npcsma.m

系统分别对无线通信中的几种重要的多址接入协议进行了仿真

% program 6-8
% npcsma.m
%
% non-persistent CSMA System
%
% Input argument
% now_time :now time but,now_time<0 initializes the access terminals
%
% Output argument
% next_time :next state change time
%
function [next_time]=npcsma(now_time)
global STANDBY TRANSMIT COLLISION      % definition of the global variable
global Srate Plen
global Mnum Mplen Mstime Mstate
global Tint Rint
global Spnum Splen Tplen Wtime

persistent mgtime mtime               % definition of the static variable
if now_time <0                        % initialize access terminals
    rand('state',sum(100*clock));     % resetting of the random table
    mgtime =-Tint*log(1-rand(1,Mnum));% packet generation time
    Mstime =zeros(1,Mnum)-inf;        % packet transmitting time
    mtime  =mgtime;                   % state change time
    Mstate =zeros(1,Mnum);
    Mplen(1:Mnum) =Plen;
    next_time =min(mtime);
    return
end

idx=find(mtime==now_time & Mstate==TRANSMIT);
                                      % finding of the terminal which
                                      % transmission succeeded
if length(idx)>0
    Spnum      =Spnum+1;
    Splen      =Splen+Mplen(idx);
    Wtime      =Wtime+now_time-mgtime(idx);
    Mstate(idx)=STANDBY;
    mgtime(idx)=now_time-Tint*log(1-rand);     % next packet generation time
    mtime(idx) =mgtime(idx);% next packet transmitting time
end

idx=find(mtime==now_time & Mstate==COLLISION);
                                      % finding of the terminal which
                                      % transmission failed
if length(idx)>0
    Mstate(idx)=STANDBY;
    mtime(idx)=now_time-Rint*log(1-rand(1,length(idx)));% resending time
end

idx=find(mtime==now_time & Mstate==STANDBY);
                                      % finding of the terminal which
                                      % carrier sensing
if length(idx)>0
    Tplen =Tplen+sum(Mplen(idx));
    for ii=1:length(idx)
        jj=idx(ii);
        if carriersense(jj,now_time)==0% channel is idle
            Mstate(jj) =TRANSMIT;      % packet transmitting
            Mstime(jj) =now_time;      % start time of transmitting
            mtime(jj)  =now_time+Mplen(jj)/Srate;% end time of transmitting
        else                           % channel is busy
            mtime(jj) =now_time-Rint*log(1-rand);% waiting time
        end
    end
end
next_time =min(mtime);                 % next state change time