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1 Optik 122 (2011) Contents lists available at ScienceDirect Optik j o ur nal homepage: Design and performance evaluation of a metro WDM storage area network with IP datagram support Bernardi Pranggono, Jaafar M.H. Elmirghani School of Electronic and Electrical Engineering, University of Leeds, Leeds, LS2 9JT, UK a r t i c l e i n f o Article history: Received 15 April 2010 Accepted 1 October 2010 Keywords: Medium access control protocol Metropolitan area network Storage area network Wavelength division multiplexing a b s t r a c t Storage area networks (SANs) are becoming an important part of optical MANs (metropolitan area networks). Growing storage and business-continuity needs; high-bandwidth, low latency requirements for SANs; storage infrastructure consolidation; and post-9/11 regulatory issues are among the several driving factors to push this trend. We, in this paper, consider a metro wavelength division multiplexing (WDM) SAN that allows the transmission of variable packet size such as Internet protocol (IP) datagram and evaluate its performance by means of discrete-event simulation. The network is based on one fixed transmitter and multiple fixed receivers. Beginning with an introduction and the context of this work, we describe the network and node architectures; and introduce the medium access control (MAC) protocols. Subsequently, using the Poisson and self-similar traffic, we present and discuss performance of the proposed network architecture in terms of throughput and queuing delay under symmetric and asymmetric traffic scenarios. The simulation results suggested that the proposed architecture is suitable for SAN applications which demand low queuing delay and high throughput Elsevier GmbH. All rights reserved. 1. Introduction In the past few years we have witnessed a tremendous growth in network traffic due to the explosive development of new services and data-intensive applications such as enterprise resource planning (ERP), customer relationship management (CRM), grid computing, backup and recovery, data warehousing and mining. In addition to this phenomenon, business-continuity needs and storage infrastructure consolidation are also generating a constant demand for a dedicated storage system with a bigger capacity of data storage. This storage system needs to be scalable, robust and able to provide data in an efficient way. Storage area networks (SANs) are one of the most suitable solutions to fulfil the requirements. Furthermore, recent demand of new services such as triple (data, voice and video) or quadruple (data, voice, video and mobile) play is also increasing significantly. In turn, this is expected to cause a massive boost in the traffic across core networks by one to two orders of magnitude which also have a significant impact on the metropolitan area networks traffic. Transport network technologies have kept in pace with the unprecedented demand on bandwidth with progress in telecommunication technologies. Photonic network technologies and wavelength division multiplexing (WDM) have become a technol- Corresponding author. address: bern@ieee.org (B. Pranggono). ogy of choice to increase network bandwidth in recent years and the trend is continuing [1]. Recent developments in WDM research have led to new technologies that offer an unparalleled bandwidth as currently available systems can support up to 600 wavelengths per fibre enabling a single fibre to transfer few terabits per second of data and information. In turn, advances in optical transport technologies could motivate the creation of new killer applications that are able to exploit the ample availability of bandwidth. Due to its superior characteristics, WDM technology is not only utilized in backbone wide area networks but has also started to be utilized in metro access networks [2,3]. This latest development makes it more and more feasible for WDM technology to be used in SANs to give SANs a more competitive, high-bandwidth, highscalability and low-latency edge. On the other hand, the legacy SAN system, based on Fibre Channel (FC) SONET/SDH technologies, has drawbacks such as, the FC distance limitation and the SONET inefficiency in transporting bursty data traffic as it is designed to support voice traffic [2]. The unprecedented users demand and the advent of photonic technologies generate great opportunities for metro WDM networks based SAN. Optical storage area networking has started to receive attention [1]. It is therefore of interest to explore systems and protocols that may enable WDM to be effectively used to enhance the capacity, access speed and capabilities of distributed SANs. Following the introduction this paper is organized as follows: Section 2 discusses background and motivation of this work. Section 3 presents our node and network architecture, and introduces /$ see front matter 2010 Elsevier GmbH. All rights reserved. doi: /j.ijleo

2 B. Pranggono, J.M.H. Elmirghani / Optik 122 (2011) the MAC protocols. Performance analysis results from discreteevent simulation are given and discussed in Section 4. Finally, the paper is concluded in Section Background and motivation SANs are devoted high-speed networks whose primary function is to interconnect computer systems and storage devices in an efficient way. In essence, SANs are composed by four primary elements, storage devices, computer systems, a communication infrastructure, which provides physical connections, and a management layer which manages the connections. Metro WDM may be considered for SANs as it has high-bandwidth, high-scalability and low-latency. A MAN type network has to be relatively flexible and scalable in the sense that it is expected to satisfy various user demands and possible configurations. In addition, the physical architecture is strongly reliant on the geographical relative positions of the access nodes (ANs). A simple slotted WDM ring network architecture intended to serve as a metropolitan access network has been proposed in [4], and is designed to interconnect several access nodes (ANs) on a regional scale. Slotted and token passing WDM ring architecture have been proposed in [5]. Early implementations of slotted rings were strictly limited by the very low bandwidth-delay product. Nowadays, this is clearly not a limitation. Indeed, in high speed networks, the duration of a slot (containing a fixed number of bits) is becoming shorter and shorter as compared to the ring propagation time and hence, a slot, is much smaller than the size of the network it moves in. In the proposed architecture in [4], a unidirectional single-fibre multi-channel time-slotted ring is implemented, with uncomplicated MAC protocol, in which the time is divided into a number of slots. The slots on the ring have a fixed size, which is equal to the Ethernet maximum transmission unit (MTU) frame size, i.e. about 12,000 bits (or 1500 bytes). An assumption was made that the packet size is always equal to the slot size. The complexity of the MAC protocol is reduced on purpose as the intelligence of the network is implemented in the access nodes. Traditionally, in order to simplify the design and operation of the network and its switching nodes, it is usually assumed that the network traffic consist of fixed size packets [6 9]. However, in reality the packet size in data communication traffic is not fixed. Based on measurement on the Sprint IP backbone, according to [10] there are mainly five major sizes of packets in data traffic, i.e. 40, 211, 572, 820 and 1500 bytes. The bytes size is for TCP ACK segments, TCP control segments, and telnet packets carrying single characters. 572 bytes and 1500 bytes are the most common default MTUs of the network interfaces on the hosts. Many TCP implementations that do not implement MTU discovery utilize either 512 or 536 bytes as the default maximum segment size (MSS) for non-local IP destinations, resulting a 552 byte or 576 byte packet size. The default packet size of 576 bytes results in fragments of 572 bytes because the length of the payload of each fragment packet except the last must be divisible by eight [11]. The most widely used Ethernet network interface utilizes 1500 bytes of MTU. The 211 bytes packets correspond to a content distribution network (CDN) proprietary user datagram protocol (UDP) application that uses an unregistered port and carries a single 211 bytes packet. The packets of around 820 bytes are generated by domain name server (DNS) and media streaming applications. Obviously, the original architecture is not suitable for this situation, in which a huge proportion of the slot space will be wasted. In our previous work, two metro WDM multi-ring network architectures and MAC protocols have been presented [12]. The two metro WDM multi-ring network architectures are designed Fig. 1. Network architecture. to accommodate variable length packet traffic, where slots of different sizes circulate along the ring. In this work, we evaluate these architecture and MAC protocols in a metro setting for SAN application where SANs co-exist with other network users. Furthermore, in addition to symmetric traffic, asymmetric traffic is also considered in order to evaluate the impact of hot-node scenarios, for example, where one server acts as a gateway, SAN access point, or as a database server. In addition, performance evaluation under the Poisson and bursty self-similar traffic is also considered. The flexibility of the network is very high and attractive and easy physical deployments can be achieved. 3. System architecture 3.1. Network architecture The architecture is a single-fibre unidirectional multi-channel slotted (synchronous) ring, similar to [13] which is designed to interconnect ANs (in this scenario some of the ANs serve SANs) on a regional scale. Each AN has add-and-drop capabilities to access the ring slots and is used to link an access network to the ring. This can be realized by means of optical add-drop multiplexers (OADM). Each node has three network ports. A Gigabit Ethernet (GbE) port is used for transmission between AN and the access network, the other two OC-like ports are used to access the WDM ring. Data sent from one access network to another is first received by an AN through the GbE link and is then transmitted through the WDM ring to the destination AN which delivers the data to the intended access network. The proposed SAN architecture using metro WDM ring network is shown in Fig. 1. In the network, the length of the ring is 138 km, which leads to a ring diameter of about 44 km. The number of wavelengths equal to four, and each wavelength is considered to have a transmission rate of 2.5 Gb/s (i.e. OC-48). This network is used to interconnect 16 nodes, and is denoted as S For the variable-size (VS) slot scheme, the slots on the ring have five different sizes, corresponding to the five different sizes of data traffic packets from the access links, which are 40, 211, 572, 820 and 1500 bytes. The total number of slots is 240. The number of slots for each size is 24, 48, 24, 48 and 96 respectively (the numbers are related to the probability distribution of packet size on the access links). The average size for a slot is bits. In the simulation, the different size slots are generated with a fixed sequence. The

3 1600 B. Pranggono, J.M.H. Elmirghani / Optik 122 (2011) Fig. 2. A generic node architecture of the slotted network. 240 slots are divided into 24 groups, in each group there are 10 slots with one slot of 40 bytes, two slots of 211 byes, one slot of 572 bytes, two slots of 820 bytes and four slots of 1500 bytes. This distribution is based on the Internet traffic distribution mentioned in Section 2. In order to simplify the calculation and rotation, the length of the ring is slightly changed to 133,240 m (the ring length was equal to 138,240 m in the original network architectures [4]). For the super-size (SS) slot scheme, the ring is divided into 16 super slots with a size of 13,500 bytes. In SS case, the length of the ring remains at 138,240 m Node architecture The generic slotted node architecture used is shown in Fig. 2. When individual optical switches build a network, packets may arrive at different times at the input ports of each node. In a slotted network, a synchronization stage is needed to ensure that packet transmission start and end at a slot boundary. Before entering the photonic switch, all the input packets arriving at the input ports need to be aligned in time with one another (see Fig. 2). Bit-level synchronization and fast clock recovery are required for packetheader recognition and packet delineation. In the optical drop part of the node, the optical signals on the node s drop channel are removed from the ring by means of an optical circulator and a fibre Bragg grating (FBG). The received packet (in case of full slot) is then either locally dropped or retransmitted onto the ring depending on the destination address. In the optical add part of the node, a fixed optical transmitter transmits the new data packet received from the local add part on the corresponding wavelength channel (depending upon slot status and the destination address of the local packet). Every node is equipped with one fixed transmitter and four fixed receivers (FT FR 4 ). The proposed FT FR 4 architecture is very flexible and more easily scalable (there can be more nodes than wavelengths) than previous proposals of WDM ring network such as FT n FR n in [5]. Because in FT FR 4 system, a node can receive packets on any wavelength, each node is assigned a different sub-carrier multiplexed tone. When a node wishes to transmit to a specific destination, it transmits the packet onto a slot of its assigned transmission wavelength and multiplexes the appropriate sub-carrier tone of the destination. Meanwhile, each node constantly monitors all wavelengths in parallel in order to detect its own sub-carrier tone, which will notify it that it is the intended destination of the corresponding packet. The absence of a sub-carrier tone indicates that there is no packet loaded on the corresponding wavelength. An illustration of sub-carrier multiplexed mechanism is shown in Fig. 3. The feasibility of such a mechanism for a slotted Fig. 3. Sub-carrier multiplexed. ring has already been demonstrated in the HORNET [14] network project. In addition, the use of fixed transmitter is more cost effective compared to tunable transmitter used in [15,16]. Each wavelength is shared both for transmission and reception since there are fewer wavelengths than nodes in the network. In the variable-size (VS) slot scheme, each node has five First-In First-Out (FIFO) buffers for transmission to store packets with five different sizes. A maximum of 20 packets can be stored in each buffer. In the super-size (SS) slot scheme, only one FIFO buffer is used for transmission with the capacity of 100 packets. In both architectures, newly arriving packets are discarded when the buffer is full. It is expected that higher layer (e.g. IP) will do the recovering task. The network parameters used in the simulation are summarized in Table MAC protocol Medium access control (MAC) protocols are necessary to prevent channel and receiver collisions and to arbitrate the access to the wavelength channels. In this paper, we do not consider Table 1 Network parameters. Wavelength rate: R W 2.5 Gb/s Number of wavelengths per fibre: N W 4 Total network rate: R N = R W N W 10 Gb/s Number of access nodes: N T 16 Light velocity in fibre: V m/s Architecture Variable slot (VS) Super slot (SS) Ring length: L R 133,240 m 138,240 m Propagation delay: D = L R/V s s Bandwidth-delay product: B DP = R W D 1,665,480 bits 1,728,000 bits Average slot size: S bits 108,000 bits Slots per wavelength: S W = B DP/S

4 B. Pranggono, J.M.H. Elmirghani / Optik 122 (2011) addressing capabilities (address resolving and format) nor optical and electronic issues (receiver synchronization, signal attenuation, chromatic dispersion and others). For the VS slot scheme, a node always inspects the activity of the channel. The node probes the destination ID of the packet in the slot. If it is the node s ID, the packet is received from the ring, the receiver then marks the slot empty once the packet is correctly received and thus makes the slot reusable immediately (destination-stripping). If the destination ID is not equal to the node ID, the slot will be released to the downstream nodes. If an empty slot is observed, the node checks the size of the slot. A packet with a corresponding size is transmitted in the slot data unit. If the relevant buffer is empty, the empty slot is released to downstream neighbors. For the SS slot scheme, the transmission is quite different. The transmitter always inspects the free slot space (the capacity of the slot minus the current load of the slot) of the current slot. While the free space of a slot is larger than the size of the current packet in the buffer, the current packet will be transferred into the slot. Thus, the super slot scheme can convey a number of packets from current nodes while it has enough space. In this case, the slot is able to make good use of the free space to carry different packets, and do not have to consider the size of the packets. Actually, the free space in the super slot is not continuous due to the removal of packets. Thus, the fibre delay line (FDL) is introduced to vary the spacing at a node in the super slot case. Here a node delays the packet stream to increase the size of an unused space within the super slot provided there is sufficient unused space within the super slot to afford packet compaction. 4. Performance evaluation Simulating Internet traffic is a complex task as it needs to consider multi-dimensional aspects. There is no single model which satisfies all its requirements. Recent trends in networking agree that Internet traffic is self-similar [17]. However, the Poisson traffic is also found to be more suitable for smallest time scale in high aggregation environment [18]. Therefore, both the Poisson and selfsimilar traffic are used for performance evaluation in this section. The Poisson traffic generator is used to simulate the traffic received from the GbE access link. The Poisson traffic has been widely used to simulate independent inter-arrival traffic. Performance evaluation under self-similar traffic is also considered. The self-similar traffic sources were simulated using aggregated ON/OFF sources using Pareto distributions. The Hurst parameter of the aggregated traffic is H = 0.8 which represents a high degree of burstiness. It is generally accepted that Hurst parameter greater than 0.7 impacts the performance of a network due to the bursty nature of the associated traffic [19]. These two traffic sources the Poisson and self-similar traffic are denoted by P and S in the graphical results which are presented later in this section. Network performance in terms of node throughput, packet queuing delay in the transmission buffer, and proportion of packets dropped for both architectures was assessed under symmetric and asymmetric traffic. In case of symmetric traffic, all source nodes generate statistically the same amount of traffic to all other destination nodes. A node cannot transfer traffic to itself. On the other hand, under asymmetric traffic, one node generates more traffic compared to all other nodes. One hot-node scenario is considered (node 0), i.e. one node receives 0.4 of the total network traffic. In the graphical results, the use of asymmetric traffic and symmetric traffic is denoted by as40 and sym respectively. Each simulation is characterized by two parameters: the normalized network load and traffic distribution model. The normalized network load, denoted by L, is used to study the per- Fig. 4. Average throughput. formance of the network against varying level of traffic volumes, ranging between zero and one, i.e. null traffic to 1 Gb/s. Since there are a total of 16 nodes in the network, the total traffic generated for the ring network with L = 1 will be 16 Gb/s. Note that the total bandwidth capacity of the WDM ring is 10 Gb/s (2.5 Gb/s 4) and therefore the normalized load of 1 will create more traffic on the ring than the total carrying capacity of the WDM ring network. This section shows the results of discrete-event simulations. The same traffic was applied to the VS slot and SS slot schemes. Fig. 4 shows the average throughput of the network. As shown in Fig. 4, in both cases S VS-P-as40 and S SS-Pas40, the maximum throughput achieved is 900 Mb/s and 855 Mb/s respectively. The use of self-similar traffic sources slightly reduces the achievable throughput when compared with the case where the Poisson traffic is used. This is expected due to the burstiness of the traffic produced by self-similar sources. Here we notice that the throughput is declining when network load is very high (L > 0.8). This is mainly due to packet dropped as the buffer being full as packet inter-arrival duration decreases. We also notice that compared to VS, the network throughput of SS is slightly lower, especially when the load is very high (L > 8). It is also clear from the graph that the use of asymmetric traffic has a negative impact on the achievable throughput of the network. The average queuing delay is shown in Fig. 5. For the queuing delay, the performance is good as the delay is always less than 500 s whenever the network load is less than 1. This is more than suitable value even in the case of real-time multimedia traffic [20]. When we compare the VS slot architecture with the SS slot architecture, the VS slot architecture achieved slightly better performance, especially under asymmetric traffic. Under symmetric traffic scenario, the achievable throughput for both S VS-S Fig. 5. Average queuing delay.

5 1602 B. Pranggono, J.M.H. Elmirghani / Optik 122 (2011) and S SS-S are roughly the same, around 950 Mb/s when the network load equal to 1. It is worth observing that the SS slot architecture does not significantly improve the slot reuse efficiency of the network, which means that the capacity of the network is not significantly increased. Therefore the maximum throughput is not improved. Moreover, in the SS slot scheme, if a 1500 bytes packet is in the buffer while the slot free space is slightly less than that, the slot cannot carry any packets from the current node, even when there are smaller packets that need to be transmitted. So nearly 1500 bytes space is wasted, i.e. 11.1% slot space is wasted (the size of slot is 13,500 bytes). However, in the VS slot scheme, the slot utilization probability is able to reach nearly 100% when the ring is saturated (L > 1). Thus, while the load is extremely high (L > 0.9), the average performance of the SS slot scheme is even worse than the VS slot scheme. For the results presented, it is clear that both VS slot and SS slot schemes achieve better performance in terms of network throughput, queuing delay, packet dropping probability and global delay, compared to the fixed-size slot scheme proposed in [4]. It has already shown in [4] that the fixed-sized implementation achieves better results compared to [5] with a simpler protocol. 5. Conclusion The article has presented and evaluated two proposed metro WDM SAN architecture that allows the transmission of variable packet size such as IP datagram. Performance evaluation under symmetric and asymmetric traffic scenarios with the Poisson and self-similar traffic sources was considered. Simulation results were presented for network throughput and queuing delay. The simulations suggested that the proposed architecture is suitable for SAN applications which demand low queuing delay and high throughput. Compared with the super-size slot architecture, the variablesize slot architecture achieved slightly better performance, especially under asymmetric traffic in the very high network load. However, as super-size slot network architecture is not based on the already known packet length distribution of certain traffic, the architecture is more suitable for general IP traffic with unknown packet length distribution. The effect of bursty self-similar traffic was also presented and discussed. Due to the nature of its burstiness, the network performance under self-similar traffic is generally worse than under the Poisson traffic. References [1] B. Mukherjee, Optical WDM Networks, Springer, New York, [2] M. Herzog, et al., Metropolitan area packet-switched WDM networks: a survey on ring systems, IEEE Commun. Surv. Tutor. 6 (Second Quarter) (2004) [3] J.M. Finochietto, et al., Towards optical packet switched MANs: design issues and tradeoffs, Opt. Switching Netw. 5 (2008) [4] C.S. Jelger, J.M.H. Elmirghani, A slotted MAC protocol for efficient bandwidth utilization in WDM metropolitan access ring networks, IEEE JSAC 21 (October) (2003) [5] J. Cai, et al., The multitoken interarrival time (MTIT) access protocol for supporting variable size packets over WDM ring network, IEEE JSAC 18 (October) (2000) [6] I. Chlamtac, et al., A contention and collision free WDM ring network for multigbit/s packet switched communication, J. High Speed Netw. 1 (1995) [7] M.A. Marsan, et al., All-optical WDM multi-rings with differentiated QoS, IEEE Commun. Mag. 37 (1999) [8] A. Bianco, et al., A posteriori versus a priori access strategies in slotted all-optical WDM rings, Comput. Netw. 32 (2000) [9] A. Cerena, et al., RINGO: a demonstrator of WDM optical packet network on a ring topology, in: Optical Network Design and Modelling (ONDM), Torino, 2002, pp [10] C. Fraleigh, et al., Packet-level traffic measurements from the Sprint IP backbone, IEEE Netw. 17 (November December) (2003) [11] C. Shannon, et al., Beyond folklore: observations on fragmented traffic, IEEE/ACM Trans. Netw. 10 (2002) [12] B. Chen, et al., A metro WDM multi-ring network with variable packet size, in: IEEE International Conference on Communications (ICC), Istanbul, 2006, pp [13] R. Randhawa, et al., Optimum performance and analysis of RingO networks, Optik 120 (2009) [14] I.M. White, et al., A summary of the HORNET project: a next-generation metropolitan area network, IEEE JSAC 21 (November) (2003) [15] I.E. Pountourakis, A WDM network architecture with multiple receivers and a synchronous transmission protocol with constraint of data channel collision, Microw. Opt. Technol. Lett. 38 (2003) [16] M.A. Marsan, et al., MAC protocols and fairness control in WDM multirings with tunable transmitters and fixed receivers, J. Lightwave Technol. 14 (1996) [17] W. Willinger, et al., Self-similarity through high variability: statistical analysis of Ethernet LAN traffic at the source level, IEEE/ACM Trans. Netw. 5 (February) (1997) [18] T. Karagiannis, et al., A nonstationary Poisson view of Internet traffic, in: IEEE INFOCOM, vol. 3, Hong Kong, 2004, pp [19] Z. Sahinoglu, S. Tekinay, On multimedia networks: self-similar traffic and network performance, IEEE Commun. Mag. 37 (1999) [20] M. Baldi, Y. Ofek, End-to-end delay analysis of videoconferencing over packetswitched networks, IEEE/ACM Trans. Netw. 8 (August) (2000)