# Introduction:

In this project, our aim is to develop a system called Data Link Simulator. There are various Data Link Control protocols that used at the data link layer which we need to understand before we start to build the system. The purpose for designing this system is because the possibility of transmission error is appeared at the data link layer. This is also because the receiver of data may need to regulate the rate at which the data arrive at his site. Hence, it is necessary to impose a layer of control in each communicating device that provides functions such as flow control, error detection and error control.

In other words, our system provides those functions mentioned earlier. In flow control function, it enables a receiver to regulate the flow of data from a sender so that the receiver’s buffers do not overflow. There are several techniques available in order to implement flow control such as Stop-And-Wait (SAW) and Sliding Windows (SW). Stop-and-Wait is a flow control protocol in which sender transmits a block of data and then awaits an acknowledgement before transmitting the next block. Furthermore, it is the simplest reliability technique and is adequate for a very simple communications protocol. While Sliding Windows is a method of flow control in which a transmitting station may send numbered packets within a windows of numbers.

While error detection function can be implemented by using calculating an error-detecting code that is a function of the bits being transmitted. Error detection function can be divided into Parity Check and Cyclic Redundancy Check (CRC). Parity Check is the simplest form of error detecting by adding or appending an extra bit, 0 or 1 called a parity bit to the message of signal or binary that is going to travel. While the Cyclic Redundancy Check is a very powerful but easily implemented technique to obtain data reliability. The CRC technique is used to protect blocks of data called Frames (blocks).

Error control is performed by retransmission of damaged/lost frames that have not been acknowledged or for which the other side requests a retransmission. There is a technique such as Automatic Repeat Request (ARQ) that can be further divided into Go-Back-N ARQ and Selective Reject ARQ. Automatic Repeat Request (ARQ) is an error-control system in which a request for re-transmission is generated by the receiver when an error is detected. Go-back-N ARQ uses sliding-window flow controls and it is an error correction technique to detect and retransmit the frames which have been corrupted due to errors in the physical link. And Selective-reject ARQ is an error control technique in which the transmitter only retransmits a specific block / frame that are damaged or lost.

The main objective of developing the system is to help the network designers and learners understand the various data link control protocols and their parameters. Besides, in this system the illustration of how the data link control protocols work will be show clearly. In this system also will show the randomness of error occurrence and the ability to allow the detection, control and reporting when errors occurred. We also concerned about the issues such as the performance and link utilization in this project.

By using our system, we can gain several benefits. The learners will understand more clearly about the concept of flow control, error detection and error control. In addition, the architecture and algorithms of those functions also can be easily understood by trying the system using the user friendly user manual.

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