Mumbai University > Information Technology > Sem4 > Computer Networks

Marks: 5M

Year: May 18

• Flow Control Flow control coordinates the amount of data that can be sent before receiving acknowledgement It is one of the most important functions of data link layer. Flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgement from the receiver.

Stop-and-Wait

1.

• Sender keeps a copy of the last frame until it receives an acknowledgement.

  -For identification, both data frames and acknowledgements (ACK) frames are numbered alternatively 0 and 1.

  -Sender has a control variable (S) that holds the number of the recently sent frame. (0 or 1)

• Receiver has a control variable ® that holds the number of the next frame expected (0 or 1).

  -Sender starts a timer when it sends a frame. If an ACK is not received within a allocated time period, the sender assumes that the frame was lost or damaged and resends it .

• Receiver send only positive ACK if the frame is intact.

• ACK number always defines the number of the next expected frame

Stop-and-Wait ARQ, lost ACK frame

2.

• When a receiver receives a damaged frame, it discards it and keeps its value of R.

• After the timer at the sender expires, another copy of frame 1 is sent.

• If the sender receives a damaged ACK, it discards it.

• When the timer of the sender expires, the sender retransmits frame 1.

• Receiver has already received frame 1 and expecting to receive frame 0 (R=0).

• Therefore it discards the second copy of frame 1.

Stop-and-Wait, delayed ACK frame

3.

• The ACK can be delayed at the receiver or due to some problem It is received after the timer for frame 0 has expired.

• Sender retransmitted a copy of frame 0. However, R =1 means receiver expects to see frame 1.

• Receiver discards the duplicate frame 0. Sender receives 2 ACKs, it discards the second ACK.

Sliding Window

Sender Sliding Window

1.

• At the sending site, to hold the outstanding frames until they are acknowledged, we use the concept of a window.

• The size of the window is at most 2 m -1 where m is the number of bits for the sequence number.

• Size of the window can be variable, e.g. TCP. The window slides to include new unsent frames when the correct ACKs are received

Receiver Sliding Window

2.

• Size of the window at the receiving site is always 1 in this protocol.

• Receiver is always looking for a specific frame to arrive in a specific order.

• Any frame arriving out of order is discarded and needs to be resent.

• Receiver window slides as shown in fig. Receiver is waiting for frame 0 in part a.