gobackn.m

本程序是用java编写的一些关于arq协议实现的小程序

function [Umas,Pak,Ack]=gobackn(amas,Nsimul,Ploss,SWS);

% function [Umas,Pak,Ack]=gobackn(amas,Nsimul,Ploss,SWS);
%
% GoBackN algoritnm simulation 
% MaxSeqNum=SWS+1
% 
%
% Input:
%
% amas - t_prop/t_transm    -  can be an array, in which case
% simulation is repeated for each value of an array and 
% cirresponding utilization is recorded in corresponding element of Umas
%
% Nsimul - number of sent frames to simulate
% time required is proportional to Nsimul
% quality of utilization estimates improves with Nsimul
% recommended Nsimul=100;
%
% Ploss - probability of packet loss (or receiving with an error)
% this probability is assumed the same for data packets and acknowledgments
%
% SWS - sender window size
% note: receiver window size - 1 in this algorith
%
% Output:
%
% Umas - vector containing channel utilization estimates for values of a
% in amas parameter. Length(Umas)=Length(amas)
%
% In case of the length(amas)>1 the following 2 output parameters
% output the result for the simulation for the last entry of vector amas
%
% Pak - matrix, each raw contains a signature of the simulated data packet sent.
% structure of the raw: (Pak#   Time_sent  Time_received  sequence#)
% Time_received = Inf   if the packet is lost
%
% Ack - matrix, each raw contains a signature of the simulated acknowledgment 
% packet sent.
% structure of the raw: (Ack#   Time_sent  Time_received  sequence#)
% Time_received = Inf   if the ack is lost
%
% 15/11/01
% A. Litvin
% e-mail: [e-mail]
%


if length(Ploss)>1
   error('Ploss size');
end

if Ploss<0 | Ploss>1 
   error('Ploss not in limits [0,1]');
end

if SWS<1
   error('wrong SWS');
end


Umas=[];

for j=1:length(amas)
   
   a=amas(j);
   
	Tprop=1;
	Ttran=Tprop/a;

	Pak=[];
	Ack=[];
	time=0;
   
   framen=0;
   U=0;
   Maxfrn=SWS+1;
   
   LFR=-1;
   LFA=-1;
   LFS=-1;
   busysender=0;
   stackarrive=[0 0];   stackarrive(1,:)=[];
   stackack=[0 0];      stackack(1,:)=[];
   stacktimeout=[0 0];  stacktimeout(1,:)=[];
   timestep=min(Tprop,Ttran)/50;
   Timeout=Ttran+2*Tprop+3*timestep;

while LFS<Nsimul
   
    % disp([time LFS LFA]);
   % disp(stacktimeout)
   
   % resend packets if timeout
   
		ind=find(stacktimeout(:,2)<=time);
      if length(ind)
         nopak=stacktimeout(1,1);
         if  time>=busysender
            busysender=time+Ttran;
            stacktimeout=[nopak time+Timeout];
            framen=rem(nopak,Maxfrn);
            gotit=rand>Ploss;
            LFS=nopak;
            if gotit
               Pak=[Pak; nopak time time+Tprop+Ttran framen];
               stackarrive=[stackarrive; nopak time+Tprop+Ttran];
            else
               Pak=[Pak; nopak time Inf framen];
            end
         end         
      end
      
   % if no timeout   we can send new packet
   
      if time>=busysender & LFS<LFA+SWS
         busysender=time+Ttran;
         LFS=LFS+1;
         stacktimeout=[stacktimeout; LFS time+Timeout];
         framen=rem(LFS,Maxfrn);
         gotit=rand>Ploss;
         if gotit
            Pak=[Pak; LFS time time+Tprop+Ttran framen];
            stackarrive=[stackarrive; LFS time+Tprop+Ttran];
         else
            Pak=[Pak; LFS time Inf framen];
         end      
      end   
      
      % check of arrival at receiver
            
      ind=find(stackarrive(:,2)<=time);
      if length(ind)
         newpak=stackarrive(1,1);   
         if newpak==LFR | newpak==LFR+1 | newpak<LFR
            if newpak>=LFR LFR=newpak; end;
            framen=rem(newpak,Maxfrn);
            gotack=1;
            if gotack
               Ack=[Ack; newpak time time+Tprop framen];
               stackack=[stackack; newpak time+Tprop];
            else
               Ack=[Ack; newpak time Inf framen];
            end          
        else
            gotack=1;
            framen=rem(LFR,Maxfrn);
            if gotack
               Ack=[Ack; LFR time time+Tprop framen];
               stackack=[stackack; LFR time+Tprop];
            else
               Ack=[Ack; LFR time Inf framen];
            end 
        end         
         stackarrive(1,:)=[];
      end
      
      % check of arrival of ack
      
      ind=find(stackack(:,2)<=time);
      if length(ind)
         newack=stackack(1,1);
         if newack==LFA | newack==LFA+1
            LFA=newack;
            ind=find(stacktimeout(:,1)==LFA);
            if length(ind)
               stacktimeout(ind,:)=[];
            end;            
         end
         stackack(1,:)=[];
      end
      
   	
   	time=time+timestep;
	end;

	Umas(j)=LFS*Ttran/time;

end;