Scott Seongwook Lee

Male Gender is a Predictor of Morbidity and Age a Predictor of Mortality for Patients Undergoing Gastric Bypass Surgery

PurposeTo determine preoperative risk factors predictive of adverse outcomes after gastric bypass surgery. Summary Background DataGastric bypass results in sustained weight loss for seriously obese patients, but perioperative complications can be formidable. Preoperative risk assessment is important to establish the risk-benefit ratio for patients undergoing these operations. MethodsData for 10 risk factors predictive of adverse outcomes were collected on 1,067 consecutive patients undergoing gastric bypass surgery at the UCLA Medical Center from December 1993 until June 2000. Univariate analyses were performed for individual risk factors to determine their potential significance as predictors for complications. All 10 risk factors were entered into a logistic regression model to determine their significance as predictors for complications. Sensitivity analysis was performed. ResultsUnivariate analysis revealed that male gender and weight were predictive of severe life-threatening adverse outcomes. Multistep logistic regression yielded only male gender as a risk factor. Male patients were heavier than female patients on entry to the study, accounting for weight as a potential risk factor. Patients older than 55 years had a threefold higher mortality from surgery than younger patients, although the complication rate, 5.8%, was the same in both groups. Sensitivity analysis demonstrated that the risk for severe life-threatening adverse outcomes in women increased from 4% for a 200-lb female patient to 7.5% for a 600-lb patient. The risk increased from 7% for a 200-lb male patient to 13% for a 600-lb patient. ConclusionsLarge male patients are at greater risk for severe life-threatening complications than smaller and/or female patients. Risk factors thought to be predictive of adverse outcomes, such as a history of smoking or diabetes, proved not to be significant in this analysis. Older patients had the same complication rate but a threefold higher mortality, suggesting that they lack the reserve to recover from complications when they occur.

QoS-aware multiple spanning tree mechanism over a bridged LAN environment

Todays emerging traffic is far removed from the traffic trends seen during the early days of Ethernet technology. As a result, the current IEEE 802.1 standards and its extensions to the spanning tree protocol fall short of providing satisfactory quality of service for traffic which has a significant amount of QoS-sensitive multimedia and VoIP traffic. In the current and near-future scenario of campus-wide networks with significantly large layer-2 clusters and numerous virtual LANs (VLANs), we show significant shortcomings of the basic spanning tree and the multiple spanning tree protocols with regard to QoS. We propose a novel, simple, and yet highly effective enhancement to the multiple spanning tree protocol to achieve a high degree of QoS by keeping in perspective the different characteristics of the various traffic types in the Diffserv framework. We discuss the problems of the current standards and present in detail our proposed extension to overcome them. Our simulation results show good improvements in throughput and significant benefits in delay for all classes of traffic to conclusively prove our claims.

Fault Tolerance and Load Balancing in QoS Provisioning with Multiple MPLS Paths

The paper presents approaches for fault tolerance and load balancing in QoS provisioning using multiple alternate paths. The proposed multiple QoS path computation algorithm searches for maximally disjoint (i.e., minimally overlapped) multiple paths such that the impact of link/node failures becomes significantly reduced, and the use of multiple paths renders QoS services more robust in unreliable network conditions. The algorithm is not limited to finding fully disjoint paths. It also exploits partially disjoint paths by carefully selecting and retaining common links in order to produce more options. Moreover, it offers the benefits of load balancing in normal operating conditions by deploying appropriate call allocation methods according to traffic characteristics. In all cases, all the computed paths must satisfy given multiple QoS constraints. Simulation experiments with IP Telephony service illustrate the fault tolerance and load balancing features of the proposed scheme.

A hierarchical multipath approach to QoS routing: performance and cost evaluation

Efforts to provide connection oriented service over the inherently best-effort Internet started almost right after its birth. Today, there exist a multitude of solutions that have been proposed but have never been implemented due to their impracticability. We propose a practical solution for fast, low cost, scalable, and yet accurate QoS routing. We propose to use hierarchical approaches to make the scheme practical and cost-effective. At the same time, we increase network utilization and decrease inaccuracy of stale information by the use of multiple paths. An extensive simulation of the various permutations of schemes over a large set of topologies and traffic conditions validate the proposed schemes and prove conclusively that hierarchical schemes with multiple path capabilities not only result in significantly lower overhead, but also give high levels of QoS performance. This paper presents the architecture of our schemes, the multiple path computation algorithm, and the simulation results validating our claims.

Internet QoS routing with IP telephony and TCP traffic

We propose the use of QoS routing to enhance the support of IP telephony. Our proposed scheme is based on QoS intradomain OSPF routing, an extension of the conventional OSPF routing protocol. A DiffServ model is used (no per flow signaling, nor per flow accounting at intermediate nodes). Processing O/H is shifted from core to edge routers, which compute routes, monitor QoS path quality and enforce call acceptance control (CAC) using the link state information advertised by OSPF. Via simulation, we show significant delay and throughput improvement over IP telephony strategies currently used in the Internet. In particular, hot spots and focussed congestion points are easily avoided. Moreover, the ability to control voice via QoS routing and CAC permits us to adjust the capacity sharing between voice traffic and TCP traffic, by reserving a fraction of link bandwidth to TCP data traffic. We also show that the added control and processing overhead is quite manageable, even in fairly large networks.

Resource Allocation and Admission Control Styles in QoS DiffServ Networks

In this paper we propose a DiffServa rchitecture for the support of real time traffic (e.g., video) with QoS constraints (e.g., bandwidth and delay) over an IP domain. The main goal of the paper is to identify solutions which provide QoS guarantees without requiring per flow processing in the core routers (as is commonly done in IntServ solutions) and which are thus scalable. We propose, and evaluate through simulation, different approaches for call admission control (CAC) and resource allocation. These approaches are all consistent with the Diff-Servmo del, but place different processing and signaling loads on edge and core routers. Paths are computed by means of a QoS routing algorithm, Q-OSPF, and MPLS is used to handle explicit routing and class separation.

Hierarchical approach for low cost and fast QoS provisioning

In order to provide practical QoS services, we propose a new traffic engineering approach for low-cost and fast QoS provisioning. The new scheme utilizes network resources more evenly by exercising an enhanced QoS path computation algorithm that finds maximally disjoint multiple QoS paths. An efficient mechanism for managing computed paths and allocating calls is also discussed as a component of the system, and this path management allows fast QoS provisioning by avoiding unnecessary path recomputation. Moreover, the naive approaches to link state information acquisition in conventional QoS routing are replaced with a more effective approach dividing the link state update process into three hierarchical subprocesses. Via simulation experiments, based on an IP telephony application, we show that we can improve the performance and at the same time reduce the link overhead.

Cost-effective multiple QoS path provisioning

This paper presents a cost-effective quality-of-service (QoS) routing scheme which deploys a heuristic algorithm to provision multiple QoS paths. Multiple QoS path provisioning is mainly aimed to provide reliable QoS guarantees for mission-critical applications. The proposed algorithm relaxes the strict disjoint path requirement and searches for more paths possibly less overlapped with each other and still satisfying given multiple QoS constraints. When multiple QoS paths are provisioned for the same destination, link/node failures become significantly less detrimental to ongoing QoS application traffic. The routing algorithm presented in this paper can be claimed to be cost-effective since a single iteration of the algorithm finds multiple QoS paths while previous research works require as many algorithm iterations as the number of searched QoS paths. This paper discusses the beneficial properties of the proposed algorithm, and simulation experiments present the reduced algorithm time complexity.