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Sunday, August 25, 2013

What is the concept of flow control?

- Flow control is an important concept in the field of data communications. 
- This process involves management of the data transmission rate between two communicating nodes. 
- Flow control is important to avoid a slow receiver from being outrun by a fast sender. 
- Using flow control, a mechanism is designed for the receiver using which it can control its speed of transmission.
- This prevents the receiving node from getting overwhelmed with traffic from the node that is transmitting.
- Do not confuse yourself with congestion control and flow control. Both are different concepts. 
- Congestion control comes in to play when in actual there is a problem of network congestion for controlling the data flow. 

On the other hand the mechanism of flow control can be classified in the following two ways:
  1. The feedback is sent to the sending node by the receiving node.
  2. The feedback is not sent to the sending node by the receiving node.
- The sending computer might tend to send the data at a faster rate than what can be received and processed by the other computer. 
- This is why we require flow control. 
- This situation arises when the traffic load is too much up on the receiving computer when compared to the computer that is sending the data. 
- It can also arise when the processing power of the receiving computer is slower than the processing power of the one that is sending the data.

Stop and Wait Flow Control Technique 
- This is the simplest type of the flow control technique. 
- Here, when the receiver is ready to start receiving data from the sender, the message is broken down in to a number of frames. 
- The sending system then waits for a specific time to get an acknowledgement or ACK from the receiver after sending each frame. 
- The purpose of the acknowledgement signal is to make sure that the frame has been received properly. 
- If during the transmission a packet or frame gets lost, then it has to be re-transmitted. 
- We call this process as the automatic repeat request or ARQ. 
- This technique has a problem which is that it is capable of transmitting only one frame in one go. 
- This makes the transmission channel very inefficient. 
- Therefore, until and unless the sender gets an acknowledgement it will not proceed further for transmitting another packet. 
- Both the transmission channel and the sender are left un-utilized during this period. 
- Simplicity of this method is its biggest advantage. 
- Disadvantage is the inefficiency resulting because of this simplicity. 
- Waiting state of the sender creates inefficiency. 
- This happens usually when the transmission delay is shorter than the propagation delay. 
- Sending longer transmissions is another cause for inefficiencies. 
- Also, it increases the chance for the errors to creep in this protocol. 
- In short messages, it is quite easy to detect the errors early. 
- By breaking down one big message in to various separate smaller frames, the inefficiency increases. 
- This is so because these pieces altogether take a long to be transmitted.

Sliding window Flow Control Technique 
- This is another method of flow control where permission is given to the sender by the receiver for continuously transmitting data until a window is filled up. 
- Once the window is full, sender stops transmission until a larger window is advertised. 
- This method can be utilized in a better way if the size of the buffer is kept limited. 
- During the transmission, space for say n frames is allocated to the buffer. 
This means n frames can be accepted by the receiver without having to wait for ACK. 
- After n frames an ACK is sent consisting of the sequence number of the next frame that has to be sent. 

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