Fast Retransmission of Real-Time Traffic in HIPERLAN/2 Systems

Transcription

1 Fast Retransmsson of Real-Tme Traffc n HIPERLAN/ Systems José A Afonso and Joaqum E Neves Department of Industral Electroncs Unversty of Mnho, Campus de Azurém Gumarães, Portugal {joseafonso, jeneves}@deumnhopt Abstract Automatc repeat request (ARQ) schemes are effectve to recover non-real-tme data corrupted by channel errors, but ther use wth real-tme traffc s seldom consdered because packet retransmssons tend to ncrease the delay beyond the deadlne However, the hgh-speed of modern wreless local area networks, together wth ther nherent low propagaton tmes, favors the use of ARQ schemes wth real-tme traffc Ths paper proposes a fast retransmsson schedulng mechansm for relable transport of realtme traffc n HIPERLAN/ systems, as well as an adaptve rate scheme desgned to reduce the delay ntroduced by retransmssons whle keepng the same level of network effcency The acheved performance s evaluated n the scenaro of a dstrbuted data acquston and control system 1 Introducton Wreless networks present many advantages over ther wred counterparts derved from the freedom from cables, such as the reducton of costs wth nfrastructure, qucker nstallaton and ncreased moblty On the other sde, these networks are subject to hgher and more varable channel error rates Several schedulng mechansms have been proposed n the lterature for use n wred networks, such as the Weghted Far Queueng (WFQ), sutable for handlng asynchronous traffc, and the Earlest Due Date (EDD), for real-tme traffc [1] However, they are not approprate for wreless networks because of the assumpton of an error free channel Moreover, the access pont does not know when a packet arrves to the queue of a remote staton, so t cannot rely on the packet taggng approach used by conventonal mechansms A practcal mplementaton of a schedulng mechansm should address these ssues, as well as some characterstcs of the wreless network, such as ts medum access control (MAC) and error control (EC) functons Many papers that deal wth schedulng n wreless networks and consder the channel errors have been publshed recently [] [3], but ther man concern s the far sharng of the avalable bandwdth among asynchronous traffc flows In ths sense, the emphass s mostly gven to the throughput nstead of the delay requrements of the flows Regardng to HIPERLAN/ systems, L et al presented a MAC performance analyss n [4], but t consders only one connecton Kadelka et al [5] presented results for a scenaro wth multple termnals; however, the error control mechansm was neglected Doufex et al [6] presented smlar results concernng the MAC layer performance These papers make smplfyng assumptons n relaton to the schedulng of data, resource request (RR) and ARQ messages Besdes, the results presented for the MAC layer are restrcted to the aggregate throughput of all connectons In [7], we proposed schedulng mechansms desgned to provde the effcent transport of real-tme traffc subject to channel errors n IEEE 8011 networks In ths paper, we make a smlar proposal concernng HIPERLAN/ systems The acheved results are provded per ndvdual connecton and address partcularly the delay performance for tme crtcal applcatons Next secton presents an overvew of the HIPERLAN/ standard; Secton 3 descrbes the proposed schedulng mechansms, whle secton 4 presents smulaton results obtaned wth these mechansms Fnally, the conclusons are presented on secton 5

2 Overvew of HIPERLAN/ HIPERLAN/ [8] s a standard for hgh-speed rado communcaton proposed by ETSI (European Telecommuncatons Standard Insttute) to connect portable devces to broadband networks based on ATM, IP and other technologes It operates n the 5 GHz band usng Orthogonal Frequency Dvson Multplexng (OFDM) and provdes an extensve QoS support Seven transmsson modes are defned, rangng from 6 Mbt/s to 54 Mbt/s The MAC protocol of HIPERLAN/ [9] s a demand based dynamc TDMA/TDD mechansm whch works under the coordnaton of the access pont (AP) The basc MAC frame format s dsplayed on Fgure 1 The MAC frame duraton s fxed ( ms), but the duraton of each phase depends on the demand BCH Broadcast Phase FCH A C H Downlnk Phase Uplnk Phase Random Access Phase Fgure 1 HIPERLAN/ MAC frame structure In order to schedule the transmssons over the wreless channel, the access pont (AP) needs to know the state of ts own buffers and the buffers of the moble termnals (MT) The termnals report ther buffer states usng Resource Request (RR) messages, ether on the uplnk phase or on the random access phase The MTs can also negotate wth the AP for a perodc allocaton of resources by means of the Fxed Capacty Agreement (FCA) procedure The resource allocaton nformaton s conveyed by Resource Grant (RG) messages nserted on the Frame control CHannel (FCH) The Broadcast control CHannel (BCH) s used by the AP to broadcast basc cell nformaton, whle the Access feedback CHannel (ACH) provdes feedback on the random access attempts made by the MTs on the prevous frame The downlnk and uplnk phases are composed of PDU trans, whch consst of a sequence of transport channels The Long transport CHannel (LCH) (54 bytes, wth 48 bytes of payload) s prmarly used to carry data messages, whle the Short transport CHannel (SCH) (9 bytes) conveys control messages, lke RR and ARQ messages Fnally, the random access phase s composed of a number of Random access CHannel (RCH) slots BCH, FCH, ACH and RCH PDUs are transmtted at the mnmum rate (6 Mbt/s) The transmsson mode used wth LCH and SCH PDUs can vary On top of the C layer s the Convergence Layer (CL), whch provdes functons as segmentaton and reassembly (SAR) and header translaton to the hgher layers Two types of CL are currently defned: a cell based one, whch s used wth ATM networks, and a packet based one, used wth IP networks 3 Proposed Mechansms 31 Typcal Retransmsson Process Before we descrbe the proposed schedulng mechansms, we present, n Fgure, an example of a typcal packet retransmsson process n HIPERLAN/ At frame x, 3 LCH channels (requested prevously by the MT) are used to carry data packets n the uplnk phase () for a partcular connecton, but packets and are receved wth errors by the AP Frame x Frame y Frame z LCHs: 3 RG LCHs: 0 SCHs: 1 LCHs: HIPERLAN/ frame perod: ms ARQ message Btmap: "010" (SCH channel) 1 RR LCHs: Fgure Typcal packet retransmsson process At frame y the AP sends an ARQ message to the MT whch performs the postve acknowledgement of packet 1 and the negatve acknowledgement of packets and In the same frame (or later), the MT requests LCH channels durng the random access phase, usng a RR message, to be able to make the retransmsson the corrupted packets As the RR message can be lost by collson, further request attempts may have to be made n the followng frames After the AP successfully receves the RR message, t allocates LCH channels at the frame z, where the lost packets are fnally retransmtted If a retransmsson fals the whole process has to be repeated agan Each retransmsson attempt ncreases the packet delay by a mnmum of frames (when y=x1 and z=y1) Ths addtonal delay can n fact be much larger, dependng on the delay between the recepton of

3 the corrupted packet and the transmsson of the ARQ message by the AP, as well as the number of collsons suffered by the RR message (whch s nfluenced by the traffc load on the random access phase) 3 Fast Retransmsson Mechansm Fgure 3 presents an example of the operaton of the fast retransmsson schedulng mechansm Frame x s dentcal to the one used on Fgure Frame x Frame x1 LCHs: 3 RG LCHs: 0 SCHs: 1 LCHs: HIPERLAN/ frame perod: ms ARQ message Btmap: "010" (SCH channel) 1 Fgure 3 Fast retransmsson schedulng mechansm Accordng to the proposed mechansm, when the AP doesn t receve a vald packet at a granted LCH channel, t schedules mmedately the transmsson of an ARQ message n the followng frame In addton, the AP grants the addtonal resources necessary for the retransmsson of the corrupted packets automatcally Thus, at the next frame (x1), the AP sends an ARQ message whch allows the dentfcaton of the corrupted packets by the MT As the uplnk phase s located after the downlnk phase on the HIPERLAN/ frame, the AP can grant the LCH channels requred for retransmsson n the same frame Ths mechansm enables the reducton of the retransmsson delay to just one frame, nstead of two or more frames as n the typcal process In addton, the mechansm reduces the contenton durng the random access phase by avodng the transmsson of RR messages 33 Adaptve Rate Scheme We also propose a smple rate adaptaton scheme whch ams to reduce the delay generated by the retransmsson process wthout compromsng the effcency The ratonale s that a lower transmsson mode s more robust aganst channel errors [5] allowng the reducton of the number of retransmssons and consequently, of the delay varaton However, the lower mode s neffcent when the channel condtons are good Accordng wth ths scheme, the orgnal transmsson of each packet uses the nomnal transmsson rate Whenever a packet s corrupted, ts retransmssons are made at the lowest transmsson mode, n order to ncrease the probablty of a successful recepton 4 Smulaton Model and Results In order to evaluate the performance of the proposed mechansms, we developed a detaled model of the HIPERLAN/ network based on the standard Ths model was mplemented usng OMNET [10], an open-source dscrete event smulaton tool Results were obtaned from steady-state smulatons The evaluaton scenaro conssts of 30 termnals whch transmt data to an access pont Each termnal generates CBR (Constant Bt Rate) traffc relatve to one ATM (Asynchronous Transfer Mode) connecton at 7067 kbt/s, whch corresponds to a cell nterarrval tme of 6 ms The applcaton envsoned wth ths scenaro s to provde wreless access to an ATM based data acquston and control system [11] Table 1 presents the values of the confgurable parameters of the network that were used n the smulatons For the fxed parameters, please refer to the standards Table 1 HIPERLAN/ parameters LCH transmsson rate 18 Mbt/s SCH transmsson rate 6 Mbt/s Uplnk / RCH guard tme µs Uplnk / RCH preamble 16 µs Turn around tme 6 µs Wndow sze 51 The LCH transport channels requred for error free transmssons were reserved usng the Fxed Capacty Agreement (FCA) procedure Addtonal channels requred for retransmssons are granted by the proposed schedulng mechansm In order to mnmze the jtter, one LCH channel s granted for each connecton every one out of 3 HIPERLAN/ frames, e, 6 ms In order to dstrbute the load among the frames, 10 out of the 30 connectons are served on each frame The followng results are relatve to the frst connecton The other 9 connectons present smlar results Fgure 5 dsplays the complementary cumulatve densty functon (CCDF) of the delay for dfferent values of the bt error rate (BER) seen by the C layer, wth constant BER When BER = 0, the delay varaton s almost zero because the cell nterarrval tme s equal to the FCA nterval and there

4 are no retransmssons In ths case, the measured channel utlzaton regardng the 30 connectons s 69 % Wth channel errors, t can be seen that the proposed schedulng mechansm forces the retransmssons to occur on consecutve frames, helpng to keep the delay wthn acceptable levels when the BER s moderate However, wth BER = 10-3, the number of retransmssons s sgnfcant, mpactng not only the delay, but also the channel utlzaton, whch rses by 803 % when compared to the error free case, whereas wth BER = 10-4 t rses only by 48 % Fgure 5 CCDF of the delay wth the fast schedulng mechansm and constant or varable BER Fgure 4 CCDF of the delay wth the fast schedulng mechansm and constant BER Gven the varyng nature of the wreless channel, more realstc results can be acheved wth the Glbert- Ellot (GE) model The parameters used n ths paper for ths model are: BER bad = 10-3, BER good = 0, T bad = 33ms and T good = 100 ms Fgure 5 compares the delay performance obtaned usng a constant BER of 10-3, the Glbert-Ellot model, and a constant BER of 48x10-4, whch corresponds to the average BER of the GE model The results show that the delay varaton wth the GE model s larger than that of a model wth a constant BER, for the same average BER Fgure 6 presents the delay performance acheved wth the adaptve rate scheme usng the Glbert-Ellot model, and compares t wth the use of a fxed transmsson rate The use of mode 4 (18 Mbt/s) all the tme results n the largest delay, whle the use of mode 1 (6 Mbt/s) results n the lowest delay However, the channel utlzaton s sgnfcantly hgher n ths case (51 %, whereas wth mode 4 s 315 %) On the other sde, the delay wth the proposed adaptve rate scheme s slghtly larger than wth mode 1, but the channel utlzaton s much smaller (3 %) Fgure 6 CCDF of the delay wth and wthout the adaptve rate scheme and varable BER 5 Conclusons In ths paper we proposed a fast retransmsson schedulng mechansm for relable and tmely delvery of real-tme traffc n HIPERLAN/ systems Ths mechansm schedules not only the data packets but also the control packets, takng nto account the characterstcs of the MAC and error control functons of the standard Wth ths mechansm, retransmssons occur on consecutve frames Together wth the proposed adaptve rate scheme, t helps to mantan the delay under control, allowng the effectve use of the HIPERLAN/ ARQ scheme wth real-tme traffc References [1] S Keshav, An Engneerng Approach to Computer Networkng, Addson-Wesley, 1999

5 [] S Lu et al, Far Schedulng n Wreless Packet Networks, IEEE/ACM Transactons on Networkng, vol 7, no 4, pp , 1999 [3] P Ln et al, A Wreless Far Schedulng Algorthm for Error-Prone Wreless Channels, ACM Internatonal Workshop on Wreless Moble Multmeda WoWMoM 000, Boston, USA, pp 11-0, August 000 [4] H L, G Malmgren and M Paul, Performance Comparson of the Rado Lnk Protocols of IEEE8011a and HIPERLAN/, IEEE Vehcular Technology Conference VTC000, Boston, USA, September 000 [5] A Kadelka, A Hettch, and S Dck, Performance Evaluaton of the MAC protocol of the ETSI BRAN HIPERLAN/ standard, European Wreless'99, Munch, Germany, pp , October 1999 [6] A Doufex et al, A Comparson of the HIPERLAN/ and IEEE 8011a Wreless LAN Standards, IEEE Communcatons Magazne, pp , May 00 [7] J A Afonso and J E Neves, Schedulng of real-tme traffc n IEEE 8011 networks, European Conference on the Use of Modern Informaton and Communcaton Technologes, Ghent, Belgum, pp , Aprl 004 [8] ETSI TR V111, Broadband Rado Access Networks (BRAN); HIPERLAN Type ; System Overvew, 000 [9] ETSI TS V11, Broadband Rado Access Networks (BRAN); HIPERLAN Type ; Lnk Control (C) Layer; Part 1: Basc Transport Functons, 000 [10] A Varga, OMNET Dscrete Event Smulaton System, User Manual, Techncal Unversty of Budapest, Department of Telecommuncatons, 000 Avalable at [11] J E, Neves, Modular Archtecture for Hgh Flexblty ATM Based Control System, Asa-Pacfc Conference on Communcatons APCC 99, Bejng, Chna, 1999