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In this laboratory programming assignment, you will be writing the sending and receiving transport-level code for implementing a simple reliable data transfer protocol. There are two versions of this lab, the Alternating-Bit-Protocol(Stop and Wait) version and the Go-Back-N version. you will implement Go Back n Sliding Window Protocol ( window size =3). This lab should be fun since your implementation will differ very little from what would be required in a real-world situation. .

Physical layer:
A physical layer object connects directly to a medium. There is only one type of physical layer. It accepts a sequence of bytes which it then sends, one bit at a time, across the medium. The receiving physical layer reconstructs the bytes, one at a time, delivering each complete byte to its data link layer. The functionality of physical layer is implemented in Channel class.

Transport layer gets data from the application layer, creates a packet from it and then passes this packet to the network layer for transmission.

public Packet(int i, int j, boolean flag, byte abyte0[])
{
        ack = false;
        seqno = i;
        size = j;
        ack = flag;
        data = new byte[size];
        data = abyte0;
}
. . .
}
[Note:We will ignore the Internet checksum field in the packet class as we will do error checking in the lower layer (Data Link Layer) in the next project(i.e., project #4)].

Where payload is 12.

Your routines will fill in the payload field from the message data passed down from application layer as explained below.

Consider the following data that are received by the transport layer from the application layer:

The routines you will write

The procedures you will write are for the sending entity (Sender) and the receiving entity (Receiver). Only unidirectional transfer of data (from Sender to Receiver) is required. Of course, the Receiver side will have to send packets to the Sender to acknowledge (positively or negatively) receipt of data. Your project will have the classes described below.

Project Classes	Description

Channel	Thread-safe shared memory for communication between the sender and receiver threads.
ErrorModel	Used to corrupt or loose packets on an imperfect channel.
Packet	Used to create a Packet
GBNProtocol	Implements the Go-Back N protocol. For lossy channel, loose the third packet.
Port	Communication port. Each contains an outgoing Channel that the thread sends data to and an incoming Channel that it reads from.
Project3	Main class. Creates two channels for communications. Two ports, each containing the two channels are created, one for the sender and one for the receiver. The sender and receiver threads are created and started. When the simulation is complete, statistics are printed.
Receiver	Thread object that reads bytes from its port's incoming Channel and sends an ACK and passes the bytes out to the higher layer or simply writes it to the output file.
Sender	Thread object that sends the bytes of a Packet object over its port's outgoing channel. It's main loop implements most of the functionality of the Go-Back N protocol. It first sends as many packets as possible to fill the window. It then checks to see if any ACK/NACKs have arrived and handles any found. Finally, it queries the window to see if any packet or packets have timed out. If so, the packets in the window are resent. At this time execution returns to the top of the loop. This continues until all packets have been sent and ACKed.
SentPacket	Wrapper class to encapsulate a single Packet and its corresponding timer. Implements EventListener to use a swing Timer object to flip a boolean when the packet times out.

Print out the following statistics after transferring a file from the sender to the receiver: