SLOTTED-RING LOCAL AREA NETWORKS WITH MULTIPLE PRIORITY STATIONS. Hewlett-Packard Company East Mission Avenue. Bogazici University

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1 SLOTTED-RING LOCAL AREA NETWORKS WITH MULTIPLE PRIORITY STATIONS Sanuj V. Sarin 1, Hakan Delic 2 and Jung H. Kim 3 1 Hewlett-Packard Company East Mission Avenue Spokane, Wasington 99109, USA 2 Signal and Image Processing Laboratory (BUSIM) Department of Electrical and Electronics Engineering Bogazici University Bebek Istanbul, Turkey 3 Center for Advanced Computer Studies University of Soutwestern Louisiana Lafayette, Louisiana 70504, USA Abstract Tree new ring access protocols, including a novel preemption sceme based on priority sceduling wic we call \soft preemption", are presented for slotted-ring networks were stations generate multiple-priority messages. Te rst of te two nonpreemptive protocols always empties te igpriority queue before releasing low-priority packets into te ring, wereas te second one assumes a desirable, but not strict, delay deadline on te low-priority waiting times. Te preemptive ring access protocol follows preemption rules based on delay deadlines imposed on te waiting times of bot priority classes. We investigate te expected delays of all te protocols via simulations and present a grapical comparison, igligting improvements in te waiting times of ig- and low-priority packets.

2 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 2 1 Introduction Slotted-ring protocols ave emerged wit te introduction of te Cambridge Ring in early 1980's [10], and gained furter recognition wit te Orwell access protocol developed by te Britis Telecom Laboratories for voice and ig-speed data integration [7]. Te generic structure of a slotted-ring network is as follows: A number of stations are connected to te ring, were several xed-lengt slots are available for te unidirectional transmission of messages around a circular topology. Eac station sends a message by writing its data into a free slot. Te availability of multiple slots reduces te queueing delays as compared to te token-ring alternative, and adds robustness to te network. In te presently considered large-scale systems, were voice and data can be integrated as ig- and low-priority respectively, it is desirable to develop ring networks were stations can generate multiple-priority messages. Tese priority assigments can be arranged to allow more urgent messages to be delivered faster under eavy trac load conditions. Anoter application is ierarcical organizations suc as te military were certain command posts must simply ave iger priority in secure and fast communications. In [13], a slotted-ring local area network (LAN) was proposed for two levels of message priorities, but te stations were replaced by two substations, eac andling a dierent class of jobs. Wile suc an assumption is valid for analysis purposes, it takes away te possibility of developing sopisticated processing algoritms tat migt perform better by sceduling alternately between te competing queues. Te nonpreemptive network model as found its usefulness in applications were te cost of resuming interrupted processes is ig. Due to te upper bounds on te performance of te nonpreemptive discipline, priority jumps metod [12] and preemption scemes ave been proposed for LANs [2], but tese are rigid and insensitive to canging load conditions of dierent priority tracs. In particular, te discretion rules in [2] are very inexible since every packet entering te system is assessed for a predened set of deadlines wic can not be altered. Witin te class of nonpreemptive ring networks, tose tat employ spatial reuse ave presented temselves as attractive designs for ig-speed LANs [3], [7]. Spatial reuse, based on te destination release principle, allows for multiple simultaneous transmissions by te same station and ence enables ig gains in trougput. In fact, tere ave been proposals to implement tis advantage in some existing ig-speed LAN standards [11], [14]. In tis paper, we propose tree ring access protocols (RAPs) for slotted-ring local area networks were eac station maintains two separate queues for te two priority classes. Te nonpreemptive protocols follow te condition tat once a packet is put on a slot, its transmission cannot be interrupted regardless of te message's priority. On te oter and in te preemptive protocol, a low-priority message may be interrupted by a ig-priority job if certain preemption rules are satised at te sceduling instant. Bot nonpreemptive protocols assure tat te low-priority packets get some sare of te slots even in te presence of eavy ig-priority trac conditions; but one protocol provides faster processing for low-priority messages. Te policies in te soft preemptive RAP are based on delay deadlines (imposed on te waiting times) wic can be slided on a time scale from zero to innity. Tey are refresed at regular intervals by a network management station after sensing te trac load conditions in te cannel. Te protocol as te exibility of going from nonpreemptive to

3 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 3 preemptive modes wit various degrees of intermediate preemption stages wit variation in delay deadlines; ence te name soft preemption. It may not be feasible for te preemptive networks proposed in te past [13] to be dynamic to suc an extent. Furtermore, te sceduling algoritm embedded into tis protocol ensures service to low-priority packet witin tolerable waiting time delays. Te organization of te paper is as follows: Section 2 presents te system model for te network under consideration in tis paper. Sections 3 and 4 introduce te nonpreemptive and preemptive access protocols, respectively. Section 5 addresses te stability issue, and Section 6 provides simulation results tat demonstrate te performance advantages of te proposed protocols in terms of average waiting times. Finally Section 7 as te conclusions. 2 System Model Te system model assumes tat stations are connected to a ring were packets ow unidirectionally from one station to te next. A preassigned station known as te monitor initializes te ring to contain a xed number of bits, and ensures tat te number of bits circulating te ring stays constant, irrespective of te number of stations on te ring [9]. Eac packet, made up of a set number of bits, occupies one constant-lengt slot, and includes te F/E (full/empty), source, destination, message, and te acknowledgement bits. Figure 1 sows te format of a typical nonpreemptive slot. F/E SOURCE DEST DATA ACK Figure 1: Slot for nonpreemptive RAPs. Te slot structure diers by an additional bit denoting te priority status of te data (H/L), in te case of preemptive sceduling as sown in Figure 2. F/E H/L SOURCE DEST DATA ACK Figure 2: Slot for preemptive RAPs. Tere are N mutually independent slots available witin te network and it is assumed tat tere is no delay between te arrival of two consecutive slots. In te beginning, all slots are marked empty wen te monitor initializes te ring. Once a station detects an empty slot, it marks F/E bit as full, and proceeds to write te source address, destination address and data information into te slot. Te slot containing tis packet ten travels around te pysical ring from one station to anoter until it reaces its destination. Te destination station, upon reception, reads te data, sets te acknowledge bits and lets te slot continue back to te source witout writing its own

4 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 4 data. Since tere are constant number of slots on te ring, source station identies te slot used for previous packet transmission by counting te number of slots passed by. Te source station reads te source address and te ACK bit from te eader of te slot to conrm correct reception of te last transmitted packet. Te source station is allowed to reuse te same slot until te information transfer is complete. However, stations are not allowed to occupy more tan one slot at any given time. Tis is necessary to avoid ogging of slots by a single station and providing additional robustness to te protocol. Te total ring delay time is R. Eac station as te capability of generating ig-priority packets. Tat is, we do not categorize users into a specic priority class, but rater, suppose tat eac user can transmit multiplepriority messages. Stations maintain two separate queues, one for low- and one for ig-priority packets, bot wit innite-size buers. Hig- and low-priority data arrive according to Poisson processes in all stations, wit cumulative rates and l, respectively. Te two data streams are statistically independent, as are te M stations. Typically, one can expect < l, as urgent messages occur less frequently tan regular messages. In general, tere are fewer slots tan stations in a network; tat is N < M. If M = N, ten te ring network is identical to a system wic contains one server per station. 3 Nonpreemptive Protocols Te most direct approac to scedule packets is to provide te ig-priority data wit straigt access to te ring as soon as a free slot becomes available. In te sequel, we will not use an index to specify te stations, but te results are valid for any station due to identical statistical assumptions across all users. Let ft n g be te sequence of consecutive time instants wen a slot becomes available for some station. Let W and W l denote te waiting times of te oldest stored ig-priority and low-priority message at time t n, respectively. If tere are no stored ig-priority (low-priority) messages at t n, ten W (W l ) is te time interval between t n and te generation instant of te ig-priority (low-priority) message tat was transmitted most recently before t n, and it is negative. Ten, te simple RAP1 can be formulated as follows: i) If at some t n, W 0, ten te corresponding ig-priority packet in te station is transmitted wit probability one, regardless of te value of W l. ii) If at some t n, W < 0 and W l 0, ten te corresponding low-priority packet is transmitted wit probability one. In oter words, wen RAP1 is in eect, a low-priority packet can not be transmitted until te ig-priority queue is emptied. Tus, RAP1 ignores te waiting times of te low-priority packets, and low-priority messages are delayed signicantly since tey always yield to ig-priority messages. However, compared to preemptive protocols discussed in [2], low-priority messages are still at an advantage in terms of average delays. Suppose now tat it is desirable tat te low-priority messages be sceduled for transmission troug te ring wen a certain time period as elapsed since te time of teir arrival into te queue. We call tis maximum desirable waiting time as te target delay deadline (TDD). End

5 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 5 of TDD does not guarantee immediate service for a low-priority packet because te slot vacation times are random variables, and terefore we consider tis as a rater loose, nonstrict deadline. However it does te serve te purpose of assuring slot sare for low-priority packets wit nonzero probability. Te imposition of te target deadline gives rise to an alternative nonpreemptive protocol, RAP2, tat employs a sceduling policy similar to te one proposed in [5]: i) If at some t n, W l TDD, ten te corresponding low-priority packet in te station is selected for transmission wit probability one, regardless of te value of W. Tis is necessary in order to meet te delay requirement imposed on te low-priority message wit reasonably ig probability. ii) If at some t n, W 0 and W l TDD, ten te corresponding ig-priority packet is selected for transmission wit probability one. iii) If at some t n, W < 0 and W l 0, ten te corresponding low-priority packet is selected for transmission wit probability one. Te protocol described above olds onto te low-priority packets until teir processing becomes inevitable if te delay deadline TDD is to be met. Wile setting a lower value for TDD would increase te probability tat low-priority packets are processed sooner, ig-priority delays would suer consequently. Still, waiting times of ig-priority trac at very low arrival rates sould be insensitive to te variations in te low-priority critical deadline. Comparing RAP1 and RAP2, one does anticipate certain dierences in delay performance. Because of te transmission privilege tey possess, ig-priority packets are expected to attain lower expected delay in RAP1. In contrast, low-priority packets are occasionally permitted to go aead of ig-priority packets in RAP2, and ence, teir expected delay performance will be better at te expense of sligtly increased ig-priority delay. For eac station, at te end of a busy period, te slot becomes unavailable for a random period of time, rater tan going back into service immediately (for te same station). Hence, te systems tat consist of te slotted-ring network wit nonpreemptive RAPs can be modeled as an M/D/1 queue wit vacations [1]. Te expected waiting time equations can be derived directly for RAP1 [6]. But te delay analysis of RAP2 is considerably more dicult because te processing decisions depend te on waiting times. Bounds on te expected delays can be derived using te regenerative teorem [8], but conditional delay distributions are required for precise analysis. Details are omitted for brevity in tis paper. 4 Soft Preemption Nonpreemptive scemes, wile avoiding retransmission costs, restrict possible inprovements in ig-priority delay performance. In tis section we propose a preemptive RAP protocol wic possesses an additional control variable along wit TDD. Suppose tat te urgency of te ig-priority packets can be quantied in terms of teir waiting times. Specically, let te critical preemptive deadline (CPD) be te maximum delay period of te leading ig-priority packet at te queue, up to wic a low-priority packet transmission on te ring will not be preempted. Once te CPD period expires, te ig-priority packet at te station preempts te rst low-priority packet in te slot passing by. Te source station of a preempted

6 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 6 packet can detect te preemption troug te status of te H/L bit, and subsequently retransmit te packet. Te dynamic variables, TDD and CPD, serve as a andle to alter service discipline properties based on network load conditions and performance requirements. At te end of CPD, service to a ig-priority packet is guaranteed wit probability very close to one since vacation times of te slots are constrained by te ig-priority load on te network only. Te soft preemptive RAP (RAP3) consists of te following rules [15]: i) If at some t n, 0 W < CPD and W l T DD, ten te low-priority packet is selected for transmission wit probability one. ii) If at some t n, 0 W < CPD and W l < TDD, ten te ig-priority packet is selected for transmission wit probability one witout preemption on te ring. iii) If at some t n, W < 0 and W l 0, ten te low-priority packet is selected for transmission wit probability one. iv) If at some t n, W CPD, ten te corresponding ig priority packet is selected for transmission wit probability one and it is allowed to preempt te rst low-priority packet passing by on te ring. Wen W 2 [0; CPD), RAP3 is similar to RAP2. Tis ensures sucient slot sare for lowpriority packets so tat teir response time is kept witin tolerable limits. Low-priority packets are never allowed to preempt any slot on te ring but ig-priority packets now acquire te autority to preempt in accordance wit te above rules. It is also possible to ave predened ig-priority performance wit respect to low-priority waiting times by sliding CPD towards or away from te origin. Te two dimensional control to acieve user-dened ig- and low-priority performance is an excellent feature of RAP3. 5 Stability Because a station can old onto a slot until it downloads its buer, te unavailability of an empty slot at times does not aect te trougput. In te presence of a packet, none of te protocols remain idle. Moreover, wen packets from bot classes are present, prioritization as no eect on trougput eiter. Hence, RAP1 and RAP2 bot maximize te ring utilization [4]. RAP3, on te oter and, imposes additional overead due to preemption and ence is less ecient. Te sucient condition for te stability of sceduling protocols suc as RAP1 and RAP2 were investigated in [5] using te teory on te ergodicity of two-dimensional discrete-time discretespace Markov cains (for renewal arrival processes), and we do not include it ere. Te necessary condition for te stability of te networks is as follows: ( + l ) R N < 1; were R is te service time. An unstable station will drive up te delays by olding onto a slot for a very long time. But tere will be N? 1 slots available for te remaining M? 1 stations and te system will continue to function. Wen te number of unstable stations reaces N, network-wide instability occurs. Protocols tat allow stations to use multiple slots (spatial reuse) are not as robust.

7 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 7 6 Simulations For simulating te protocols, te system model was coded in `C' on a Sun SparcStation ,000 iterations were performed for eac set of low- and ig-priority arrival rates. Average waiting times for bot low-and ig-priority packets were calculated assuming four slots on te ring. Total number of stations present on te ring were equal to ten (M = 10 and N = 4). Simulation results depicting performance as a function of arrival rates are furnised in Figures 4 troug 8. RAP2 simulations were run for TDD = 8, and RAP3 experiments also assumed a value of CPD = 4. Wen bot TDD and CPD were made innite, RAP3 obtained te same performance as RAP1. Comparing te results, it can be noted tat average low-priority waiting times are uniformly te igest for RAP3, despite te target delay deadline set on te low-priority packets. Tis was to be expected since TDD is really a secondary stipulation of te protocol. Te low-priority performance sows an improvement wen te priority queues are competitively sceduled in a nonpreemptive mode. At very low arrival rates of low-priority packets ( l 0:2), te RAP2 yields a tolerable deterioration in ig-priority delay performance wit a noticeable decrease in average low-priority waiting times. However, for iger arrival rates, te RAP2 switces te roles of te two priority classes wit regard to teir average delays, and ence becomes useless. Te latter eect is a result of coosing a very low TDD value. As Figure 3 indicates, RAP2 does yield more controlled delays for bot priority classes wen TDD is selected appropriately ig. For instance, for a coice of TDD = 40 at = 0:3 and l = 0:4, we can obtain bounded average low-priority delays wile keeping te average ig-priority delays muc lower. As can be seen in Figure 8, for moderate TDD values and low ig-priority arrival rates, low-priority packets acieve marginal delay gains by using RAP2 over RAP1, because wen te network load is low, te TDD parameter goes underutilized. For iger arrival rates, low-priority average delay does go down signicantly wen RAP2 is used. RAP3 is very eective in reducing ig-priority waiting times for ig at te expense of signicant rise in low-priority delays. Also, ig-priority delay performance is virtually insensitive to te variations in TDD wen is so ig tat low-priority packets are often preempted. It is of great interest to see tat average ig-priority delay is uniformly bounded for all arrival rates tat fall witin te stability region. Tis suggests tat RAP3 enables eective control of ig-priority performance. Figure 9 quanties te level of control provided by CPD for TDD = 16 slots and l = 0:1. Dramatic reduction in ig-priority delays is possible under loaded network conditions by increasing te CPD parameter. Te average ig-priority delays are lower for RAP1 (compared to RAP3) at very low arrival rates, due to infrequent preemptions in RAP3. Ironically, te RAP1 average low-priority delays are also lower for exactly te same reason.

8 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations 8 15 Average waiting time in slots b a TDD Figure 3: Average waiting times of RAP2 as a function of TDD, for = 0:3 and l = 0:4 (a: ig-priority; b: low-priority). 7 Discussion and Conclusion Sliding te values of TDD and CPD on te time axis, as a function of te varying load rates, dictates te ig- and low-priority waiting times. We can interface RAP3 to a network manager wic, in turn, can slide tese deadlines according to user-dened requirements of multiple-priority packet transmission. Te ring networks proposed in tis paper ave several more advantages in addition to teir suitability for voice/data integrated applications. Tey are simple and deterministic in teir operations. Moreover, low-priority messages are guaranteed processing even wen te ig-priority messages represent a large volume, unlike te algoritm in [13]. In te absence of a network manager or management agent, te coice between te tree protocols largely depends on te respective trac loads from te two priority classes, as well as te urgency of te ig-priority messages. If te low-priority arrival rate is muc iger tan te ig-priority arrival rate, and if te ig-priority packets can tolerate a certain waiting time delay, ten RAP2 is desirable to keep te low-priority delays manageable. On te oter and, if te ig-priority arrival rate is ig and tey need to be transmitted as fast as possible, RAP1 is more appropriate. Tat RAP3 can perform as RAP1 and RAP2 under dierent values of its deadline may suggest tis protocol as a compromise, owever one as to be alert wit te ig delays for low-priority packets. RAP3 is also suitable for designing ring networks were users branc into more tan two priority classes. Tat is, between M stations, it is possible to impose delay constraints suc tat D 1 > D 2 > : : : > D M were D i is te CPD for station i; i = 1; :::; M. Suc a system is suitable for ierarcical network structures, were users do assume a normal mode (low-priority) wen tere is no emergency. Future researc will investigate protocols tat allow for multiple slots per station at any time, were networks wit asymmetric loads can be accommodated more fairly at te expense of system robustness. Hybrid protocols, utilizing RAP2 and RAP3 depending on te network load, can also be devised to improve performance. Finally, te exact dependence of te protocols' performance on TDD and CPD needs to be

9 Sarin, Delic and Kim: Slotted-ring local area networks wit multiple priority stations Low priority waiting time Hig priority waiting time (RAP1) 80 Average waiting time in slots RAP1 RAP2 RAP Average Low priority arrival rate ( λ l ) 0.50 waiting time in slots 1.00 (λ = 0.4) l (λ l= 0.2) Figure 4: Performance of RAP Hig priority arrival rate ( ) Low priority waiting time Hig priority waiting time (RAP2) Figure 7: Waiting time comparison of ig priority packets. Average waiting time in slots Low priority arrival rate ( λ l ) 100 Figure 5: Performance of RAP2. (λ l= 0.4) RAP1 RAP2 RAP Average 100 Low priority waiting time waiting Hig priority waiting time (RAP3) time in slots 1.00 (λ l= 0.2) Average waiting time in slots Hig priority arrival rate ( ) λ 0.50 Figure 8: Waiting time comparison of low priority packets. Low priority arrival rate ( λ l ) Figure 6: Performance of RAP3.

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