Difference between Stop and Wait, GoBackN, and Selective Repeat

Flow Control In the Network

The handling of data flow between different nodes, such as computers, printers, and wireless devices, is referred to as flow control in the network. It is an essential mechanism for effectively transmitting data in the network. Data flow is a serious concern when transmitting data from one node to another. It is caused when the receiving node is unable to handle the data. The data transmitted is more than the receiving node can manage. This leads to data loss and re-transmission of the =data frames.

In order to prevent data loss and effectively transmit data in the network, flow control instructs the sending node about how much data should be retransmitted to the receiving node. The node transmitting the data should wait for an acknowledgment from the receiving node before sending additional data frames into the network.

The flow control protocols that determine the mechanism used to send acknowledgment and retransmit data are Stop and Wait, GoBackN, and Selective Repeat.

Stop And Wait Protocol

• This is the most simple protocol that ensures guaranteed transmission of the data in the network.
• Here the stop and wait refer to the two steps performed by the protocols. The sender node transmits the data into the network. The data frame is sent sequentially in one by one manner. Once a data frame is transmitted in the network. The sending node waits for the receiving node to send an acknowledgment in the network.
• The stop and wait protocol is implemented as a flow control mechanism when the data link layer in the network provides the flow control services. This data link protocol enables the sender to transmit the data over noiseless channels.
• This protocol supports unidirectional data transmission in the network. This means that there is only one sender or receiver in the network. Both nodes cannot simultaneously send or receive the data in the network. The nodes cannot utilize the resources of the network. The protocol ensures that the data flow is maintained in the network, but it lacks any error control mechanism to the network.
• The primary concept behind implementing this protocol is that whenever the sender node transmits a data frame in the network, it waits for an acknowledgment from the receiver. Only when the acknowledgment is received it transmit to the other node in the network. The transmission is halted if the acknowledgment is received.
• The idea behind using this frame is that when the sender sends the frame, he waits for the acknowledgment before sending the next frame.
• Before discussing GoBackN and Selective Repeat protocols, the user must have prior knowledge of another important protocol: the sliding window protocol.

Sliding Window Protocol

The Sliding Window Protocol is a mechanism that allows the data-transmitting node to transmit multiple data frames simultaneously in the network. This protocol manages the data packets shared between two nodes where reliable and guaranteed data frame delivery is required. This protocol is implemented in the TCP (Transmission Control Protocol).

In this protocol, each data frame is assigned a sequence number. This protocol is implemented to ensure that the successfully transmitted data is not transmitted again by the sender node. This is useful as it increases the efficiency of the network and avoids duplicate data. The sequence number is unique for each data frame and used to confirm whether the receiving node successfully receives the data frame.

Two types of Sliding Window Protocols can be implemented in the network:

• Go-Back-N ARQ
• Selective Repeat ARQ

Go-Back-N ARQ

• In Go-Back-N ARQ, N stands for the Window size of the sender node, and ARQ stands for Automatic Repeat Request. Let us consider an example to explain this protocol where the sender window size is 3. this means that the sender node can transmit three data frames before waiting for an acknowledgment from the receiving node.
• The nodes use protocol pipelining that allows the nodes to transmit multiple data frames in the network simultaneously without waiting for the acknowledgment of the first frame. Suppose the sender needs to send 5 data frames in the network. If the protocol Go-Back-3 is implemented, the user can send frame 1, frame 2, and frame 3 simultaneously without waiting for the acknowledgment of frame 1.
• Since multiple data frames are transmitted in this protocol, each data frame sent is numbered sequentially. This step is performed to ensure that each data frame can be differentiated. These numbers are referred to as sequential numbers.
• The number of frames that can be sent simultaneously may differ with the sending node. The sender window size determines it. We can say that N data frames can be sent simultaneously in the network before the sender node receives the acknowledgment for the first frame sent to the receiver.
• If the sender does not receive an acknowledgment from the receiver node within an agreed-upon time, then all the data frames transmitted and stored in the current window are retransmitted by the sender. Suppose the sender has transmitted frame 1, frame 2, and frame 3 and does not receive the acknowledgment for frame 1; then all three frames will be retransmitted.

Selective Repeat ARQ

• Selective Repeat ARQ is also called the Selective Repeat Automatic Repeat Request. This protocol is implemented in the data link layer of the OSI model. This protocol implements a sliding window method to transmit the data frames in the network.
• If there are limited mistakes during the data frame transmission, then the user can implement Go-Back-N ARQ, but if there are a lot of errors, then implementing Selective Repeat ARQ is the best option. It works well in a limited bandwidth.
• The window size for both the sender and receiver nodes is kept equal. This is done by making the window size for the sender node equal to the receiver node window. The sliding window is always more than 1.
• Suppose a corrupt frame is transmitted in the network. When this corrupt data frame reaches the receiver node, it is not directly rejected by the receiver; instead, the receiving node transmits a negative acknowledgment to the sender. Upon receiving the negative acknowledgment, the sender resends the data frame instantly. There is no waiting for any time out to transmit that data frame.

Difference Between Stop and Wait Protocol and Sliding Window Protocol

These two protocols act as a flow control mechanism and can be implemented in the network that ensures that the data is successfully transmitted from the sender node to receiving node since Go-Back-N and Selective Repeat protocols are sliding window protocols.

Let us discuss the difference between Stop and Wait Protocol and Sliding Window Protocol.

Difference Between Go-Back-N and Selective Repeat

The two sliding window protocol also differs in their function and working. The difference between Go-Back-N and Selective Repeat protocols are as follows:

Difference between Stop and Wait, Go-Back-N, and Selective Repeat

The following table differentiates between all three flow control protocols that are Stop and Wait, Go-Back-N, and Selective Repeat protocols:

Conclusion

Go-Back-N and Selective Repeat both use Sliding Window Protocol that enables the sender node to transmit multiple data frames simultaneously in the network. The sender node does not require waiting for the acknowledgment from the receiver node to send several data frames.

The Stop-and-Wait protocol is different from the sliding window protocol as this protocol cannot transmit multiple data frames simultaneously in the network. It only allows the sender to node sends one data frame at a time. The sender waits for an acknowledgment after transmitting each data frame in the network. If it does not receive the data frame in a given time, it then retransmits the frame.