In a transmission system, any inconvenient may arise such as overflow of data, error, lost or damage frame. Therefore, many functions have been established in order to assure the reliability and efficiency of data transmission. These functions are flow control, error control and error detection which is defined in the Data Link layer. Many problems have been occurred during the development of networking in the past. Developers kept asking questions during their research to find out the solutions for those problems that they had to face with. They want to know the way how the network handles data.
Some of the problems may be about the sender and receiver. The receiver should receive data at an efficient pace that will allow enough time to handle data. Another problem may be about the speed of the network. The total time to transmit data from point A to point B depended on many different factors. Some of these factors that affect the speed are transmission time defined as “the time it takes to emit a bit onto the medium” and propagation time defined as “the time it takes for a bit to traverse the link between source and destination” ().
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Developers also want to make sure that the sender and receiver can understand each other by using the same protocol and receiver has the ability to minimize the error occurring during transmission of data.
Therefore, some flow control has been created as the solution. Stop and Wait is a simple technique that is used to control flow of data bits. In this technique one data frame at a time is in transit and the source will wait for acknowledge before proceeding other transmission frame. This technique has the advantage of forcing the source to stop and wait for acknowledge, but is quite ineffective in a long bit length or in a very high data rate network. Sliding-Window technique is introduced to overcome the weakness of Stop and Wait technique.
It provides the opportunity to transit multiple frames without waiting any acknowledgements. To prevent frames from being lost, the sender keep track of frame in buffer in case any frame needs to be retransmitted. The receiver also allocates an amount of buffer space equal to the of the buffer memory to be received. In this scheme, many frames can be confirmed with one acknowledgement. This allows data frame to be transferred faster compare to Stop-and-wait technique by having less amount of overhead. Flow control assumes its function by managing data flow, however, a link free error is not guaranteed.
Error control function must be created to detect and correct error during the frame transmission. The most common strategy of error control in data transmission is Automatic Repeat Request (ARQ), which is classified into three different techniques: Stop-and-wait, Go-Back-N, and Selective-Reject. In Stop-and-Wait ARQ, the source sends a data frame and then waits for a response. Once a frame is received, an acknowledgment is sent to the source indicating that the destination has gotten the last frame and is ready to receive the next one. The sequence number that follows the acknowledgment is the next sequence number expected by the receiver. A timer is set to limit time delay if the receiver does not acknowledge or in case where data is lost or damaged.
The most efficient and most commonly used error control technique is Go-back-N ARQ. In this technique the sender can transmit many data frames without an acknowledgement. If the receiver rejects a frame then the sender must retransmit the frame and all subsequent frames that were sent. Selective-reject ARQ is the most complex error control and recovery. As in the previous technique the receiver or the node can transmit a quantity of data frame without an ACK, but it needs to keep in buffer the non-acknowledged frames. However, if a frame is rejected by the receiver, then the receiver only retransmits that concerning frame.
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Figure one depicts a good example of recovering error in a transmission system. The source sends four data frames in sequence. The first one does not reach destination. When the receiver receives packets two, it detects that frame one was lost and immediately sends a selective request to sender. Because frames that is not acknowledged are keep in buffer, therefore the sender responds by sending the missing frame, while it continues to transmit frame 5 to 7. So error control is an additional tool for flow control in order to ensure the correctness and efficiency of data transmission.
However, data reliability cannot be accomplished without the presence of error detection function. The purpose of this function is to ensure that data is being check for errors, since any transmission media are all subject to noise. There must be a need to establish some technique to detect error. Some of those are check sum, parity check, and cyclic redundancy check (CRC).
The CRC is known as the most common and the most powerful in data communication because it presents some interesting advantages such as data reliability, protection of blocks, frames with little overhead and the ease of implementation. The CRC algorithm is to calculate a number that is unique for every chunk of data.
First, the original message is given and fragmented into frames. Each frame is divided by the pre-defined CRC-polynomial. The remainder is the Frame Check Sequence (FCS).
The FCS is appended to the frame to be transmitted.
Once the receiver receives the frame, a check process is initiated. It will divide the frame by the pre-defined CRC polynomial. If the remainder is zero then transmission is accurate, otherwise there is an error (figure two).
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Figure three represents a shift register circuit through which CRC process is implemented in hardware. To create the CRC, data bits are accepted through the register one at a time and using the “xor” function. The content of the register will be the FCS.
Once again, the presence of anything else results in error. All the functions provided by data link layer against transmission impairments contribute greatly to the accuracy and reliability of data across a physical link. Many techniques have been defined; one appears more efficient or offers more advantages than others. The estimation of each technique is based on many factors such as: transmission time, bit error rate, reliability, flexibility, and robustness…
In addition, the application of each technique depends on the maximum utilization efficiency offers and also the protocol that provides the means to connect node. 1. Stallings, William. Data and computer communication, 6 th Ed.
New Jersey, Prentice Hall, Inc, 2000 2. URL: web > 3. R. E. Newman-Wolfe, 1995. URL: web.
