All-Optical Packet-Switched Networks: A Study of Contention- Resolution Schemes in an Irregular Mesh Network with Variable-Sized Packets

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1 All-Optical Packet-Switched Networks: A Study of Contention- Resolution Schemes in an Irregular Mesh Network with Variable-Sized Packets Shun Yao a, Biswanath Mukherjee b, S. J. Ben Yoo a, and Sudhir Dixit c a Department of Electrical Engineering University of California Davis, Davis, CA b Department of Computer Science University of California Davis, Davis, CA c Nokia Research Center, 5 Wayside Road Burlington, MA ABSTRACT This paper presents a comparative study of contention-resolution schemes based on wavelength, time, and space domains in an unslotted optical packet-switched network with a large irregular mesh topology consisted of 15 nodes. For the first time, to the best of our knowledge, we investigated the effect of selective deflection and limited wavelength conversion. Features and performances of different combinational schemes are listed and compared. While simulation results show the effectiveness of wavelength conversion for resolving contentions over optical buffering and space deflection, physical explanations of the different effectiveness in resolving contentions of these schemes are also discussed. Keywords: Wavelength division multiplexing, Packet switching, Photonic switching systems, Optical delay lines 1. INTRODUCTION With the rapid growth of data traffic, packet-switched network will be more desired by service providers to meet such versatile traffic demand. Migrating the switching functionality from electronics to optics can resolve the electrical-opticalelectrical conversion bottleneck in future networks, together with the use of wavelength-division multiplexing (WDM) technology. In the foreseeable future, an all-optical packet-switched layer will offer us a finer granularity, compared with circuit-switched networks, to close the gap between the electrical (IP/MPLS) layer and the optical (WDM) backbone, and provide transparency to data-rate and data-format [1]. In a packet-switched network, contention occurs at a switching node whenever two or more packets are trying to leave the switch from the same output port, on the same wavelength. In electrical packet networks, contention is usually resolved with the store-and-forward technique, which requires the packets in contention to be stored in a memory bank and sent out at a later time when the desired output port is free. This is possible because of the availability of electronic random-access memory (RAM). There is no equivalent applicable optical RAM technology; therefore, different approaches have to be used. Meanwhile, WDM networks provide one new additional dimension, namely wavelength, for contention-resolution. In this paper, we explore three dimensions of contention-resolution schemes: wavelength [2], time [3], and space [4] in an irregular mesh network consisted of 15 nodes. Many previous works on optical packet networks employ optical delay lines to resolve contentions [3][5][10]. The deflection approach to resolve contentions has been studied in [4] for electronic networks, while the wavelength-conversion approach has also started to receive attention recently [6][11][12]. Our objective is to comprehensively study the tradeoffs in Correspondence author: Shun Yao, Tel: Fax: shyao@ece.ucdavis.edu This work is partly supported by Nokia Research Center. 1

2 performance and cost of the three approaches and find out other factors, such as network topology, that can impact the effectiveness of these approaches. In general, optical packet-switched networks can be divided into two categories: time-slotted networks with fixed-size packets [7] and unslotted networks with fixed-size or variable-size packets [8]. A fair amount of work has been conducted on slotted networks with fixed-length packets, in terms of network architecture and performance [13]. In an unslotted network, the packets are not aligned and they are switched one by one 'on the fly'. While unslotted networks generally have lower cost (no expensive synchronization stages), higher flexibility (variable packet length), and robustness (simpler controls), they have lower overall throughput than synchronous networks, because of the increased contention probability. Since today's WDM networks offer high aggregate bandwidths, we are interested in finding out the performance of unslotted networks with different contention-resolution schemes. This paper will focus on an unslotted irregular mesh network since most of the published work has been carried out on slotted networks or unslotted networks with regular topology [9]. In our previous work [14], a small network with 6 nodes and a simple topology was studied. Basic contention-resolution schemes using one of the three approaches (i.e. wavelength conversion, optical buffering and space deflection) were compared. In this work we simulated a much larger network with a number of more complicated contention-resolution schemes, such as different combinations of wavelength conversion, optical buffering and space deflection. We also investigated the impact of selective deflection and limited wavelength conversion using parametric wavelength conversion technology. The characteristics and performances of different schemes are listed and compared after the discussion of physical explanations of some of the network behaviors under these schemes. In Section II, the network topology of a fifteen-node network and node architectures for different contention-resolution schemes are described. Section III presents the simulation results of these schemes and explains the network behavior observed under these schemes. We also provide an overall comparison of all the aspects, in terms of network performance and cost, for these approaches. Section IV concludes the paper. 2. CONTENTION RESOLUTION SCHEMES In our study we employ a part of a telco's metropolitan-area network topology (Fig. 1), on which different approaches are applied. In our previous work a smaller network was studied [14], as to avoid unnecessary complexity in order to study the basic network behavior under different contention-resolution schemes. In this paper, after studying a much larger, irregular network we confirm the conclusions made in our previous work and gain more insights of the impact of different schemes with regard to a more realistic network topology Fig. 1. Topology of the network under study. In the network shown in Fig. 1, there are fifteen nodes. Each link is L km long and consists of two fibers to form a bidirectional link. Every fiber contains W wavelengths, each of which carries a data stream at data rate R. Each node is 2

3 equipped with an array of W transmitters and an array of four receivers, each operating at one of the W wavelengths independently at data rate R. We assume that packet arrivals at each node follow the Poisson process, packet lengths are exponentially distributed with average length B bits, and the intensity of traffic between all node-pairs is the same (uniformly distributed traffic matrix). By default, all packets are routed from source to destination by the shortest path. add (a) drop add (b) drop add drop add drop (c) (d) Fig. 2. Node architectures for different contention-resolution schemes: (a) buf1wav; (b) buf4wav; (c) wc; (d) wc+buf4wav. The node architectures for the three different contention-resolution schemes are shown in Fig. 2. Every node has a number of input / output fibers, depending on the nodal degree. Each switch fabric has W input / output ports connected to the local transmitters / receivers, since there are W wavelengths in each fiber. The architecture is different for each of the contentionresolution schemes under investigation. For optical buffering, the switching block has one (or more) additional port, which leads to the local optical delay line. This delay line can hold the packet for the duration of the transmission delay of integer number of average-sized packets. In our study, the delay line is equal to one average packet length. When contention occurs, the packet is switched into the delay line as it comes out of the switch fabric, and re-appears at the input of the switch after traversing the entire delay. The delay line being of one average packet-size long does not imply that longer packets cannot be buffered. In the case where a packet longer than average is being buffered, when the packet head emerges from the other end of the delay line, its 'tail' will still be passing through the input port of the switch, therefore, the stream of bits from this packet forms a loop inside the switch and delay line. We consider two cases of buffering: single-wavelength delay line and multi-wavelength delay line. In the singlewavelength case (Fig. 2(a)), the delay line can only take one packet at a time, i.e., there is no multiplexer or demultiplexer on both ends of the delay line. If the buffer is busy, the packet in contention will be dropped. In the multi-wavelength case (Fig. 2(b)), the delay line is equipped with a multiplexer and a demultiplexer on either end. It can at most accommodate W packets, each on a different wavelength. This requires the switch fabric to have more input / output ports, which can impose a considerable increase on the cost of the switch, since typically the number of switch elements inside a switch fabric grows 3

4 hyperlinearly with the number of input /output ports. The cost of the multi-wavelength buffering scheme can be much higher than that for the single-wavelength buffering. We also set a maximum hop count H to limit how many hops a packet can travel (each time the packet goes through the buffer or the packet is transmitted from one node to another, it is counted as one hop). This is similar to the time-to-live (TTL) field in the IP packet header. By doing this we take into account the physical impairment that a packet suffers whenever it travels through any physical device, such as a fiber link, switch element, a multiplexer, a coupler, etc. f f 1 f 2 f P /2 f 3 f 4 (a) f P f 1 f P1-2 /2 f 2 f P1-3 /2 f 3 f 4 f P1-2 f P1-3 (b) Fig. 3. Parametric wavelength conversion. f For wavelength conversion (Fig. 2(c)), the signal on each wavelength from the input fiber is first demultiplexed and sent into the space switch, which is capable of recognizing the contention and select a suitable wavelength converter leading to the desired output fiber. The wavelength converters can operate at full range (i.e. can convert one incoming wavelength to any of the other wavelengths) or at limited range (i.e. can convert one incoming wavelength to one or several pre-determined wavelengths). Before the packets leave the node, they are multiplexed back together. input λ 1 input λ 1 λ 2 P 1-2 P 1-3 output λ 2 output λ 3 P 2-3 P 2-4 λ 4 (a) Fig. 4. Architecture for limited wavelength conversion. (b) 4

5 The majority of wavelength conversion techniques demonstrated to date are for on single wavelength channel. Parametric wavelength conversion is a promising technique offering multi-channel wavelength conversion without measurable crosstalk [15]. The wavelength interchanging crossconnect architecture (WIXC) incorporating parametric wavelength converters have been designed to provide scalability and modularity [11]. Further, the scaling of the architecture has been demonstrated without increasing the number of wavelength converters by virtue of limited multi-channel wavelength conversion [16]. The nonlinear interaction in the device results in generation of the converted wave at a wavelength corresponding to the frequency difference between the pump and signal waves (f 2 =f p -f 1 ) [15], [16]. In Fig. 3(a), a pump laser at frequency f P is used to convert between signals f 1 and f 4, with the frequency mirror of f P (f P /2, which corresponds to half of the pump s frequency) located between f 1 and f 4. The same pump can also convert between f 2 and f 3. Fig. 3(b) shows that two pump lasers, f P1-2 and f P1-3, are used to convert between f 1 / f 2 and f 1 / f 3. Fig. 4(a) shows a limited wavelength conversion architecture where one packet can either bypass the wavelength converter (thus stay on the same wavelength), or be converted to another fixed wavelength. Fig. 4(b) shows the architecture where one packet can either stay on the same wavelength, or be converted to one of the two pre-assigned wavelengths. In both architecture 1x2 switching elements are used. In space deflection (which can work with any architecture in Fig. 2), when contention occurs, the switching node chooses the available output port (with a vacant wavelength same as the incoming packet's wavelength if no wavelength converters are used, or with another vacant wavelength if wavelength converters are used) that leads to the substitute path. If there are no such ports that the packet can be deflected to, the packet will be dropped. Worth mentioning here is that, in this network, because some nodes have many neighbors, the deflection scheme can avoid the 'blind deflections'. Deflection is only carried out in hub nodes (nodes that have much higher degrees than others and serve as major routing nodes, i.e. nodes 1, 6, 7, and 13 in Fig. 1). Because in this topology most of the routing is done by four hub nodes, it is possible for us to have a deflection policy with which a packet in contention would only be deflected to another specific node that can lead the packet to arrive at its original 'next hop' node passing no more than two extra nodes. If no such node exists, the packet will be dropped. This type of deflection requires the node to have a deflection table that contains preferred deflection ports for each destination. For example, if a packet from node 3 is destined for node 9 via node 1 and there is contention at the output port in node 1 leading to node 7, the packet will be deflected to the port leading to node 11; if the port leading to 11 is also busy, the packet will be dropped immediately instead of being deflected to node 13 or node 6. Table 1 is an example of deflection table in node 7. Although in most deflection networks certain mechanisms have to be implemented to prevent loops (a packet being sent back to a node it has trespassed before), such as setting a maximum hop count and discarding all the packets that have passed more hops than this number. However, in our study, the network topology and the above particular deflection policy can automatically eliminate looping (since the shortest path between any source-destination pairs involves no more than two hub nodes and we can make sure the deflection table will not cause any looping). Although we do set the maximum hop count to 8 in the simulation, the purpose of doing so is to limit physical impairment (signal to noise ratio degradation, accumulated crosstalk, accumulated insertion loss, etc.) of packets. Next-hop node Deflect to Drop Table 1. Deflection table for node 7. Fig. 2(d) shows the node architecture for wavelength conversion combined with multi-wavelength buffering. Note that a packet can be dropped at any node under all of these schemes due to (1) unavailability of a free wavelength, (2) unavailability of a free buffer, and / or (3) the fact that the packet may have reached its maximum hop count (under buffering and / or deflection). In addition to the contention-resolution schemes using only one of the approaches, we also simulated some combinations of these three approaches. We first define the notations for these approaches and their variations: 5

6 1. baseline: No contention-resolution is used. Packet in contention is dropped immediately. 2. buf1wav: Buffering. The delay line can only take one wavelength at any time. 3. buf4wav: Buffering. The delay line can take four wavelengths at a time. 4. def: Deflection. 5. wc: Wavelength conversion at full range. 6. wc_lim: Limited wavelength conversion. One wavelength can stay the same or be converted to only another fixed wavelength, e.g. λ 1 to λ wc_lim_cyc: Limited wavelength conversion. One wavelength can stay the same or be converted to one of the two fixed wavelengths in a cyclic fashion, e.g. λ 1 to λ 2 or λ 3, λ 2 to λ 3 or λ 4, etc. We have simulated a number of combinations of the above approaches. In the following section, these notations are used to indicate the different approaches and their priorities used in one particular scheme. For example, wc+buf1wav+def means a combination of full range wavelength conversion, single wavelength buffering and deflection; with the order of wavelength conversion first, buffering second and deflection last. 3. ILLUSTRATIVE PERFORMANCE COMPARISON 3.1. Comparison of the basic contention-resolution schemes We have chosen four metrics to evaluate network performance with different contention-resolution schemes: network throughput, packet-loss ratio, average end-to-end delay and average hop distance. They indicate the network utilization, reliability, latency and physical impairment to the signal respectively. The packet-loss ratio is the total number of packets lost in transit divided by the total number of packets generated. The network throughput is defined as: total number of bits successfully delivered Network throughput = network transmission capacity simulation time ideal average hop distance Network transmission capacity = total number of links number of wavelengths data rate Network throughput is the fraction of the network used to successfully deliver data. Because packets can be dropped, a part of the network capacity is always wasted in transporting the bits that are dropped later. In an ideal situation where no packets are dropped, the network will be fully utilized and the throughput will reach unit 1. Average hop distance is the hop distance a packet can travel, averaged by all the possible source-destination pairs in the network. The ideal average hop distance (i.e. no packet-dropping) of this network is Table 2 shows the values of the parameters used in the simulations. Link length L Data rate R Number of Average packet Max hop distance wavelength W size B 20 km 2.5 Gb/s 4 12,000 bits 8 Table 2. Network parameters used in simulation. All the results are plotted against, i.e. the franction of full transmission capacity. (If the source is generating 0.5 giga bits of data per second and the transmitter / line cpacity is Gb/s, the would be 0.2.) Our simulation did not consider higher layer protocol behaviors such as acknowledgement and retransmission. Fig. 5 compares the network throughput of five most basic approaches, namely baseline, buf1wav, buf4wav, def, and wc. We find that wc provides the best result, which is about 50% higher than that of the other four schemes. This indicates the high effectiveness in resolving contentions using wavelength converters. The capacity of contention-resolution for the rest of the 6

7 schemes are ranked in the following order: buf4wav>buf1wav>def>baseline. buf4wav gives approximately 5-10% improvement over buf1wav, showing that larger buffer capacity leads to higher network throughput. Meanwhile, it is important to consider the extra cost introduced by multi-wavelength optical buffering. The switch fabric needs to support three more input / output ports to accommodate all four wavelengths in the optical buffer (Fig. 2(d)). Typical switch fabric architecture requires hyperlinear increase in the number of switch elements as the number of ports increases. Therefore, the increased number of optical buffers induces significant increases in the cost and the size of the switch fabric. We also observe that def does not resolve the contention very effectively. In our previous study of a simpler network [14], which consisted of six nodes, we had observed that deflection resolves contention better than buf1wav and buf4wav with very light traffic load. There are two reasons for this phenomenon. First, the deflection uses the unused capacity of the whole network as a much larger buffer to store the packets in contention. Second, in the topology of our previous work, two thirds of the nodes can carry out deflection; therefore the chance of a packet being deflected is high. With the network topology in this paper, only about one fourth of the nodes can perform deflection. Furthermore, in this particular topology, any source node can reach any destination node through at most two hub nodes, therefore the chance of a packet being deflected is much lower than that in the topology studied in [14]. Space deflection can be a good approach in a network with high-connectivity topology, such as ShuffleNet [4][9], but is less effective with a low-connectivity topology, such as inter-connected rings. throughput baseline buf1wav buf4wav def wc Fig. 5. Basic schemes: throughput comparison. packet loss ratio baseline buf1wav buf4wav def wc Fig. 6. Basic schemes: packet-loss ratio comparison. average end-to-end delay (sec) baseline buf1wav buf4wav def wc Fig. 7. Basic schemes: average delay comparison. average hop distance baseline buf1wav buf4wav def wc Fig. 8. Basic schemes: average hop distance comparison Fig. 6 compares the packet-loss ratio (represented in fraction) of these schemes. It is a good complement to Fig. 5 because the network's throughput is statistically in reverse proportion with the packet-loss ratio. All the phenomena mentioned above are more prominent in this figure. While wc shows low packet-loss ratio (less than 1%) under the light load (where offered load at transmitter is less than 0.14), other schemes' packet-loss ratios are many times higher. This indicates that wavelength 7

8 conversion can greatly help us achieve good quality of service (QoS). Furthermore, the figure shows that wc has a slow increase of packet-loss ratio with the offered load, which makes the network more stable over a larger operating range of packet-arrival rate. Schemes involving deflection or buffering experience rapid increase in packet-loss ratio, because both buffering and deflection are very sensitive to a certain load threshold. This is especially true under uniform traffic because all the hub nodes become congested nearly simultaneously, when the traffic load reaches the threshold value. Once this load threshold is reached, neither deflection in time domain (optical buffering), nor deflection in space domain seems to be effective, since the chance of contention is the same everywhere in time and space. Moreover, in wc, there is a certain degree of intelligence: the switching node automatically looks for a vacant wavelength along the shortest path; therefore, it is able to allocate the available free network capacity in the wavelength domain, which will definitely assist the packet to come closer to its destination. Fig. 7 compares the average end-to-end delays. With increasing load, the average delay actually decreases for schemes other than def. For def, the end-to-end delay increases with load in the beginning. This is because, when the load increases, the packet-loss ratio also increases. Packets with closer destinations are more likely to survive, while packets that have to travel long distances are more likely to be dropped in the network. The overall effect is that, when only survived packets are being considered in the statistics, the delay decreases in baseline, buf1wav, buf4wav and wc. For def under light load, the scheme is resolving contentions efficiently by mis-routing packets to nodes outside the shortest path, thus introducing extra propagation delay. Although the local buffering also brings in extra delay in buf1wav and buf4wav, the delay is negligibly small compared with the propagation delay in each link. This extra propagation delay from deflection can diminish the effect of increasing packet-loss ratio and as a whole the end-to-end delay increases with more traffic load. It can be noticed that the interpretation of the average end-to-end delay should be done along with a consideration of the packet-loss ratio. A decrease in end-to-end delay does not necessarily imply improved network performance; the packet-loss ratio should reduce as well for improved network performance. Fig. 8 shows the average hop distance of packets for all our schemes. In this figure, the effect of buffering and deflection to resolve contention is more obvious. In the light-load region, buf1wav and buf4wav are working very well, therefore there is an increase in the average hop distance (note that every time a packet passes the delay line and re-enters the switch it is counted as one hop, in order to take in account of the physical impairment occurred in the process), which implies more packets are being buffered and more packets with far destinations have survived. Although buffering does not introduce a large delay, it still brings physical impairment to the signal, because the packet must pass various devices such as splitters and switch elements. The curve of def does show a similar pattern, yet it is much less noticeable, because the chance for a packet being deflected in one of the four hub nodes is lower than that of a packet being buffered in any of the fifteen nodes. We observe that all the schemes have a decreasing average hop distance under heavy load. This indicates that after their capability of resolving contention is reached at a certain amount of load, packets with far destinations are less favored Comparison of combinational schemes We choose five scenarios to study the different combinations of contention-resolutions: buf4wav+def, wc+buf1wav, wc+def, wc+buf1wav+def and wc+buf4wav+def. Fig. 9 shows the throughput comparison of these schemes. The benefit of using wavelength converters can be easily observed. All the four schemes involving wavelength conversion offer at least 25% more throughput than buf4wav+def. Also the figure shows wc+buf1wav is better than wc+def, which indicates single-wavelength buffering is more effective than deflection in this topology. wc+buf4wav+def offers less than 5% more throughput than wc+buf1wav+def. Considering the high cost of the much larger switch fabric for four-wavelength buffering, buf4wav does not appear to be very cost effective. Our simulations also show a similar result in the packet-loss ratio comparison in Fig. 10. Fig. 11 and Fig. 12 show the end-to-end delay and average hop distance respectively. The end-to-end delay shows a general trend of decrease for all the five schemes, due to the dropping of packets with far destinations with increasing traffic load. The slight increase in the light-load region of the curves is due to def, buf1wav and buf4wav resolving contentions as described before. In the average hop distance comparison, buf4wav+def shows a rapid increase with load, indicating that a large number of packets are being buffered or deflected. wc+buf4wav+def only shows an increase trend at the heavy-load part of the curve, the reason is that with light load, wavelength conversion is resolving the contention well, thus very few packets are being buffered. When the load is higher, more and more packets are being buffered successfully, resulting in a larger average hop distance. The saem phenomenon should be expected in wc+buf1wav+def, but it is actually much less 8

9 obvious because with single-wavelength buffers, less packets can be stored in the buffer, and thus delay lines have less impact on the average hop distance. Similar reasoning can be applied to wc+buf1wav and wc+def. throughput buf4wav+def wc+buf1wav wc+buf1wav+def wc+buf4wav+def wc+def Fig. 9. Combinational schemes: throughput comparison. packet-loss ratio buf4wav+def wc+buf1wav wc+buf1wav+def wc+buf4wav+def wc+def Fig. 10. Combinational schemes: packet-loss ratio comparison. average end-to-end delay (sec) throughput Fig. 11. Combinational schemes: average delay comparison buf4wav+def wc+buf1wav wc+buf1wav+def wc+buf4wav+def wc+def wc wc_lim wc_lim_cyc Fig. 13. Limited wavelength conversion: throughput comparison. average hop distance packet-loss ratio 3 Fig. 12. Combinational schemes: average hop distance comparison buf4wav+def wc+buf1wav wc+buf1wav+def wc+buf4wav+def wc+def wc wc_lim wc_lim_cyc Fig. 14. Limited wavelength conversion: packet-loss ratio comparison 9

10 3.3. Limited wavelength conversion From the throughput and packet-loss ratio comparison (Fig. 13, 14) of three schemes with only wavelength conversions (wc, wc_lim and wc_lim_cyc), it is noticed that full range wavelength (wc) performs approximately 20% better than wc_lim and 3% better than wc_lim_cyc. wc_lim_cyc appears to be a good choice because its performance is nearly as good as full range wavelength conversion and has a reasonably lower cost. Our extensive studies include comparisons of nine schemes involving full range and limited wavelength conversions, as shown in (Fig. 15, 16). Fig. 15 shows the packet-loss ratio comparison (the curves in the throughput comparison are not distinct enough, therefore not shown in this paper). The lowest four packet-loss ratios belong to wc+buf4wav+def, wc_lim_cyc+buf4wav+def, wc+buf1wav+def and wc_lim_cyc+buf1wav+def. The fact that both wc_lim_cyc+buf1wav+def and wc+buf1wav have lower packet-loss ratio than wc_lim+buf4wav+def indicates that the benefit of using more buffering is not as great as using more wavelength conversion in this network. The highest three packet-loss ratios belong to wc_lim+buf1wav, wc_lim+buf1wav+def and wc_lim_cyc+buf1wav. The results are compliant with our former assumption: the ranking of three domains of contention-resolution in term of effectiveness is wavelength conversion > optical buffering > deflection. In addition, wc_lim_cyc is more effective than buf4wav in this case. Fig. 16 shows the average hop distance of these nine schemes, which helps us better understand the different network behaviors. These nine curves can be divided into three groups: 1. (wc, wc_lim, wc_lim_cyc)+buf1wav 2. (wc, wc_lim, wc_lim_cyc)+buf1wav+def 3. (wc, wc_lim, wc_lim_cyc)+buf4wav+def In group 1, wc_lim+buf1wav reaches its max average hop distance the earliest with increasing load, indicating that the network congestion occurs the earliest. wc_lim_cyc+buf1wav and wc+buf1wav both have lower max average hop distance and reach network congestion point more slowly, indicating that less buffering is used with more wavelength conversion implemented in the network. In group 2, similar trend can be observed as in group one except that the overall average hop distance is higher because deflection in involved. In group 3, the curves show a much higher average hop distance than those of the other two groups because four-wavelength buffers and deflection combined together provide much higher chance for packet to travel extra hops toward their destinations. wc_lim+buf4wav+def has the highest average hop distance because of less wavelength conversion capacity in the network, i.e. more packets have to rely on buffering or deflection. packet loss ratio wc+buf1wav wc_lim+buf1wav wc_lim_cyc+buf1wav wc+buf1wav+def wc_lim+buf1wav+def wc_lim_cyc+buf1wav+def wc+buf4wav+def wc_lim+buf4wav+def wc_lim_cyc+buf4wav+def. Fig. 15. Combinational schemes with limited wavelength conversion: packet-loss ratio comparison. mean number of hops wc+buf1wav wc_lim+buf1wav wc_lim_cyc+buf1wav wc+buf1wav+def wc_lim+buf1wav+def wc_lim_cyc+buf1wav+def wc+buf4wav+def wc_lim+buf4wav+def wc_lim_cyc+buf4wav+def Fig. 16. Combinational schemes with limited wavelength conversion: average hop distance comparison. 10

11 3.4. Further discussions In our simulation we set the maximum hop count to 8, which is not fair for packets that have to travel a longer distance to their destinations. One way to avoid this problem is to use maximum extra hop count, instead of total maximum hop count. This will require the source node to have knowledge of the whole network and the minimum required hop count. The source node will stamp the packet with the sum of the minimum required hop count and maximum hop count; this sum will be decremented at each node. As stated in Section II, the maximum hop count will affect deflection much less than buffering due to the fact that deflection only occurs at the four hub nodes. Another variation involves the deflection policy. In the simulation presented above the deflection is next-hop oriented, in which the hub node will deflect a packet based on the packet s original next hop so the packet can reach this next hop by traveling extra hops. This is not necessarily the most efficient deflection because sometimes a packet can have a shorter or less crowded alternative path without going through the original next hop. For example, if a packet arrives at node 7 and its final destination is node 11. According to the shortest path the next hop is node 1. In case of contention, with next-hop oriented deflection, it will be deflected to node 6, counting on that node 6 will eventually send it back to node 1 and then it can follow the original shortest path. A better way to do deflection is to examine the packet s final destination. In this case the packet should be deflected to node 9 because it is much less likely to be dropped by node 9 and node 10. This type of destination-oriented deflection will require the hub nodes maintain knowledge of the whole network topology and calculate the optimal deflection table. However, our simulation results showed very little improvement from destination-oriented deflection policy, due to the small number of nodes able to carry out deflection in the network Overall comparison Table 3 shows a summary of the simulation results. wc+buf4wav+def gives the best overall performance, followed by wc+buf1wav+def. When the network has a large number of wavelengths, the cost of all the wavelength converters could be an issue. In this particular case, wc+buf1wav+def seems to be a good choice because it offers satisfactory performance without introducing high cost. Using large amount of buffering, i.e. buf4wav, can be very expensive because the extra number of switch ports it requires. If we want to further lower the network cost we can use limited wavelength conversion to reduce the number of wavelength converters. Implementing deflection introduces no extra hardware cost. But, in general, deflection is only effective in a network with high connectivity. Scheme Latency Packet-loss Ratio Cost buf1wav low high low buf4wav low high medium def high high lowest wc low medium medium buf1wav+def high medium-high low buf4wav+def high medium-high medium wc+buf1wav low medium-low medium-high wc+buf1wav+def low Low medium-high wc+buf4wav+def low Low high Table 3. Comparison of different contention-resolution schemes. 4. CONCLUSION This paper presented simulation results on contention-resolution schemes in an unslotted all-optical packet-switched network model with variable-length packets. We compared the advantages and limitations of contention-resolution in time, space, and wavelength dimensions. Limited wavelength conversion and selective deflection are investigated for the first time. Among all the schemes, wavelength conversion combined with buffering and deflection is found to be the most efficient. It can be more costly and complex because of the extra device, switch elements and control software. Minimum optical buffering 11

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