Sanghyun Ahn

Virtual path layout design on ATM networks

The paper examines the efficient layout of virtual paths (VPs) in an ATM network. The ATM network consists of ATM switches and their attached network end users, which may be gateways, routers, and hosts. The physical topology, the offered traffic, and call setup matrices of the network end users are assumed to be given. The problem is formulated as a flow-based optimization problem. A heuristic approach is presented which (i) establishes VPs according to physical network-specific and application-specific constraints and a cost function, (ii) provides multipaths between each source destination user pair to minimize the cell blocking probability and to increase network resilience, and (iii) uses a novel VP combining process which is guaranteed to always satisfy the switching constraints. Simulation results are presented for the proposed VP planning policy. Guidelines for the design of robust VP layouts and the efficient establishment of VCs are also presented.<<ETX>>

A simple load-balancing approach in cheat-proof ad hoc networks

Ad hoc routing protocols such as AODV and DSR do not try to search for new routes until the network topology changes. Hence, with low node mobility, traffic may concentrate on just a few nodes, which results in long end-to-end delay due to congestion at the nodes. Furthermore, since some specific nodes are used for a long time, their battery power may be rapidly exhausted. To resolve this traffic concentration problem, we propose a load balancing scheme called simple load-balancing approach (SLA) that allows each node to drop RREQ or to give up packet forwarding depending on its own traffic load. Meanwhile, mobile nodes may deliberately give up packet forwarding to save their own energy. To make nodes volunteer in packet forwarding we also suggest a payment scheme called protocol-independent fairness algorithm (PIFA).

Adaptive routing considering the number of available wavelengths in WDM networks

In most existing studies of wavelength-division multiplexing networks, the problems of routing and wavelength assignment are generally treated separately, since it is NP-complete to produce the optimal solutions for the two problems at the same time. The four presented adaptive routing algorithms, however, consider the availability of wavelengths during the routing process. Our algorithms favor paths with the near-maximum number of available wavelengths between two nodes, resulting in improved load balancing. Simulations show that our algorithms reduce call blocking by nearly half when compared with the least-loaded and the k-fixed routing algorithms in some small networks using the first-fit wavelength assignment policy. In addition, simulation and analysis show that the path length of our algorithms is almost the same as those of the other algorithms.

Long-term outcomes after revascularization for advanced popliteal artery entrapment syndrome with segmental arterial occlusion

OBJECTIVES There are few long-term follow-up studies about the result of revascularization surgery for the treatment of popliteal artery entrapment syndrome (PAES). We performed this retrospective study to analyze the long-term result of revascularization surgery in patients with advanced PAES during the last 16 years. METHODS Twenty-two limbs in 18 consecutive patients with PAES were treated surgically at Seoul National University Hospital between January 1994 and December 2009. The preoperative diagnosis of PAES was made by duplex ultrasonography, three-dimensional computed tomography angiography, magnetic resonance imaging, or conventional angiography. The method of surgical approach was determined by the extent of arterial occlusion in preoperative images. RESULTS The mean age was 31 years old and the majority of patients were men (94%). The chief complaints were claudication in 18 limbs, ischemic rest pain in three limbs, and toe necrosis in one limb. All 22 limbs underwent revascularization for advanced PAES with segmental arterial occlusion. Fourteen limbs underwent musculotendinous section and popliteo-popliteal interposition graft (13 posterior approaches, one medial approach), five femoropopliteal (below-knee) bypasses, one femoro-posterior tibial bypass, and two popliteo-posterior tibial bypasses. All revascularization surgeries were performed with reversed saphenous veins. The overall primary graft patency rates at 1, 3, and 5 years were 80.9%, 74.6%, and 74.6%, respectively. Comparing 5-year graft patency according to the extent of arterial occlusion, patients with occlusion confined to the popliteal artery (n = 14) showed a better patency rate than patients with occlusion extended beyond the popliteal artery (n = 8) with no statistical significance (83.6% vs 53.6%; P = .053). Comparing 5-year graft patency according to the inflow artery, superficial femoral artery inflow (n = 6) showed a worse patency rate than popliteal artery inflow (n = 16) (30.0% vs 85.9%; P = .015). CONCLUSION In advanced popliteal entrapment syndrome, longer bypass with superficial femoral artery inflow showed poor long-term graft patency rate. The graft patency rate was excellent in patients whose arterial occlusion was confined to the popliteal artery and treated by popliteal interposition graft with reversed saphenous vein. With these data, we suggest that longer bypass extending beyond the popliteal artery might only be indicated in patients with critical limb ischemia when the extent of disease does not allow short interposition graft.

Multiple shared backup cycles for survivable optical mesh networks

This paper proposes a backup network planning method for survivable WDM mesh networks. The proposed method centers around multiple backup cycles where each network link is assigned m backup cycles and each cycle protects 1/m of the working capacity of a target link. Distributed link restoration is performed using preplanned cycles, in which both the backup paths and the spare capacity can be shared. The preconfiguration of the cycles and the spare capacity placement are derived directly from the network topology off-line, which is independent of the primary traffic status or its dynamic changes over time. The proposed method provides efficiency and simplicity to survivable network design and management, and also to runtime recovery operation. Experimental results show that the proposed method needs on average under 60% of spare capacity redundancy for single link failure while preserving the speed of cycle-based restoration.

QoS-driven wireless broadband home networking based on multihop wireless mesh networks

Wireless mesh networks are a suitable technology for wireless broadband home networks which can expand into ubiquitous networks. In this paper, we introduce a home network architecture which is based on the multihop wireless mesh networks and provide a general overview of the architecture. The system model introduced might be helpful to set up self-organizing and self-configuring network for consumer homes. The wireless mesh home network architecture can reduce the deployment cost and provide a much simpler deployment process by wireless home mesh routers. The home mesh routers provide various functions. Thus, home mesh router discovery and selection problems pose a primary challenge in the wireless mesh home network architecture. To deal with these problems, we propose a scoped-proactive home mesh router discovery scheme and a QoS-driven home mesh router selection mechanism. The scoped-proactive discovery protocol limits the flooding area. Among the candidates, a serving mesh router is selected based on a QoS-driven selection metric. Through simulations, we show that our schemes perform better than existing mechanisms in terms of various performance measures

A VANET routing based on the real-time road vehicle density in the city environment

The intelligent transportation system (ITS) can enhance the drivers safety by providing safety-related information such as traffic conditions and accident information to drivers. The vehicular ad hoc network (VANET) is an essential technology for the deployment of ITS. And, for the reliable delivery of safety-related information to vehicles in the VANET, a reliable VANET routing protocol is required. VANET routing is challenging since it is fundamentally different from conventional ad hoc networks; the vehicles move fast, and the network topology changes rapidly causing intermittent link connectivity. In this paper, we propose a routing protocol that works based on the real-time road vehicle density in order to provide fast and reliable communications so that it adapts to the dynamic vehicular city environment. In the proposed routing mechanism, each vehicle computes the vehicle density of the road to which it belongs by using beacon messages and the road information table. Based on the real-time road vehicle density information, each vehicle establishes a reliable route for packet delivery. In order to evaluate the performance of the proposed mechanism, we compare our proposed mechanism with GPSR through NS-2 based simulations and show that our mechanism outperforms GPSR in terms of delivery success rate and routing overhead.

A Simple Load-Balancing Approach in Secure Ad Hoc Networks

Most ad-hoc routing protocols such as AODV and DSR do not try to search for new routes if the network topology does not change. Hence, with low node mobility, traffic may be concentrated on several nodes, which results in long end-to-end delay due to congestion at the nodes. Furthermore, since some specific nodes are continuously used for long duration, their battery power may be rapidly exhausted. Expiration of nodes causes connections traversing the nodes to be disrupted and makes many routing requests be generated at the same time. Therefore, we propose a load balancing approach called Simple Load-balancing Approach (SLA), which resolves the traffic concentration problem by allowing each node to drop RREQ or to give up packet forwarding depending on its own traffic load. Meanwhile, mobile nodes may deliberately give up forwarding packets to save their own energy. To make nodes volunteer in packet forwarding we also suggest a payment scheme called Protocol-Independent Fairness Algorithm (PIFA) for packet forwarding. To evaluate the performance of SLA we compare two cases where AODV employs SLA or not. Simulation results show that SLA can distribute traffic load well and improve performance of entire ad-hoc networks.

Cold ischemic time is critical in outcomes of expanded criteria donor renal transplantation

The outcomes of expanded criteria donor (ECD) kidneys have been reported to be inferior compared with standard criteria donor (SCD) kidneys. However, the graft survival rate of ECD is not so inferior to SCD in Korea. The purposes of this study were to compare the outcomes of ECD kidneys with SCD kidneys and identify the influencing factors. We retrospectively studied 143 deceased donor transplants from August 2006 to June 2010. The patients were divided into SCD (n = 117) and ECD (n = 26) by UNOS criteria. The one‐ and three‐yr graft survival rates of SCD and ECD (99.1% and 94.4% vs. 100% and 92.9%, respectively, p = 0.15) were not significantly different between groups. The mean cold ischemic time (CIT) was 3.8 ± 2.2 h. When compared the outcome of ECD kidneys with data reported by Organ Procurement and Transplantation Network and the Scientific Registry of Transplant Recipients (OPTN/SRTR) (one‐ and three‐yr graft survival rate: 86.7% and 73.2%), the graft survival rate of our center was superior. In OPTN/SRTR data, transplant with CIT shorter than 11 h was only 20%. The outcomes of ECD grafts are outstanding and comparable with SCD grafts in our center, and the only distinguishing factor is markedly short CIT. Finishing the allocation before organ recovery and immediate operations after recovery could shorten the CIT.

Impact of a simple load balancing approach and an incentive-based scheme on MANET performance

Most reactive mobile ad hoc network (MANET) routing protocols such as AODV and DSR do not perform search for new routes until the network topology changes. But, low node mobility does not affect the MANET connectivity and the same routes may be used for a long time. This may cause concentration of traffic on few mobile stations (MSs), which results in congestion and hence longer end-to-end delay. In addition, continuous use of MSs may cause their battery power to get exhausted rapidly. Expiration of MS energy causes disruption of connections traversing through the MSs and could generate many simultaneous new routing requests. Therefore, we propose a load balancing approach called Simple Load Balancing Approach (SLBA), which can be transparently added to any current reactive routing protocol such as AODV and DSR. SLBA minimizes the traffic concentration by allowing each MS to drop RREQ or to give up packet forwarding depending on its own traffic load. Meanwhile, MSs may deliberately give up forwarding packets to save their own energy. For encouraging MSs to volunteer in forwarding packets, we introduce a reward scheme for packet forwarding, named Protocol-Independent Fairness Algorithm (PIFA). We compare the performance of AODV and DSR with and without SLBA and PIFA. Simulation results indicate that SLBA can distribute traffic very well and improve the MANET performance. PIFA is also observed to prevent MANET partitioning and any performance degradation due to selfish nodes.