Caching of popular content during off-peak hours is a strategy to reduce network loads during peak hours. Recent work has shown significant benefits of designing such caching strategies not only to deliver part of the content locally, but also to provide coded multicasting opportunities even among users with different demands. Exploiting both of these gains has been shown to be approximately optimal for caching systems with a single layer of caches.Motivated by practical scenarios, we consider a hierarchical content delivery network with two layers of caches. We propose a new caching scheme that combines two basic approaches.The first approach uses a decode and forward strategy to provide coded multicasting opportunities within each layer; the second approach uses strategic forwarding without decoding to provide coded multicasting opportunities across the two layers. By striking the right balance between these two approaches, we show that the proposed scheme achieves the optimal communication rates to within a constant multiplicative and additive gap. We further show that there is no tension between the rates in each of the two layers up to the aforementioned gap. Thus, both layers can simultaneously operate at approximately the minimum rate.
Nikhil Karamchandani is a postdoctoral researcher in the Department of Electrical Engineering at the University of California Los Angeles. He received the B.Tech degree from the Department of Electrical Engineering at the Indian Institute of Technology Bombay in 2005, and the M.S. and Ph.D. degrees from the Department of Electrical and Computer Engineering at the University of California at San Diego in 2007 and 2011 respectively. His research interests are in networks,communications, and information theory. Mr. Karamchandani received the California Institute for Telecommunications and Information Technology (CalIT2) fellowship in 2005.