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Les Mécanismes De Fiabilisation (Protocoles ARQ) Et Leur Adaptation Dans Les Réseaux Radiomobiles De 3G Robert Bestak

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Les Mécanismes De Fiabilisation (Protocoles ARQ) Et Leur Adaptation Dans Les Réseaux Radiomobiles De 3G Robert Bestak Les mécanismes de fiabilisation (protocoles ARQ) et leur adaptation dans les réseaux radiomobiles de 3G Robert Bestak To cite this version: Robert Bestak. Les mécanismes de fiabilisation (protocoles ARQ) et leur adaptation dans les réseaux radiomobiles de 3G. Traitement du signal et de l’image [eess.SP]. Télécom ParisTech, 2003. Français. tel-00005738 HAL Id: tel-00005738 https://pastel.archives-ouvertes.fr/tel-00005738 Submitted on 5 Apr 2004 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Thèse présentée pour obtenir le grade de docteur de l’Ecole Nationale Supérieure des Télécommunic ations Spécialité : Informatique et Réseaux Robert BESTAK Les mécanismes de fiabilisation (protocoles ARQ) et leur adaptation dans les réseaux radiomobiles de 3G Soutenue le 18 décembre 2003 devant le jury composé de Xavier LAGRANGE Président Khaldoun AL AGHA Rapporteurs Laurent TOUTAIN Philippe MARTINS Examinateurs Boris SIMAK Jérôme BROUET Invités Rémy ROGACKI Philippe GODLEWSKI Directeur de thèse A mes parents v Remerciements Tout d’abord, je tiens à remercier mon directeur de thèse le professeur Philippe Godlewski. Ses conseils et ses critiques constructives m’ont beaucoup apporté tout au long de ces années. Je lui suis très reconnaissant pour sa patience et ses efforts à me guider durant ma formation scientifique. Un grand merci aux maîtres de conférences Philippe Martins et Elie Najm qui ont contribué largement à ma formation. J’ai beaucoup apprécié nos discussions techniques qui m’ont orienté vers le but de ma recherche. Je remercie les rapporteurs de ma thèse, Khaldoun Al Agha et Laurent Toutain, d’avoir consacré de leur temps à ma thèse et d’avoir apporté des remarques constructives. Je tiens à remercier les autres membres du jury dont Xavier Lagrange, Boris Simak, Jérôme Brouet et Rémy Rogacki de leurs commentaires pertinentes. Je remercie tous les enseignants et thésards du Département Infres de l’ENST pour avoir créé une ambiance amicale et sympathique. Je remercie tout particulièrement mes amis avec qui j’ai partagé le bureau durant la thèse Tatiane Aubonnet, Arnaud Bailly, et Alexandre Tau- veron qui m’ont donné leur soutien durant cette thèse. Un grand merci aux étudiants de l’ENST qui m’ont permis d’enrichir mes connaissances au cours de l’encadrement des projets. Je voudrais exprimer ma gratitude à mon entourage personnel, mes parents et mes amis qui m’ont encouragé tout au long de mes études en France, sans lesquelles je n’aurai pas finalisé cette thèse. vi vii Résumé Les réseaux radiomobiles de 3ème génération (3G), tel que l’UMTS (Universal Mobile Tele- communications System), ont été prévus, dés le début de leur conception, pour offrir un mode paquet aussi bien qu’un mode circuit; le mode circuit étant le mode unique de la 2G (cas du système GSM, Global System for Mobile communications). Pour fiabiliser les transmissions de données en mode paquet, il faut mettre en œuvre des protocoles disposant de la technique ARQ (Automatic Repeat reQuest). L’objet de cette thèse est l’étude de protocoles ARQ qui peuvent être activés dans l’architecture protocolaire globale de l’UMTS. Cela nous amène à étudier les couches contenant les protocoles ou entités suivan- tes : RLC (Radio Link Control), TCP (Transmission Control Protocol) et MAC-hs (Medium Access Control-high speed). L’étude des nouvelles caractéristiques des protocoles ARQ de l’UMTS ainsi que la possibilité de leur enrichissement est effectuée. L’empilement des méca- nismes ARQ présente des risques d’interactions dont certains sont analysés. Le chapitre 1 présente le contexte de notre étude. Le lecteur est familiarisé aux innovations et notations de l’UMTS qui sont reprises dans les chapitres suivants. L’architecture de l’interface radio de l’UMTS et ses différents aspects sont présentés dans le paragraphe 1.2; la présentation concerne principalement l’architecture d’un point de vue protocolaire. Suite à cette présentation de l’architecture de l’UMTS dans son ensemble, le paragraphe 1.3 introduit les trois protocoles ARQ ainsi que les éléments réseaux entre lesquels ces protocoles fonctio n- nent (figure 4). L’étude détaillée de ces protocoles est faite dans les chapitres suivants. Suivant l’exigence aux délais et la sensibilité aux erreurs, les services de l’UMTS sont divisés en quatre classes. Celles-ci sont présentées dans le paragraphe 1.4. La transmission fiable de données sur l’interface radio de l’UMTS est principalement réali- sée par le protocole RLC, plus précisément par les entités RLC qui fonctionnent en mode ac- quitté (AM, Acknowledged mode). Le protocole est largement adaptable et des primitives spé- cifiques permettent de l’ajuster confo rmément à la qualité de service requise. Dans le paragra- phe 2.3, nous analysons différents aspects des mécanismes du protocole RLC, comme la fonc- tion SDU discard. Le protocole RLC de l’UMTS a été amélioré et enrichi par rapport au proto- cole RLC du (E)GPRS (Enhanced, General Packet Radio Service). Certaines des différences entre elles sont mentionnées dans le paragraphe 2.5. Le chapitre 2 est terminé par des remar- ques concernant l’implementation des protocoles (le paragraphe 2.6). Résumé viii Nombre de services prévus pour l’UMTS sont proches des services offerts par l'Internet fixe (comme le web-browsing, la messagerie électronique, etc.). Ces services sont fournis via la pile protocolaire TCP/IP, base traditionnelle et obligatoire du monde Internet. L'utilisation de TCP/IP sur une interface radio a donné lieu à de nombreuses études mais seulement une partie considère une couche liaison avec un ARQ. Les mécanismes TCP sont comparés avec ceux du RLC de l’UMTS dans le paragraphe 3.4. Le paragraphe 3.5 analyse l’utilité de certaines propo- sitions du TCP pour les réseaux sans fil comme Split-TCP, Wirless-TCP, Snoop , etc., dans l’environnement de l’UMTS. L’introduction du protocole RLC au-dessous de la pile protocolaire TCP/IP est utile pour la performance globale. La grande flexibilité du RLC conduit à une longue liste de paramètres ajustables, tels que les temporisateurs, la fenêtre d’émission, le nombre maximum de tentatives de réémissions d’un bloc RLC, etc. Un de ces paramètres spécifie la taille de la mémoire tam- pon (buffer) en entrée de la couche RLC. Nous étudions l’impact des différents soft states du protocole TCP sur l’occupation du tampon de l’entité RLC doit être pris en compte. L’occupation de ce tampon affecte le TCP RTT (Round Trip Time). Il faut distinguer les transmissions de données : (i) en voie montante et (ii) en voie descendante. Dans le premier cas, l’occupation du tampon de l’entité RLC (situé dans l’UE, User Equipment) est déterminé par le système d'exploitation d’UE et le contrôle interne de flux entre les protocoles RLC et TCP en plus des soft states du TCP. Notre étude considère uniquement le deuxième cas (voie descendante) où le contrôle interne de flux entre les deux protocoles n’existe pas. L’occupation du tampon de l’entité RLC (RNC, Radio Network Controller), en considérant différentes va- leurs du RTT dans le réseau fixe et du BlER (Block Error Rate, taux d’erreurs par bloc) sur la liaison radio sont analysées dans le paragraphe 4.3. De plus, les diverses tailles du tampon de l’entité RLC sur la performance du protocole TCP sont étudiées. Pour surmonter quelques inconvénients de l’empilement TCP/RLC, dans le paragraphe 4.4.1, nous proposons d’introduire entre les deux protocoles un mécanisme spécifique appelé Early Error Notification (EEN). Celui-ci est basé sur la fonction SDU discard du RLC et sur la possibilité d’échanger des messages de contrôle entre les protocoles TCP et RLC. Les résultats de simulations sans et avec l’EEN sont présentés dans le paragraphe 4.4.2. Le mécanisme EEN est suggéré pour les transmissions de données en voie montante. La possibilité d’utilisation de l’EEN en voie descendante est étudiée dans le paragraphe 4.4.3. Pour améliorer les performances des transmissions de données en voie descendante, la Re- lease 5 de l'UMTS propose un nouveau mode d'accès, HSDPA (High Speed Downlink Packet Access). Le mode HSDPA introduit un mécanisme hybride ARQ (HARQ) qui concerne les ni- veaux physique et MAC (localisé au Node B). Ce mécanisme, basé sur le Stop&Wait ainsi sur une numérotation spécifique, est analysé dans le paragraphe 5.2. Résumé ix Le mode d’allocation du HSDPA est très flexible. Néanmoins, des services comme le streaming n’ont pas besoin d’une telle allocation dynamique. Les services de streaming génè- rent un flux sortant régulier de données. Pour un tel flux de données, il est plutôt envisageable d’allouer périodiquement des ressources radio. L’impact de l’allocation périodique sur l’allocation dynamique est étudié dans le paragraphe 5.3. La majorité des études sur l’HSDPA concerne l’adaptation rap ide du lien radio, la perfor- mance d’Error Correction Code ou les politiques d’allocation de ressources radio. En ce mo- ment, peu d’études s’intéressent aux mécanismes du protocole MAC-hs. Les mécanismes du protocole MAC-hs sont liés avec la procédure de l’adaptation rapide du lien radio à la couche physique.
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