Analyzing the performance of localization protocols for underwater acoustic sensor networks

Sualtı Duyarga Ağları (SDA) insan eliyle tehlikeli veya olanaksız sayılabilecek sualtı görevlerinde kullanılırlar. SDA’lar okyanusbiliminde, deprem ve tsunami tahmininde, askeri uygulamalarda, okyanus petrol platformlarının gözetlenmesinde ve çeşitli birçok alanda kullanılabilirler. Sualtı duyarga düğümleri sabit bir platforma tutturulmuş olabilir veya su içersinde serbest halde yüzebilir. Su yüzeyinden metrelerce aşağıda yüzen duyarga düğümler, gezgin sualtı duyarga ağını oluştururlar. Duyarga düğümler okyanuslardan sıcaklık, akıntı hızı, tuzluluk ve görüntü kaydı gibi veriler toplarlar. Gezgin bir SDA’da, duyarga düğümler su yüzeyinin altında, akıntıyla birlikte hareket eder ve belirli bir olayı izlerler. SDA’larda en ciddi sorunlardan biri konumlandırmadır. Konum bilgisine, veri etiketleme ve konum-tabanlı yönlendirme protokollerinde ihtiyaç duyulur. Geniş ölçekli, üç boyutlu SDA’lar için, literatürde az sayıda konumlandırma protokolü önerilmiştir. Bu makalede, İner-Çıkar düğümlerle Konumlandırma (İÇK) ve Vekil Konumlandırma (VK) yöntemlerini tanıtıp, sözkonusu yöntemlerin başarımını önceden önerilmiş olan bir başka yöntemle karşılaştırmaktayız. Bu yöntem, Geniş Ölçekli Konulmandırmadır (GÖK). Bu üç tekniğin avantaj ve dezavantajlarını gezgin bir SDA için göstermekteyiz. Benzetim sonuçlarımız GÖK’ün yüksek konumlandırma başarısına sahip olduğunu, ancak bu yöntemin beraberinde yüksek enerji tüketimi ve ek haberleşme yükü getirdiğini göstermektedir. İÇK ise, yüksek konumlandırma başarımı, yüksek kesinlik, düşük enerji tüketimi ve düşük haberleşme maliyetine sahiptir. VK ise, kabul edilebilir konumlandırma başarımı, düşük enerji tüketimi ve daha az ek yük getirmekte, buna karşılık diğerlerinden daha düşük kesinlik sağladığı görülmektedir.  Anahtar Kelimeler: Sualtı duyarga ağları, konumlandırma, duyarga ağlar.Underwater Sensor Networks (USNs) can improve naval defense, earthquake/tsunami forewarning, water pollution detection, ocean life monitoring systems, etc. Stationary Underwater Sensor Networks are ideal for securing or monitoring a fixed target region, e.g. monitoring oil drilling platforms for spill detection, harbor entrances for surveillance, ocean bottom for seismic activity observation, etc. On the other hand, mobile untethered Underwater Sensor Networks are flexible and better alternatives for short term exploration of moving targets. For instance, untethered, free-floating underwater sensors can track a chemical spill or a pollutant that may be dangerous to human health or sea life. In a sensor network, sensor nodes collect data from their surrounding and tag these data, in order to transmit them to a more powerful node for processing. Therefore, it is crucial to know the location of the sensor nodes. Location is required for data tagging, as well as, target detection, node tracking, etc. In addition, localization is essential for position-based routing algorithms which are powerful alternatives to classical routing approaches in Mobile Ad Hoc Networks (MANET). Localization is a well studied topic in terrestrial sensor networks. Nevertheless, in Underwater Sensor Networks, localization is still challenging due to several reasons: i) unavailability of the GPS; ii) low bandwidth, long delay and high bit error rate of the acoustic links; iii) necessity of high amount of sensor nodes to cover the three dimensional region. The use of GPS is limited to surface nodes because the GPS signal does not propagate through the water. In sensor networking literature, several GPS-less (GPS-free) positioning schemes have been proposed however they usually have high overhead. The underwater sensors use acoustic links and the bandwidth of those links is low even for very short distances. Moreover, acoustic communications has high propagation delay and high bit error rate. In Underwater Sensor Networks, localization protocols are expected to avoid excessive overhead and establish localization with the least possible messages. This is also enforced by the limited battery life of the underwater sensor nodes and the difficulty of recharging or replacing the batteries in an underwater application. Usually, an underwater application requires a large number of sensor nodes because the data rate of the acoustic links increases with decreasing distance and shorter ranges between nodes, means that more sensor nodes are needed to cover the three dimensional oceanographic zone. In addition, in a mobile Underwater Sensor Network localization should be repeated and stale location information should be cleared periodically. Considering all these challenges, it is essential to develop novel localization protocols tailored for mobile Underwater Sensor Networks. In this article, we introduce two distributed, scalable localization techniques; Dive and Rise Localization (DNRL) and Proxy Localization (PL). In DNRL, mobile beacons ascend and descend in the water to deliver their GPS driven coordinates. In PL, the already localized nodes act like beacons likewise multi-stage localization which is a preliminary version of PL (Erol et al., 2008). Unlike the previous work, in PL the non-localized nodes use a different metric to choose the best possible proxies among the candidates which enhances the performance of the protocol. Here, we compare the performance of PL, DNRL and Large-Scale Localization (LSL). LSL is a technique from the literature (Zhou et al., 2007). We evaluate the performance of these schemes in terms of localization success, accuracy, overhead and energy consumption. Since we compare the performance of the localization techniques for a mobile Underwater Sensor Network, a realistic underwater mobility model is essential. Recently, the works of Caruso et al. (2008) and Erol et al. (2008) have applied the real ocean current behavior to Underwater Sensor Networks. We use the "Meandering Current Mobility with Surface Effect" (MCM-SE) model to compare the performance of the three localization schemes for a mobile underwater sensor network. The main aim of the article is to provide a comparison between recently proposed localization schemes for Underwater Sensor Networks. Based on the simulation results, we compare and analyze the performance of three recent methods that are developed for distributed localization in large-scale Underwater Sensor Networks in terms of localization ratio, accuracy, protocol overhead and energy consumption.  Keywords: Sensor networks, underwater sensor networks, localization

Availability constrained routing and wavelength assignment techniques for optical WDM networks

Dalgaboyu bölmeli çoğullama (WDM) tekniği ile optik ağlar tarafından sunulan yüksek bandgenişliği, optik hatlarda veya ağ bileşenlerinde oluşabilecek hatalar karşısında da yoğun miktarda veri kaybı riskini beraberinde getirmektedir. Bu durumun önüne geçmek için, bağlantılar belirli bir sürdürülebilirlik politikası ile korunarak kurulmaktadır. Ağda oluşabilecek hata durumlarınnda da bağlantının kullanılabilir ve sunulan hizmetin kesintisiz olması kullanıcılar tarafından beklenmektedir. Bu nedenle, bağlantı istekleri kurulurken, yol ve dalgaboyu atamasında, ilgili sürdürülebilirlik politikası altında kullanılabilirlik kısıtının göz önünde bulundurulması gerekmektedir. Bu çalışmada, paylaşımlı yol koruma politikası altında kurulan bağlantıların kullanılabilirlik isteklerini göz önünde bulundurarak yol ve dalgaboylarını atayan iki farklı teknik önerilmektedir. Bu tekniklerden ilki, G-DAP (Global Differentiated Availability-Aware Provisioning) sezgisel olarak yedek dalgaboyu kanalları üzerinde, her bir kullanılabilirlik sınıfı için global bir paylaşım derecesi kestirir. Diğer teknik LBL-DAP (Link-By-Link Differentiated Availability-Aware Provisioning) ise bir optimizasyon modeli kullanarak, her bir kullanılabilirlik sınıfı için yedek kanallar üzerindeki paylaşım derecesini, her bir optik hat için ayrıca hesaplar. Bağlantı isteklerinin %98, %99, %99.9, %99.99% ve %99.999 kullanılabilik düzeyinin birinden geldiği ortamda yapılan testlerde, önerilen teknikler yaygın olan CAFES (Compute-A-Feasible Solution) algoritmasıyla NSFNET ve EON topolojilerinde karşılaştırılmıştır. Bağlantıların sınıflar arasında düzgün ve heterojen dağıldığı ortamlarda toplanan sonuçlar, önerilen tekniklerin daha yüksek bağlantı kabul oranı ve kullanılabilirlik sağladığını göstermektedir. Ayrıca, yedek kaynak kullanım oranını düşürmesi nedeniyle LBL-DAP’ın en iyi başarımı sağladığı görülmüştür. Anahtar Kelimeler: Optik ağlar, dalgaboyu bölmeli çoğullama, kullanılabilirlik, yol atama.As a result of the increase in the bandwidth demand of the next generation Internet applications, Optical Wavelength division Multiplexing (WDM) networks seem to be the most appropriate technology that can be deployed in the backbone. Optical WDM networks introduce the advantage of offering bandwidth partitioned into a number of gigabits per second wavelength channels. However, the advantage introduced by the huge bandwidth offer also introduces a disadvantage when the network experiences a failure. Service interrupts on any component along the lightpath may lead to significant amount of data loss since the fiber capacity is huge. Factors like multiple errors, long fault recovery duration, and component failure characteristics introduce availability constraint for the network elements, and also for the connections. Therefore, connections are required to be provisioned by taking availability constraint into consideration. In short, availability stands for the probability of a network component, a channel or a link being in the operational state at any time t. Significant amount of the previous work is concerned with availability aware routing and wavelength assignment (RWA) under shared backup protection. The first and the most common availability aware routing scheme is compute-a-feasible-solution (CAFES). In this scheme, a number of candidate working paths are selected. For each working path, a corresponding backup path is selected by forcing the backup channels to be shared. The working and backup path pair that leads to the highest availability or another lowest cost metric is selected, and assigned to the incoming connection request. In this work, we present two dynamic connection provisioning schemes for differentiated availability-constrained RWA. Both of the schemes are derived from the conventional reliable provisioning scheme CAFES. In the dynamic environment, connections arrive with the availability requirements of 98% (class-1), 99% (class-2), 99.9% (class-3), 99.99% (class-4), and 99.999% (class-5). First scheme is called Global Differentiated Availability-aware Provisioning (G-DAP). This scheme monitors the average availability per connection for each class and resource-overbuild throughout the network. In order to enhance the performance of the connection provisioning, G-DAP also takes the advantage of the trade-off between resource overbuild and connection unavailability where resource overbuild is the ratio of the number of backup channels to the number of working channels in the network, and unavailability is one's complement of the availability. Based on the change in these two parameters it attempts to specify a feasible global sharing degree for all wavelength channels per availability class. The trade-off function is defined as the product of these two parameters. Hence, if the tradeoff is monitored to be decreasing for the related availability class, the last action (increment or decrement) taken on the sharing degree of that class is repeated; otherwise, it is reversed. The second scheme is called Link-by-Link Differentiated Availability-aware Provisioning (LBL-DAP). LBL-DAP estimates a separate feasible sharing degree per class for the channels of each link. It periodically runs an integer linear programming (ILP) function to obtain the feasible sharing degrees on each link. When searching for a backup RWA configuration, both schemes modify the link costs based on the feasible sharing degree obtained for the availability class of the incoming connection and current load for the connection?s class on the link respectively. Since we aim to improve the performance in terms of resource overbuild, connection availability, and blocking probability, we use the conventional reliable provisioning scheme, CAFES as a base in our simulations. Moreover, since connections arrive with differentiated availability requirements, we also modify CAFES to enable a connection to be provisioned unprotected if a selected working path can meet its availability requirement. Thus, resource consumption overhead of this scheme is modified for its favor. Performance of G-DAP and LBL-DAP are compared to that of CAFES by simulation under NSFNET and EON topologies. Simulation results are collected under two different conditions where the connection requests are distributed uniformly and heterogeneously among the availability classes. It is shown that the proposed schemes lead to enhanced blocking ratio and connection availability. Moreover, by taking the advantage of optimization and considering the feasible sharing degrees for the links separately, LBL-DAP also introduces significant decrease in resource overbuild. Keywords: Optical networks, wavelength division multiplexing, availability, routing