ECOC 2009, 20-24 September, 2009, Vienna, Austria
Paper 1.6.4
Online Upstream Scheduling and Wavelength Assignment Algorithms for WDM-EPON Networks (1)
Konstantinos Kanonakis , Ioannis Tomkos (1)
(1)
High-Speed Networks and Optical Communications Group, Athens Information Technology, Athens, Greece. e-mail:
[email protected]
Abstract We show that efficiency problems arise when online scheduling and wavelength assignment is employed in WDM-EPON networks and propose algorithms for minimizing their effect, leading to improved capacity utilization and consequently reduced frame delay. Introduction Wavelength division multiplexed passive optical networks (WDM-PONs) have received a significant amount of attention lately as one of the solutions for boosting the performance of traditional PON networks in terms of scalability, available bandwidth and flexibility. A WDM-EPON adopts essentially the same architecture as a TDMA-EPON, however the sharing of resources among ONUs is achieved also in the wavelength domain. Static assignment of wavelengths does not provide efficient utilization of resources (i.e. wavelengths), since each ONU will only consume a small portion of a wavelength. Hence the OLT should dynamically assign the wavelength on which each ONU will transmit its upstream data based on channel availability, leading to a hybrid WDMA/TDMA MAC. Significant performance benefits can be achieved due to the resulting statistical multiplexing. However efficient algorithms are needed for the process of upstream scheduling and wavelength assignment (USWA). There exist two general approaches: In the offline one, the OLT waits until it has received all the reports from ONUs (or part 1 of them ) and then performs some algorithm to find the best USWA scheme for the corresponding grants. In the online case, upon the arrival and processing of a report from an ONU, the OLT immediately decides on the USWA for the corresponding grant. Although the offline approach can address fairness and QoS 1 issues among different ONUs, it has been shown that the online one always outperforms it in terms of frame delay. Therefore, the motivation for algorithms to improve wavelength utilization under the online case is strong. This work identifies the inefficiency problems that arise in online USWA and proposes algorithms to minimize them. Network Architecture and Control Operation Since no standardized architecture for WDM-PON networks has appeared yet, we assume a typical PON one, whereby the passive coupler/splitter is replaced by a WDM multiplexer/demultiplexer. We will assume that ONUs are either colourless (which facilitates the management and distribution of ONU equipment, while allowing for dynamic wavelength assignment schemes), or able to operate within a specific wavelength range. This can be achieved by
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using either tunable lasers or Reflective Semiconductor Optical Amplifiers (RSOA) at the 2 ONUs . Regarding the MAC layer operation, we 3 follow an approach similar to the SIPACT concept . The framework is still based on the EPON Multi-Point Control Protocol (MPCP), with the control frames modified to include information about the supported wavelengths by each ONU and convey the upstream wavelength on which each ONU is supposed to send their granted data. Problem Definition All existing USWA algorithms consider a wavelength as free only after the end of the last transmission scheduled within it. However, the need for supporting a large user base, combined with the difficulties of optimal placement of remote nodes especially in sparsely populated or rural areas and the targeted long-reach of next generation access networks, inevitably lead to a considerable spread of round-trip values among the ONUs. In any case, the end result is what we refer to as scheduling voids. These are periods in the OLT upstream schedule, before or between already scheduled transmissions, which the OLT could still use for upstream transmissions of certain ONUs (e.g. ones with short round-trip times). The complete problem of bandwidth allocation in WDM-EPONs consists both of grant sizing and grant scheduling. In order to facilitate the comparison of the following algorithms, only grant scheduling is considered, while sizing is assumed to be based on a simple gated approach. Algorithms for Online USWA First Available Wavelength (FAW): This is a relatively straightforward, low-complexity algorithm for online USWA. Upon the arrival of a report message from an ONU, the OLT schedules the corresponding grant in the first wavelength that has no transmission programmed after the current time plus the round-trip of the ONU, the latter sum being the arrival time of the ONU data at the OLT. If no such wavelength exists, then the wavelength with the earliest finishing last transmission is selected and the grant for the new reservation must be delayed accordingly. Since the OLT only keeps track of the last scheduled transmission in each wavelength, the complexity of
ECOC 2009, 20-24 September, 2009, Vienna, Austria
Paper 1.6.4
the algorithm is very low. However, FAW does not efficiently utilise upstream channels due to the formation of scheduling voids which can not be filled. First Available Void (FAV): In order to make up for the low channel utilization and increased frame delay in FAW we propose the FAV algorithm which works as follows: The OLT keeps track of all scheduling voids in all upstream wavelengths (including the ones defined by the last transmission and the positive infinity) and schedules the upstream transmission for an ONU to take place within the first void which is large enough to contain the granted upstream data plus the accompanying report message. The complexity of this algorithm is clearly higher than that of FAW and is dependent on the number of voids in the schedule. It is easy to see though that the maximum amount of voids at any time is upper bounded by W+N, where W is the number of upstream wavelengths and N is the number of ONUs. Moreover, the intermediate gap of back-to-back grants is not considered as an eligible void, reducing thus the amount of searches. FAV is expected to always perform better than FAW in terms of frame queuing delay due to its full exploitation of all possible upstream scheduling opportunities.
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Fig. 2: Delay reduction of FAV compared to FAW for (a) W = 4, D = 20 to 100 km and (b) W = 4, N = 32.
978-3-8007-3173-2 © VDE VERLAG GMBH
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References 1 M. P. McGarry et al., J. Lightwave Technol. 26, 1204-1216 (2008). 2 A. Banerjee et al., J. Opt. Netw. 4, 737-758 (2005). 3 F. Clarke et al., Proc. OFC’06, OThK3 (2006).
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Conclusions We have shown that by using a void filling algorithm (FAV) the delay performance of online upstream scheduling and wavelength assignment in WDM-EPONs can be significantly improved. In the examined scenarios, the delay reduction compared to a simpler scheme (FAW) can reach up to 25% and it becomes more significant when the split ratio and the differential distances among ONUs increase. This work has been partly supported by the E.U. Project SARDANA.
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to FAW at any offered load. In Fig. 2 we plot the delay reduction offered by FAV compared to FAW. Fig. 2(a) shows that the performance of FAV scales very well with the split ratio, since the greatest reduction (by almost 25%) is achieved for N = 64, while in all scenarios FAV behaves the best at the load region between 0.7 and 0.8. Finally, Fig. 2(b) demonstrates the effect of differential ONU delays on the efficiency of FAV: The best performance is achieved when ONUs are more sparsely located (D = 20 to 100 km). FAV and FAW perform exactly the same when all ONUs are at the same distance (D = 60 km), since in that case no scheduling voids are expected.
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Fig. 1: Average end-to-end frame delay for N = 64 and D = 5 to 15 km.
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Performance Evaluation To evaluate the performance of the proposed USWA algorithms, a simulation model was developed in OPNET Modeler. The metric of interest was the average frame delay since it perfectly mirrors the degree of utilization of each algorithm. Performance was measured against the OLT-ONU distance D, the number of ONUs (N) and wavelengths (W) and the offered network load L. Each wavelength had a capacity of 1Gbps and each ONU was connected via a GE interface. For simplicity we assumed that all ONUs support all wavelengths. Fig. 1 depicts the average ONU end-to-end frame delay for the case of 64 ONUs located randomly at distances between 5 and 15 km from the OLT. The effect of the WDM statistical multiplexing is obvious by the much lower delay achieved when W is increased from 1 to 4. FAV manages to offer improved perforamance compared
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