IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 2, JANUARY 15, 2013 183 Traffic Grooming and Spectrum Assignment for Coherent Transceivers in Metro-Flexible Networks Cristina Rottondi, Massimo Tornatore, Achille Pattavina, and Giancarlo Gavioli Abstract— Novel distance-adaptive optical transmission technologies have been proposed to boost transceiver datarates and to enable more flexibility in the allocation of traffic flows. The application of this new class of transceivers is being widely investigated in core networks, while their suitability in the metro area is still an open issue. On one hand, the short metro distances enable the utilization of higher spectrally efficient modulation formats, on the other hand, the lower bitrate suggests to employ lower baud rate with respect to core networks. In this letter, we perform traffic grooming and spectrum assignment using transceivers with fixed baud rate of 28 and 14 GBd and distance-adaptive modulation formats in optical metro networks. Comparisons with the wavelength-division multiplexing systems running over a fixed grid show that 1) significant savings in terms of spectrum occupation can be achieved, and that 2) such savings can be effectively achieved also using lower baud rate transceivers (e.g., 14 GBd). Index Terms— Metro network, optical coherent transmission, ring network, traffic grooming. I. I NTRODUCTION T HE METRO network conveys the traffic generated by the access segment of the telecom network to the backbone portion. Its extension normally ranges from tens to hundreds of kilometers and it is traditionally based on well-known tech- nologies such as SONET/SDH, Metro Ethernet and Resilient Packet Ring (RPR). In the last decades, the sharp increase of metro traffic demand has been accommodated thanks to the introduction of wavelength-division multiplexing (WDM) technology. Cur- rently, WDM technology standards [1] define a division of the fiber spectrum in channels with fixed 50 GHz or 100 GHz bandwidth. Inside these channels, signals are transmitted using transceivers at fixed rate (typically 10, 40 or 100 Gbps). The recent development of innovative transmission tech- nologies based on optical coherent detection and digital signal processing is paving the road towards software defined trans- ceivers [2] which adapt their modulation format according to the reach to be covered and/or to the amount of traffic to be transmitted [3]. The introduction of finer-grained frequency slots (e.g., 6.25 or 12.5 GHz, as suggested in [4]) is expected to Manuscript received September 7, 2012; revised October 24, 2012; accepted November 7, 2012. Date of publication November 20, 2012; date of current version January 9, 2013. C. Rottondi, M. Tornatore, and A. Pattavina are with the Dipartimento di Elettronica e Informazione, Politecnico di Milano, Milano 20133, Italy (e-mail: rottondi@elet.polimi.it; tornator@elet.polimi.it; pattavina@elet. polimi.it). G. Gavioli is with Alcatel-Lucent, Optics Division, Vimercate 20059, Italy (e-mail: giancarlo.gavioli@alcatel-lucent.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2228479 increase spectrum utilization, by reducing/eliminating unnec- essary guard bands and by flexibly adapting the spectrum allocation to the traffic demand (flexible grid). In our previous paper [5], we have analyzed the benefits achieved by the introduction of such elastic transceivers in metro networks in terms of spectral occupation and discussed the impact of having distance-adaptive modulation techniques and reconfigurable baud rate. Here, we expand our analysis to consider the following novel assumptions: 1) we consider coherent transceivers with fixed (specifically 28 and 14 GBd) baud rates; 2) we consider a flexible spectrum grid with granularity of 12.5 and 6.25 GHz, according to [4]; 3) we consider the possibility of fitting such transceivers in channels with bandwidth of 18.75 1 , 25, 31.25, 37.5, or 50 GHz, in order to identify the most effective trade-off between spectral efficiency and reach; 4) we consider/model the formation of superchannels [7]. Under these new assumptions, we perform traffic grooming and spectrum assignment for a set of given traffic demands and we investigate if, in a metro network operating with narrow flexible grid, lower baud rate transceivers can provide a more effective spectrum utilization. The remainder of the letter is as follows: Section 2 describes the metro network architecture and the design problem we address. Numerical results are discussed in Section 3, while Section 4 concludes the letter. II. NETWORK ARCHITECTURE AND MODEL 1) Network Topology, Spectrum Grid,Transceiver Model: In our work, the metro network physical topology is modeled as a bidirectional ring network with N nodes and 2 N fiber links divided in a grid of frequency slots having M GHz slot width. Each node is equipped with coherent transceivers which can support multiple modulation formats, but using a fixed baud rate B . Given a transceiver with baud rate B , the corresponding channel bandwidth or spectrum occupancy F (i.e., the exact quantity of spectrum to be reserved to serve the signal emitted by the transceiver) is evaluated on a flexible grid as an integer multiple k of the slot width M, such that F = k · M ≥ B . Note that increasing the ratio B / F between the baud rate and the channel bandwidth of a transceiver leads, on one hand, to higher spectrum efficiency but, on the other hand, to additional crosstalk due to adjacent channel coherent interference and thus reduces the maximum transmission reach, especially for 1 Note that using odd multiples of 6.25 GHz requires optical carriers to be placed on a grid of 3.125 GHz granularity. 1041–1135/$31.00 © 2012 IEEE