Flow Control Techniques

Question 1

Flow Control - general

Answer 1

• technique ensuring the sending entity does not overwhelm the receiving entity (i.e. the receiver’s data buffer does not fill up and overflow)
• temporal parameters used for transmission control: transmission time (emit) & propagation time (traverse)
• resides in the Data Link OSI layer

Question 2

Stop-and-Wait Flow Control Protocol

Algorithm & Characteristics

Answer 2

• Algorithm:
  1. Source transmits a frame
  2. Destination receives frame and replies back with an acknowledgement (ACK) for that frame
  3. Source waits for ACK before sending next frame
• Characteristics:
• destination can stop the data flow by not sending ACK
• works well for large frames (use of frame fragmentation = large block split into smaller frames)
• inefficient

Question 3

Sliding Window Flow Control

general characteristics

Answer 3

• allows for multiple frames to be in transit on the link
• efficient algorithm for full duplex links and speedy transmissions

Question 4

Sliding Window Flow Control

Algorithm

Answer 4

1. receiver has buffer W long
2. transmitter can send up to W frames without waiting for ACK
3. each frame is numbered by the sender to keep track of ACK frames
4. the receiver sends ACK frame, including number of next frame expected
5. sequence number is bounded by the size of control field in the frame (k), so frames are numbered modulo 2^k
6. sender maintains a list with the sequence numbers it is allowed to transmit
7. receiver maintains a list with the sequence numbers it is prepared to receive