Performance Analysis of a Managed Bandwidth Service for ATM Networks

Transcription

1 Performance Analyss of a Managed Bandwdth Servce for ATM Networks Chrstos Bouras Chryss Chantz Vaggels Kapoulas Alexandros Panagopoulos Ioanna Sampraku Afrodte Sevast Research Academc Computer Technology Insttute, Patras, Greece, and Computer Engneerng Dept., Unv. of Patras, Greece E-mal: bouras@ct.gr Keywords: Managed Bandwdth Servce Abstract In ths paper we are consderng methods to mprove the performance of a bandwdth control scheme (.e. Managed Bandwdth Servce) for an ATM network nfrastructure (wth applcaton to the Greek Research and Technology Network - GRNET). These methods try to ncrease the effcency of the system and the utlsaton of the avalable bandwdth. More specfcally, we consder: a) a bandwdth-reszng algorthm for vrtual paths n order to (constantly) keep the allocated bandwdth very close to the bandwdth actually used, b) a smple method to estmate the effectve bandwdth for VBR paths that can be used n call admsson, and c) a sem-offlne call admsson scheme where requests are gathered and consdered for acceptance n regular ntervals (wth a further mprovement, of allowng connectons to be allocated a lttle before or after the tme ntally requested). All these methods were tested n a smulaton settng (wth the ATM-TN smulator), and results ndcate that they lead to an ncreased number of accepted requests and better network utlsaton. INTRODUCTION In the last few years, great attenton has been gven to the desgn and mplementaton of mechansms for network bandwdth management. The number of applcatons wth remarkable demands n terms of network resources s constantly rsng. In cases of nadequate resources, the servce provded does not meet the desred, promsed or expected qualty levels. Therefore, t s becomng ncreasngly vtal to ensure hgh-qualty network servces for bandwdth crtcal applcatons. At the same tme, lack or neffectve allocaton of resources results n lmtatons to the number of users that can be smultaneously served. In ths case, effcent management of bandwdth can contrbute to the mprovement of network utlsaton from an economcal pont of vew. One of the man network resources s the avalable bandwdth of the communcaton channels. In order to use the network, the user requests a vrtual connecton to be establshed between the source and destnaton node. Although the rate of nformaton flowng through such a connecton may vary n tme, the network has to guarantee that the connecton wll support at least the bt rate that was agreed upon the admsson of the requested connecton. In ths paper, we study methods to mprove the performance of a Managed Bandwdth Servce (.e. ncrease the utlsaton of the network and/or ncrease the number of accepted requests etc.). The study s based on smulaton experments on the ATM-TN smulator. The smulatons were run for the network topology of the Greek Research and Technology Network (GRNET) and the requests and traffc patterns are based on traces of actual request sequences and actual transmssons for dfferent knds of applcatons (e.g. vdeoconference, steamng vdeo etc.). We mplement and study algorthms that allow us to address both the bandwdth management and admsson control ssues (.e. whch requests to satsfy and whch ones to reject). In both cases there are some strct performance guarantees: transmsson starts at a specfc tme, ends at a specfc tme, uses a predefned amount of bandwdth, and s guaranteed to be successfully accomplshed, once admtted. Currently the Managed Bandwdth Servce (MBS) of the GRNET uses a greedy on-lne approach n servng requests for vrtual connectons wth guaranteed bandwdth. Requests are placed n advance and concern connectons that wll be ntated n the future (reservaton n advance). Requests are processed n a Frst- Come-Frst-Serve (FCFS) bass, are accepted f resources are gong to be avalable durng the requested tme perod, and are notfed mmedately. Comparng the negotaton of mmedate and advance reservaton, one of the dfferences s the specfcaton of addtonal parameters for settng up a reservaton n advance. As the resources for advance reservatons wll be for a gven tme n the future, one of the necessary parameters s the tme at whch the resources wll be needed and the duraton of the connecton whch descrbes for how long wll the reservaton be alve. Based on ths nformaton the admsson control scheme s able to recognse whether the reservaton s overlappng wth others or not. More specfcally, we assume that requests for the establshment of connectons arrve on-lne; each request specfes the source and destnaton nodes, the requested bandwdth, and the duraton. The algorthm, based on the avalablty of resources, ether rejects the request, or accepts t. In the latter case, t makes an a-pror reservaton for the establshment of the connecton, by reservng the requred bandwdth along some path between the source and the destnaton nodes for the specfed duraton. In fact, the system calculates and vrtually reserves the requred resources for the specfed tme nterval n the future, however wthout mmedately blockng them. We mplemented ths algorthm n a smulaton envronment so that t can be used as the bass for comparsons. In order to mprove the call admsson module we use the results of the smulaton to get a smplfed emprcal formula for the effectve bandwdth of VBR connectons. The admsson control module can use ths formula to better calculate the allocated bandwdth (and thus the stll avalable bandwdth). The current MBS of GRNET uses an over-estmaton for ths effectve bandwdth n order to guarantee the Qualty of Servce (QoS). However, part of ths bandwdth s wasted snce t s allocated but hardly used.

2 In order to maxmse the utlsaton of the network resources (especally bandwdth) and be able to accept more calls, we then enhance our algorthm by mplementng the bandwdth reszng mechansm descrbed n [1]. The purpose of bandwdth reszng s to make better use of trunk capacty by dynamc adjustment of allocated bandwdth for each vrtual path. We adjust the allocated bandwdth of VPs every tme a VP s establshed or termnated, accordng to some bandwdth adjustment rules. We tested the enhanced algorthm on the ATM-TN smulator to see the effectveness of the mprovements. Results ndcate that some bandwdth can be saved for use by addtonal requests. Fnally, we test varatons of the admsson control algorthm manly by lftng the requrement for on-lne decsons. Requests are examned at regular (short) tme ntervals leadng to a semofflne algorthm where notfcaton of acceptance or rejecton s delayed. Ths leads to an ncrease of the utlsaton and/or the number of the accepted requests dependng on what we are tryng to maxmse. In an attempt to further mprove the MBS, we allow the system to shft the tme that a connecton begns n order to avod overlappng and, thus, serve more connectons. PREVIOUS WORK Bandwdth allocaton has been a popular research subject durng the last years. In order to solve the bandwdth avalablty problems of today s networks, varous managed bandwdth servces, smlar to the MBS of GRNET, have been developed n several natonal research networks. One of the most known servces of ths knd has been developed by TEN-155. The TEN-155 Managed Bandwdth Servce [2], s an end-to-end servce whch enables connectvty among research projects n countres connected to TEN-155. Varous natonal net works have also developed servces for bandwdth control. E.g. the SWITCHlan develops the SWITCH Managed Bandwdth Servce ( The VCCS (Vrgna Communty College System) Intranet has adopted Polcy-Based Management that allows for central control of bandwdth management and the enforcement of QoS polces [3]. A smlar servce mplemented on an IP network, s the 'Reserved Bandwdth' servce of the vbns backbone network [4]. Ths servce allocates bandwdth to applcaton flows usng RSVP as the sgnallng protocol. Concernng the best way to mplement a bandwdth allocaton servce and flow control, a varety of related algorthms have been proposed. These can be classfed n fve levels: cell level (prortes, cells flow control), flow level (burst control, etc.), connecton level (Connecton Admsson Control CAC), Vrtual Path level (bandwdth allocaton), and network level (routng of Vrtual Paths). As far as the connecton level s concerned, [5] presents a mult-class CAC polcy for hgh-speed ATM swtches. Accordng to ths polcy, traffc s descrbed wth respect to the usage parameter control (UPC) parameters. The authors present formulas for determnng the bandwdth needed for mantanng the QoS guarantees that can be ncorporated n ther CAC for Constant Bt Rate (CBR) and Varable Bt Rate (VBR) types of servce. In [6], the authors propose three algorthms for mplementng effcent bandwdth allocaton and call admsson control for VBR servce usng UPC parameters. The frst algorthm, called TAP, takes nto account s Cell Loss Rato (CLR). The second algorthm, called MEB, s based on the dea of effectve bandwdth. The thrd algorthm, called MSM, s based on Lucent s CAC. Concernng the Vrtual Path and network levels, the authors of [1] deal wth Vrtual Path management n ATM networks and propose a Vrtual Path Bandwdth Reszng Algorthm that conducts utlsaton drven vrtual path bandwdth adjustment. Alternatvely, the algorthm proposed n [7] adjusts the bandwdth of a VP when the number of VCCs reaches a certan lmt. Fnally, the authors of [8] and [9] present the subject of Bandwdth Allocaton for Vrtual Paths (BAVP) that ams at the optmal bandwdth allocaton (capacty, servce rate) n Vrtual Paths based on the overall network status. OUR APPROACH In ths paper we mplement and test two smple, yet effectve, algorthms n order to make the approprate decsons upon the acceptance of requests, mprove the number of accepted calls, and maxmse lnk utlsaton. The frst one takes place n the call admsson phase, and the second durng the actual servce of the VP connecton. The model The ATM network, G = (V, L), L = m, consdered here conssts of a set of nodes, V, and a set of lnks, L, connectng all these nodes. A user located at one end node makes a request for a PVC orgnatng from ths node S (source), and endng at another end node D (destnaton). Each PVC request must defne the QoS characterstcs for ths PVC, the tme of creaton of ths PVC, and the duraton. We denote: S - the source of VP. D - the destnaton of VP. L - the network lnk, = 1,..,m. C - the total capacty of lnk, = 1,..,m. CBR Bw - the bandwdth of a request for CBR connecton. VBR Bw - the peak cell rate (PCR) of a request for a VBR connecton. Vp _ bw - the ntally bandwdth for VP, = 1,..,n, where n s the number of VPs, for whch reservatons have already been made. Vp _ bw _ temp - the new bandwdth that wll be for VP, durng the next perod of tme, after a bandwdth adjustment of ths VP. arr t - the arrval tme of PVC (VP) request. cr t - the creaton tme of reservaton of VP. T f - the future perod (duraton) for whch the reservaton of VP s made. C ( T f ) - the capacty of lnk n perod T f, durng whch a number of VPs have been belongng to ths lnk, = 1,..,m. P - the path of the VP connecton.

3 The CAC and Routng Algorthm Frst we mplement a method for decdng whether to admt or reject a connecton request. Users make connecton requests wth the QoS characterstcs they desre ther connecton to have. In case of a CBR connecton request, we examne whether the network can CBR guarantee an amount of Bw bandwdth, whch s the requested amount of bandwdth for the CBR connecton. In case of a VBR connecton request, at the present we only consder the Peak Cell Rate (PCR) requrement as the amount of bandwdth the network VBR must be able to guarantee to ths connecton, Bw, n order to accept the requested connecton. Fnally, n case of an acceptance the ntally allocated bandwdth wll be Vp _ bw, and ths CBR VBR wll be equal to ether Bw or Bw. In the sequel we present a more detaled descrpton the frst algorthm, whch has already been mplemented n the GRNET. Upon an arrval of a PVC request from S to D for a T f future perod, we retreve the data of the VP reservatons already made for part or the whole of that perod. We then fnd those network lnks that cannot accommodate the partcular connecton (e.g. CBR ( f Bw > C T ) for CBR connectons, or VBR ( f Bw > C T ) for VBR connectons), and consder an abstract network where these lnks are not ncluded. For the remanng network topology, we calculate values (weghts) of the lnks that represent ther workload at that perod. Then we execute the Djkstra algorthm for the current topology, startng from the source node of the requested connecton. If the destnaton node appears n the emergng routng table, the request s accepted. If not, t s rejected. In case t s accepted, the system makes a PVC reservaton for that perod, whch means that n tme t cr we wll establsh a PVPC (VP), for T f perod, whch has the same characterstcs as the accepted PVC request. The values (weghts) that wll express the workload of each lnk n the remanng topology, are used as nput to the Djkstra routng algorthm, n order to fnd the "optmal" feasble path between the source and destnaton of the requested PVC. More specfcally, whenever the Djkstra algorthm s executed n context of the decson-makng module's algorthm, the algorthm tself assgns a numercal value to every lnk of the network topology. Ths value represents the load of the lnk for the future perod T f. The computaton of the value of each lnk s based on the avalable () capacty C ( T f ) of each partcpatng lnk, and on the mnmum bandwdth a user can reserve for that perod, mn_ Vp _ bw j. The value of C ( T f ) for each lnk, s computed by removng the Vp _ bw j of every VP j n lnk, from the total capacty of ths lnk, C. To be more specfc, the value for lnk, s calculated as follows: C ( T f ) = C - Vp _ bw j, j where j ndcates all VPs for T f n lnk. Thus the value of each lnk of the remanng topology s computed as follows: lnk_value = mn_ Vp _ bw j / C ( T f ). After the executon of ths algorthm, a decson s reached, as well as a selecton of the route for the VP. The bandwdth reszng algorthm The algorthm used for bandwdth reszng s based on the algorthm presented n [1]. It controls (and modfes) the allocated bandwdth of each VP, based on measurements of the utlsaton of the VPs. Ths adjustment mechansm attempts to stablse the network utlsaton and part of the allocated bandwdth so that t s avalable for other requests. Ths leads to an ncreased number of calls that can be accepted, and ncreased bandwdth utlsaton. Ths algorthm s executed every tme there s a VP creaton cr (.e. at the t tmes of the VPs), or VP release event. We examne the utlsaton of each VP n the prevous tme nterval, and we decde whether a VP needs a bandwdth ncrease, or a bandwdth decrease. The utlsaton s every tme consdered w.r.t. the prevously bandwdth of each VP, Vp _ bw _ temp. Frst we dstngush those VPs that need a decrease. We defne low a lower threshold, thres, for VP bandwdth utlsaton. If the mean utlsaton of a VP was below ths lower threshold, ths VP wll get a decrease. Therefore, we compute a decrease step, - dec_step, for every VP that needs a decrease. The lower the utlsaton s, the bgger the decrease wll be. We also defne three other low thresholds below the predefned thres that wll be used n order to defne the approprate dec_step for each one of those VPs. These low low low thresholds are denoted as thres 1, thres 2, and thres 3, and are such that low low low low thres > thres 1 > thres 2 > thres 3 > We, then, defne the four dfferent possble values of dec_step, that correspond to VP utlsaton n one of the four possble regons that are defned between the above four thresholds. The best threshold values and the correspondng decrement steps are determned based on smulaton results and are presented n the next secton. The new decreased bandwdth that wll be for each VP s computed based on the dec_step decded for each VP, and s the followng: dec_bw = Vp _ bw _ temp dec_step * Vp _ bw _ temp, and the bandwdth whch wll be becomes Vp _ bw _ temp = dec_bw. After ths, we proceed wth those VPs that need a bandwdth ncrease n the next nterval. In computng the C for the next nterval, we use the new decreased values of those VPs. upper We defne an upper threshold, thres, for VP bandwdth utlsaton. If the mean utlsaton of a VP was above ths upper threshold, ths VP s a canddate to get a bandwdth ncrease. Therefore we compute an ncrease step for each one of the canddate VPs. The hgher the utlsaton of a VP s, the hgher prorty

4 wll ths VP get. Therefore, we sort the canddate VPs from the one wth the most urgent need for an ncrease, to the one wth the less urgent need. We adjust the bandwdth of VPs begnnng from the most urgent (hgher utlsaton) to the least urgent (lower utlsaton) one. For every VP, we frst fnd the lnk that belongs to the path of ths VP and has the mnmum C n the next nterval. If ths mnmum C cannot suffer an ncrease (mnmum C =), or the prevously allocated bandwdth to ths VP s equal to the ntally bandwdth of ths VP,.e. Vp _ bw _ temp = Vp _ bw j, then we do not resze ths VP. If nether s the case, we proceed n fndng the approprate ncrease step, nc_step for ths VP. We defne one more threshold upper above the predefned thres that wll be used n order to choose an approprate nc_step for each one of those VPs. Ths upper threshold s denoted as thres 1, and s such that upper upper thres < thres 1 < 1. We, then, defne the two dfferent possble values of nc_step, for VP utlsaton n one of the two possble regons that are defned between the above three thresholds. The best values are also determned based on smulaton results. The new ncreased bandwdth, nc_bw, wll then be computed smlar to the decreased bandwdth. However f the ncrement calculated s greater than the mnmum C, then the ncrement becomes equal to the mnmum C. Now, f the ncreased bandwdth calculated s greater that the ntally requested bandwdth, then the bandwdth, whch wll be, does not ncrease beyond the ntally requested bandwdth. The sem-offlne call admsson algorthm In the on-lne case, all decsons are taken wth no knowledge of future requests. Ths can lead to bad decsons. In order to mprove the call admsson procedure and maxmse some revenue (e.g. the utlsaton, the number of accepted calls etc.), we can postpone for a whle the procedure of decson makng, and queue the requests. Then, we can take the approprate decsons for all the queued requests takng nto account the parameters of all of them. Ths procedure takes place at regular ntervals (hence, the semofflne charactersaton). The approach used to decde whch requests to accept and whch to reject s a greedy one. We opted for smplcty because the purpose s to examne whether the ntroducton of ths sem-offlne procedure can lead to substantal ganng. In the future, a more effcent algorthm can be ntroduced to replace the greedy one. More specfcally all the queued requests are sorted n descendng order based on ther revenue. The requests are then examned n ths order, usng the prevously mentoned CAC algorthm. The tme shftng of connectons Smulaton results showed that sometmes some requests were rejected because of a small overlap wth other accepted connectons and the consequent non-avalablty of bandwdth for ths perod of tme. However, most of the tmes the requests concern events (e.g. vdeoconferences) that can be re-scheduled to start a lttle earler or a lttle later f ths overlappng s to be avoded. In order to examne whether ths re-schedulng can be benefcal, we use a smple algorthm to shft the startng tme of the requested connectons a lttle (n order to 'pack' them better). Agan, a smple approach s followed. Requests are queued and sorted as before. However, when a request gets accepted t s scheduled to start as early as possble. On the other hand, f a request cannot be accepted for the specfed startng tme, we examne whether t can be accepted ether earler or later. In case t can, t s accepted. SIMULATION STUDIES In ths secton, we provde our smulaton results to llustrate the performance gans of the varous mprovements that we have adopted for the MBS of GRNET. For our smulatons we used the ATM-TN smulator. The smulator s not capable of handlng VP constructon and destructon durng a run. Ths was essental for our study. So, we extended the smulator wth a wrapper that handled all the changes n the allocaton of VPs and used the ATM-TN smulator as a sub-routne. In addton we provded more traffc sources by mplementng more realstc traffc sources that follow patterns of real connectons (e.g. vdeoconference connectons and Vdeo-on-Demand connectons) In our smulatons we consder the exact topology of the GRNET. Requests are consdered to come at random ntervals followng the geometrc dstrbuton, and the holdng tme s also geometrcally dstrbuted. The requred bandwdth of PVCs follows traces of real requests. Calculaton of the effectve bandwdth One of the problems of the current MBS mplementaton n GRNET s that t over-estmates the effectve bandwdth for the VBR requests. Ths s done to avod the problem of gvng to these requests less QoS that requested. However, ths over-estmaton causes the servce to allocate more bandwdth than needed and thus some bandwdth s wasted. In order to mprove the call admsson control algorthm of the MBS we used the results of the smulaton to come up wth a smplfed formula for makng an estmaton of the effectve bandwdth. Ths smplfed formula s of the form: Effectve bandwdth = SCR + c (PCR-SCR) Our objectve s to calculate the value of the constant c (usng smulaton results) so that the above formula closely estmates the effectve bandwdth (n fact we want the formula over-estmate effectve bandwdth but as close as possble to the actual value). The result of the calculaton of the effectve bandwdth for varous connectons shows that dfferent connectons correspond to a dfferent constant c n the above formula. Fgure 1 shows ths constant c for varous connectons. As we can see, most of the tme the effectve bandwdth does not exceed SCR by more than 5% of the dfference between PCR and SCR. In fact, n almost 9% of the connectons the effectve bandwdth does not exceed SCR by more than 4% of the dfference between PCR and SCR. In average, the effectve bandwdth s equal to SCR plus 15% of the dfference between PCR and SCR.

5 ,7 25,6 2,5,4 (a) 15 1 Int. Alloc. Cur. Alloc. total,3 5,2,1 45 Fgure 1. Constants c for varous connectons Smulatons show that a general value of c =.3 can be safely used to calculate the effectve bandwdth (.e. t does not cause stuatons where connectons receve less QoS than guaranteed). Therefore the effectve bandwdth can de calculated as: Effectve bandwdth = SCR +.3 (PCR-SCR) The effectve bandwdth calculated ths way s less than the currently used over-estmaton, and the adaptaton of ths formula leads to an ncreased number of accepted requests and ncreased bandwdth utlsaton. Bandwdth Reszng Our next step s to study the mprovements ganed from the mplementaton of bandwdth reszng. We set the upper threshold of utlsaton equal to.8 and the lower threshold equal to.5. Through several tests, we decded to choose the above values as the base parameters n the followng analyss because they produce a hgher call acceptance rate and a hgher utlsaton, whle they do not cause any problems. such as allocatng less bandwdth than needed and causng cells to drop. Then, we select the ncrease and decrease steps as follows (BU denotes bandwdth utlsaton): If.8 < BU <=.9, the ncrease step s set to.25 If BU >.9, the ncrease step s set to.35 If.4 < BU <=.5, the decrease step s set to.2 If.3 < BU <=.4, the decrease step s set to.25 If.2 < BU <=.3, the decrease step s set to.3 If BU <=.2, the decrease step s set to.35 The above values were found to be better after varous smulatons. (b) (c) Total allocated Cur. allocated Total used Total allocated Cur. allocated Total used Fgure 2. and used bandwdth for deal traffc sources Frst we experment wth traffc sources that follow closely the parameters they advertse (.e. the traffc produced s shaped as expected). Some results of the smulatons are shown n fgure 2. Fg.2a corresponds to a 2 Mbps lnk, fg. 2b to a 4 Mbps lnk and fg 2c to a 2 Mbps lnk. In each subfgure the parameters depcted are: the bandwdth that would have been allocated wthout VPM, the currently allocated bandwdth usng VPM, and the actual bandwdth used. As expected the type of the traffc sources selected makes the actually used bandwdth to closely follow the requested one, and thus t does not allow VPM to change the ntally requested bandwdth too much durng the run. Obvously n ths case there s no gan. However, most of the traffc sources n real applcatons do not follow closely ther advertsed behavour. In order to test bandwdth (a) (c) (e) (b) (d) Fgure 3. and used bandwdth for realstc traffc sources (f)

6 reszng n these cases, we experment wth traffc sources that are more realstc and follow the patterns of observed actual transmssons. Some results of smulatons for ths case are shown n fgure 3. As above each subfgure corresponds to a dfferent lnk. We observe that, n the case of realstc traffc sources, bandwdth management can save bandwdth,.e. allocate less than requested (but more than actually needed). Ths saved bandwdth can be allocated to other requests (.e. more requests can be accepted). Notce that most of the tme the total allocated bandwdth (as decded by the VPM algorthm) s more than actually used by the realstc traffc sources. But sometmes the actual traffc gets hgher than the bandwdth allocated at that tme. However, ths does not happen often and more mportantly t causes no cell drops. As a concluson, bandwdth management can mprove the performance of MBS by makng the allocated bandwdth follow more closely the actually used bandwdth, and by ng bandwdth to be used by other requests (.e. t ncreases the number of accepted requests). Sem-onlne call admsson The way the current MBS works s that every request s examned mmedately after t s submtted. Ths usually leads to some bad decsons. However, the servce requres that reservatons are made n advance. In such a settng the on-lne requrement may be relaxed a lttle bt, snce the requested connecton does not have to be mmedately avalable. Therefore t makes sense to delay a lttle bt the decson (and the notfcaton) n order to get more requests and then decde based on ths addtonal knowledge. Based on ths observaton our next step was to study the case where requests were queued and then examned n a sem-batch mode at regular tme ntervals. The algorthm used s a greedy one and at each tme tres to accept these requests (among the queued ones) that maxmse the network utlsaton. Results from the smulatons are shown n fgure 4. The bandwdth usage s shown for two dfferent szes of the tme nterval. Also the bandwdth usage for the on-lne algorthm s shown for comparsons. Results ndcate that servng requests n batches ncreases the utlsaton of the network. Tme-shftng of connectons Our fnal step s to try to mprove even more the utlsaton of the network by allowng the shftng of the tme a connecton takes place (actually up to 1% of ts duraton). Some results for ths case are shown n fgure 5. As we can see tme-shftng seems to offer mprovement only when the overall usage of the network ncreases or decreases (e.g. durng early n the mornng or late n the afternoon of a workng day). When the usage of the network remans hgh the mprovements are not very bg. CONCLUSIONS FUTURE WORK We have presented few mprovements for the MBS of GRNET. These mprovements were studed usng smulaton and varous results were presented. Most of the results are promsng and ndcate that the mprovements are worth beng mplemented. Our future plans are: a) to further study varous other mprovements, b) to substtute the greedy algorthms used wth more effcent ones and c) to make smlar studes for networks that offer IP-based Qualty of Servce no batchng batchng(nt.2sec) batchng(nt.8sec) Fgure 4. Bandwdth usage for sem-batch request servng batchng(nt.8sec) var.tmes(b.nt.8sec) Fgure 5: Bandwdth usage when tme-shftng of connectons s allowed Acknowledgement The authors would lke to thank Gorgos Mourlas for hs valuable help n debuggng the code wrtten for the smulaton experments. REFERENCES [1] Y.-D. Ln, W.-J. Su, and C.-C. Lo, "Vrtual Path Management n ATM Networks", n Proc. of the Int. Conference on Communcatons, Dallas, USA, June 1996 [2] DANTE, "External Operatonal Procedures for the TEN-155 Managed Bandwdth Servce", 2 August 1999, QUA OPS Verson 1, [3] Roney E. Boyd Jr., "VCCS Network management Whte Paper", Vrgna Communty College System, August 1999, [4] Laura Cunnngham, "QoS Development n the vbns", n the Communcatons Magazne, Vol. 36, No. 5, May 1998, [5] G. Ramamurthy and Q. Ren, "Mult-Class Connecton Admsson Control Polcy for Hgh Speed ATM Swtches," n Proc. of the Conference on Computer Communcatons (IEEE Infocom), Kobe, Japan, Aprl 1997, pp [6] D. Wu and H. J. Chao, "Effcent Bandwdth Allocaton and Call Admsson Control for VBR Servce Usng UPC Parameters," Internatonal Journal of Communcaton Systems, Vol. 13, no. 1, February 2, pp [7] S. Ohta, and K.-I. Sato, "Dynamc Bandwdth Control of the Vrtual Path n an Asynchronous Transfer Mode Network", IEEE Transactons of Communcatons, Vol.4, No.7, July 1992, pp [8] A. Ptslldes, G. Stylanou, C. Pattchs, A. Sekercoglu, A. Vaslakos, Bandwdth Allocaton for Vrtual Paths (BAVP): Investgaton of Performance of Classcal Constraned and Genetc Algorthm Based Optmsaton Technques, INFOCOM 2, Tel-Avv, Israel, March 2, pp [9] A. Ptslldes, G. Stylanou, C. Pattchs, A. Sekercoglu, A. Vaslakos, "Investgaton of the Performance of EP-BAVP (Evolutonary Programmng for Vrtual Path Bandwdth Allocaton)", Int. Conf. On Telecommuncatons, Port Carras, Greece, 1998, pp