Latency in TCP communication via WiFi between Android tablet and laptop

Question

In my setup, I use a PC (laptop) with Win7 that creates a hosted network (WiFi). I program it in C++/CLI (.net). On the other side, I have a tablet with Android (ICS). The tablet is the server and the PC is the client. I chose TCP for accuracy, I exchange commands, I don't want to loose packets.

On the tablet side, I use a ServerSocket that waits for communication with the method accept(). Then it returns a Socket, etc.

On the PC side, I use a Socket then I connect it to the tablet using connect(). After it is done, I send() bytes to the tablet that get them through its InputBuffer... The it returns bytes through its OutputBuffer and they are retrieved in the PC by calling receive().

The problem is that sometimes it takes few seconds for a send/receive cycle, other times it takes few tens of milliseconds. So here are my questions:

1. Is it a normal behavior that we can expect for such communications?
2. Can we fine tune this behavior by setting, say, buffer sizes or other things?
3. What is the longest step: connecting or transferring data?

I would appreciate if someone could give me some clues. I'm pretty new in network programmation and sometime "I'm loosing my latin!" (well... a french expression!)

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Code examples: (skipping trivial code and comments...)

PC side (C++/CLI)

Socket^ lpSocket = gcnew Socket( SocketType::Stream, ProtocolType::Tcp );

lpSocket->Connect( Addr, Port ); // PROBLEMS HERE

SocketError err;
array<Byte>^ lOutBuffer = Encoding::UTF8->GetBytes( "Here's a sentence.\r\n" );
lpSocket->Send( lOutBuffer, 0, lOutBuffer->Length, SocketFlags::None, err );
array<Byte>^ lInBuffer = gcnew array<Byte>( 1024 );
int N = lpSocket->Receive( lInBuffer, 0, lInBuffer->Length, SocketFlags::None, err );
String^ lpOutString = Encoding::UTF8->getString( lInBuffer, 0, N );

Tablet side (Android ICS) The code is a little more complicated due to threading. I will skip the mandatory try/catch...

// ... We are in a background thread
ServerSocket mSS = new ServerSocket();
mSS.bind( new InetSocketAddress( mPort ) );
mSS.setReuseAddress( true );

Socket lCS = mSS.accept();
lCS.setSoTimeout( 1 );
lCS.setTcpNoDelay( true );

BufferedInputStream lIn = new BufferedInputStream( lCS.getInputStream() );
PrintStream lPS = new PrintStream( lCS.getOutputStream() );
byte[] lBuf = new byte[1024];
String lInStr="";

while ( true )
{
    try
    {
        if ( lInStr.endsWith( "\r\n" ) ) { break; }

        int N;
        if ((N=lIn.read( lBuf,0,1024))!=-1) { lInStr += new String( lBuf, 0, N, "UTF-8" );}
        else { break; }
    }
    catch ( SocketTimeoutException e )
    { /* Can occur because timeout is set to 1ms */ }
}

lOut.print( lInStr ); // Just echo input to output
lOut.close();
lCS.close();

mSS.close();

So, it is a simple app where I can accept one connection. Once it is accepted, I process it, then I close the sockets and that's it.

The problems occur (it seems) in the PC side with connect(). I often get error Timeout or ConnectionRefused. If I redo the connect() in a loop, I obtain a connection. But even when there is no timeout, the connection can take few seconds to establish.

Answer 1

The WiFi in the tablet I was working with is bugged. I tried with a phone and other tablets and it works quite fine.

I don't know actually if it is a bug with the version of Android (4.0.3) or if it is a hardware issue because I could not test two different tablets having the same Android.

Anyway...

I appreciate the help you gave me when saying that the connection delay looked too long. This is what sent me to the right path to the solution!