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Routing, Code, and Spectrum Assignment (RCSA) in Elastic Optical Networks N. Sambo1,2 , G. Meloni2 , F. Cugini 2 , A. D’Errico3 , L. Pot`ı2 , P. Iovanna3 , P. Castoldi1 1: Scuola Superiore Sant’Anna, Pisa, Italy; 2: CNIT, Pisa, Italy; 3: Ericsson, Pisa, Italy
[email protected]
Abstract: This paper introduces the concept of code selection in RSA for EONs. Simulations show that code-adaptive time frequency packing reduces blocking probability by one order of magnitude with respect to format-adaptive Nyquist WDM. OCIS codes: 060.0060, 060.4251.
1.
Introduction
Elastic optical networks (EONs) or flex-grid optical networks may leverage on distance adaptive transmission techniques able to trade-off between spectral efficiency (i.e., the information rate transmitted over a unit of bandwidth) and the all-optical reach. Typically, distance adaptation has been achieved in the literature by relying on different modulation formats [1–3]. Such technique may require transponders able to support several modulation formats such as polarization multiplexing quadrature phase shift keying (PM-QPSK) and polarization multiplexing 16 quadrature amplitude modulation (PM-16QAM). Recently, distance adaptation through code adaptation has been proposed [4–7]. An example is the time frequency packing (TFP) transmission technique also capable of achieving very high spectral efficiency (e.g., 5.1 b/s/Hz with PM-QPSK) [4] . In particular, code redundancy is tuned based on the optical reach (the larger the length the more the redundancy). Code adaptation can be applied with a transponder supporting a single modulation format (e.g., PM-QPSK) [4] or with a transponder supporting multiple modulation formats (e.g., PM-QPSK and PM-16QAM) [7]. This paper, for the first time, introduces the concept of routing, code, and spectrum assignment (RCSA) in EONs. RCSA is applied to TFP super-channels (i.e., optical connections obtained with multiple sub-carriers). Coding is selected to satisfy quality of transmission (QoT), also considering detrimental filtering effects, an important issue in EONs. Simulation results show the benefits in terms of blocking probability with respect to modulation-formatdistance-adaptive Nyquist wavelength Division Multiplexing (NWDM) techniques (including PM-QPSK, PM-8QAM, and PM-16QAM). Code-adaptive transmission is able to reduce blocking by more than one-order of magnitude with respect to modulation-format adaptive NWDM. 2. Routing, code, and spectrum assignment (RCSA) Consider a lightpath at information rate R transmitted on a super-channel consisting of a number N of PM-QPSK optical sub-carriers. TFP is exploited to reduce the bandwidth Bi of each sub-carrier and their spectral separation below the Nyquist limit [4], leading to high spectral efficiency (e.g., 5.16 b/s/Hz) and enabling code adaptation. Coding and coherent detection are properly designed to account for signal degradations (e.g., amplified spontaneous emission, optical filtering effects). Low-density parity-check (LDPC) code is used to approach the maximum information rate achievable with PM-QPSK. A set of LDPC code rates ci = f /b can be exploited, where b − f bits of code are transmitted each f bits of information. Code rate (i.e., redundancy) affects the ability to correctly receive the information transmitted over an all-optical path, also traversing a certain number of nodes, thus considering filters (spectrum selective switches —SSSs). It is assumed that the sub-carrier baud rate is fixed by the electronics. This imposes a maximum rate Ri transmitted per sub-carrier. Such rate includes information and coding. Consequently, based on the selected code ci , the number of sub-carriers has to be selected in order to satisfy the requested information rate R. Fig. 1a shows the flow chart of the proposed RCSA. After connection request between nodes s, d at an information rate R, a set of k paths is computed between s and d. Then, the more spectral-efficient code ci (i.e., less redundant) satisfying QoT on at least a path within the k paths is selected. ci is selected accounting for noise, fiber non-linear effects, and number of traversed filters (which depends on the number of path hops). An empirical model will be shown later. Then, a new set of paths obtained by removing all the paths not satisfying the QoT with ci is considered. The number N of sub-carriers is computed considering that the sub-carrier information rate is Ri × ci . Consequently, B = N × Bi imposes the width of the ITU-T flex-grid slot to switch the super-channel, defined as m × 12.5 GHz [8]. In particular, the computation of the width, thus of m, has to account for B (the width must be larger or at least equal to
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W1I.1.pdf
s,d connection
request at rate R
(a)
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Tab. 1 Relation between ci, all-optical reach and traversed node, N, B, and m
set of k pre-computed paths
between s,d
ci
Reach [km]
Traversed nodes
N
B [GHz]
8/9
320
4
7
140
m=12; 150
8/9
400
5
7
140
m=13; 162.5
4/5
640
8
8
160
m=14; 175
3/4
720
9
9
180
m=15; 187.5
1500
12
9
180
m=16; 200
3/4
computation of best code
ci satisfying QoT on at least a path
of the k paths
10
Blocking probability
eliminates
unacceptable paths
with ci from the k paths
Compute N
satisfying R
considering ci
0
10
-1
10
-2
10
-3
Format adaptive (PM-QPSK,PM-16QAM) Code adaptive
(b)
10-4 10
Compute super-channel
bandwidth B
m and width [GHz]
-5
10-6
50
100
150
200
250
300
350
400
Offered traffic load [Erlang] 0
10
Spectrum assignment YES
Is available
spectrum? YES Connection set up
NO
Is available
a more redundant
code ci?
Blocking probability
ITU-T m computation accounting for filtering effects
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
Format adaptive (PM-QPSK, PM-8QAM, PM-16QAM) Code adaptive
(c)
NO Connection blocked
50
100
150 200 250 300 Offered traffic load [Erlang]
350
400
Fig. 1. RCSA flow-chart (a); blocking vs. offered load when NWDM includes: PM-16QAM and PM-QPSK (b), PM-16QAM, PM-8QAM, and PM-QPSK (c). B) and has to account for filtering effects. Indeed, because of the not-ideal flat passband of the SSSs, the width could not be selected to a value too close to B since the filter transition bands may introduce distortions on the transmitted signal, in particular on the two side sub-carriers. Thus, m could require to be over dimensioned. Once ci , N, and m are computed, routing and spectrum assignment is performed on the set of paths available for the selected code ci . Then, connection is set up. In case spectrum with width m satisfying continuity constraint is not found, a more redundant code is assumed. This results in the possibility to consider for routing and spectrum assignment a larger set of paths within the k paths, thus increasing the probability to establish the connection. If no more code rate can be considered, the connection is blocked. Tab. 1 shows the relation between the code rate, the optical reach and the number of traversed nodes (thus, including filtering), N, and m. Reported values satisfy error free transmission. Such relations have been identified through measurements on a 1 Tb/s information-rate super-channel considering the re-circulating loop as in [4], by adding a SSS Finisar wave shaper inside the loop of 80 km. Bi and Ri were set to 20 GHz and 160 Gb/s, respectively. As an example, it is possible to see that a ci = 8/9 guarantees QoT for a path of four nodes when m = 12 (150 GHz is larger than B = 140 GHz). If five nodes instead of four have to be traversed, the same redundancy can be kept, but filters have to be enlarged, thus m = 13. If six nodes have to be traversed more redundancy is required (ci = 4/5) enabling up to eight nodes path. Finally, we clarify that the QoT is mainly affected by filtering effects, indeed, without filters in the loop in [4], 5000 km were reached. Regarding the complexity of code-adaptive TFP, PM-QPSK TFP requires a transponder only supporting PM-QPSK, thus it does not need digital-to-analog converter (DAC) at the transmitter. At the receiver, TFP requires a sequence detector [4]. Conversely, format-adaptive NWDM requires a DAC at the
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transmitter (e.g., to support 16QAM), while symbol-by-symbol detection at the receiver. 3. Simulation results A custom built event-driven C++ simulator has been exploited. The network topology used in [7] with 30 nodes and 55 bi-directional links is considered. Code-adaptive TFP transmission at 1 Tb/s information rate is assumed with transmission characteristics shown in Tab.1 and explained in the previous section. Code-adaptive TFP transmission is compared with format-adaptive Nyquist Wavelength Division Multiplexing (NWDM) transmission [9] considering up to three modulation formats: PM-QPSK, PM-8QAM, PM-16QAM. All-optical reach of PM-16QAM, PM-8QAM, PM-QPSK is provided by [9] and it is 270, 630, and 1890 km, respectively. The model in [9] does not account for filtering effects, thus, for format adaptation, we assumed ideal filtering or effective pre-distortion/pre-emphasis against filtering. From the same paper, we derived m, that is 11, 14, 20 for PM-16QAM, PM-8QAM, and PM-QPSK, respectively. Inter-arrival process of 1 Tb/s requests is Poissonian. The holding time follows a negative exponential distribution with mean 500 s, with requests uniformly distributed among all node pairs. The set of k paths is composed of all the paths within one hop from the shortest path in terms of hops. Fig. 1b shows the comparison between code-adaptive TFP and format-adaptive NWDM when the latter may exploit PM-16QAM and PM-QPSK. Even by considering filtering effects, TFP is able to reduce blocking probability with respect to format-adaptive NWDM, e.g. by more than one-order of magnitude at 150 Erlang. Fig. 1c shows the comparison between code-adaptive TFP and format-adaptive NWDM with PM-16QAM, PM-8QAM or PM-QPSK. Exploiting PM-8QAM provides benefits to format adaptation since PM-8QAM achieves larger distances than PM16QAM and it is more spectral efficient than PM-QPSK (m = 14 instead of 20). Code-adaptive TFP still achieves lower blocking probability than format-adaptive NWDM. This is due to the combinations of two factors. First, in the considered scenario, code-adaptation provides more flexibility than format-adaptive NWDM by enabling up to five possibilities (see the set of codes in Tab. 1) to trade off between reach-nodes and spectral efficiency, thus spectrum can be better used. Second, code-adaptive TFP is a faster-than-Nyquist transmission technique achieving high spectral efficiency with a low order format as PM-QPSK: e.g. 6.1 b/s/Hz (considering the second line of Tab. 1 where 1 Tb/s is switched in 162.5 GHz) with respect to the maximum of 4 b/s/Hz achievable with NWDM PM-QPSK. 4. Conclusions Routing, code, and spectrum assignment (RCSA) has been introduced and investigated in this paper with the aim of trading off between the robustness of the quality of transmission and the spectral efficiency, while accounting for detrimental filtering effects. Simulations have shown that code-adaptive time frequency packing reduces blocking probability by one order of magnitude with respect to format-adaptive Nyquist wavelength division multiplexing. Acknowledgment: This work was supported by the FP-7 IDEALIST project, grant agreement 317999. References 1. F. Cugini, G. Meloni, F. Paolucci, N. Sambo, M. Secondini, L. Gerardi, L. Poti, and P. Castoldi, “Demonstration of flexible optical network based on path computation element,” JLT, 2012. 2. O. Gerstel, M. Jinno, A. Lord, and S. Yoo, “Elastic optical networking: a new dawn for the optical layer?” Communications Magazine, IEEE, vol. 50, no. 2, pp. s12 –s20, february 2012. 3. X. Cai, K. Wen, R. Proietti, Y. Yin, D. Geisler, R. Scott, C. Qin, L. Paraschis, O. Gerstel, and S. Yoo, “Experimental demonstration of adaptive combinational qot degradation restoration in elastic optical networks,” JLT, 2013. 4. N. Sambo, G. Meloni, F. Paolucci, F. Cugini, M. Secondini, F. Fresi, L. Poti, and P. Castoldi, “Programmable transponder, code and differentiated filter configuration in elastic optical networks,” JLT, 2014. 5. D. Mello, A. Barreto, T. de Lima, T. Portela, L. Beygi, and J. Kahn, “Optical networking with variable-code-rate transceivers,” JLT, 2014. 6. I. Cerutti, F. Martinelli, N. Sambo, F. Cugini, and P. Castoldi, “Regenerator placement in code-rate-adaptive flexi-grid networks,” in Proc. of ECOC 2014, sept. 2014. 7. N. Sambo, A. D’Errico, C. Porzi, V. Vercesi, M. Imran, F. Cugini, A. Bogoni, L. Pot`ı, and P. Castoldi, “Sliceable transponder architecture including multiwavelength source,” JOCN, 2014. 8. “Draft revised G.694.1 version 1.3,” Unpublished ITU-T Study Group 15, Question 6. 9. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,” JLT, 2011.
