This document discusses the Go Back N protocol. It explains that Go Back N is used when transmission times are large or bandwidth is high. It allows the sender to transmit multiple frames before waiting for acknowledgments. This increases efficiency but requires buffering of frames at the sender. An error may require retransmission of multiple correct frames received after the erroneous one. Advantages are increased efficiency and low waiting times by varying the sender window size, but disadvantages include high buffer needs and resending correct frames after errors.

1. COMPUTER NETWORKS AND
DATA COMMUNICATIONS
BY-
SHIVANI MONGA
(3033)
TOPIC- GO BACK N PROTOCOL

2. Sliding Windows
 Sliding window protocol (SWP):
 A protocol that allows several data units to be in transition
before receiving an acknowledgement.
 Sending window
 Maintain by the sender
 A set of sequence number
 Represents a range of permissible sequence numbers for
transmitted but not-yet-acknowledged frames
 As the protocol operates, the window slides forward over
the sequence number space.
 Receiving window:
 Maintain by the receiver
 A set of sequence number
 Represents a range of frames it is permitted to accept
2

3. We have three sliding window protocols-
A one bit sliding window protocol
A protocol using go back n
A protocol using selective repeat

4. Go back N
It is used when the transmission time for frames is
large or the bandwidth is too large.
When either the bandwidth or the round trip delay
is large,we need to increase the size of sending
window.
In networking if a task is often began before the
previous task ended. This is known as
PIPELINING.

5. Assume that, in a Stop-and-Wait ARQ system, the bandwidth
of the line is 1 Mbps, and 1 bit takes 20 ms to make a round
trip. What is the bandwidth-delay product? If the system data
frames are 1000 bits in length, what is the utilization
percentage of the link?
Solution
The bandwidth-delay product is
Example-1 -

6. The system can send 20,000 bits during the time it takes for
the data to go from the sender to the receiver and then back
again. However, the system sends only 1000 bits. We can say
that the link utilization is only 1000/20,000, or 5 percent.
For this reason, for a link with a high bandwidth or long
delay, the use of Stop-and-Wait ARQ wastes the capacity of
the link.
Example-1 (continued)

7. What is the utilization percentage of the link in Example 1 if
we have a protocol that can send up to 15 frames before
stopping and worrying about the acknowledgments?
Solution
The bandwidth-delay product is still 20,000 bits. The system
can send up to 15 frames or 15,000 bits during a round trip.
This means the utilization is 15,000/20,000, or 75 percent. Of
course, if there are damaged frames, the utilization percentage
is much less because frames have to be resent.
Example 2 -

8. In the Go-Back-N Protocol, the sequence numbers are
modulo 2m,
where m is the size of the sequence number field in
bits.
Note

9. Figure- Send window for Go-Back-N

10. The send window is an abstract concept defining an
imaginary box of size 2m − 1 with three variables: Sf,
Sn, and Ssize.
Note
The send window can slide one
or more slots when a valid acknowledgment arrives.

11. Figure Receive window for Go-Back-N ARQ

12. The receive window is an abstract concept defining an
imaginary box of size 1 with one single variable Rn.
The window slides when a correct frame has arrived;
sliding occurs one slot at a time.
Note

13. a) Pipelining and error recovery. Effect
of an error when
b) (a) receiver’s window size is 1

14.  Pipelining and error recovery. Effect of an
error when
 (b) receiver’s window size is large.

15. ADVANTAGES OF GO-BACK N
 The sender can send many frames at a time
 Efficiency is more.
 Waiting time is pretty low.
 We can alter the size of the sender window

16. DISADVANTAGES OF GO-BACK
 Buffer requirement
 Transmitter needs to store the last N packets
 Scheme is inefficient when round-trip delay
large and data transmission rate is high
 Retransmission of many error-free packets
following an erroneous packet