Dynamic Routing and Frequency Slot Allocation in Elastic Optical Path Network Using Adaptive Modulations with Consideration of both Spectrum Availability and Distance Hui Dinga, Min Zhanga, Jiuyu Xiea, Ying Wanga, Fei Yeb, Lifang Zhanga, Xue Chena aState Key Lab. oflnformation Photonics and Optical Communications(Beijing Univ. of Posts & Telecom.), P.O.B 201, 10 Xi Tu Cheng Road, Beijing 100876, China bWuhan National Lab. for Optoelectronics, Huazhong Univ. of Science and Technology, 1037 Luo Yu Road, Wuhan 430074, China
[email protected],
[email protected]
ABSTRACT
We proposed a dynamic routing and frequency slots allocation scheme which adopted the adaptive modulation for a novel Spectrum-sLICed Elastic optical path network (SLICE). In our dynamic routing scheme, both spectrum resource availability (SRA) and distance (hop number) are considered at the modulation format selection phase for adaptive modulation. We then conducted numerical simulation to compare our Spectrum Resource Adaptive and Distance Adaptive (SRA-DA) mechanism with previous DA in both small network topology and 7x7 mesh topology. Results shown that SRA-DA mechanism achieved lower blocking rate and higher slots utilization compared to DA in bigger networks. Key words:
Routing and spectrum allocation; Elastic Optical Network; Adaptive modulation 1.
INTRODUCTION
A spectrum-efficient and scalable spectrum-sliced elastic optical path network (SLICE) using Optical Orthogonal Frequency Division Multiplexing (OFDM) was recently proposed to support various client data rate varied from several gigabits per second to more than 100Gb/s [1-12]. Authors in [10] proposed a novel distance-adaptive (DA) modulation to allocate "spectrum slices" to an end-to-end path according to the path distance, in which the modulation level was chosen with respect of the transmission characteristics. This approach was then employed in a DA routing and spectrum assignment algorithm, in which spectrum efficient and complex modulation fonnat are used for short distance while robust and simple modulation fonnat are used for long distance [6]. There are some papers on static planning in flexible OFDM optical network [2, 9]. Christodoulopoulos, K etc. proposed the static planning in SLICE using Integer Linear Programming (lLP). However, so far there are few papers discussing the dynamic routing and spectrum allocation in SLICE network. Authors in [1] investigated this problem by adopting DA modulation; the modulation fonnat is chosen according to the source-destination pair distance (hop number) to satisfy the Bit Error Rate (BER) limitations for the paths. The spectrum resource availability in each link that constructed the route is not considered in previous routing and frequency slot allocation yet.
Network Architectures, Management, and Applications IX, edited by Lena Wosinska, Ken-ichi Sato, Jing Wu, Jie Zhang, Proc. of SPIE-OSA-IEEE Asia Communications and Photonics, SPIEVol. 8310, 83101E . © 2011 SPIE-OSA-IEEE . CCC code: 0277-786X/11/$18 . doi: 10.1117/12.904215 SPIE-OSA-IEEE/Voi. 8310 83101E-1
In this paper, we present a dynamic routing and frequency slot allocation scheme considering both spectrum availability (SA) and DA modulation in SLICE. By dynamically updating spectrum resource database (SRB) like a simplified Traffic Engineering Database [13], the modulation level of a computed least crowded route can be adjusted. After that, the allocated spectrum can fully accommodate the client request. We hence evaluate the requests blocking rate and necessary slots comparing the proposed SRA-DA with DA in SLICE in two topologies. 2.
DYNAMIC ROUTING AND FREQUENCY SLOT ALLOCATION CONSIDERING BOTH SPECTRUM RESOURCE AVAILABILITY AND DISTANCE-ADAPTIVE MODULATION
We assume that a preliminary network planning phase is completed before service requests arrival, during which the modulation level is set for each path with respect to only the length of the path by distance-adaptive modulation. We also assume that no wavelength conversion is offered at any node and all connection requests are full-duplex. The procedure of the proposed algorithm is described below. Step 1. When the client request arrives, choose the route that offers the most available frequency slots according to SRB from the candidate paths of the source-destination node pair. Try to apply the pre-set modulation format to the selected route based on the hop number. Step 2. After the fust step modulation selection, if there are enough consecutive slots available on every link of the path, select those slots that started from the lowest frequency for all the links. The route and the slot set are assigned to the request and the path is established. Update the SRB. Step 3. If there are not enough consecutive slots after the first step modulation selection, check the client request data rate. When the request is a low data rate (ie. several gigabits per second) service, shift the modulation format to higher level in order to allocate less slots. Assign the slot set to the request and establish the path. Otherwise, block the request. Update the SRB. Step 4. Tear down the connections whose holding time expire, release the frequency slots on their routes and update the SRB again. Go to step 1. For example, consider the 6-node network as shown in Fig.3. The numbers along edges represent the available frequency slots. Consider a client request A to F requesting low data rate. It can only be routed on A-B-C-E-D-F route using slot set (6, 7, 8). Under only DA modulation, the five-hop route [10] should adopt QPSK and four consecutive slots are needed to establish this route. However, there are only three available now. In our algorithm, since the A to F request is a low data rate request, it fits the spectrum availability adaptive modulation condition and the modulation format can be shifted to 16QAM with acceptable QoT. In this case, three consecutive frequency slots are enough for the request and the A to F path can be finally established. Hence, now more requested paths are setup thanks to adjusting modulation format according to both spectrum availability and path distance.
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NUMERICAL SIMULATIONS AND DISCUSSIONS
We perfonned some numerical simulations to compare the blocking rate and necessary frequency slots using SRA-DA to DA. In the simulations, we assume all the channels are capable of transmitting up to 112Gbps bit rate data at two different modulation levels with all the linear impainnent compensated with respect to different path distance [1, 6, 10]. OFDM adopted in this paper generates 8 subcarriers which divide the ITU-T grid 100GHZ into 8 frequency slots. So, the frequency slot width is set at 12.5GHz while the modulation fonnats is to be chosen from 16QAM and QPSK. The dynamic connection requests follow as the Poisson process and the source-destination node pair is generated randomly from all the nodes. The traffic is unifonnly distributed among all the node pairs. The holding time of each connection follows a negative exponential distribution. Physical topologies used in the experiments are a 7x7 mesh topology and NSFNET topology where all physical link lengths are fixed at 50km. According to [14], the capacity of the Broadband DCS layer, IP layer and high rate private line contribute 42%, 43% and 15% respectively, of the total demand of the optical layer. So it's reasonable to set the majority of the service demand low data rate while the minority of it is set high data rate. The hop number of each least crowded path route versus path number under the same traffic load is shown in FigA. The three horizontal lines plot the number of frequency slots needed for the hop number when DA modulation is used. Table 1 shows the statistical results of necessary slots distribution using SA-DA-SLICE when no request is blocked, for the NSFNET topology and 7x7 mesh topology respectively. Table I Distribution of necessary slots nsfnet
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We evaluate the blocking rate when different requested data rate is considered and the traffic distribution remains the same. The requested data rate is divided into two categories: high data rate, which includes the Broadband DCS traffic and the IP traffic, and the low data rate, which is high rate private line traffic. Fig.5 and Fig.6 show respectively, the blocking rate (bars) and necessary frequency slots (dots) using SA-DA compared with only DA at different requested data rate distribution for 7x7 mesh topology and NSFNET topology. Nearly zero blocking is achieved in both topologies using SA-DA with about 20% extra necessary frequency slots needed. SA-DA outperforms DA in term of blocking rate, for the reason that SA-DA algorithm computes the least crowded route according to SRB. We can also observe that blocking rate decreases as the proportion of low data rate requests increases in Fig.5. This is because when there are not enough frequency slots to serve the low data rate request, the modulation level on this path can be lifted to higher level, hence the path is setup, more frequency slots are necessary then. The same pattern can be also seen with DA. On the other hand, in NSFNET topology, the blocking rate is not relevant with the variation in proportion of low data rate requests, since nearly all paths in NSFNET topology are less than five hops so only the higher modulation format of the two are used. So in NSFNET topology, even when it is the low rate service that short for frequency slots, there is no other modulation format it can be shifted to, so the service is blocked anyway. 4.
CONCLUSION
We have proposed a dynamic routing and frequency slots allocation algorithm in SLICE networks using spectrum availability and distance adaptive modulation. Numerical experiments have been conducted; the results showed that our algorithm can largely reduce the blocking rate in large scale networks with reasonable necessary frequency slots increase compared to adopting only DA modulation. ACKNOWLEDGEMENTS
This study is supported by 863 Program "Technologies & Prototype of Gridless, Rate-Variable Optical Switching", Fund amental Research Funds for Central Universities (2009GYBZ, 2009RC0401), Beijing Municipal Education Commission Sci.
&
Tech. Programs (KM2006 10009014, KM201110009001). REFERENCE
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