  Caching is an efficient way to reduce peak-hour network traffic congestion by
storing some contents at user's local cache without knowledge of later demands.
Maddah-Ali and Niesen initiated a fundamental study of caching systems; they
proposed a scheme (with uncoded cache placement and linear network coding
delivery) that is provably optimal to within a factor 12. In this paper, by
noticing that when the cache contents and the demands are fixed, the caching
problem can be seen as an index coding problem, we show the optimality of
Maddah-Ali and Niesen's scheme assuming that cache placement is restricted to
be uncoded and the number of users is not less than the number of files.
Furthermore, this result states that further improvement to the Maddah-Ali and
Niesen's scheme in this regimes can be obtained only by coded cache placement.
