Aggregated Multicast — A Comparative Study

Though IP multicast is resource ef£cient in delivering data to a group of members simultaneously, it suffers from scalability problem with the number of concurrently active multicast groups because it requires a router to keep forwarding state for every multicast tree passing through it. To solve this state scalability problem, we proposed a scheme, called aggregated multicast. The key idea is that multiple groups are forced to share a single delivery tree. In our earlier work, we introduced the basic concept of aggregated multicast and presented some initial results to show that multicast state can be reduced. In this paper, we develop a more quantitative assessment of the cost/bene£t trade-offs. We propose an algorithm to assign multicast groups to delivery trees with controllable cost and introduce metrics to measure multicast state and tree management overhead for multicast schemes. We then compare aggregated multicast with conventional multicast schemes, such as source speci£c tree scheme and shared tree scheme. Our extensive simulations show that aggregated multicast can achieve signi£cant routing state and tree management overhead reduction while containing the expense of extra resources (bandwidth waste and tunnelling overhead). We conclude that aggregated multicast is a very cost-effective and promising direction for scalable transit domain multicast provisioning.

Diagnosing the Prodromal State of Alzheimer's Disease

Mild Cognitive Impairment- Amnestic Subtype (MCIa) is a putative prodromal stage of Alzheimer’s Disease (AD) characterized by focal deficits in episodic verbal memory. Less is known about relative deficits in visuospatial learning, although there is ample evidence indicating involvement of the hippocampus in visuospatial learning, as well as hippocampal degeneration in early AD. The aim of this study was to better characterize the components of working memory dysfunction in people with MCIa to increase the ability to reliably diagnose this disease. Fifty-six elderly adults diagnosed with MCIa and 94 healthy elderly completed a hidden maze learning task. Results indicated similar functioning between groups on measures of reasoning, problem solving, and accuracy. However, MCIa subjects were less efficient at learning the hidden path, making more errors per second on average (Cohen’s d= -.78) and requiring a longer time to complete the maze (Cohen’s d=.77). The learning curve between the first two trials was four times as steep for healthy elderly compared to MCIa (slopes = 4.9 vs. 1.24, respectively), indicating that MCIa subjects exhibited relative difficulty in holding and making effective use of an internal spatial map in order to improve performance. Our results suggest that MCIa patients have focal deficits in visuospatial working memory, with relative preservation of functioning on other more global measures of cognitive functioning. This particular pattern of results may be specific to the amnestic variant of MCI.