Flow control is a speed matching mechanism used to regulate the flow of data between devices. It ensures smooth and efficient communication. It is a responsibility of the data link layer of the TCP/IP protocol.
We live in a information based world where data is constantly exchanged between various devices. This makes ensuring efficient and reliable communication crucial. Imagine a scenario where a high-speed server (sender) sends data to a slower client (receiver) without any control mechanism.
The receiver has a limited speed at which it can process incoming data and a limited amount of memory in which to store the data. If this receiver’s memory is filled faster than its processsing speed then it would be overwhelmed. This would result in dropped packets and the need for retransmission. This, in turn, would lead to inefficiencies, delays, and poor performance.
Thus, this receiver must inform the sender before it’s memory limit is reached and request that the transmitter send fewer frames or stop temporarily.
This mechanism used to regulate the speed of data transmission is called flow control. It allows devices to coordinate and adjust the rate of data transfer to ensure smooth communication. By implementing this, devices can prevent overwhelming the receiving device, avoid congestion, and maintain reliable data transmission.
Importance:
Flow control plays a critical role in network communication for several reasons:
Preventing Data Loss
Flow control prevents data loss by ensuring that the receiving device can handle the incoming data at its own pace. It enables the receiving device to signal the sender when it is ready to receive more data. This avoids overflow conditions that may lead to dropped packets.
Avoiding Congestion
In networks with varying speeds or traffic patterns, flow control helps prevent congestion. By regulating the flow of data, it ensures that devices with limited buffering capabilities or slower speeds are not overwhelmed with excessive data. This prevents unnecessary congestion.
Balancing Speed Mismatch
Flow control is crucial when devices in a network have different transmission speeds. It helps in matching the transmission rate of the sender to the capacity of the receiver. This avoids situations where the sender overwhelms the receiver with data.
Providing Reliable Communication
Flow control ensures reliable communication by providing feedback and acknowledgments between the sender and receiver. It allows the sender to verify that the data has been successfully received and enables retransmission of lost or corrupted packets if necessary.
There are two types of flow control protocols
1) Noisless Channells
This includes protocols like simplest and Stop ‘n’ Wait.
2) Noisy Channels
This includes protools like Stop ‘n’ Wait ARQ, Go-Back-N ARQ and selective repeat ARQ.
Flow control techniques in the Data Link Layer have a direct impact on the overall performance and efficiency of data transmission. They help balance the data flow between sender and receiver. This helps prevent congestion, minimize data loss, and optimize network utilization.
Some Important Terms
Before studying various flow control protocols, there are some important terms you should familiarize yourself with.
1) Efficiency
Efficiency is a key metric in flow control. It represents the percentage of time spent on transmitting actual data compared to the total time (this includes any overhead or idle time). Higher efficiency indicates a more effective use of network resources and faster data transfer.
2) Timeout (TO)
Timeout is a predefined duration during which a sender waits for an acknowledgment from the receiver. If the acknowledgment is not received within the timeout period, the sender assumes that the packet was lost or corrupted and initiates retransmission. The timeout value is set based on network conditions. This ensures a balance between responsiveness and allows for variations in network delays.
3) Window Size
Window size refers to the number of data packets that can be sent by the sender without receiving acknowledgments from the receiver. It determines the amount of unacknowledged data that can be in transit at any given time. A larger window size allows for increased throughput by overlapping transmission and acknowledgment processes, reducing the effects of network latency.
4) Throughput
Throughput is the rate at which data is successfully delivered from the sender to the receiver over a network. It is measured in bits per second (bps) and depends on various factors, including network bandwidth, latency, and flow control mechanisms. Higher throughput indicates more efficient and faster data transfer.
5) Sequence Numbers
Sequence numbers refer to unique identifiers assigned to data frames transmitted from the sender to the receiver. These sequence numbers serve the purpose of distinguishing and ordering the frames to ensure reliable and orderly delivery.
Each frame transmitted by the sender is assigned a specific sequence number, which is typically a numerical value. The sequence number allows the receiver to identify and keep track of the frames it receives. By using sequence numbers, the receiver can detect any missing or out-of-order frames and request retransmission if necessary.
6) Congestion
Congestion occurs when the network becomes overloaded with more data traffic than it can handle effectively. It leads to decreased throughput, increased delays, and packet loss. Flow control mechanisms help prevent or manage congestion by regulating the rate of data transmission and ensuring that the network resources are utilized optimally.
7) Congestion Control
Congestion control techniques are employed to prevent or manage network congestion. They regulate the flow of data to avoid overwhelming the network resources and ensure optimal performance. Congestion control mechanisms dynamically adjust the transmission rate based on network conditions, preventing packet loss and maintaining smooth data flow.
8) Receiver Window:
The receiver window, also known as the receive buffer, is a designated memory space in the receiver’s system that stores incoming data packets. It determines the number of unacknowledged packets the receiver can handle at a time. The receiver window size is communicated to the sender, allowing it to control the flow of data based on the receiver’s capacity to process incoming packets.
9) Buffer:
A buffer is a temporary storage area used to hold data packets during transmission. Buffers are used in flow control to accommodate variations in the transmission rate between the sender and the receiver. They help smooth out any differences in the rate at which data is produced and consumed.
10) ACK/NACK:
ACK (Acknowledgment) and NACK (Negative Acknowledgment) are control signals used in flow control to indicate the successful receipt or failure to receive a data packet, respectively. The receiver sends an ACK or NACK signal to the sender to confirm the receipt of a packet or request retransmission if the packet is lost or corrupted.
It’s important to note that flow control in the Data Link Layer operates independently of flow control mechanisms implemented at higher layers, such as the Transport Layer’s congestion control mechanisms in TCP/IP. Each layer has its own techniques tailored to its specific requirements and objectives.