Data Unit) with a maximum size predefined by the RRC (Radio Resource Control)functions [13].


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The MAC (Medium Access Control) [14] layer selects a number of Transport Blocks (TB)to be sent in one TTI (Transmission Time Interval). Each TB contains one RLC-PDU and thenumber of TBs sent in one TTI is predefined by the RRCfunctions. TTI is the duration of a radioframe and it is a multiple of 10 ms. It may be typically 10, 20, 40 or 80 ms. At this time, allMAC-PDUs are given a CFN (Connection Frame Number) which is a kind of timestamp indica-ting on which radio frame this data should be sent. This is why, as already mentioned above,all flows (voice and data) get time constraints on the UTRAN interfaces even though a tolerantmargin may be chosen for non real-time traffic. If this data arrives too late in the Node B (e.g.case of downlink), the frame is discarded.

The FP (Frame Protocol) layer assembles in the same frame the different TBs sent in thesame TTI and adds some more information which allows the receiving entity to know how theframe is built (number and size of TBs). In the uplink, some information about the quality of


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FIG. 4 – UTRAN general architecture.

Architecture générale de l’UTRAN.

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the received frame is added, so that theS-RNC has an idea of the reliability of the data fromone link compared to another one.

The AAL 2 layer segments FP-PDUs (FP - Packet Data Unit) when their size exceed 45 bytes(optionally 64 bytes) in the SS-SAR sub-layer and then inserts the CPSpackets (the mini-cells)in the ATM cells.

V. PERFORMANCE ISSUES

Within UTRAN, two major categories of flows are transported on the Iub and Iur interfaces :real-time applications (e.g. voice) which require low delay and non real-time applications(e.g. web browsing) which are tolerant of transfer delay. Even if all flows become delay sen-sitive in UTRAN because of the air interface synchronization, non real-time applications maytolerate a higher delay than real-time applications. In the case of mixed traffic (real-time andnon real-time flows), we can privilege real-time packets in order to meet stringent delayrequirements [15]. Non real-time packets are delivered with a higher delay but without vio-lating the time constraints of the air interface. Therefore, differentiation between services isneeded at the AAL 2 layer.

To transport these two types of flows on AAL 2 connections, two schemes are foreseen:1. Mono-service VC scheme : in this scheme, real-time flows are aggregated in one ATM VC

with stringent class of QoS and treated separately from non real-time applications which areaggregated in another ATM VC with tolerant class of QoS. In this scenario, real-time VC trans-ports one type of traffic and it is treated at ATM layer with higher priority than the non real-time VC.

2. Multi-service VC : in this scheme, real-time and non real-time applications are aggre-gated in the same VC. In order to differentiate between services, a scheduling mechanism is


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FIG. 5 – Iub protocol stack.

Pile protocolaire de l’interface Iub.

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needed at the AAL 2 layer to manage priority between different queues. At the ATM layer, onetype of VC is needed with a stringent class of QoS.

In the first scheme, all connections in the same VC have the same class of service. Thus,they have the same priority and a simple fair scheduling algorithm is recommended. We pro-pose the classic FCFS(First Come First Served) and the RR (Round Robin) algorithms. TheFCFSor FIFO (First In First Served) consists of providing one queue for all the packets comingfrom all the AAL 2 connections in the VC. Since all the packets have the same time require-ments because the traffic is homogeneous, the first packet arriving in the queue must be ser-ved first and the FCFSalgorithm is suitable. The RR algorithm consists of providing one queuefor each AAL 2 connection, and the scheduler selects one packet from each queue in eachcycle.

In the second scheme, different classes of service are handled in the same VC. An appro-priate scheduling algorithm should be implemented in the CPSmultiplexer which selects thepacket that should be sent on the link. The FCFS policy is not an appropriate solution formulti-service VC. The data flows may consume a large amount of the bandwidth and destroythe QoS of flows with stringent delay constraint (e.g. voice). The Priority of voice packetsover data packets is suitable for voice flows, but it is not appropriate for data flows especiallywhen voice traffic is very important and may consume all the available bandwidth. A trade-off between these two policies should be implemented. We should take into account a fairshare of the bandwidth between different services and the time constraint of each service.WRR (Weighted Round Robin) and EDF (Earliest Deadline First) are two proposed algorithmsto arbitrate between the data units that are ready for transmission on a link. The WRR algo-rithm consists of serving a predefined number of packets from each queue in a periodicmechanism. This number of packets divided by the cycle length of the algorithm representsthe weight assigned to the queue. The basic idea of the EDF scheduling is to assign a deadlineto every packet as it arrives, by adding the delay guarantee associated with the correspondingconnection to the arrival time of the packet. The EDF scheduler selects the packet with thesmallest deadline for transmission on the link. This scheme implies a dynamic priority as thepriority of a packet increases over time spent in the system.

Since all flows on the Iub and Iur interfaces become delay sensitive because of the airinterface synchronisation, the AAL 2 layer must guarantee the delivery delay of the traffictransported between the UTRAN nodes. Thus, once the traffic passes trough the air interface,the wired network should not be the bottleneck. The AAL 2 connections must be well dimen-sioned in order to guarantee the delay requirements.

The Timer-CU is an important parameter of the AAL 2 protocol. Its value is critical in thecase of low bit rate traffic. A very small Timer-CU value leads to a poor bandwidth utilization.A very high Timer-CU value optimises the bandwidth utilization, but leads to a higher pac-ketization delay and consequently to a deterioration in the quality of service of the transpor-ted traffic. An optimal Timer-CU value should be chosen carefully to optimise the bandwidthutilization and keep an acceptable delay for the transported packets.

The mini-cell transport as defined by the AAL 2 protocol leads to the idea of AAL 2 mini-cell switching. The main difference with the ATM switching is that the switched entities havevariable lengths. Within the first UTRAN networks, the choice between an ATM switch and anAAL2 switch will be an important issue. For example, in the case of a D-RNC, all AAL2 connec-tions in the same ATM VC are not directed to the same S-RNC. In this case, an ATM switch is notsufficient and the AAL 2 switching technology is necessary in theD-RNC. However, in the caseof a traffic concentrator between several Node Bs and a RNC, the switching entity may be an


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ATM switch or an AAL 2 switch. In this case, the choice between the two switching technolo-gies will be based on a trade-off between performance, implementation complexity and costof each solution. In this paper, we consider a concentrator node between several Node Bs andone RNC and we compare the performance of an AAL 2 switch and an ATM switch.

In order to evaluate the capacity of an ATM VC supporting an AAL 2 traffic, we define theequivalent bandwidth of each flow as the mean bit-rate of the flow at the ATM level includingall overheads. In other words, the equivalent bandwidth of an AAL 2 connection is defined asthe ratio of the total ATM bit-rate of all active AAL 2 connections in the VC and the number ofthese connections. In order to guarantee the delay requirements of the AAL2 flows, the sum ofall equivalent bandwidths of all active AAL 2 connections must be less than or equal to a cer-tain threshold that is a percentage of the PCR(Peak Cell Rate) value of the VC. This thresholddepends on the type of flows and the RCR value.

VI. TRAFFIC MODEL

The traffic pattern at the AAL 2 level is different from its pattern at source level. In fact,data flow goes through different protocol layers [11] and its characteristics change beforeentering the AAL 2 layer. In order to have an accurate traffic model at the AAL 2 level, weshould analyse the behaviour of the traffic coming from upper layers. This traffic goesthrough a protocol stack which has the architecture described in Figure 6.

The RLC is transparent for voice flows. For data flows, the RLC layer segments higher layerdata units into a number of RLC-PDUs and sends them to the MAC layer. The MAC layer putseachRLC-PDU in one TB and selects the TBs that shall be sent in the same TTI on the air inter-face. The FP layer assembles all the TBs transmitted in the same TTI in one frame called FP-PDU. This FP-PDU is transmitted to the AAL 2 layer.


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FIG. 6 – Protocol stack architecture

Architecture de la pile protocolaire.

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In this paper, we consider two types of flows : voice (AMR: Adaptative Multi Rate) anddata flows (UDD: Unconstrained Delay Data).

At the source level, a voice model is defined by the 3GPP for the AMR codec [16,17].This model consists of an ON/OFF model with exponential distribution of the ON and OFF

periods. The time interval between two packets and the packet size are constant values. ASID (SIlence Descriptor) is sent immediately after the ON period to inform the receiverabout the beginning of an OFF period. The SID is then sent during the OFF period only whenthe background noise level is changed in order to inform the receiver about this change.In our model,SID is sent every 160 ms during the OFF period. Table I represents the modelparameters.

There are different AMR coding mode. Table II represents these different modes with theircorresponding packet size. In this paper, the AMR 12.2 coding mode is used.

The traffic coming from the voice source passes through the protocol stack describedabove. The traffic pattern is slightly changed when entering the AAL 2 layer. In fact, the RLC

protocol is transparent for voice traffic but at the MAC layer, one voice packet is transmittedon the radio channel in each TTI. The TTI value for voice connections is 20 ms. Thus, the FP

layer receives one voice packet each 20 ms. It adds the FP overhead and sends this packet to


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TABLE I – Voice model parameters.

Paramètres du modèle de voix.

Parameter Distribution Value

ON period length Exponential 3 sec

OFF period length Exponential 3 sec

Time-interval between two packets Constant 20 ms

AMR Packet size Constant Depending on the AMR coding mode

SID packet size Constant 39 bits

TABLE II – AMR coding modes.

Les modes du codeur AMR.

AMR coding mode Throughput Packet size

AMR12.20 12.20 kbps 244 bits

AMR10.20 10.20 kbps 204 bits

AMR7.95 7.95 kbps 159 bits






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the AAL2 layer. The traffic entering the AAL2 layer is similar to the source traffic model with adifferent packet size (native packet size + MAC and FP headers). Ten signaling bytes are sentevery 300 ms.

For the data traffic, we consider in our study UDD flows which represent web browsingsources. A web browsing traffic model is defined by the 3GPP[17] : a web browsing sessionconsists of a sequence of packet-calls corresponding to the download of pages. The numberof packet-calls in a session is a geometrically distributed random variable with a mean of 5.Each packet-call represents the download of an internet file that has a Pareto with cut-offdistributed size with a minimal file size of 1858 bytes and a maximal file size of5 000 000 bytes. The shape parameter of the normal Pareto distribution function is α = 1.1.the packet-calls are separated by a time interval called reading-time which is a geometricallydistributed random variable with a mean of 12 seconds. This model is applicable on the Iuinterface but it is not directly applicable on the Iub and Iur interfaces. In fact,RRC flowcontrol mechanisms shape the traffic coming from upper layers so that the data throughput onthe air interface does not exceed the defined bit-rate for the chosen UDD mode. Furthermore,the TTI parameter gives a periodic pattern to the traffic transported on radio channels andentering in the AAL 2 layer. The RLC protocol splits upper layer packets into TBs with a prede-fined size and adds a 2-byte header. In each TTI, the MAC layer sends a certain number of TBson the radio channel without exceeding the bit-rate of the UDD mode (in our model). Thisnumber is determined by a dynamic resource allocation algorithm at the RRClayer. The RRCissimplified in our model and we consider that the number of TBs to be sent within a TTI isfixed and the RLC-PDU size is also predefined. At the FP layer, all TBs transmitted in the sameTTI for one user are assembled in one FP-PDU that is transmitted to the AAL 2 layer after addingthe FP overhead. The UDD traffic entering the AAL 2 layer has the pattern represented inFigure 7.

In each TTI, one FP-PDU is received at AAL 2 layer. The FP-PDU size depends on the UDD

mode and on the number of TBs sent in the TTI. The number of TBs sent in each TTI dependson the radio link utilization : if the radio link is low loaded, the RRC algorithm increases thenumber of TBs allowed to each user in the TTI because there is a free bandwidth. If the radiolink is very loaded, the RRC algorithm decreases the allowed number of TBs for each user inorder to share the bandwidth between all data users.

In our model, we considered UDD sources at the AAL 2 level with a bit-rate correspondingto the maximum bit-rate of the UDD type. In fact, we consider that the bandwidth on the radio


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FIG. 7 – UDD traffic pattern at AAL -2 layer.

Comportement du trafic UDD au niveau AAL2.

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interface is available and the AAL 2 transport layer should not be a bottleneck. In our simula-tions, the TTI value used for data channels is 40 ms and the TB size is 40 bytes. Thus, each 40there is an FP-PDU to be sent to the AAL 2 layer. This packet has a size corresponding to theUDD type. For example, for UDD 64kbps sources, there are 8 TBs sent in each TTI. Each TB hasa size of 42 bytes (40 bytes + 2 bytes header). All these TBs all assembled in one FP-PDU over-loaded by a 3-bytes header.

VII. SIMULATION MODEL

We consider an ATM link between a Node B and a RNC. In the Node B, an AAL 2 multi-plexer is implemented in order to aggregate several AAL 2 connections into one ATM VC. EachAAL 2 connection corresponds to one radio channel. The different layers are implemented inthe simulation model as shown in Figure 8.

Two scenarios are considered in the simulations : two different mono-service VCs, one foreach type of traffic, and one multi-service VC for both types of traffic.

In the scenario of traffic concentration, all Node Bs are connected to the concentrator (ATM or AAL 2 switch) through ATM VCs and all flows coming from Node Bs are aggrega-ted and sent to the RNC via one ATM VC. Figure 9 represents the scenario of trafficconcentration.


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FIG. 8 – Simulation model.

Modèle de simulation.

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VIII. SIMULATION RESULTS

We studied by simulation the impact of the Timer-CU on traffic performance, differentscheduling algorithms at the AAL 2 level, the equivalent bandwidth, the capacity of a VC withdifferent PCRvalues and a comparison between AAL 2 and ATM switching technologies.

VIII.1. Timer-cu

The Timer-CU is an important parameter of the AAL 2 protocol. Its value may affect theperformance of the traffic transported on an AAL 2 link. In a low loaded VC, if the Timer-cuvalue is very large, it may lead to a long packetization delay and consequently to a QoSdegradation. Very low Timer-cu value may lead to a poor bandwidth utilization. Its valueshould be chosen carefully to obtain a trade-off between QoS requirement and bandwidthefficiency. In [18], we studied in depth the impact of this parameter. In this paper, we willpresent the most important results.

Figure 10 represents the filling ratio in the case of a mono-service VC supporting voicetraffic. The filling ratio is defined as the ratio between the number of bytes used for data inthe ATM cell and the number of bytes available for AAL 2 traffic in each ATM cell (47 bytes).Figure 11 represents the 95-percentile delay and the Standard Deviation of delay (StdDev)


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FIG. 9 – Traffic concentration.

Concentration de trafic.

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for voice packets. Figures 12 and 13 represent respectively the filling ratio and the 95-per-centile delay in the case of a mono-service VC supporting data traffic.


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FIG. 10 – Filling ratio for voice VC.

Taux de remplissage pour un circuit virtuel de voix.

FIG. 11 – Delay and StdDev for voice packets.

Délai et Ecart-Type pour les paquets de voix.

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We observe that for high Timer-CU values, the packetization delay decreases when thenumber of streams increases. The filling ratio increases with the number of streams in thecase of a small Timer-CU value. At high load, the Timer-CU does not have an importantimpact on performance. In [18], we showed that the optimal Timer-CU value chosen in thecase of mono-service VC is suitable for multi-service VC. The transfer delay depends on thevalue of the Timer-CU regardless the PCRvalue of the VC. On the other hand, the filling ratiodepends on the Timer-CU value that is a function of the PCRvalue. However, the delay is themore critical parameter in UTRAN. Thus, a Timer-CU value chosen to satisfy delay require-ments is needed.


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FIG. 12 – Filling ratio for data VC

Taux de remplissage pour un VC de données.

FIG. 13 – Delay for data packets

Délai pour les paquets de données.

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VIII.2. Scheduling mechanisms

In the case of a mono-service VC, all AAL 2 connections have the same priority. Thus, theFCFSpolicy is suitable. Another possible algorithm is the RR (Round Robin). This algorithm isa cyclic mechanism that serves periodically one packet from each connection.

A mono-service VC (CBR VC – Constant Bit Rate, with PCR = 2 Mbps) supporting voicechannels is considered and the two algorithms (FCFSand RR) are compared. The utilization ofthe VC depends on the number of voice streams (between 0.1 and 0.85). Figure 14 representsthe 95-percentile delay in the case of the FCFSand RR algorithms and Figure 15 represents thestandard deviation of delay (StdDev).


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FIG. 14 – Voice delay (FCFSvs RR).

Délai pour la voix (FIFO vs RR).

FIG. 15 – StdDev of delay (FCFSvs RR).

Écart-Type du délai (FIFO vs RR).

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We observe that the simple FCFSmechanism is very suitable in the case of a mono-serviceVC for voice streams. It gives the same performance of the RR mechanism. The same expe-rience was repeated with different Timer-CU values and we observed the same result.

In the case of a multi-service VC where voice channels and data channels are aggregated,the implementation of a scheduling algorithm in the CPSmultiplexer is mandatory. In fact,data packets are more tolerant to delay than voice packets. If a voice packet and a data pac-ket are presented at the CPSmultiplexer, we can serve the voice packet first, then the datapacket. The FCFS(or FIFO) policy is not an appropriate solution to arbitrate between voiceand data packets because data flow may disturb the voice flow and consequently leads to aQoS degradation for real-time traffic. A scheduling mechanism based on the priority of thereal-time flow over the non real-time flow gives better performance for voice traffic butincreases considerably data delays. A trade-off between these two solutions is needed. Inthis paper, we studied the EDF algorithm (Earliest Deadline First) based on a transmissiondeadline assigned for each packet and the WRR algorithm (Weighted Round Robin) based ona weight assigned for each flow. These two alternatives may be a compromise if the dead-lines and the weights are appropriately chosen. In our simulations, we considered differentcombinations of flows :

• 20% voice and 80% data• 50% voice and 50% data• 80% voice and 20% data

Three Types of UDD traffic are considered :UDD 64 kbps,UDD 144 kbps and UDD 384 kbps.Different weights for the WRR algorithm are considered:• 1/5 for voice and 4/5 for data• 1/2 for voice and 1/2 for data• 4/5 for voice and 1/5 for data

Different deadlines for the EDF algorithm are considered :• 2ms for voice and 20ms for data• 2ms for voice and 50ms for data• 5ms for voice and 100ms for data

For each type of flow, the buffer size is supposed to be infinite which means that the sys-tem is without loss. A CBR VC with a PCRvalue of 2 Mbps is considered. The load of the VC isabout 0.7.

We calculated the complementary distribution of delay for voice and data packets. In thispaper, we present few simulation results for some flow combinations and some UDD types.Figures 16, 17 and 18 represent the complementary distribution of delay for different flowcombinations and for the UDD 64kbps mode. Figures 19,20 and 21 represent the same resultsfor the UDD 384kbps mode.

In special cases,WRR gives better performance than EDF. This result is very clear when thedata flow has a very bursty pattern. For example, when the UDD 384kbps traffic is used, theFP-PDU has a greater size and consequently the number of mini-cells arriving at the same timein the CPSmultiplexer is higher and the bursty pattern is clear. Thus, when a long data packetarrives in the AAL 2 layer, it will be segmented into several minicells that have the same dead-lines. When the CPSmultiplexer gives access to the data queue, it will serve all data minicellsbefore giving access to the voice queue. At this time, if a voice minicell arrives in the CPS

multiplexer, it will wait for the end of service of all data mini-cells for which the deadline hasexpired, and the waiting time may be very large depending on the data source type and the


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data packet length. The WRR algorithm is a good solution in this case because it shares thebandwidth between all flows. The weights of WRR depend on the traffic configuration. It isnot easy to determine these weights if we do not have an idea of the traffic entering the AAL 2layer. In the case where we know the traffic configuration (the percentage of the enteringtraffic : Pi) and the mean packet size of each service (Si), we may use these parameters withthe maximum delay allowed for each service (Ti) in order to determine the weight of eachqueue (Wi) :


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FIG. 16 – Complementary distribution : 20% voice and 80% data UDD 64kbps

Distribution complémentaire de probabilité : 20% voix et 80% données UDD 64kbps.



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Distribution complémentaire de probabilité : 20% voix et 80% données UDD384kbps.

FIG. 20 – Complementary distribution : 50% voice and 50% data UDD 384kbps.


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= 3 3

VIII.3. Equivalent bandwidth and vc capacity

The capacity of a VC is an important issue. In fact, the simultaneous number of the AAL 2connections in an ATM VC with a given PCRvalue should be controlled to guarantee the requi-red quality of service for the traffic supported by the AAL2 connections. The equivalent band-width as defined above gives an idea of the capacity needed for each AAL 2 connection. Inorder to guarantee the delay requirements, the sum of the equivalent bandwidths for all activeconnections must be less than or equal to a certain threshold. This threshold is a percentageof the PCR value of the VC. This percentage factor is computed by simulation for differentPCRvalues and different flow types. The equivalent bandwidth may be computed analyticallyor by simulation.

Equations 1 and 2 give the equivalent bandwidth for AMR flow and UDD flow respectively.

Equation 1 :

BW= 1 + O_MAC + O _FP + O _CPS2 3 }T8TI} 3 3

Equation 2 :

BW= 1 3 O_CPS + FP _PDU_size2 3 }T8TI} 3 3

ON}}ON + OFF

53}47

FP_PDU_size}}

45

ON}}ON + OFF

53}47

D3 TTI}

8

Sj}Si

Tj}Ti

Pi}Pj

Wi}Wj


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FIG. 21 – Complementary distribution : 80% voice and 20% data UDD 384kbps.


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where :

FP_PDU_size =

3 (RLC_ PDU_size+ O_ RLC+ O_ MAC) + O_ FP

BW : equivalent bandwidth.

D : source bit-rate [bps]. For AMR traffic, D is the bit-rate of the AMR mode used. For UDD

traffic, D is bit-rate of the corresponding mode.TTI : Transmission Time Interval of the channel [sec]O_MAC, O_FP, O_CPS: Overhead size for MAC, FP and CPSlayer respectively.ON, OFF : mean value of ON and OFF periods [sec].53 : size of an ATM cell with header.47 : number of bytes in an ATM cell offered to the AAL 2 traffic.RLC_PDU_size : the size of the RLC-PDU predefined by the RRC functions.x : is the first integer greater than or equal to x.

The equivalent bandwidth computed by simulation is slightly greater than the value given by these equations. In fact, the ATM cells may be filled by padding if the Timer-CU

expires before the traffic fills the ATM cells. The output throughput is greater than the calculated equivalent bandwidth because of the additional bandwidth added by the paddingbytes.

Figures 22, 23, 24 and 25 represent the equivalent bandwidth and the maximum utilization of the VC (the threshold) for voice traffic (AMR12.2) and UDD traffic (UDD64) respectively.

D3TTI}}}83RLC_PDU_size


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FIG. 22 – Equivalent bandwidth for AMR12.2 traffic.

Bande passante équivalente pour le trafic AMR12.2.

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FIG. 23 – Threshold for AMR12.2

Seuil pour AMR12.2.

FIG. 24 – Equivalent bandwidth for UDD64 traffic.

Bande passante équivalente pour le trafic UDD 64.

FIG. 25 – Threshold for UDD64.

Seuil pour UDD64.

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The equivalent bandwidth is stable for the VCs with a PCRgreater than 1 Mbps. It is about10 kbps for AMR12.2 traffic and about 9 kbps for UDD64 traffic.

It is clear that for low PCR values, the maximum utilization of the VC is small especiallyfor UDD traffic. In fact,UDD traffic is very bursty and at low PCR values, we should keep alarge bandwidth free in order to be used by the instantaneous bursts. In this case, the maxi-mum VC bandwidth is used only in a short time period when several bursts are active at thesame time. The rest of the time, the VC capacity has a poor utilization. When the PCR valueincreases, the free bandwidth needed becomes lower and thus, the threshold increases andreaches 82% for AMR12.2 traffic and 63% for UDD64 traffic.

VIII.4. Comparison between AAL 2 and ATM switching technologies

The performance of the AAL 2 switching technology is treated in this section in the case ofa concentration point between several Node Bs and one RNC. The concentration point is


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FIG. 26 – TAAL 2 and ATM switching.

Commutation AAL2 et ATM.

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considered to be an AAL 2 switch or an ATM switch in order to compare these two switchingalternatives. All the CBR VCs coming from the Node Bs to the switch are aggregated in thesame output CBR VC. The PCRvalue of each VC entering the switch is 2 Mbps. The PCRvalueof the VC between the switch and the RNC is 2 Mbps. The load of a VC between the Node Band the switch is 13% (low loaded VC). We consider two types of traffic :AMR12.2kbps andUDD64kbps with a combination of 50% voice and 50 % data. The scheduling mechanismused in the AAL 2 multiplexer is the absolute priority of voice packets over data packets. Weconsider that the links between the Node Bs and the switch are low loaded in order to studythe impact of the AAL 2 switching on the multiplexing gain. Figure 26 represents the 99.9-percentile delay for voice and data packets in the case of an AAL 2 switch and an ATM switch.This is the delay between the SAP(Service Access Point) of the AAL 2 layer in the Node B andthe other SAP of the AAL 2 layer in the RNC including switching delay.

We observe that in the case of an AAL 2 switch, we can aggregate a larger number of NodeBs than the case of the ATM switch with acceptable delay. In fact, when multiplexing at theAAL2 layer, we can differentiate between different types of flows and implement a schedulingmechanism in order to give priority for stringent delay packets (e.g. voice packets). Further-more, in the case of an AAL2 switch, we may benefit from the CPSmultiplexer to eliminate thepadding in the cells coming from the low loaded VCs and reduce the bit-rate of the outgoingflow. In the case of an ATM switch, the bit-rate of the outgoing flow is the sum of all incomingbit-rates. If the entering VCs are high loaded, the AAL 2 switching technology will not have animportant advantage and an ATM switch is sufficient.

IX. CONCLUSION

In this paper, we presented some issues in performance of the AAL 2 protocol studied bysimulation. The Timer-CU is a very important parameter of the AAL 2 protocol. A high Timer-CU value leads to high packetization delay and consequently to a QoS degradation especiallyin the case of low loaded VCs. A small Timer-CU value leads to a poor bandwidth utilization.A trade-off between delay and bandwidth efficiency may give the optimal value of the Timer-CU and this value may be chosen in the case of mono-service VC.

In the case of low loaded VC between a Node B and a RNC, the Timer-CU may not have animportant impact because we can choose a small value in order to guarantee the packetiza-tion delay and we do not care about the bandwidth efficiency because the VC is low loaded.But this is not true if there is a concentration node between the Node B and the RNC, espe-cially if this node is an ATM switch. In fact, if the VCs entering the ATM switch are low loadedand if we choose a small Timer-CU value in order to guarantee the packetization delay, theATM cells will be partially filled. This is not a problem for a VC entering the concentratorbecause it is low loaded but the output VC may be high loaded and since in the ATM switchthere is no multiplexing at the AAL2 level, the cells in the output VC will be partially filled andconsequently lead to a bandwidth loss. In this case, we should carefully choose the value ofthis timer. A Timer-CU between 1 ms and 2 ms may be an optimal value.

The scheduling mechanism is very important in the CPSmultiplexer especially in the caseof multi-service VC. We showed that the FCFS(or FIFO) mechanism is appropriate for mono-


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service VCs and gives good performance. Furthermore, it is less complicated than the RoundRobin algorithm which requires a timer management for the periodic mechanism.

In the case of multi-service VCs,FCFSis not an appropriate solution. Priority penalizes datatraffic especially when voice traffic is heavy. EDF and WRR are a trade-off between these twosolutions. The implementation of the EDF algorithm is more complicated than WRR because itrequires the management of several timers, while WRR requires only the management of oneperiodic timer for all queues. Furthermore,EDF may lead to a QoS degradation for voice traf-fic in some specific situations as showed above. WRR may be used as a scheduling mechanismin the CPSmultiplexer and the weights should be chosen appropriately.

The equivalent bandwidth is an important parameter which gives an idea of the bandwidthneeded by each AAL 2 connection and of the maximum number of AAL 2 connections that canbe supported by a given VC with delay guarantee.

Finally, the problem of the AAL2 switching is considered and we concluded that when swit-ching at the AAL 2 layer, we can obtain a large multiplexing gain especially when we haveconcentration points.

Within the framework of release 5 of the 3GPP, the IP solution has been selected as anothertransport technology within UTRAN [17]. Many manufacturers and operators defend this solu-tion in the perspective of the deployment of global IP networks. This technology becomesvery attractive if future data traffic represents a significant part of the whole traffic transpor-ted by the network. The IP transport technology in UTRAN is the subject of our current studies.

Manuscrit reçu le 3 octobre 2002Accepté le 10 février 2003

REFERENCES

[1] ITU-T I.363.2,B-ISDN ATM Adaptation Layer Specification:AAL Type 2.[2] ITU-T I.366.1, Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2.[3] 3GPPwebsite, http://www.3gpp.org.[4] MINICEL PROJECT, ENST-Paris, France Télécom R&D, Mitsubishi ElectricITE, Technical Reports

1,2,3,4,5,6,7,8,9.[5] ITU-T I.366.2,AAL type 2 service specific convergence sublayer for narrow-band services.[6] ETSI TS125.401,UTRAN Overall Description.[7] ETSI TS125.413,UTRAN Iu Interface RANAP Signalling.[8] 3G TS 25.435,UTRAN Iub Interface User Plane Protocols for Common transport Channel Data Streams.[9] 3G TS 25.427,UTRAN Iub/Iur Interface User Plane Protocol for DCH Data Streams.

[10] ETSI TS125.420,UTRAN Iur Interface General Aspects and Principles.[11] 3G TS 25.301, Radio Interface Protocol Architecture.[12] 3G TS 25.322,RLC Protocol Specification.[13] 3G TS 25.331, Radio Resource Control (RRC) Protocol Specification.[14] 3G TS 25.321,MAC Protocol Specification.[15] 3G TS 23.107, QoS Concept and Architecture.[16] ETSI TR 101.112, Selection Procedure for the choice of radio transmission technologies of the UMTS.[17] 3G TR 25.933,IP Transport in UTRAN.[18] MAKKÉ (R.), TOHMÉ, (S.), COCHENNEC(J-Y.), PAUTONNIER (S.), Impact of the Timer-CU of the AAL 2 Protocol

on Traffic Performance within the UTRAN, IFIP Personal Wireless Communications 2001, 8-10 August 2001,Lappeenranta, Finland, pp. 3-22.


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