1th WEA International Conference on COMMUICATIO, Heraklion, reece, July 3-5, 8 Unavoidable Constraints and Collision Avoidance Techniques in Performance Evaluation of Asynchronous Transmission WDMA Protocols I.E. POUTOURAKI, D.A.PAVLIDI chool of Electrical and Computer Engineering, ational Technical University of Athens 157 73 Zografou, Athens REECE Abstract: - This study extends the analysis in [1] where the effect of the round trip propagation delay on performance measures of an Asynchronous Transmission WDMA protocol is examined for infinite population and Poisson arrivals. An other attribute, associated with the WDM networks, that affects their behaviour is the receiver collision phenomenon. Especially this paper examines the influence on throughput and total delay measures of the transmitted information as function of the propagation delay latency and the receiver collision phenomenon characteristics []. o this presentation accomplishes the analysis for a complete study and a substantial examination of the real performance behaviour of the optical Passive tar WDM networks. Key-Words: - Wavelength Division Multiplexing (WDM), Collision Avoidance Technique, Asynchronous transmission, propagation delay latency, receiver collisions. 1 Introduction WDMA protocols can be divided into two categories, a) synchronous and b) asynchronous transmission techniques. In the first category the data channels are slotted and stations are obliged to transmit at the beginning of each time interval, denoted as cycle [3]. In the second category the data channels are unslotted and there is no synchronization among the stations for data transmission [1]. The adopting procedures introduce three causes of packet loss. The first is the control packets collision on the control channel, the second the collision of data packets overlapping in time in the same data channel and the third is receiver collision. Receiver collision corresponds to the case of a successfully (re)transmitted data packet over a data channel and finds the destination station tunable receiver busy, when it arrives []. Therefore the capacity utilization is limited due to channel contentions and destination conflicts that consequently result in low throughput. In an unslotted network the packets may or may not be of the same size. Packets arrive and enter the network system without being aligned. Therefore, the packet (re) transmission could take place at any point in time. Obviously, in unslotted networks, the chance for contention is larger because the behaviour of the packets is more unpredictable and less regulated. On the other hand, unslotted networks are easier and cheaper to build, more robust, and more flexible than slotted networks. The roundtrip propagation delay plays a key role in the performance evaluation of the WDMA protocols. In optical networks, the propagation delay is higher than the data packet transmission time. The close relation and the difference between propagation delay and data packet transmission time is a very important factor in designing multiple access WDM networks [1]. In this paper, the proposed architecture is a Passive tar network and the transmission access scheme belongs to the asynchronous transmission protocols. This work can be considered as an extension of the study in [1] for infinite population where the time axis is slotted for the control channel competition while the data channel contention is based on the Unslotted Aloha scheme adopting the tell and wait asynchronous transmission policy for fixed length data packets. The proposed protocol exploits the large propagation delay as a useful attribute to develop suitable multiple access algorithms based on the knowledge about the availability of the data channels and the status of tunable receivers (idle or busy) of the end stations. In other words, the proposed access method is based on the decision of a station for transmission or not at the end of the propagation delay after a I: 179-5117 147 IB: 978-96-6766-84-8
1th WEA International Conference on COMMUICATIO, Heraklion, reece, July 3-5, 8 successful transmission on the control channel. In this way only one data packet among those that are destined to the same data channel during the critical period of time is transmitted, given that the receiver tunable receiver is idle at the time instant where the successfully transmitted data packet arrives at destination. With this update method, at any time unit the availability of data channels and the status of tunable receivers is known for any destination. o each station can schedule its (re)transmission avoiding data channel and receiver collision. The contribution of this analysis is: 1) The system model uses a simple scheduling algorithm that can be implemented in real time, ) Preserves without changing the complexity of network architecture. 3) Reduces the undesirable characteristics of the asynchronous and unslotted transmission protocols by improving the throughput performance. The paper is organized as follows: In section, we describe the network architecture model and assumptions. In section 3, we analyze the model for throughput and delay measure evaluations with and without receiver collision analysis. Also simulation analysis verifies the analytical obtained results. We present numerical results in section 4, and we make some concluding remarks in section 5. multichannel transmission system. Thus, the proposed network model can be described as a CC TT FR TR structure. It has a common control channel CC. TT means that each station has a tunable transmitter tuned at λ, λ 1,,λ Ν. The outcoming traffic from a station is connected to one input of the passive star coupler. Additionally, every station uses a fixed tuned receiver FR for the control channel and one tunable receiver to any of data channels λ 1,,λ Ν, indicated by TR. The incoming traffic to a user station is splitted into + 1 wavelength by a WDMA splitter from which one of wavelengths can be selected by the tunable receiver. The transmission time of a fixed size control packet is used as time unit (control slot) and the data packet transmission, normalized in control slot time units is L (L > 1), which is called data slot. We assume the existence of a common clock, obtained by distributing a clock to all stations. The round trip propagation time between any station to the hub of the star coupler and to any other station is assumed to be equal to R data slots (R L time units) and is the same for all the stations. Control channel is considered to be slotted with control packet transmission time. The control packet consists of the transmitter address, the receiver address, the wavelength λ k and the transmission instant t th over the control channel as it is shown in Fig.. The speed and the range of tunable transmitters set major limitations affecting WDM operation. In our study we are not taking into account these limitations assuming negligible tuning times and very large tunable bandwidths. Fig.1. Passive tar Coupler etwork Model and Assumption The system under consideration as Fig. 1 shows is a passive star network. The system uses +1 wavelengths, λ, λ 1,,λ Ν, and assumes infinite population. The system at wavelength λ operates as the control channel while the remaining Ν channels at wavelengths λ 1,,λ Ν, constitute the data.1 Transmission part We apply data packet collision avoidance algorithm as following. If a station has to send a data packet to another, first chooses randomly a wavelength on which the packet will be transmitted. Then, informs the other stations by sending a control packet on the control channel to compete according to the lotted Aloha protocol to gain access. The station continuously monitors the control channel with its fixed tuned receiver. The outcome of its control packet will be known R L time units later (acknowledgement period of time) because of the broadcast nature of the control channel. The station monitoring the control channel gives emphasis for the last L time units before the end of the acknowledgement period. After the end of this period the station is aware of the status of the data channels. Especially the station recognizes if the specified data channel on its control packet will be I: 179-5117 148 IB: 978-96-6766-84-8
1th WEA International Conference on COMMUICATIO, Heraklion, reece, July 3-5, 8 idle or busy at the end of the acknowledgement time period and on the other hand ascertains the destination station tunable receiver status. In case of idle data channel and successful (re) transmission of the corresponding control packet in conjunction with the availability of the destination tunable receiver at the end of next (R+1)L time units, the station transmits immediately the data packet without data channel collision and destination conflict. In the opposite case the station postpones the data packet transmission waiting for a number of time units according to the adopted retransmission policy.. Reception part In the receiving part, all stations continuously monitor the control channel with their fixed tuned receiver examining all control slots. At the beginning of each control slot every station knows the status of the data channels (idle or busy) and the time instant that a busy channel will be idle. When a destination station sees its address announced in a successful transmitted control packet, reads first the transmission time t th in the control packet and examines if the sender will find idle the prescribed data channel at the next time instant after the end of its acknowledgement period. Moreover the prospective receiver knows the status case of its tunable receiver (idle or busy) at the next time instant after the passage of (R+1)L time units, for the suitable decision. amely, if the receiver knows that the sender wins the right to transmit and it is ready at the subsequent (R+1)L time units, then adjusts its idle tunable receiver to the channel specified in the control packet for data packet reception. Fig.. Data and Control packets structure Finally according to collision avoidance algorithm the sending and the receiving part receive the same information simultaneously, at the end of the critical period of time, and synchronize their action for transmission or postponement of a data packet. 3 Analysis We use the following notations: = the number of data channels in the system; L = the length of data packets in control packet transmission time units; =the average number of transmitted control packets per control slot time on the control channel; C e =, the output rate of successful transmitted control packets; d = the average rate of successfully transmitted data packets through one of the data channels per data slot. According to the reference protocol in [1], the is given by equation (4) which we repeat here: d L e = 1 e (1) T = the total system throughput per data slot without the effect of receiver collision is: T = d () 3.1 Delay D= the interval between the generation time of a data packet and the time of successful reception at destination. Data packet delay D, is composed from three parts as follows: D = D w + D r + (R+1)L +1 (3) D W = the waiting time from the generation of a packet until the begin of the next control slot. D r = the delay from the transmission of a data packet until the begin of successful reception at the destination. F r = the probability of successful transmission of a data packet. From (1) we take: Fr = e 1 e ( 4) Q = the average number of trials for successful transmission of a data packet: e Q = (5) 1 e D b = the average delay between successive retransmissions given by: K + 1 D b = (6) K is a number representing time units. After a station s unsuccessful transmission, it schedules the next retransmission choosing randomly a number between 1 and K time units. o: d I: 179-5117 149 IB: 978-96-6766-84-8
1th WEA International Conference on COMMUICATIO, Heraklion, reece, July 3-5, 8 [ D] E[ D ] + E[ D ] + (R + 1)L 1 E = w r + (7) 1 e K + 1 E[ D] = + 1 RL 1 + L 1 + + (8) 1 e + (R + 1)L + 1 3. Receiver Collision In this subsection is analyzed the case of finite number, M, of stations in the system. We approximate the total input traffic rate of control packets into the data channels transmission system, by a Poisson arrival process. According to the equation (6) in [] the total network throughput, RC, conditional upon the receiver collision phenomenon is: T RC = T 1 ML (9) ubstituting T in the above equation with (), we take: [ 1 ] e RC = Le 1 e 1 e M (1) According to the Delay analysis of subsection it is taken: F = e r e 1 e 1 1 e M (11) And finally: 1 e D + e 1 e 1 1 e M K + 1 + RL + 1 + (R + 1)L + 1 [ ] E = L 1 1 (1) 3.3 Average Rejection Probability We define P rej, as the average rejection probability at destination, in steady state, the ratio of the average number of data packets rejected at destination due to active receivers, to the average number of successfully transmitted packets over the multichannel system during a data packet transmission time, then: T RC Prej = (13) T 4 umerical results The results from the analysis are illustrated in Fig. 3, 4, 5 and 6. Fig. 3 depicts the throughput D versus. Analysis without and with receiver collision and simulation for = {5, 1, } data channels, M=5 stations, L=5 time units, R=1 data slots time and K=1 time units. This figure shows how the receiver collision effect, affects the throughput performance. The curves divergence is obvious in the two cases (with and without receiver collision) as the number of channels increases. The network and the protocol behaviour are accomplished in Fig. 5. This figure shows the average rejection probability versus for = {5, 1, } data channels and M= {5, 1} stations. Rejection probability, P rej, is a very interesting magnitude and shows how this quantity changes in two cases, first as the number of stations increases and second as the number of data channels varies. The P rej reduction was expected as M increases for fixed values of and. d (packets/data slot).4.35.3.5..15.1.5 =5 =1 M=5 R=1 L=5 K=1 without receiver collisions... with receiver collisions x simulation.1..3.4.5.6.7.8.9 1 (control packets/time unit) Fig.3. Throughput d (packets/data slot) versus (control packets/time unit) without and with receiver collision analysis and simulation for = {5, 1, } data channels, M=5 station, L=5 time units, R=1 data slots time and K=1 time units. Reversely, for fixed values of M and, as increases, P rej grows up. Fig. 4 illustrates the average Delay, E[D], versus d. Analysis without and with receiver collision and simulation, for = {5, 1, } data channels, M=5 stations, R=1 data slots and K=1 time units. The unstable distinctiveness of the Aloha protocols follows the unslotted multichannel Aloha case too. I: 179-5117 15 IB: 978-96-6766-84-8
1th WEA International Conference on COMMUICATIO, Heraklion, reece, July 3-5, 8 E[D] (time units) 1 1 8 6 4 M=5 R=1 L=5 K=1 - - - - without receiver collisions. with receiver collisions simulation =1.5.1.15..5.3.35.4 d (packets/data slot) Fig.4. Average Delay E[D] (time units), versus throughput, d (packets/data slot) without and with receiver collision analysis and simulation for = {5, 1, } data channels, M=5 stations, R=1 data slots and K=1 time units. Average Rejection Probability.15.14.1.1.8.6.4. =5 M=5 - - - - M=1 =5 =1.1..3.4.5.6.7.8.9 1 (control packets/time unit) Fig.5. Average Rejection Probability versus (control packets/time unit) for = {5, 1, } data channels and M= {5, 1} stations. M axim um A verage Rejec tion P robabilities.14.1.1.8.6.4. M=5 M=1 M=15 =5 =1 Fig.6. Maximum Average Rejection Probabilities for = {5, 1, } data channels and M= {5, 1, 15} stations. evertheless the figure shows the operation range of the proposed Protocol algorithm which associates with higher throughput and lower delay. A remarkable point is the major unstable attributes when we take into account the effect of receiver collision. An other notation is that as increases the system becomes more stable. Finally, Fig. 6 presents the P rej (max) for = {5, 1, } data channels and M= {5, 1, 15} stations. The conclusions from this figure verify the ascertainments from the previous comments and explanations. 5 Conclusion This paper is an attempt to represent the basic characteristics of WDM networks, as propagation delay latency and receiver collision phenomenon and the way these parameters affect the performance measures behaviour of an Asynchronous transmission protocol. The study in [1] proposes an algorithm for exploitation of the propagation delay latency to improve the system throughput characteristics but ignores the receiver collision phenomenon. This effort, analysis and simulation, demonstrates how the conjunction of the two basic aforementioned optical network parameters, especially the receiver collision occurrences, influences the total performance characteristics of the proposed Passive tar network architecture, I: 179-5117 151 IB: 978-96-6766-84-8
1th WEA International Conference on COMMUICATIO, Heraklion, reece, July 3-5, 8 giving a complete and real treatment of our protocol consideration. References: [1]Ioannis E. Pountourakis, Propagation Delay Latency and Data Channel Collision Avoidance Protocol Journal of Information cience Vol.157, o.1-, 3, pp. 111-11. []I.Pountourakis, Performance Evaluation with Receiver Collisions Analysis in Very High- peed Optical Fiber Local Area etworks Using Passive tar Topology, IEEE J. Lightwave Technology, Vol.16, o.1, 1998, pp. 33-31. [3]I.Pountourakis, P.Baziana, Markovian Receiver Collision Analysis of high-speed multi-channel networks, Mathematical Methods in the Applied ciences J., Vol.9, o.5, 6, pp. 575-593. I: 179-5117 15 IB: 978-96-6766-84-8