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GO BACK N ARQM S RAMAIAH INSTITUTE OF TECHNOLOGY
AUTONOMOUS INSTITUTE AFFILIATED TO VTU
Information Science and Engineering Department
Group no: 10
Group name : THE KING MAKERS
Group members:
Chetan .S (1ms07is021)
Deepak.P.J (1ms07is022)
Pavan.R (1ms07is062)
Phaneesh.L.N ( 1ms07is066)
Introduction:
ARQ is a method of data transmission which uses timeouts and acknowledgement signals to achieve reliable transmission over an unreliable service i.e the internet. An acknowledgement is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame or packet. A timeout is a reasonable point in time after the sender sends the frame/packet; if the sender does not receive an acknowledgement before the timeout, it usually re-transmits the frame/packet until it receives an acknowledgement or exceeds a predefined number of re-transmissions.
There are mainly 3 types of ARQ
Stop and wait ARQ Go back n ARQ Selective repeat ARQ
Here we deal about go back n ARQ
Go back n ARQ is a protocol in which the sender continues to send a number of frames without receiving the ACK (acknowledgement) signal from the receiver. The receiver receives the next frame in accordance with the sequence number and sends that number with every ACK it sends. The receiver will not accept the frames that don’t have the SEQ number it expects.
Once the sender has sent all of the frames in its window, it will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received from the receiver process after time out it sends the outstanding and the subsequent frames.
LITERATURE SURVEY
The present invention relates to a data transmission system using an automatic repeat request (hereinafter is referred to as abbreviation "ARQ") and more particularly to a data transmission system for realizing a procedure of retransmission of error detected frames, which is adapted to a transmission channel such as radio communication with relatively high transmission error-rate.
The recovery of transmission error in data transmission has been conducted by automatic retransmission using ARQ and/or by error-correction technique using error-correction code. ARQ is a method of recovering an error in a data transmission in which any blocks of data found to contain errors at the receiving terminal are automatically requested for repeat transmission from the transmitting terminal. In response to this request, the required blocks are transmitted again from the transmitting terminal to the receiving terminal. A Go-back-N ARQ method and a selective repeat ARQ method are representative.
An example of procedure of the Go-back-N ARQ method is as follows:
In this method, the transmitting terminal transmits data with additions of an error detection code and transmission sequential number, and the receiving terminal performs error checking on every frame of the received data. When any frame containing an error is detected, the receiving terminal informs the transmitting terminal of the sequential number of the frame concerned and requests repeat transmission of the data from the transmitting terminal. The transmitting terminal searches back the sequential number of the frame and repeats transmission of the sequence of data starting from the required frame.
The detailed procedure of retransmission of frames by the Go-back-N ARQ is as follows:
The transmitting terminal transmits data with error detection codes and transmission sequence numbers N(S). At the receiving terminal, aframe with transmission sequence number N(S)=2 is now found to contain an error.
In this case, the receiving terminal transmits a REJ frame with a numeral 2 previously set in a received sequence number N(R) for requesting repeat transmission of the frame having the sequence number 2. Upon receipt of the REJ frame, the transmitting terminal returns to transmission number N(S)=2 and repeats transmission of data in the sequence beginning from the specified sequence number. However, this method has relatively low efficiency and reduces a throughput in data transmission since it requires repeat transmission of the already transmitted correct frames following the requested frame.
ALGORITHM
SENDER
Sw = 2 ^m -1;Sf = 0;Sn = 0;
While ( true){Waitforevent ( );
If (event (requesttosend) ) {
If (Sn-Sf >=Sw)Sleep ( );
Getdata ( ) ; Makeframes (Sn); Storeframes (Sn); Sendframes (Sn); Sn =Sn+1; If ( timer not running) Starttimer ( ); }
If ( event ( arrivalnotification ) )
{ Receive (ACK); If ( corrupted (ACK) ) Sleep ( ); If ( ( ackno > Sf ) && ( ackno <= Sn ) ) ; While ( Sf <= ackno ) {
Purgeframe ( Sf ); Sf = Sf + 1; } Stoptimer ( ); }
If ( event ( timeout ) )
{Starttimer ( ) ; Temp = Sf ; While ( temp < Sn ); { Sendframe ( Sf ); Sf = Sf + 1;
}
}
}
RECEIVER
Rn = 0;
While ( true )
{
Waitforevent ( ) ;
If ( event (arrivalnotification ) )
{
Receive ( frame ) ;
If ( corrupted ( frame ) )
Sleep ( ) ;
If ( seqno == Rn )
{
Deliverdata ( ) ;
Rn = Rn + 1 ;
sendACK ( Rn ) ;
}
}
}
PROGRAM :
#include<iostream>#include<cstring>#include<string>#include<fstream>using namespace std;int flag=0;int flag1=0;int flag2=0;int flag4=0;int flag5=0;int flag6=0;int loop2;char ch1;char ch2;int loop;char sender[50]="How many wants to live after 2012 ?";char recive[50];int lensender;int senderack[50];int curpos;int i;int k;int trav=0;int Ws;int recnext;int errack[10]={-1};int errno;int errpck[10]={-1};int waste;char ch='r';void dispsending();void send();void shift();void dispsimple();void dispreceiving();int timer(char str[50],char value[50],int colval) //function takes read, to be searched string and field, size of str1{int k,count=0,i=0,flag=2; //outer loop to travers through ':'for(k=0;k<40;k++) //timer started to check the sender and receiver {
if(str[k]==':') //to start search only after required position got
count++;else
if(count==colval) //start positon{count++; //count incremented so that loop wont search again
while( isdigit(value[i]) ) //loop till the key element traversed
{if(!isdigit(str[k+i])) //this returns if
key element is greater size wise{
flag=1;return flag+10;}
if(str[k+i]!=value[i]) //if values are not matching find if its greater
{if(flag==2) //this condition makes flag change
only once its lifetimeflag=-1;
if(str[k+i]>value[i]){if(flag==-1) //same as above usageflag=3; //assign if data is
greater}elseif(flag==-1)flag=1;
}i++; //increment the searching loop for size of key
element}if(isdigit(str[k+i])) //if next element after the search is
digit then data is greaterflag=3;
} }return flag+10;}
//start of pgm
int main(){
curpos=0;recnext=0;cout<<"enter the sliding window size\n";cin>>Ws;lensender=strlen(sender);cout<<"how many packets to miss?";
cin>>errno;for(i=0;i<errno;i++){cin>>errpck[i]; //read the packets to miss}
cout<<"how many ack's to miss?";cin>>errno;
for(i=0;i<errno;i++){cin>>errack[i]; //read the acks to miss}
while(curpos<lensender){
cout<<" press Y to miss ack or packet: ";while(ch=='!'){cin>>ch;}
if(ch=='y'){ //reads the second time
cout<<"how many packets to miss?";cin>>errno;
for(i=0;i<errno;i++){cin>>errpck[i]; //read the packets to miss}
cout<<"how many ack's to miss?";cin>>errno;
for(i=0;i<errno;i++){cin>>errack[i]; //read the acks to miss}
}cout<<"\n\n---------------THE NEW WINDOW POSITION------------\n";
dispsimple();
send();int use=timer(sender,recive,Ws);if(use==-1) //if timed out the packets are resentsend();dispsending();dispreceiving();
//cout<<"the acknowledge\n\n";//for(i=0;i<lensender;i++)//cout<<senderack[i];//cout<<endl;loop2=curpos;
for(;senderack[loop2]==1;loop2++) //slide the window till a acknowledge is recieved properly
{curpos++;}
ch='!';cout<<"\n\n----------------- x ------ x -----------------\n";
}
return 0;} //end of main
void send(){
for(loop=0;loop<Ws;loop++) //if packet is missing then entire ack's from that packet sould n be sent
{
if((curpos+loop)!=errpck[flag4]){
if(recnext==(curpos+loop)){ //to find the first
defect packetrecive[recnext]=sender[curpos+loop];recnext++;}
}else{break; //flag 5 used to detect if packet failure
is happen}
}int temp=loop;
for(loop=0;loop<Ws;loop++) //if packet is missing then entire ack's from that packet sould n be sent
{
if((curpos+loop)!=errpck[flag4]){
if(recnext==(curpos+loop)){recive[recnext]=sender[curpos+loop];recnext++;}
}else{flag4++;flag5=1; //flag 5 used to detect if packet failure
is happen}
}for(loop=0;loop<temp;loop++){
if((curpos+loop)==errack[flag2]){senderack[curpos+loop]=-1;flag2++;flag1=1; //this is set so that we will come to know if
error in ack is occured}else{senderack[curpos+loop]=1;}
}}
/*int flag=0,flag1=0,flag2=0,loop2,loop;char sender[50]="bhaskar is very good person";char recive[50];int lensender,senderack[50],curpos,i,k,Ws,recnext,errack,errpck;*/
void dispsending(){int temp;int q;q=trav; //used to identify the packet currently intemp=(Ws/2)+curpos+1;cout<<" ";
for(k=0;k<temp;k++)cout<<" "; //print the top partcout<<"|";for(;k<(lensender+10+(strlen(recive)/2));k++)cout<<"`";cout<<"|";k++;waste=k;cout<<endl;
cout<<" ";
for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<" "; //for packet sendingfor(;k<(curpos+Ws) && k<lensender;k++){
if(errpck[q]==k){cout<<" ";q++;}elsecout<<"|";
}cout<<" ";for(;k<waste-2;k++)cout<<" ";cout<<"| (sending...)";cout<<endl;
q=trav;cout<<" ";
for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<" "; //for packet sending line 2for(;k<(curpos+Ws) && k<lensender;k++){
if(errpck[q]==k){cout<<" ";q++;}elsecout<<"|";
}
cout<<" ";for(;k<waste-2;k++)cout<<" ";cout<<"| ";
cout<<endl;
q=trav;cout<<" ";
for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<" ";ch1='&'; //for packet sending line 2for(;k<(curpos+Ws) && k<lensender;k++){
if(errpck[q]==k){cout<<ch1;ch1++;q++;}elsecout<<"|";
}
cout<<" ";for(;k<waste-2;k++)cout<<" ";cout<<"| ";for(k=curpos+1;k<(curpos+Ws);k++){ for(i=0;i<10;i++)
if(flag5==0){
if(errpck[i]==k){cout<<"packet-"<<errpck[i];flag5=1;errpck[i]=-1;}
}elseif(errpck[i]==k){cout<<","<<errpck[i];errpck[i]=-1;}
} cout<<endl;
trav=q;cout<<" ";
for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<",";for(;k<(curpos+Ws) && k<lensender;k++)cout<<"|";cout<<",";for(;k<waste-2;k++)cout<<" ";cout<<"V ";
if(flag6==1)cout<<" missing :-(";
cout<<endl;
cout<<" ";for(i=0;i<curpos;i++)cout<<sender[i];cout<<"|";for(;i<(curpos+Ws) && i<lensender;i++)cout<<sender[i];cout<<"|";for(;i<lensender;i++)cout<<sender[i];for(;i<waste-2;i++)cout<<" ";cout<<"V";cout<<endl;
cout<<" ";for(k=0;k<curpos;k++)cout<<" ";cout<<"`";for(;k<(curpos+Ws) && k<lensender;k++)cout<<"`";cout<<"`";for(;k<lensender+10;k++)cout<<" ";cout<<recive;cout<<endl;cout<<endl<<endl;
}
void dispsimple(){
cout<<" ";for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<",";for(;k<(curpos+Ws) && k<lensender;k++)cout<<",";cout<<",";cout<<endl;
cout<<" ";for(i=0;i<curpos;i++)cout<<sender[i];cout<<"|";for(;i<(curpos+Ws) && i<lensender;i++)cout<<sender[i];cout<<"|";for(;i<lensender;i++)cout<<sender[i];for(;i<lensender+3;i++)cout<<" ";cout<<"-->";for(;i<lensender+10;i++)cout<<" ";cout<<recive;cout<<endl;
cout<<" ";for(k=0;k<curpos;k++)cout<<" ";cout<<"`";for(;k<(curpos+Ws) && k<lensender;k++)cout<<"`";cout<<"`";
cout<<endl<<endl;}
void dispreceiving(){int temp;
temp=(Ws/2)+curpos+1;cout<<" ";
for(k=0;k<temp;k++)cout<<" "; //print the top partcout<<"|";for(;k<(lensender+10+(strlen(recive)/2));k++)cout<<"`";
cout<<"|";k++;waste=k;cout<<endl;
cout<<" ";for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<" "; //for packet sendingfor(;k<(curpos+Ws) && k<lensender;k++){ if(senderack[k]==1)
cout<<"|";elseif(senderack[k]==0)cout<<" ";elseif(senderack[k]==-1)cout<<"|";
}cout<<" ";for(;k<waste-2;k++)cout<<" ";cout<<"|";cout<<endl;
cout<<" ";for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<" ";
ch2='@'; //for packet sending line 2for(;k<(curpos+Ws) && k<lensender;k++){ if(senderack[k]==1)
cout<<"|";elseif(senderack[k]==0)cout<<" ";elseif(senderack[k]==-1){cout<<ch2;ch2--;}
}cout<<" ";for(;k<waste-2;k++)cout<<" ";cout<<"| (received)";cout<<endl;
cout<<" ";for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<" "; //for packet sending line 2for(;k<(curpos+Ws) && k<lensender;k++){ if(senderack[k]==1)
cout<<"|";elseif(senderack[k]==0)
cout<<" ";elseif(senderack[k]==-1)cout<<" ";
}cout<<" ";for(;k<waste-2;k++)cout<<" ";cout<<"| ";
flag6=0;for(k=curpos;k<(curpos+Ws);k++)
{if(flag6==0)
{if(senderack[k]==-1){cout<<"ACK-"<<k;flag6=1;}
}elseif(senderack[k]==-1){cout<<","<<k;}
} cout<<endl;
cout<<" ";for(k=0;k<curpos && k<lensender;k++)cout<<" ";cout<<",";for(;k<(curpos+Ws) && k<lensender;k++){ if(senderack[k]==1)
cout<<"|";elsecout<<",";
}cout<<",";for(;k<waste-2;k++)cout<<" ";cout<<"| ";
if(flag6==1)cout<<" missing :-(";
cout<<endl;
cout<<" ";for(i=0;i<curpos;i++)cout<<sender[i];cout<<"|";for(;i<(curpos+Ws) && i<lensender;i++)
cout<<sender[i];cout<<"|";for(;i<lensender;i++)cout<<sender[i];for(;i<waste-2;i++)cout<<" ";if(flag6==1 || flag5==1)cout<<"| TIME OUT (resending...)";elsecout<<"|";flag6=0,flag5=0;cout<<endl;
cout<<" ";for(k=0;k<curpos;k++)cout<<" ";cout<<"`";for(;k<(curpos+Ws) && k<lensender;k++)cout<<"`";cout<<"`";for(;k<lensender+10;k++)cout<<" ";cout<<recive;cout<<endl;cout<<" ";for(k=0;k<curpos-1;k++)cout<<" ";cout<<curpos<<",0 <-";for(;k<curpos+Ws-6;k++)cout<<" ";cout<<"-> "<<Ws-1;cout<<endl;}
RESULTS : ( OUT PUT )
LIMITATIONS
Even the transmitted frames should be sent again if there is failure in successful transmission of the previously sent frames.
The receiver can’t accept any frames which is out of order. The size of the receiver window is 1 so the efficiency is
not that great.
CONCLUSION
We can conclude by saying that GO BACK N ARQ overcomes the short coming of stop n wait ARQ and also increases the efficiency of transmission by utilizing the bandwidth to the maximum possible.
Implementation of go back is comparatively simpler that is it’s algorithm is easy to understand and efficient.
The only disadvantage being the necessity to send even the transmitted frame again due to the failure in the successful transmission of the previously sent frame. This is over come by a more efficient ARQ (selective repeat ARQ) which is more complex.
