my table does not fit with page margin

Question

 \documentclass[graybox]{svmult}
\usepackage{sidecap}
% choose options for [] as required from the list
% in the Reference Guide
\usepackage{breakcites}
\usepackage{graphicx}
\usepackage{tabularx,booktabs}
\newcolumntype{C}{>{\centering\arraybackslash}X} % centered version of "X" type
\setlength{\extrarowheight}{1pt}
\usepackage{mathptmx}       % selects Times Roman as basic font
\usepackage{helvet}         % selects Helvetica as sans-serif font
\usepackage{courier}        % selects Courier as typewriter font
\usepackage{type1cm}        % activate if the above 3 fonts are
                            % not available on your system
\usepackage{makeidx}         % allows index generation
\usepackage{graphicx}        % standard LaTeX graphics tool
                             % when including figure files
\graphicspath{{Figures/}}
\usepackage{multicol}        % used for the two-column index
\usepackage[bottom]{footmisc}% places footnotes at page bottom

% see the list of further useful packages
% in the Reference Guide

\makeindex             % used for the subject index
                       % please use the style svind.ist with
                       % your makeindex program

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}   
 \begin{table*}[htbp]
    \centering
    \caption{Comparison of protocols used in recent researches in Airborne Networks}
    \scalebox{.75}[.75]{
        \begin{tabular}{|p{4.2em}|p{4.5em}|p{7em}|p{4em}|p{5em}|p{8em}|p{8em}|p{2em}|}
            \toprule
            \textbf{Protocol Name} & \textbf{Type of Protocol} & \textbf{Method of Neighbour discovery } & \textbf{Simulator used } & \textbf{Mobility Model} & \textbf{PDR} & \textbf{E2E Delay} & \multicolumn{1}{p{13.285em}|}{\textbf{Overhead}} \\
            \midrule
            DOLSR & Proactive &  Broadcasting Hello messages to one hop neighbours and then select MPR nodes. & OPNET 4.5 & Random Waypoint Mobility model & More than AODV, DSR and OLSR. & Less than AODV, DSR and OLSR. & \multicolumn{1}{p{13.285em}|}{Lesser than OLSR, AODV and DSR due to reduction in no. of MPR’s~\cite{r7}.} \\
            \midrule
            A-GR & Geographic & It makes use of location and mobility information.& Qualnet 5.0 & Gauss-Markov Model & A-GR provides more PDR with little degradation as the no. of nodes increases to 50 or more. & At high node density, A-GR provides lesser delay compared to GPSR \& GRAA. & \multicolumn{1}{p{13.285em}|}{For GPSR \& GRAA, overhead increases exponentially\cite{r15}.} \\
            \midrule
            Improved reactive and geographic (IRG) & Geographic & Reactive,Greedy Forwarding & NS-2.35 & Random waypoint model &  IRG outperform the AODV \& GPSR with increase of speed because selection of next hop is based upon relative velocity between nodes.& IRG provides slightly lesser delay than GPSR. AODV has highest delay with increase in node velocity.& \multicolumn{1}{p{13.285em}|}{Routing overhead increases for all three i.e. IRG , GPSR \& AODV when speed is increased. GPSR performs better at low velocity compared to IRG and AODV. But at high speeds, IRG overcomes GPSR.\cite{r28}. } \\
            \midrule
            Aero RP & Geographic & The packet forwarding decisions are made hop by hop. A velocity dependent parameter called Time to intercept (TTI) provides an idea about relative speed of the potential neighbour w.r.t receiving node. & NS-3  & Random Waypoint Model & The PDR increases for Aero RP as the no. of nodes are increased. & Aero RP in ferry or buffer modes, holds the packet for some specified time which at the same time ensures more packet delivery. But the other two modes of Aero RP i.e. Drop Beacon \& Drop Beaconless have least delay & \multicolumn{1}{p{13.285em}|}{Aero RP \& OLSR creates less no. control packets.~\cite{r14}.} \\
            \midrule
            Geographic Routing Protocol for Aircraft Ad-Hoc network (GRAA)  & Geographic  & Three dimensional up-to-date information about the location of aircraft and its direction of movement to calculate Euclidean distance between nodes. & Qualnet  & Random waypoint Mobility model \& Two Ray Ground radio propagation model  &For predetermined route and increase in speed, PDR is higher for GRAA than GPSR & Due to its hybrid approach, GRAA provides faster delivery of packets to destination than GPSR. Because in case of GPSR, once the packet is delivered to unexpected zone, it increases end-to-end delay.& ~\cite{r29} \\
            \bottomrule
    \end{tabular}}
    \label{table}
    \end{table*}%
\end{document}

Answer 1

the best what i can do with your table is:

(red lines show page layout)

you really need to reconsider about table content (make text in cells shorter, maybe with use of abbreviations and their explanations in table notes).

from your preamble i consider only necessary packages and add makecell for thead in column headers. also i use tabularx environment and reduce font size in table to \footnotesize. this should work with your document class too (but table can be narrower and consequently higher).

 \documentclass[twocolumn]{article}
\usepackage{mathptmx}       % selects Times Roman as basic font
\usepackage{booktabs, makecell, tabularx}
\renewcommand\theadfont{\bfseries}
\renewcommand\theadgape{}
\newcolumntype{L}{>{\raggedright\arraybackslash}X}

%-------------------------------- show page layout, only for test
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%

\begin{document}
\begin{table*}
    \centering
    \footnotesize
    \setlength\tabcolsep{3pt}
\caption{Comparison of protocols used in recent researches in Airborne Networks}
\begin{tabularx}{\linewidth}{@{} >{\hsize=0.5\hsize}L
                                c X c
                                >{\hsize=0.5\hsize}L
                                LLL @{}}
    \toprule
\thead[b]{Protocol} & \thead[b]{Type of\\ Protocol} & \thead[b]{Method of\\ Neighbour\\ discovery}
    & \thead[b]{Simulator\\ used } & \thead[b]{Mobility\\ Model} & \thead[b]{PDR} & \thead[b]{E2E\\ Delay} & \thead[b]{Overhead} \\
    \midrule
DOLSR   & Proactive     &  Broadcasting Hello messages to one hop neighbours and then select MPR nodes & OPNET 4.5 & Random Waypoint Mobility model & More than AODV, DSR and OLSR. & Less than AODV, DSR and OLSR. & Lesser than OLSR, AODV and DSR due to reduction in no. of MPR’s~\cite{r7} \\
    \midrule
A-GR    & Geographic    & It makes use of location and mobility information. & Qualnet 5.0 & Gauss-Markov Model & A-GR provides more PDR with little degradation as the no. of nodes increases to 50 or more. & At high node density, A-GR provides lesser delay compared to GPSR \& GRAA. & For GPSR \& GRAA, overhead increases exponentially \cite{r15}. \\
    \midrule
Improved reactive and geographic (IRG) & Geographic & Reactive,Greedy Forwarding & NS-2.35 & Random waypoint model &  IRG outperform the AODV \& GPSR with increase of speed because selection of next hop is based upon relative velocity between nodes.& IRG provides slightly lesser delay than GPSR. AODV has highest delay with increase in node velocity. & Routing overhead increases for all three i.e. IRG , GPSR \& AODV when speed is increased. GPSR performs better at low velocity compared to IRG and AODV. But at high speeds, IRG overcomes GPSR.\cite{r28}.  \\
    \midrule
Aero RP     & Geographic    & The packet forwarding decisions are made hop by hop. A velocity dependent parameter called Time to intercept (TTI) provides an idea about relative speed of the potential neighbour w.r.t receiving node. & NS-3  & Random Waypoint Model & The PDR increases for Aero RP as the no. of nodes are increased. & Aero RP in ferry or buffer modes, holds the packet for some specified time which at the same time ensures more packet delivery. But the other two modes of Aero RP i.e. Drop Beacon \& Drop Beaconless have least delay & Aero RP \& OLSR creates less no. control packets.~\cite{r14}. \\
    \midrule
Geographic Routing Protocol for Aircraft Ad-Hoc network (GRAA)  & Geographic  & Three dimensional up-to-date information about the location of aircraft and its direction of movement to calculate Euclidean distance between nodes. & Qualnet  & Random waypoint Mobility model \& Two Ray Ground radio propagation model  & For predetermined route and increase in speed, PDR is higher for GRAA than GPSR & Due to its hybrid approach, GRAA provides faster delivery of packets to destination than GPSR. Because in case of GPSR, once the packet is delivered to unexpected zone, it increases end-to-end delay. & ~\cite{r29} \\
    \bottomrule
\end{tabularx}
    \label{table}
\end{table*}%
\end{document}