Refferences are not shown in slide. Why?

Question

MWE

\documentclass[usenames,dvipsnames]{beamer}
\usepackage{xcolor}
\usepackage{tikz}
\usepackage{listings}
\usepackage{algpseudocode}
\usefonttheme[onlymath]{serif}
\usepackage{amsmath}
\usetheme{Madrid}


\algnewcommand\algorithmicinput{\textbf{Input:}}
\algnewcommand\algorithmicoutput{\textbf{Output:}}
\algnewcommand\algorithmicinitialization{\textbf{Initialize:}}
\algnewcommand\Input{\item[\algorithmicinput]}%
\algnewcommand\Output{\item[\algorithmicoutput]}%
\algnewcommand\Initialize{\item[\algorithmicinitialization]}%


\logo{%
  \includegraphics[width=1cm,height=1.5cm,keepaspectratio]{example-image-a}%
  \hspace{\dimexpr\paperwidth-2cm-5pt}%
  \includegraphics[width=1cm,height=1cm,keepaspectratio]{example-image-a}%
}
\begin{document}
  \title{Resource  Allocation in Cloud Radio Access Network}
  \author[alhelal \& Jobayed Ullah]{
  \parbox{2.5cm}{
\centering Md.Al-Helal\\Roll:SH-51}\hspace{1cm}
\parbox{2.5cm}{
{\centering Jobayed Ullah\\Roll:EK-107}}
}
\institute[CSEDU]{Computer Science \& Engineering\\CSEDU}
\date{February 25, 2018}
\begin{frame}
  \maketitle
\end{frame}
\begin{frame}
  \frametitle{Background}
  Traditional cellular, or Radio Access Networks (RAN), consist of many standalone base stations (BTS) have some limitations-
  \begin{itemize}
    \item Each BTS is costly to build and operate.
    \item When more BTS are added to a system to improve its capacity, interference among BTS is more severe as BTS are closer to each other.
    \item Because users are mobile, the traffic of each BTS fluctuates (called 'tide effect'), and as a result, the average utilization rate of individual BTS is pretty low.
  \end{itemize}
\end{frame}
\begin{frame}
  \frametitle{Architechture Overview}
  \includegraphics[scale=0.3]{example-image-b}
\end{frame}
\begin{frame}
  \frametitle{Architechture Overview}
  \begin{itemize}
    \item Large scale centralized deployment: Allows hundreds of thousands of RRHs to connect to a centralized BBU pool.
    \item Native support to Collaborative Radio technologies: Any BBU can talk with any other BBU within the BBU pool with very high bandwidth (10Gbit/s and above) and low latency(10us level).
    \item  C-RAN BBU pool is built on open hardware, like x86/ARM CPU based servers. Real-time virtualization makes sure the resources in the pool can be allocated dynamically to base station software stacks, say 4G/3G/2G function modules from different vendors according to network load.\cite{Gill}
  \end{itemize}
\end{frame}
\begin{frame}
\bibliography{references}
\bibliographystyle{plain} 
\end{frame}
\begin{frame}{\phantom{}}
 % \color{Brown}
  \color{Sepia}
  \centering \Huge\textbf{Thank You}
\end{frame}
\end{document}

and references.bib is

@ARTICLE{Gill,
   author    = "A. E. Gill",
   title     = "Some Simple Solutions for Heat-Induced Tropical Circulation",
   journal   = "Quart. J. R. Met. Soc.",
   volume    = 106,
   year      = 1980,
   pages     = "447--462",
}
@InProceedings{conf/wiopt/AlabbasiC17,
  title =   "Delay-aware green hybrid CRAN",
  author =  "Abdulrahman Alabbasi and Cicek Cavdar",
  publisher =   "IEEE",
  year =    "2017",
  bibdate = "2017-07-06",
  bibsource =   "DBLP,
         http://dblp.uni-trier.de/https://doi.org/10.23919/WIOPT.2017.7959942;
         DBLP,
         http://dblp.uni-trier.de/db/conf/wiopt/wiopt2017.html#AlabbasiC17",
  booktitle =   "WiOpt",
  crossref =    "conf/wiopt/2017",
  ISBN =    "978-3-9018-8290-6",
  pages =   "1--7",
  URL =     "http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=7951216",
}

output shows