Data-centric networking using multiwavelength headers/labels in ...

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Data-Centric Networking Using Multiwavelength Headers/Labels in Packet-Over-WDM Networks: A Comparative Study C. Skoufis, S. Sygletos, N. Leligou, C. Matrakidis, I. Pountourakis, Member, IEEE, and A. Stavdas, Member, IEEE

Abstract—In this paper, various issues related to packet transportation in dynamically reconfigurable networks are studied and techniques for transmitting the associated signaling are discussed. A novel transportation frame based on multisymbol coding is presented, its functionality is analyzed, and the various alternatives for transmitting multiwavelength label/headers are reviewed and benchmarked. A physical layer performance comparison between two multiwavelength schemes, i.e., the bit-parallel and the multilevel ( -ary) coded, is carried out using analytic modeling as well as simulation tools. It is shown that the multilevel coding is an interesting solution, particularly in a wide-area network environment deploying line rates higher than 10 Gb/s. Index Terms—Bit-parallel, data-centric networks, framing, -ary modulation, optical packet switching, signaling, wavelength-division multiplexing (WDM).

I. INTRODUCTION

T

HE DEPLOYMENT of wavelength-division-multiplexed (WDM) systems allowed an unprecedented enhancement in the attainable product of transported capacity times overall transmission length. As the momentum of the optical networking revolution is growing, WDM is advancing toward the network periphery [metropolitan area networks (MAN), access networks], while it is now imposing a major rethinking regarding the requested functionality in Layer-Two and Layer-Three of the OSI model. These developments are interrelated since MANs and access networks have characteristics not previously witnessed in core networks. Their most striking difference, compared with core networks, is that they are both cost-sensitive. Another feature worth mentioning is that “hot spots” are emerging in MANs in a nonpredictable manner, flooding the network with dissimilar types of protocols and bit rates, while the traffic profile is bursty. In parallel, wide-area networks (WANs) maintain many of the aforementioned MAN characteristics with the added constraint that the average intra-WAN distance is much larger. Consequently, the introduction of WDM in MAN/WAN is proceeding for different reasons and under different assumptions with core networks. In this new environment, the adopted WDM technology should primarily ensure that the optical layer is dynamically reconfigurable for several reasons. Manuscript received December 17, 2002; revised April, 25 2003. This work has been supported in part by the European Union’s IST-DAVID project. The authors are with the Institute of Communication and Computer Systems (ICCS), Department of Electrical and Computer Engineering, National Technical University of Athens, Athens 157 73, Greece (e-mail: astavdas@ cc.ece.ntua.gr). Digital Object Identifier 10.1109/JLT.2003.816897

First, performing statistical multiplexing directly in the optical layer is the only alternative to WDM channel overprovisioning for gratifying peak-rate demand. In addition, in contrast to synchronous digital hierarchy/synchronous optical network (SDH/SONET)-over-WDM solutions, the exploitation of the subwavelength granularity provides a service transparent and future proof platform. Second, it allows real-time service provisioning toward fully automated networks. Both features allow the reduction of the capital and operational cost of optical networks. The same cause is served through “delayering,” which paves the way toward network simplifications. The final goal is a two-layer communication infrastructure where the synchronous transfer mode (ATM) functions will be absorbed in Internet protocol (IP), and the SDH-based functions will be absorbed in the optical layer [1]. The emerging “intelligent” optical layer incorporates switching and protection, providing simultaneously an agile transportation mechanism. The next generation of dynamically reconfigurable multiterabit–per-second MANs/WANs are generally addressed as packet-over-WDM (P-o-WDM) [2], [3]. This new framework has its own conceptual and technological challenges. For example, the interoperability between IP and WDM control planes is still an open issue, while the associated signaling is confronted with many challenges, ranging from concept to physical implementation. The aim of this paper is to propose and study a novel transportation format, as well as to propose and benchmark a transmission scheme aimed exclusively to signaling transportation in the framework of P-o-WDM networks. The remainder of the paper is organized as follows. In Section II, various transportation and signaling issues of dynamically reconfigurable P-o-WDM networks are discussed. In Section III, a novel transportation scheme based on a multi- approach is presented, and its functionality is analyzed. Finally, in Section IV, a comparative study in terms of physical layer performance of the multiwavelength headers is carried out, followed by the conclusions. II. TRANSPORTATION AND SIGNALING CONSIDERATIONS IN THE NEW ERA A. The New Transportation Layer The reasons plesiochronous (PDH) systems have been replaced by synchronous systems (SDH/SONET) are well understood today. In synchronous systems, large chunks of data can be dropped/inserted from a synchronous frame (of

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SKOUFIS et al.: DATA-CENTRIC NETWORKING USING MULTIWAVELENGTH HEADERS/LABELS

125 sec duration; a value stemming from voice-oriented legacy) without excessive bit-by-bit processing like bit stuffing in a long chain of hierarchical (de)multiplexing. The add/drop function is completed via the use of pointers, which reduce the processing burden. An additional advantage of synchronous systems is the introduction in the frame of various overhead sections (OHs) with administrative and control signaling. Given that the connections were semistatic, the use of a cumbersome two-way reservation mechanism that introduces high latency in the connection setup process is not an obstacle. In the emerging data-centric MAN/WAN, the traffic volume is expected to range between 2.5 and 10 Tb/s for the WAN, while it could scale between sub- and 1.5 Tb/s for the MAN. It is important to note that the traffic profile will be variable and hard to predict. To efficiently transport, and even more, to reconfigure dynamically, such capacity is a formidable task. The SDH/SONET platform cannot cope in this environment for many reasons: First, SDH/SONET offers a more transparent networking environment compared with PDH, but it significantly lags on future requirements for service transparency; it is an inherently nontransparent solution. Second, in synchronous networking, the entire capacity of the traffic volume needs to be processed—something that is unrealistic in the aforementioned MAN/WAN environment. Finally, the two-way reservation mechanism, although it offers stability and robustness, is overwhelmed in a dynamic networking environment with frequent connection setup requests. Therefore, besides two-way reservation, a complementary mechanism is sought for the dynamic phases. The emerging entity, although not yet fully identified, is expected to be a simplified infrastructure [1], [5]–[7]. In order to transport capacity well into the terabit-per-second regime and simultaneously provide statistical multiplexing in the optical layer, the WDM platform should be explored in parallel at both the wavelength and subwavelength level. The former, when deployed in the context of wavelength-routed networks, facilitates to avoid processing the entire traffic volume, and in particular, the transit traffic through the node. Moreover, advances in optical switching technology make it feasible to exploit the subwavelength granularity and, thus, to reconfigure the optical layer with a speed ranging from seconds down to tens of nanoseconds. As a result, the network can be designed assuming two transporting and switching granularities: a fast one consisting of single-wavelength packets (with a duration at the order of a sec) and a multiwavelength packet/slot cascade with a much larger duration (typically 20–30 ms) that is called frame (super-packet) [4]. This scheme allows introducing statistical multiplexing purely in the optical layer without breaking transparency and without introducing, at the initial deployment phase, expensive optical technology. The proposed concept in [4] is primarily addressing hubbedring networks. For networks with meshed connectivity, the scope and the format of the frame should be modified to reflect the new functionality. This is presented in Section III. Still, the trend that was inaugurated with SDH/SONET, i.e., to include administrative and signaling sections within the transportation frame, becomes even more important in a dynamic networking environment, and this topic is addressed in the following section.

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B. Network Control Signaling Beyond Circuit Swithcing Certainly, a data-centric networking solution should be at least as robust as its synchronous predecessor was, while it should have lower latency than SDH/SONET. Therefore, the role of such a frame will be not only to allocate, groom, and efficiently transport short-duration packets, but also to convey the associated control information related to both dynamic routing as well as that of a cumbersome two-way reservation mechanism. To address the dynamic signaling problem, many mechanisms have been proposed, such as e the deployment of MAC protocols (e.g., using a separate channel in ring networks) [8], a one-way reservation signaling mechanism [optical burst switching (OBS)] [9]–[11] or the deployment of a simplified circuit switching two-way signaling, implemented using “light” hardware [12]. The scheme we are proposing here is based on the principles of optical packet switching, something that delegates higher functionality to the optical layer. However, in order for the signaling to be able to support this higher functionality, the interplay between: a) the nominal information transfer rate; b) the adopted processing system; and c) the physical layer transmission scheme should be analyzed in more detail, and this is done in the remaining part of this section. Regarding the transmission scheme, an important consideration is whether the information should be conveyed using an in-band or an out-of-band mechanism [5]. With in-band signaling, the total optical bandwidth dedicated for signaling purposes is minimized. However, in order to carry out various networking operations, bit-level synchronization between the header and the payload needs to be maintained, and well-defined guard bands between them are necessary. This complicates the implementation in a WDM environment. It is important to note that in the framework of transparent networks, the header suffers from the same degree of signal degradation with the payload due to amplified spontanerous emission (ASE) accumulation, dispersion, and fiber nonlinearities, etc., unless all-optical label/header swapping techniques are used. To minimize the packet loss rate, highly reliable labels/headers are needed [5], [11], and for this reason, it is imperative to limit the impact of transmission impairments on them. In all-optical networks, out-of-band signaling can be constructed either transmitting the payload and the header at different subcarrier frequencies [3], [13] or the header can be transmitted using separate wavelength(s). Unlike the previous in-band case, bit-by-bit synchronization circuitry between header and payload is not needed, and since the header is dropped/reinserted in every node, it is regenerated. As a result, the corresponding physical impairments are not accumulated. However, in out-of-band schemes, optical bandwidth is wasted in one form or the other. The second factor that affects signaling is the identification of the appropriate processing system, which is inextricably linked with the amount and the frequency of information that needs to be exchanged between nodes. Regarding the latter, the views in the literature differ significantly simply because different reconfiguration speeds are assumed. In any case, the complexity of these tasks, even in slowly reconfigurable P-o-WDM net-

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Fig. 1. Schematic illustration of the available processing platforms and the associated physical implementation principle.

works, is enormous [5], [6], [14]. The information field in the label/header should includes the following: 1) source and destination addresses; 2) information for synchronization and framing for identifying a particular slot (wavelength) in a multiwavelength frame; 3) header error correction (HEC); 4) packet priority including preferred (shortest) routing paths for high-priority messages or other quality enforcement fields; 5) information regarding network status and restoration parameters for lightpath provisioning and operation, administration, and maintenance (OAM) functions. In addition, information related to network topology, as well as neighbor and resource discovery, needs also to be transported. In Fig. 1, a summary of all existing information processing platforms is presented. As it becomes obvious, the complexity of the aforementioned operations indicates that, despite the advances in optical signal processing, information systems based on electronic devices are the uncontested candidates. Therefore, the role of “optics” is to simplify the associated electronic processing burden. In this paper, we propose to use out-of-band signaling coded so that the electronics deployed to process the information are operating at a speed lower than the line rate, while the information transfer rate is still the same or higher than the line rate. How this is achieved is analyzed in the next section. III. MULTIDIMENSIONAL TRANSPORTATION SCHEMES A. Principle of Operations In Fig. 1, it is shown that there are schemes that allow the electronics to operate at speed lower than the aggregate rate without compromising the information transfer rate. In this paper, like in [15]–[17], the multi- approach is investigated, which is based

on a suitable exploitation of the wavelength and time domains. This approach is a special case of multidimensional transportation where two or more physical properties like time, wavelength, the frequency-amplitude phase of a subcarrier signal, or the polarization state are concurrently used for forming up a sort of a -ary (multisymbol/multilevel) coding scheme. The information capacity of such a system is given by (1) is the number of discrete symbols used for transmitwhere is the symbol rate of the source. ting the information, and is the product of the overall discrete physical states In (1), that could be used for transporting information. It is pointed out that from information capacity point of view, the important factor is the number of symbols used and not the way these are physically implemented. As a result, the various configurations are equivalent. For example, the transported information could be increased four times, for a source with a steady 16 pulse rate, if the transmitted pulses are conveyed by wavelength channels (provided that a maximum a posterior (MAP) receiver is available). Equally, the same result could be achieved using four wavelength channels where each channel is modulated using a clearly distinguished frequency per time slot deploying an -ary frequency-shift-keying/phase-shift-keying (FSK/PSK) scheme. Moving toward the other extreme, the -ary modulation format could be generated using a single wavelength channel and deploying a 16-level FSK or any other subcarrier modulation. It is worth pointing out that many other schemes deploying multi- headers have been proposed in the literature [18]–[24], although the scope is quite different. The objective in [18]–[21] is to construct an optical correlator using either passive [18]–[20] or active [21] elements. In [22]–[24], the multiwavelength principle is applied to form a bit-parallel transportation

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(a)

(b)

(c) Fig. 2. (a) Schematic illustration of a packet roaming an optical network. The packet is comprised from a payload and a multi- header, forming up an out-of-band signaling scheme. (b) Schematic illustration of a multi- transmitter. The M -ary case is shown where only one wavelength is transmitted during a time slot. (c) Hierarchical WDM demultiplexing (the signaling wavelengths are very densely packed). One from M orthogonal symbols is transmitted/received per time slot.

scheme. In [22], the serial-to-parallel conversion occurs only at the receiving end implying that the transmitting part has to operate at the aggregate rate. In [23] and [24], there is a serial-to-parallel conversion in the sense that a single-wavelength high-bit-rate system can be decomposed to a band of lower rate information bearers. The corresponding electronics at the receiving end would also need to operate at the same rate. In [18]–[24], there are also different approaches in terms of the technology to be used. In [18]–[20], the devices are pas-

sive [fiber Bragg gratings (FBGs)], while in [21] and [22], semiconductor optical amplifier (SOA)-based devices are used. Furthermore, the mode of operation of the optical correlator in [18], [19],and [20] is different. In [18] (as well as in [21] and [22]), a reconfigurable and modular networking scheme is implemented, but the underlying bit-level synchronization turns out to be a critical issue. In [19] and [20], there are no synchronization problems at the expense of reconfigurability and optical hardware.

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Fig. 3. Constructing a multi-terabit-per-second capacity frame out of multi- header-based packets. The corresponding switching granularities are illustrated. Packets are dropped/inserted in every optical node without breaking transparency.

In the multi- header concept adopted here, a wavelength channel group is reserved for constructing the packet header, forming an out-of-band signaling mechanism. It is important to note that the wavelengths used for constructing a header are different from those used for transporting payloads. Such a packet is schematically shown in Fig. 2(a). In our scheme, only one wavelength could be present during a given time slot, something that facilitates in forming up an -dimensional space of orthogonal symbols. The cost of the additional transceivers to be used only for signaling purpose is of concern, given that these systems are addressing metropolitan or WANs. Currently, efforts in the optoelectronic industry are aiming to transform the optical components from precious commodity to widely available products through improvements of integration and packaging. Obviously, these advances will have decisive effect for the deployment of systems like those proposed in this paper. In any case, the transmitter side could be implemented using any of the aforementioned techniques [18]–[24], or it could simply be a multiwavelength laser comb integrated with a set of external modulators (ILM), as shown in Fig. 2(b). On the receiver side, the particular concern is to deploy an optical configuration that preserves the order in which the multiwavelength pulses have been transmitted (assuming that dispersion effects have been compensated). We have already proposed such a scheme in [16]. For both multiplexing/demultiplexing, a hierarchical configuration could be introduced, facilitating to minimize spectral waste. This is illustrated in Fig. 2(c). In [16], it is shown that the total bandwidth dedicated for header/label transportation could be minimized through ultradense WDM multiplexing. In particular, two holographic concave grating to serve as WDM (de)multiplexers were proposed; one for 2.5-Gb/s systems (with 8- GHz channel spacing) and one for 10-Gb/s systems (0.15-nm channel spacing). The losses and crosstalk performance of these devices is satisfactory, given that these two gratings will serve a point-to-point system (the header is dropped/reinserted in every node). Hence, despite that a non-ITU-T grid is chosen and that thermal stabilization of the lasers is mandatory, the total bandwidth reserved for signaling is minimized.

B. From Single Packets to Frames The principle of a packet employing a multi- header can be extended to construct a label preceding a mutlipacket frame. A schematic representation of such a frame is shown in Fig. 3. Issues related to delineation and indication of packet slot availability (voids) within a frame are analyzed in [15] and will not be repeated here. It is pointed out that variable-length packets that are integer multiples of the minimum packet/time slot might be added/dropped from the frame when this is routed within the optical network. This allows offering real-time packet grooming in the optical layer, something that improves the wavelength utilization factor. From a systems perspective, the proposed scheme can be seen as a converging point between the all-optical equivalent of a synchronous frame, e.g., STM-256 (or higher) and an OBS frame, even though one can identify important differences between these two frames and the one proposed here. In our paper, the role of this frame is twofold. First, it allows introducing three transporting and switching granularities: one at the entire frame level, one at a single wavelength level, and one at a subwavelength (packet/slot) level. For example, performing pure space switching, the entire frame could be routed from one fiber to the other while wavelengths, and within them individual packets/time slots, could be shared between nodes. Second, a packet within a frame, as well as the header and the payload within a packet, can be identified using passive devices, like a WDM grating demultiplexer or a filter, since header bits are “self routed” to the appropriate detectors. In this process, in contrast to SDH/SONET framing, no dedicated hardware, pointers and/or signal processing is required. The fact that bit-by-bit processing to separate the header from the payload is not necessary alleviates the overall synchronization requirements [(in contrast, in optically time-division-multiplexed (OTDM) in-band packet switching, complex optical and electronic circuitry and bit-by-bit processing is mandatory, even for just separating the header from the payload]. This allows dropping all headers in every node, processing them electronically, and reinserting them before onward transmission, while allowing the payloads to be routed within the optical node’s switch fabric transparently. At the same time, the headers are

SKOUFIS et al.: DATA-CENTRIC NETWORKING USING MULTIWAVELENGTH HEADERS/LABELS

fully regenerated removing all degradation due to channel imperfections (noise, dispersion, and crosstalk, etc.). Finally, unlike synchronous systems, encapsulation restrictions in terms of hierarchy and protocol are not in use, while only the payloads destined to the particular node are dropped and processed. This allows retaining the main advantage of the wavelength-routed networks, i.e., that transit traffic simply bypasses the optical node. These features are common in schemes employing multiheaders, regardless of the way this is implemented. In our proposal, the fact that an -ary coding is employed gives two additional benefits. First, as one can directly deduce from (1), in the proposed scheme, a high information transfer rate is accomplished, while the speed of the electronic devices is retained at a much lower rate. This is an important feature considering the complex processing tasks that need to be executed (Section II-B). In particular, this method becomes the only viable option when the information transfer rate exceeds 10 Gb/s, since it allows deploying commercially available electronic systems. Second, the inherent coding associated with -ary modulation allows achieving low error rate, as it is shown in Section IV-C. Finally, it is worth mentioning that a multi- header can serve as an effective method for routing packets in a meshed network, after suitable network partitioning. The nodes are partitioned in clusters (area and subarea within them and so on) identified by a unique wavelength. In the field dedicated to accommodate source/destination address in the header (Section II-B), this unique combination of “colored” pulses could be directly mapped to domains to facilitate packet routing to the final destination. This scheme is analogous to IP routing; the different wavelengths in the optical header would imply different network domains, as it is indicated in Fig. 4(a), avoiding prefix matching. The simultaneous exploitation of both wavelength (wavelength routing) and subwavelength granularity in the proposed frame is shown in Fig. 4(b), where an entire wavelength is routed from node A to node B while packets have been dropped/added from node A toward node C. Also given that, e.g., in the generalized multiprotocol label switching (GMPLS) framework, the network control cannot set up end-to-end light paths across multiple domains, the proposed frame could be used to support interdomain routing, as shown in Fig. 4(c). In this perspective, the frame will be the optical equivalent of a border gateway protocol to add/drop and route packets from one domain to the other. C. At What Speed Labels/Headers Should Be Transmitted As shown in Fig. 3, since the header wavelengths are the same for all payloads, these headers can be transmitted only serially. For transporting information reasonably efficiently, the overall duration of the concatenated headers should be a small fraction of the duration of the minimum length packet. This requires the headers to be transmitted at the highest possible speed, for example, at a 40-Gb/s nominal information rate. On the other hand, due to back plane restrictions and limitations on the total number of input/output ports, the next-generation terabit-persecond-capacity IP routers is very likely to have fewer ports operating at a line rate of 40 Gb/s. Hence, there might be a need for the entire packet to be transmitted at this rate.

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In this context, a platform offering P-o-WDM at 40 Gb/s is really a challenging topic. This is not only because optical packet switching technology is still under development but also because all-optical networking at 40 Gb/s is still an open question, even for core semistatic networks. Currently, the proliferation of 40-Gb/s optical networking using standard single-mode fiber (SSMF) and binary transmission has not been proven yet for many reasons. 1) Polarization-mode dispersion (PMD) becomes an important limitation at this speed, 2) Second-order dispersion is a bottleneck that currently is tackled using special fiber (NZ-DSF), while the thirdorder dispersion compensation requires a rather complex mechanism. 3) The role of fiber nonlinearities is even more critical, while the OSNR requirements are more stringent. Therefore, it is interesting to explore a scheme where both payload and header/label transportation could be based on -ary modulation (see the discussion in Section III-A). An important observation is that under the proposed scheme, a 40-Gb/s information transfer rate could be achieved using the existing fiber infrastructure, avoiding complex PMD and dispersion compensation mechanisms. Last, but not least, upgrading a transparent optical network from 10 Gb/s to 40 Gb/s would require replacing only the transmitter and the final receiver. Regarding Fig. 1, two mutli- configurations will be investigated, i.e., 1) the bit parallel and 2) the multilevel scheme, to be called -ary hereafter. It is pointed out that the bit-parallel transmission is a special case of an -ary system, as this can be easily deduced from [16, eq. (1)]. In fact, these are two extreme expressions of the same phenomenon. For the remainder of this paper, the various transmission schemes having an information transfer capacity of 40 Gb/s are benchmarked. IV. COMPARATIVE STUDY OF VARIOUS TRANSPORTATION METHODS The main difference between the -ary and the bit-parallel schemes is the way the corresponding systems operate. In the former case, only one out of transmitters can be active, while transmitters could be active during a given in the latter, all time slot. Thus, the bit parallel for fixed number of wavelength channels allows maximum binary data parallelism prior to transmission and, hence, electronics operating at the lowest possible speed. In recent years, bit-parallel transportation has attracted attention (e.g., see [24] amongst many). On the other hand, the -ary case is based on the creation and transmission of orthogonal symbols. The main difference between the two approaches can be found in the information content transmitted during a time slot and the inherent coding embedded in the case orthogonal symbols. with Here, three transportation mechanisms, namely, the binary serial, binary bit parallel, and -ary modulation format are benchmarked against 40-Gb/s information transfer capacity. The assessment of the latter against the two former ones is presented in the following sections. In our analysis, issues related to hardware complexity and complications due to

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(a)

(b)

(c) Fig. 4. (a) Routing scheme compatible to multi- header where the network is partitioned into domains designated by “colors” that can be directly mapped into the source/destination field in the header. (b) Routing through the optical cloud using the proposed frame, demonstrating statistical multiplexing within the optical layer. (c) Optical interdomain routing using the proposed frame.

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multiprocessor parallelism for the bit-parallel case are ignored. The comparative study is solely restricted to transmission performance associated with physical-layer limitations. A. The Influence of Dispersion The impact of different propagation group velocities affects the two transmission schemes that are based on time-slotted multiwavelength headers in a different way. In -ary modulation, dispersion might change the time sequence (the chronological order) of the mutli- pulses, and in this case, a different symbol, compared to the one transmitted, will be received. In bit parallel, a serial binary word is parallelized, and dispersion will affect the reverse process, i.e., from parallel to serial at the receiving end. Therefore, the role of dispersion needs to be studied for both configurations. The system under test is illustrated in Fig. 5, and the corresponding parameters are listed in Table I. The length of the dispersion-compensating fiber (DCF) was chosen to exactly cancel the dispersion for the middle channel of the band reserved for signaling. The symbol with the worst possible dependence on dispersion is considered, i.e., the symbol consisted of alternate and wavelengths. It has been assumed pulses of 6.4 nm, which is a rather extreme case, since, as mentioned in Section III-A, dense channel spacing configurations may apply. The power penalty on the bit-error rate (BER) with respect to the corresponding binary single wavelength channel (adjacent pulses on the same wavelength) is illustrated in Fig. 6(a) and (b) for an single-mode fiber (SMF) length of 80 and 160 km, respectively. In both configurations, the impact of dispersion has been evaluated for a 10-Gb/s system modulated in the return-to-zero (RZ) format with 0.5 duty cycle. The BER curves were obtained using a commercial simulation tool. One can conclude that the compensation provided by the DCF is adequate for negating the dispersion, since the penalty is less than 1 dB for all practical applications. B. The BER Due to Transmission Impairments Following the analysis presented in [16], the symbol error rate for an -ary coded signaling scheme is derived. The cor) at the responding BER versus the electrical SNR per bit ( receiver according to [16, eq. (13)] is (2), shown at the bottom is the number of wavelength channels of the page, where . Equation (2) facilitates the (symbols) in use and comparison of binary and -ary systems. In the derivation of , something (2), it has been assumed that , where designates the symbol, that leads to and the binary bit. A justification of this relation and the condition apply is given in Appendix A. -ary signal and a MAP receiver, the electrical For the , where SNR is defined as

Fig. 5. Transmission scheme for evaluating the dispersion penalty due to the deployment of a multi- header.

, with , , , and being the standard deviations due to thermal noise, shot noise, signal–spontaneous, and spontaneous–spontaneous beating terms. The last expression is substituted in (2) to obtain the BER as a function of bit rate and peak P-o-WDM at the receiver . The BER for the bit-parallel scheme is calculated from the [26] as follows: (3) , and where , , , and are the currents and the standard deviations of noise for 1 and 0, respectively. In principle, given that, in the bit-parallel scheme, the aggrewavelength channel, each gate rate is equally divided over rate, forward-error-correction (FEC) one operating at the codes could be used for improving the BER performance. In practice, the extensive parallelism might be restrictive on the attainable rate. Nevertheless, a simple Hamming code is considered here in order to investigate the tolerance over physical layer impairments. A Hamming code can correct a single error is caland, from [25], the resulting BER for each channel culated from

(4) is the total length of the codeword (data bits where coding bits), is the numbers of bits used for data transmisis the BER of the binary channel prior coding, sion, is the expected number of decoded errors given and primary errors in the received codeword. is calculated from [25] or

(5)

Therefore, by using (3)–(5), the BER is calculated for a system with Hamming code. It is pointed out that from a transmission performance point of view, each channel bit-parallel system suffers the same -factor degradation with that of a channel in a WDM system consisting of equal number of channels. Given that many degradation facWDM tors are bit-rate dependent, and since each one of the

(2)

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TABLE I LIST OF PARAMETERS USED IN THE PHYSICAL-LAYER SIMULATIONS

channels is operating at the of the aggregate rate, OSNR, dispersion, and fiber nonlinearities constraints are relaxed. Regarding dispersion, in Section IV-B, it has been shown that the power penalty is limited. C. Performance Evaluation

(a)

(b) Fig. 6. Dispersion penalty due to (a) 80 km of SMF and (b) 160 km of SMF. The latter is assumed to be the longest point-to-point lengths of a WAN network.

For comparing the three transmission systems, (2)–(4) will be used. The comparison will be made in two steps. In the first stage, the BER performance is deduced ignoring transmission impairments. The fiber sections are represented by their corresponding loss figure (SMF/DCF links). In addition, the receivers are more or less ideal in the sense that the thermal noise Hz for is bit-rate independent with a value set at all cases. In all cases, shot noise is ignored, while the receiver responsivity is one. Still, however, the corresponding electrical Bessel filters have a 3-dB bandwidth that is 0.7 of the corresponding bit rate. In order to compare equal information transfer schemes, for the serial binary configuration, the line rate is 40 Gb/s, the bit parallel consisted of 16 wavelength channels at a line rate of 2.5 Gb/s each, and the -ary scheme consists of 16 wavelength channels with a symbol rate (source) at 10 Gb/s for each wavelength. It is reminded that all these schemes have 40-Gb/s transmission capacity. The optical filter bandwidth is 25 GHz for the systems with a source rate of 2.5 Gb/s and 10 Gb/s, while it is 100 GHz for the 40-Gb/s line-rate system. In the case of the Hamming code (15, 11), only 11 transmitters were sending data, and therefore, the corresponding bit rate was adjusted. The power emitted from each transmitter was 3 dBm on average. The remaining parameters are listed in Table I. In Fig. 7(a) and (b), the BER is calculated for a single 40-km span and for a 4 40-km total length, respectively, versus the received power (a variable attenuator before the p-i-n has been

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Fig. 8. factor versus channel performance comparison between the fiber nonlinearity model of Appendix B and the one obtained using a commercial simulation tool.

(a)

(b) Fig. 7. Comparative assessment of four configurations assuming an ideal receiver for (a) 40 km of SMF and (b) 160 km (4 spans 40 km). A: Serial binary 40-Gb/s link. B: Bit-parallel binary system with 16 wavelengths at 2.5 Gb/s each. C: Bit-parallel binary system with FEC at 3.6 Gb/s per wavelength. D: Optically -ary coded system.

2

M

assumed). The main conclusion was that, in the case of -ary modulation, the lowest error rates are attained. At the same time, the performance of the bit-parallel systems (with or without FEC) makes them an interesting alternative. Given the superior performance of both multi- configurations over the serial binary transmission, the latter scheme is excluded from the remaining studies. For the final comparison, a trace of reality is inserted. The thermal noise is bit-rate dependent, and the influence of fiber nonlinearities is taken into account. Given that nonlinear effects will play a decisive role in assessing the two transportation formats, attention has been paid to model the nonlinearities accurately. The adopted methodology and main assumptions are presented in Appendix B. Up-to-date fiber nonlinearities like XPM, FWM, and SRS are primarily investigated independently of each other when analytical solutions are sought. Here, the nonlinearities are treated collectively. In Fig. 8, the factor of a WDM system is assessed versus the channel index number for 16 channels, spaced by 25 GHz, using both the model of Appendix B and a simulation tool. The average power was 3 dBm, the total length was 160 km (four spans of 40 kKm of SMF/DCF sections), and the source rate was at 2.5 Gb/s. The

DCF length was chosen to exactly compensate the dispersion of the middle channel. The predominant nonlinarity in this case is FWM. Comparing the graphs of Fig. 8, one can conclude that there is a good agreement among them. The bit parallel and -ary systems are compared for different values of the launched power per channel i.e., 6 dBm, 3 dBm, 0 dBm, and 3 dBm average power, for different transmission lengths. It is assumed that the longest intranode distance for a MAN and WAN is 40 and 160 km, respectively. The assumptions used in the simulations are listed above and in Table I. The channel spacing was 25 GHz in all cases for the bit parallel and the -ary systems that are operated at a line rate of 2.5 Gb/s and source rate of 10 Gb/s, respectively. The BER curves versus the level of attenuation to be incurred at the input of the corresponding receiver were obtained and illustrated in Fig. 9(a)–(d). In the MAN case, Fig. 9(a) and (b), the performance of both systems is the same for any practical case for power per channel less than 0 dBm. For higher power per channel, a BER floor is observed in the bit parallel which, however, does not restrict the system since the value of the floor is low. Nevertheless, due to saturation of the optical amplifiers, the maximum power per channel is inherently limited in the bit-parallel scheme, something that is not the case in the -ary scheme (in the latter case, only a single pulse per time slot is transmitted). This implies that the transmitting power per channel in the -ary case can be higher should this be needed. In the WAN case, Fig. 9(c) and (d), the nonlinearities create a fundamentally new situation for values of channel power 0 dBm and above. Therefore, in a WAN where the links have a nominal information transfer rate of 40 Gb/s, the proposed -ary scheme not only allows using the same electronics with that of a 10-Gb/s system but also demonstrates a superior transmission performance. These studies indicate that the both -ary and bit-parallel systems, seen as two extreme cases of a unique phenomenon, show a superior transmission performance with respect to a serial binary system. The choice between the -ary and bit-parallel configuration strongly depends on the network type under consideration (MAN or WAN), as well as on the particular physical-layer constraints that apply.

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of control electronics operating at speeds lower than the line rate without sacrificing the information transfer rate, which can be at the line rate or higher. This is achieved introducing multi- headers/labels in one of the proposed bit-parallel or -ary schemes. Physical-layer modeling studies indicate that the choice between the two configurations strongly depends on the network type under consideration as well as on the particular physical-layer constraints that apply. APPENDIX A RELATION BETWEEN

(a)

(b)

(c)

AND

Assuming the number of symbols in use is , the corre. When the inforsponding information content is mation content is the same, the symbol rate ( ) and the bit rate , which ( ) are connected through the expression . That is, when comparing leads to the expression equal energy pulses, the duration of the symbol should be times longer than the duration of the binary bit. Here, the binary symbols are no longer 1 and 0. Instead, the two binary states are designated by a pulse from two distinctive wavelengths, i.e., and . The two configurations are distinguished in the sense that, in the latter case, there is an absence of a threshold for comparing power levels. factor in an From [16], the expression connection the -ary multi- transmission scheme with a MAP receiver is slightly different from the classical expression [26]

The same relation stands for the SNR per bit (

(A1.1) ). Therefore (A1.2)

. The expression with substituted in (A1.1), leading to

is

(d) Fig. 9. BER performance for (a)-(b) a MAN and (c)-(d) a WAN, assuming: 6 dBm, 3 dBm, 0 dBm, 3 dBm of average P-o-WDMer for the bit-parallel (A, C, E, G) and -ary (B, D, F, H) systems, respectively.

+

+

M

0

(A1.3) ), then

(

When

V. CONCLUSION There are still many questions related to how transportation and signaling will be carried out in the data-centric era. A critical issue is the requested reconfiguration speed in dynamic P-o-WDM networks. The scheme proposed in this work allows transportation in the terabit-per-second regime and, at the same time, exploitation of the subwavelength granularity by adding/dropping the payload/header of a packet to/from the proposed frame using passive devices. The proposed out-of-band signaling alleviates many synchronization problems that are inherent in legacy framing. In addition, it allows introducing a feature-rich signaling schem, while it facilitates the deployment

(A1.4) Consequently, when . then

MODELING

and

APPENDIX B OF FIBER NONLINEARITIES ANALYTIC EXPRESSIONS

,

USING

To study the effect of transmission impairments in WDM periodically amplified dispersion compensated links, an analytical

SKOUFIS et al.: DATA-CENTRIC NETWORKING USING MULTIWAVELENGTH HEADERS/LABELS

model for studying the collective influence of FWM, SRS, and XPM on -factor degradation has been developed. Each fiber link is comprised of an SMF segment of length , followed by a DCF segment of length , and an optical amplifier that exactly compensates the fiber losses. For calculating theXPM-induced crosstalk, the analysis presented in [27] and [28] has been adopted. The XPM is treated as a noise term added on the “mark” symbol. The corresponding noise variance is calculated and used for evaluating the factor. The noise variance at the end of a link comprised of dispersion-compensated sections is given by

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channels creating the crosstalk are simultaneously in the “mark” condition. Regarding SRS, the analysis of [31] and [32] is adopted where the impact of SRS is studied through its mean value as well as the associated statistical variation. Assuming that equally powered channels are launched in the system, the mean power depletion at the end of the fiber link on the th channel is given by

(A2.3)

(A2.1) where represents the responsivity of the receiver and the transfer function of the electrical filter. and are the average power and the intensity spectrum of the probe and , , and the interfering channel, respectively. In addition, represent the dispersion parameter, the nonlinear coefficient, and the attenuation coefficient of the SMF section at the coris responding wavelength . In addition, and the interfering the walk-off parameter between the probe wavelengths, , 2 refers to SMF and DCF, respectively. Following the statistical analysis of [29], the variance of the at the FWM power generated on the channel at wavelength end of the th segment of an -channel WDM system is given by

, , and are the cross-sectional area, The parameters, the effective length, and the Raman gain slope of the fiber section, respectively. Since the channels are statistically independent, the total SRS crosstalk variance is showin in (A2.4), shown at the bottom of the page, where

where is the channel spacing in frequency. For evaluating the performance of the transmission system, the factor has been used as a figure of merit function. Therefore, the assumption that each nonlinearity impacts separately the factor as an independent Gaussian perturbation term has been made. Ignoring interaction terms between then nonlinearities, it is (A2.5)

(A2.2) is the FWM efficiency factor, which is defined in represents the propagation constant difference between the carriers for the SMF is the degeneracy factor that takes and DCF, respectively, ) or 6 (if ). The summation in (A2.2) a value of 3 (if is over all relevant combinations satisfying the relationship for ( ) and ( ). The factor represents the probability that two ( ) or three ( )

where [30].

( 0,1, denoting the space and mark bit, where respectively) are the signal power levels of the th channel. The noise-like-induced degradations is defined as the equais the tion shown at the bottom of the page, where the receiver-induced thermal and shot noises and signal–ASE and ASE–ASE beating noise terms. Finally, , , and represent the optical power crosstalk of the respective nonlinear effects and calculated according to (A2.1), (A2.2), and (A2.4).

(A2.4)

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ACKNOWLEDGMENT

[20]

The simulation tool used in the physical-layer modeling was VPITransmissionMaker from Virtual Photonics Incorporation.

[21] [22]

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C. Skoufis, photograph and biography not available at the time of publication.

S. Sygletos, photograph and biography not available at the time of publication.

N. Leligou, photograph and biography not available at the time of publication.

C. Matrakidis, photograph and biography not available at the time of publication.

I. Pountourakis (M’90), photograph and biography not available at the time of publication.

A. Stavdas (M’00), photograph and biography not available at the time of publication.