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An All-Optical WDM Packet-Switched Network Architecture with Support for Group Communication Marcos Rogério Salvador, Sonia Heemstra de Groot, and Diptish Dey Centre for Telematics and Information Technology (CTIT), University of Twente P.O. Box 217, 7500 AE, Enschede, The Netherlands Tel +31 53 4898013, Fax +31 534894524 {salvador, heemstra, dey}@cs.utwente.nl

Abstract. The so-called media convergence to the Internet is foreseen. As a consequence of this convergence, MANs will face new demands not only in terms of bandwidth, but also in terms of services. To meet these new demands, new MAN architectures are required. WDM-based MAN architectures that tackle the first problem are available not only in the literature, but also on the market. In this paper we deal with the second problem. Specifically, we describe a novel WDM-based MAN architecture that supports group communication, a service that is expected to increase considerably as new applications converge to the Internet. Based on packet switching, the architecture supports both point-to-point and point-to-multipoint communication in the optical domain.

1 Introduction The convergence of various media, applications and networks to the Internet in the near future is foreseen. Such a convergence will affect metropolitan area networks (MANs) directly. Firstly, the demand for bandwidth is expected to increase enormously. It is arguable whether MAN architectures based on electronic technologies will cope with such a foreseen demand. Secondly, Internet traffic will have characteristics that will differ even more from those that drove the design of MAN architectures currently in operation. MAN architectures were mostly designed to cope with long-lived voice flows and, therefore, are circuit-switched. Internet Protocol (IP) traffic, however, is inherently bursty and consists mostly of short-lived data flows and small packet sizes. The bandwidth demand problem has been tackled with the deployment of wavelength division multiplexing (WDM). WDM is an optical technology that exploits the frequency spectrum of the light, thus enabling several distinct optical channels within a single optical fiber, each carrying up to tens of Gbps. MAN architectures based on WDM are a fact today. However, they are still based on circuit switching and the store-and-forward (i.e., multihop) communication paradigm. To cope with the characteristics of the next generation Internet, MAN architectures should pursue all-optical (i.e., single-hop) packet switching. Besides the robustness and better resource utilization that are typical of packet switching, all-

optical packet switching eliminates queuing delays at intermediate nodes and provides bit rate and protocol transparencies. All-optical WDM packet-switched network architectures are described in [1], [2]. Common to these architectures is the fact that they rely on the slotted-ring concept and the use of tunable transmitter (TTx) and fixed receiver (FRx) node architectures. In such networks, the total capacity of each channel is divided into (time) slots of fixed length. Upon arrival of an empty slot, a node tunes its laser onto that slot’s wavelength and transmits. At the destination, the payload is obtained from the slot and the slot is released, being reused by either the node itself or downstream nodes. Whilst TTx/FRx node architectures provide very high performances, they have some drawbacks. Firstly, tunable lasers are too expensive and very difficult to control. Secondly, the use of a single FRx per node makes the support of group communication somewhat inefficient and the demands for group communication are expected to increase considerably as a result of the foreseen convergence. For instance, a source node may have to transmit a given information W times, where W denotes the number of distinct wavelengths that the destination nodes can receive from. This is a considerable drawback. Dey et al., in [3], propose a node architecture that deals with these problems. Unlike other node architectures, the node architecture relies on a fixed transmitter (FTx), array of receivers (ARxs) configuration. The main benefits of this node architecture are: i) management complexity due to TTxs is eliminated, ii) cheap cost of FTxs and iii) support of group communication is more efficient. The node architecture has different characteristics and requirements and, hence, requires medium access control (MAC) mechanisms specifically designed for it. We discuss such mechanisms in this paper. We also discuss some performance results that were obtained via simulation activities. The rest of this paper is organized as follows. In Sect. 2 the network and the node architectures are described. In Sect. 3 the MAC protocol that has been designed for the network and the node architectures described in Sect. 3 is described. In Sect. 4 some performance results are shown and analyzed. In Sect. 5 we conclude the paper.

2 The Network and the Node Architectures The network architecture is based on an adaptation of the slotted-ring concept to the multi-channel nature of WDM. In this architecture, W wavelength channels are used to carry payload information. A single extra wavelength channel is used to carry control information. The total bandwidth of each channel, including the control one, is divided into (time) slots of fixed length. Slots across the W payload wavelength channels, herein called payload slots, are synchronized in parallel so as to reach each node all at the same time. Slots on the control wavelength channel, herein called control slots, are sent slightly ahead of their corresponding payload slots. This is to account for the configuration time of the fabrics at the nodes. Fig. 1 illustrates how payload slots and control slots are synchronized.

∆t Control slot

Payload slots

Fig. 1. Slot alignment

The network is partitioned into S = N / W segments1, where N is the number of nodes in the network. If N ≤ W then each node is assigned (via management operation) an exclusive transmission wavelength channel. Otherwise, S nodes, each on a different segment, share the same transmission wavelength. A node can transmit on only one wavelength. On the other hand, a node can receive on all wavelengths simultaneously. Fig. 2 illustrates the basic network architecture. Although out of the scope of this paper, scalability can also be achieved via an interconnected ring structure. N0 N7

N1 w3 w0

N6

N2

Slots

N5

N3 N4

Fig. 2. Example of Network Architecture; F = 1 optical fiber; N = 8; W = 4.

The node architecture is shown in Fig. 3. Each node is equipped with one FTx and an array of W FRxs, each tuned on a distinct wavelength, for payload transmission purposes. Each node is also equipped with one FTx and one FRx, both operating on the same wavelength, for control information transmission purposes. Essential to the support of group communication is the adoption of three-state switches and tapcouplers in the node architecture.

1

For the sake of simplicity we assume N to be an integer multiple of W.

Fig. 3. Node Architecture [3]

A slowly tunable λ-drop is used to separate the wavelength channel carrying the control slot from the wavelength channels carrying the payload slots. The control slot is converted to electronic domain and processed by the header processor. To account for the time to process a control slot, payload slots are delayed in fiber loops. Based on the information contained in the control header, the header processor sets the three-state switch to either bar, split or cross state. If the slot is not destined to that node then the switch is set to the bar state. If the slot is destined to that node only then the switch is set to the cross state. If the slot is destined to that node and others then the switch is set to the split state. A second slowly tunable λ-drop is used to separate the transmission wavelength channel of a node from the other wavelength channels. When a slot on the transmission wavelength reaches the switch, the latter has already been set to the appropriate state. The slots on the remaining wavelengths are de-multiplexed and sent each through a tap-coupler, which drops a very low percentage of the signal to the connected receiver. The signal is converted to electronic domain and either selected or discarded, as determined by the header processor after processing the control slot. The signal that passes through each tap-coupler is again multiplexed. The process ends with the slot on the transmission wavelength and the slot on the control wavelength being added via two slowly tunable λ-add in tandem.

3 The MAC Protocol A MAC protocol is required to coordinate access in the network. The protocol differs from MAC protocols of conventional networks in that it aims at minimizing

processing delays rather than at optimizing network utilization. In all-optical networks bandwidth is plentiful. Protocol processing is the bottleneck. The MAC protocol relies on the label switching forwarding paradigm. Label switching provides for traffic engineering (TE) and virtual private networking (VPN), both essentials to the provision of next generation services. Although TE makes no sense in the basic network architecture described in Sect. 2, MANs are usually laid out as counter-rotating ring topologies or interconnected ring topologies. In these topologies2, TE is extremely important. Specifically, the protocol follows the Multi-Protocol Label Switching (MPLS) architecture [4], which is under development within the Internet Engineering Task Force (IETF). Slots follow a previously established label-switched path (LSP). Each node along a LSP maintains a cross-connect table known as label information table (LIT). Each LIT entry identifies an input triple of the form to an output triple of the same form. Each control slot is assigned a label at an ingress node. At each subsequent node, the label is used as an index to a LIT to determine whether the corresponding payload slot should be either received, forwarded or discarded. Since control slots are processed at every node along a LSP, the layout of the control slot is very important to minimizing protocol latency. Fig. 4 depicts the control slot layout. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O| EXP | TTL |F|P| Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |S| CRC-8 | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ O = Occupancy (free/busy) EXP = Experimental TTL = Time-to-live F = Fairness P = Parity S = Stack CRC = Cyclic Redundancy Check

Fig. 4. Control slot layout

The MAC protocol provides a transport service. The payload slot layout is an opaque structure in which the MAC protocol has no interest whatsoever. The protocol simply transports a frame received from a source node’s high-level data link control (HDLC) sub-layer to a destination node’s HDLC sub-layer. This transport service is unreliable. Error probabilities in all-optical networks are so low that reliability can be better achieved at end nodes’ HDLC sub-layers. Trying to achieve it in a hop-by-hop basis just adds death weight (i.e., useless overhead) to the protocol. To achieve high throughputs and low delays, unicast slots are released at destination nodes and can be reused by the destination node itself or by any downstream node. This implies that transmissions on a given channel can occur simultaneously (as long as they take place on distinct links). The term spatial reuse is also used to characterize networks that employ destination release. 2

We do not consider these topologies in this paper.

Multicast slots can be released either at the last destination or at the source. In the former, a multicast slot is forwarded over a point-to-multipoint LSPs. This is more suitable for networks running dense mode multicast routing. In such networks, receivers are densely distributed geographically and, therefore, releasing multicast slots at last destination nodes may result in considerable performance improvements. Salvador et al., in [5], propose a protocol to construct point-to-multipoint LSPs in alloptical WDM networks running dense mode multicast routing. In the latter, a multicast slot is simply broadcast. This approach is simpler and more suitable for networks running sparse mode multicast routing. In such networks receivers are sparsely distributed geographically and, therefore, improvements that can be achieved in terms of performance are not worth the cost of complexity that is required to enable last destination release. Destination release may introduce access unfairness under certain traffic patterns. A node that is immediately downstream to a node that is the destination of most of the traffic in the network may monopolize the channel and prevent other nodes from transmitting. To achieve access fairness, the transmission quota-based mechanism proposed in the MetaRing architecture [6] is adopted. Under such a mechanism, transmission only takes place if the three following conditions are met: i) there is at least one packet waiting for transmission; ii) an empty slot arrived and iii) the node still has some transmission quota left. Otherwise, the node refrains from transmitting and forwards the slot to the next node. The fairness mechanism works as follows. Upon two visits of the so-called SAT signal, a node is allowed to transmit up to l + k 0 data units, where l and k are multiples of the slot size. If upon visit of the SAT signal (i.e., the F bit of the incoming control slot is set to 1) a node has transmitted at least l data units, the node’s quota is renewed and the SAT signal is immediately forwarded to the next node. Otherwise, the node holds the SAT signal (i.e., sets the F bit of the outgoing control slot to 0) until it has transmitted at least l data units. The node’s quota is then renewed and the SAT signal is forwarded to the next node (i.e., the F bit of the outgoing control slot is set to 1). Fairness is enforced on a node basis. However, there is one SAT signal regulating transmission on each channel. This is to prevent nodes transmitting on highly loaded channels from affecting nodes transmitting on lightly loaded channels. If only a single signal is used in the network, nodes transmitting on highly loaded channels will hold the SAT signal and nodes transmitting on lightly loaded channels will be prevented from transmitting while the SAT signal does not arrive. Each node maintains one queue per LSP that the node can transmit over (i.e., each node maintains at least N-1 queues, where N is the number of nodes in the network). In the Internet, packet sizes are variable and mostly small [7] while the slots are of equally fixed size. Thus, with a single queue it may not be possible to achieve reasonably good slot utilization due to head-of-the-line (HOL) blocking. Queues store frames rather than packets. Frames are formed in advance because performing framing on the fly may constitute a bottleneck at certain bit rates. A frame may contain one or more packets depending on the slot size and the packet sizes. Although concatenating packets at the source and separating them at the destination are expensive operations in terms of processing, we believe that this is acceptable considering that slots are of fixed size while packets are of variable size.

One could argue that because packets in the Internet are mostly small [7], small slots could be used so that concatenation and separation (CAS) operations would not be required. The price to pay for this simplicity, however, is higher control slot forwarding rates. This may constrain the network in terms of scalability [8]. A frame also contains a node’s 48-bit MAC address. The source node’s address can be used by the destination node’s HDLC to inform that certain frames or even packets are missing. The protocol works as follows. Upon arrival of a slot, a node first verifies if P is correct. If not, the corresponding payload slot is discarded and marked as free. If the node can transmit on the payload slot’s wavelength then it attempts transmission. If no error is detected, the node proceeds by checking O to find out whether the slot is free or busy. If the slot is free and the node has sufficient transmission quota left, the MAC layer informs HDLC about the arrival of a free slot along with the slot’s fiber link and wavelength. Based on the slot’s fiber link and wavelength, HDLC moves the packet scheduler to the appropriate queue and selects the frame at the head of the queue. HDLC then returns the frame together with label that describes the LSP over which the frame must be sent. The MAC protocol updates the control slot’s label field with the received label. The other fields are updated accordingly as well. The control slot is forwarded to the next node. Transmission of the selected frame is delayed sufficiently to keep payload slots synchronized in parallel3. This assures that misalignment of slots due to dispersion is corrected at least once every ring revolution. At each subsequent node, the slot’s label is matched to a LIT. If no match is found, the corresponding payload slot is discarded and marked as free. If the node can transmit on the payload slot’s wavelength then it attempts transmission. If a match is found that determines that the slot must be forwarded, the slot’s label is swapped with the matched entry’s outgoing label and the slot is forwarded over the outgoing interface and the outgoing wavelength. If a match is found that determines that the node is a destination of a multicast session, the payload slot’s content is sent up to HDLC. The slot is forwarded to the next node according to the matched entry. If a match is found that determines that the node is the destination (in case of unicast) or the last destination of that slot (in case of multicast), the payload slot’s content is sent up to HDLC and the slot is marked as free. If the node can transmit on the slot’s wavelength, it attempts transmission. Otherwise, it forwards the empty slot to the next node.

4 Performance Results We now present some performance results that were obtained via simulation activities. The simulations considered a 50km long network with N = 16 nodes, each equally spaced from one another, and W = {2, 4, 8, 16} wavelengths. Slots are 552byte long and packets are assumed to fit exactly in the slots. Transmission quota Q(l, 3

Advanced signal dispersion is assumed.

k), where l = 100 and k = 200, is chosen. Each node generates the exact same amount of unicast traffic to each other node. Packet arrival is Poisson and is such that every queue in each node has always at least one packet ready for transmission. Node 0 also generates multicast traffic at the same rate to nodes 1, 3, 5, 7, 9, 11.

Per node Throughput (%)

Per Node Throughput x Number of Wavelengths 150 100 100 50

68.2

W =2 W =4 W =8 W = 16

41.7 23.5

0 Number of Wavelengths

Fig. 5. Per node average Throughput

Fig. 5 plots per node throughput. Per node throughput is considered as the number of successfully transmitted slots divided by the total number of processed slots by a node. Fig. 5 shows that nodes achieve maximum throughput when N W. Two are the reasons for this: i) there is no access contention since each node is assigned an exclusive transmission channel; and ii) l the bandwidth * latency product, which guarantees that a node will never be prevented from transmitting upon arrival of an empty slot. As the number of nodes sharing a given channel increases, the throughput of each of these nodes gets worse. 



Channel Utilization (%)

Channel Utilization x Number of Wavelengths 100 80 60

93.7

83.3 68.2

W =2 50.1

40 20

W =4 W =8 W = 16

0 Number of Wavelengths

Fig. 6. Average Channel Utilization

Fig. 6 plots channel utilization. Channel utilization is considered as the number of links traversed by non-empty slots divided by the total number of links traversed by both non-empty slots and empty slots (plus inter-slot gap). As expected, the utilization

of a channel improves as the number of nodes transmitting on that channel increases. The explanation for this statement is that under uniform and symmetric traffic conditions the average number of hops traversed by a (non-busy) slot from a source to a destination is N / 2. Thus, once a slot is released is has to traverse H hops in the average before being reused, where H is given by N divided by the number of nodes transmitting on that channel times 2. This explains why, for instance, average wavelength utilization approximates 50% when N = W. Note that this figures change in the presence of multicast traffic as there might be more than one destination. In this case, average channel utilization will depend on the distance in number of hops from a source to the last destination. The bigger the distance the higher the channel utilization is.

Per Queue Access Delay (in number of slots)

Per Queue Access Delay x Number of Wavelengths 80

69 W =2

60 39 40 20

23

15

W =4 W =8 W = 16

0 Number of Wavelengths

Fig. 7. Per Queue Average Access Delay

Fig. 7 plots per queue average access delay. Again as expected, per queue average access delay gets higher as: i) the number of possible destinations increases; and ii) the number of nodes transmitting on a given channel increases. When N = W, per queue average access delay is given by the number of possible destinations. This is why, for instance, when N = W = 16 and there are no multicast receivers, per queue average access delay equals 15 (a node does not transmit to itself and, therefore, the number of possible destinations or number of destination queues equals N-1). Per queue average access delay gets higher as the number of nodes transmitting on a given channel increases. This is because access contention increases. Consequently, it takes longer for the packet scheduler to complete its cycle (i.e., to return to a given queue).

5 Concluding Remarks This paper focused on a WDM MAN architecture that has been designed to support packet switching in the optical domain. Unlike others, this architecture supports not only point-to-point communication, but also point-to-multipoint communication. This is an important feature in the support of next generation services.

The network strives mainly for simplicity. Simplicity is fundamental in our network (even more than in conventional networks) because it leads to low processing delays. Fiber loops are directly proportional to processing delays of control slots. Thus, simplicity leads not only to scalability in terms of bit rate, but also to low cost due to the need for shorter fiber loops. Simulation activities showed that the network provides high performance. Certainly, channel utilization is not the strength of the architecture, specially when W=N. However, MANs may contain up to a couple of hundred nodes while the number of wavelengths is likely to be smaller. Furthermore, under certain traffic patterns (e.g., multicast), channel utilization improves. Finally, bandwidth is expected to be abundant and cheap and, therefore, channel utilization at the levels showed in this paper may not be an issue.

Acknowledgements This research is supported by the Dutch Technology Foundation STW, applied science division of NWO and technology programme of the Ministry of Economic Affairs of The Netherlands. The authors are also grateful to Kishore Kumar Sathyanandam for his cooperation to this work.

References 1. Marsan M. A., Bianco A., Leonardi E., Meo M., Neri F.: MAC Protocols and Fairness Control in WDM Multi-Rings with Tunable Transmitters and Fixed Receivers. IEEE/OSA Journal of Lightwave Technologies, Vol. 14, No. 6, June 1996. 2. Chlamtac I., Elek V., Fumagalli A., Szabo C.: Scalable WDM Access Network Architecture Based on Photonic Slot Routing. IEEE/ACM Transactions on Networking, Vol. 7, No. 1, February 1997. 3. Dey D. Koonen A.M.J., Salvador M. R.: Network Architeccture of a Packet-switched WDM LAN/MAN. In Procs. of IEEE/LEOS Symposium, Benelux chapter, Delft, The Netherlands, October 2000. 4. Rosen E. C., Viswanathan A., Callon R.: Multiprotocol Label Switching Architecure. IETF RFC 3031, January 2001. 5. Salvador M. R., Heemstra S. de G., Dey D.: Supporting IP Dense Mode Multicast Routing Protocols in WDM All-Optical Networks. In Procs. of SPIE/IEEE/ACM Optical Communications and Networking Conference, Dallas, Texas, US, October 2000. 6. Ofek Y.: Overview of the MetaRing Architecture. Computer Networks and ISDN Systems, 26, pp. 817-829, 1994. 7. Thompson K., Miller G. J., Wilder R.: Wide-Area Internet Traffic Patterns and Characteristics. IEEE Network Magazine, November 1997. 8. Karn P. et al.: Advice for Internet Subnetwork Designers. IETF Internet Draft , February 2001.

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