Cross-Layer Spectrum Defragmentation for IP over Elastic Optical Network
Yunrong Zhang, Ya Zhang, Yongcheng Li, Gangxiang Shen* School of Electronic and Information Engineering
Soochow University, P. R. China *corresponding email: [email protected]
Yonghu Yan, Wei Chen Key Lab. of New Fiber Tech. of Suzhou City
Jiangsu Hengtong Fiber Science and Technology Corporation Suzhou, Jiangsu Province, P. R. China
Abstract—Spectrum defragmentation is important for an elastic optical network (EON) to enhance its spectrum utilization. However, the cause of spectrum fragmentation is the dynamicity of client-layer services. Thus, it is of significance to also consider the reconfiguration of the client-layer services when carrying out spectrum defragmentation. We expect that this effort can further improve the network spectrum utilization. Motivated by this, we propose cross-layer spectrum defragmentation for an IP and EON network so as to achieve better spectrum utilization by this joint effort. Simulation results show that the proposed cross-layer scheme is beneficial to greatly improve bandwidth blocking performance (BBP) compared to the other schemes that do not consider such joint effort.
Keywords—IP over EON, cross-layer spectrum defragmentation, spectrum utilization, BBP
I. INTRODUCTION
Elastic optical network (EON) has received extensive interest in recent years due to its flexibility in spectrum allocation and high spectrum efficiency. Setting up and tearing down lightpaths with different numbers of frequency slots (FSs) dynamically will lead to severe spectrum fragmentation, which would degrade the overall network spectrum utilization of an EON [1]. Various spectrum defragmentation strategies have been proposed to smooth the network spectrum usage for future lightpath service connections [2]. However, most of the exisitng studies have focused on the optical layer, not jointly considering the IP layer. Actually, the service request arrival and departure in the IP layer is the root cause leading to the spectrum fragmentation in the optical layer since the latter provides transport capacity for the former. Thus, there is an important open question: can we further improve the network service provisioning performance by jointly considering the reconfiguration of IP traffic flows in addition to implementing the spectrum defragmentation?
This paper aims to answer the above question. We for the first time propose cross-layer spectrum defragmentation for the IP and optical layers. Specifically, we consider not only rerouting traffic flows in the IP layer but also changing the route and spectrum of each established lightpath in the optical layer. To implement the defragmentation, we have employed the triggering mechanism of defragmentation upon blocking; that is, a defragmentation process is triggered whenever a service connection is blocked [2]. Also, when implementing the defragmentation, two defragmentation strategies are employed, i.e., defragmentation with sequentially releasing and re-establishing service connections (SR-D) and defragmentation with jointly releasing and re-establishing service connections
(JR-D) [2]. Under these defragmentation strategies, the proposed cross-layer spectrum defragmentation scheme is evaluated in comparison with other two schemes, i.e., IP-layer reconfiguration only and optical-layer defragmentation only. Simulation results show that the proposed cross-layer scheme is efficient to significantly outperform the other two schemes in terms of bandwidth blocking probability (BBP).
II. DEFRAGMENTATION SCENARIOS
This study evaluates three defragmentation schemes, i.e., IP-layer (traffic) reconfiguration only, optical-layer (spectrum) defragmentation only, and cross-layer (spectrum) defragmentation. We first explain the scheme of cross-layer defragmentation, based on which we further introduce the other two schemes. When the cross-layer defragmentation is triggered, we first release all the IP traffic flows. If this release leads to some lightpaths free of IP traffic, we further release these lightpaths. Then we employ an auxiliary graph (AG)-based multi-layer traffic grooming algorithm (introduced later) [3] to re-establish the released services and the new service.
Fig. 1 uses an example to illustrate the key idea and the benefit of cross-layer defragmentation. Given a six-node network with the state of network resources shown in Fig. 1a. We assume that each link has 10 FSs and there are three service connections S1 (B-D), S2 (C-D), and S3 (B-C) that require 160-Gb/s, 170-Gb/s, and 20-Gb/s bandwidths, respectively. Their corresponding IP traffic flows and the lightpaths provisioning capacity for these IP flows are as shown. Specifically, the lightpath (B-A-C-D) is assigned with the modulation format of BPSK and FSs from 1 to 7, which corresponds to the virtual link B-D in the IP layer. With the accommodation of S1, the remaining capacity of the virtual link B-D is 65 Gb/s. Similarly, we can see the modulation formats and FSs assigned for the other lightpaths and their corresponding virtual links. The virtual links C-D and B-C have 30-Gb/s and 130-Gb/s remaining capacities, respectively.
After the cross-layer defragmentation, the state of the service connections and the network resource usage are shown in Fig. 1b. Here the lightpath (B-D) replaces the lightpath (B-A-C-D) to use a higher level of modulation format 8-QAM and require fewer FSs due to its shorter lightpath distance, and it is similar for the lightpath (C-D) that replaces the lightpath (C-E-D). Also, the lightpath (B-A-C) is released without the re-establishment of a corresponding lightpath. This is because the service connection S3 (B-C) can be established by using the remaining capacity on the virtual links B-D and C-D, thereby saving network spectrum resources.
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Fig. 1. Examples of different defragmentation schemes.
The other two defragmentation schemes are similar to the cross-layer one except that they carry out the defragmentation or reconfiguration only in a single layer. In the scheme of IP-layer reconfiguration only, we only reroute the IP-layer traffic flows, but keep the lightpaths in the optical layer untouched (see Fig. 1c). In contrast, in the scheme of optical-layer defragmentation only, we only reroute the lightpaths in the optical layer, but keep the traffic flows in the IP layer untouched (see Fig. 1d). Comparing to the network resource usage of cross-layer spectrum defragmentation, the other two
schemes consume more network spectrum resources, which therefore demonstrates the efficiency of the cross-layer defragmentation scheme.
III. ALGORITHM FOR CROSS-LAYER DEFRAGMENTATION
The primary objective of spectrum defragmentation is to smooth network spectra for future arriving requests so as to reduce service blocking probability. With this goal, we propose the scheme of cross-layer (spectrum) defragmentation, in which the defragmentation upon blocking triggering mechanism [2] and the JR-D and SR-D strategies [2] are employed. Whenever a service request is blocked, the defragmentation process is triggered. For this, we implement the JR-D strategy, after which we try to establish the blocked request again. If successful, we establish the service connection; otherwise, it is blocked. In JR-D, when defragmentation is triggered, we first release the network resources used by all the released service connections and then jointly re-establish them. Specifically, in the re-establishing process, we sort the service connection according to their traffic demands from the highest to the lowest (i.e., establish the services with the highest traffic demand, and if two services have the same traffic demand, first establish the one with a longer remaining service time). In SR-D, we first sort all the established service connections in a descending order based on their traffic demands and then release and re-establish these service connections one by one. Next, we present the algorithm for this cross-layer defragmentation scheme as follows, which is based on the JR-D strategy.
Cross-layer spectrum defragmentation Step 1: Release the used resource of all the established service
connections altogether. Step 2: Sort the established service connections in a descending
order based on their traffic demands and remaining service times.
Step 3: Get the first service from the ordered list and try to re-establish it.
Step 4: Employ an auxiliary graph (AG) which is built for the IP and optical layers to carry out multi-layer traffic grooming for the services. If the service connection can be established, move to Step 6; otherwise, move to Step 5.
Step 5: If the service connection cannot be established, we release the resource of all the previous re-established service connections and recover the used resource of all the established service connections to their original status, then implement the SR-D algorithm (introduced later).
Step 6: Repeat steps from 3 to 5 for all the remaining services.
Here for efficient network resource allocation under the multi-layer environment, we employ the AG-based traffic grooming algorithm [3]. Specifically, an AG (as shown in Fig. 2) is constructed when a service request arrives. In the AG, the virtual nodes of the same name correspond to the physical nodes in the physical topology. There are three types of directed edges, i.e., lightpath edges, transponder edges, and fiber edges. The fiber edge corresponds to a physical link in the physical topology, the lightpath edge corresponds to an existing lightpath, and the transponder edge is constructed to connect the virtual nodes of the fiber edge and the lightpath edge [4]. In Fig. 2, based on such an AG, we can employ Dijkstra’s shortest path algorithm to find the shortest route with the minimum sum weight between A and D, i.e., A-A1-
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B1-B-B2-D1-D. The details of AG-based traffic grooming algorithm are not presented here due to the page limit.
A1 B1
A DB
B2
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D1Lightpath egde
Transponder egde
Fiber egde
Fig. 2. An example of an auxiliary graph.
Also, in Step 5 of the cross-layer defragmentation algorithm, when it is impossible to re-establish all the released service connections, we would refer to the SR-D algorithm, which is described as follows.
SR-D Algorithm Step 1: Sort the established service connections in a descending
order based on their traffic demands. Step 2: Get the first service connection from the ordered list,
release its used resource, and try to re-establish it. Here the IP-layer traffic flow is released first and if this release leads to some lightpaths free of IP traffic, also release these lightpaths.
Step 3: Employ the AG-based multi-layer traffic grooming for the released services and re-establish it.
Step 4: Repeat steps from 2 to 3 for the remaining service connections in the ordered list.
IV. PERFORMANCE EVALUATION
The performance of the proposed defragmentation schemes was evaluated via simulations. We considered 11-node 26-link COST239, 14-node 21-link NSFNET, and 21-node 25-link ARPA-2 as our test networks in which each fiber link has 320 FSs. A dynamic service request model is assumed, under which request arrival follows a Poisson process and their holding time follows a negative exponential distribution. The traffic demand of each service request is uniformly distributed within the range of [10, 400] Gb/s. A total of 106 service requests were simulated to calculate BBP, which is defined as the ratio of the total amount of blocked service bandwidth to the total amount of arriving service bandwidth.
Fig. 3 shows how the BBP changes with an increasing traffic load (Erlang) per node pair, where the legend “IP_only” corresponds to the scheme of IP-layer reconfiguration only, “Opti_only” corresponds to the scheme of optical-layer defragmentation only, and “Cross-layer” corresponds to the scheme of cross-layer defragmentation. We can see that the cross-layer defragmentation scheme can always achieve the best performance. This implies the effectiveness of the proposed joint effort. Also, comparing the results of the different test networks, we see that with the decrease of network nodal degree, the performance of the scheme of optical-layer defragmentation only seems to improve significantly. For the highest nodal degree (i.e., COST 239), the scheme of optical-layer defragmentation shows the highest BBP, higher than the scheme of IP-layer reconfiguration only; however, for a lower nodal degree (e.g., NSFNET), the performance of optical-layer defragmentation only is significantly improved to even outperform the scheme of IP-layer reconfiguration only, while for an even lower nodal degree (e.g., ARPA-2), the performance of the optical-layer
defragmentation only is very close to the cross-layer defragmentation scheme. This is reasonable since at a lower nodal degree, a lightpath may go via the second shortest route, which could be much longer than the first shortest route, and the defragmentation process in the optical layer can help the lightpath revert to the shortest route, thereby significantly reducing spectrum resource consumption.
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pair.
V. CONCLUSION
We for the first time proposed cross-layer spectrum defragmentation for the IP and EON networks. Simulation results showed that the proposed scheme is effective to significantly improve BBP compared to the other schemes that do not consider such joint effort. Acknowledgment: This work was supported by NSFC (61671313).
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