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Flow control protocols- sliding window, Lecture notes of Data Communication Systems and Computer Networks

Madan Mohan Malaviya University of TechnologyData Communication Systems and Computer Networks

The pdf includes Flow control protocols- sliding window.

Typology: Lecture notes

2025/2026

Available from 05/21/2026

ambika-mishra
ambika-mishra 🇮🇳

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Sliding window protocols
Flow Control Protocols-
Sliding Window Protocol-
Sliding window protocol is a flow control protocol.
It allows the sender to send multiple frames before needing the
acknowledgements.
Sender slides its window on receiving the acknowledgements for the sent frames.
This allows the sender to send more frames.
It is called so because it involves sliding of sender’s window.
Maximum number of frames that sender can send without acknowledgement
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Sliding window protocols

Flow Control Protocols-

Sliding Window Protocol-

 Sliding window protocol is a flow control protocol.  It allows the sender to send multiple frames before needing the acknowledgements.  Sender slides its window on receiving the acknowledgements for the sent frames.  This allows the sender to send more frames.  It is called so because it involves sliding of sender’s window. Maximum number of frames that sender can send without acknowledgement

= Sender window size

Optimal Window Size-

In a sliding window protocol, optimal sender window size = 1 + 2a Derivation- We know, η = 1 Tt / (Tt + 2Tp) = 1 Tt = Tt + 2Tp Thus,  To get 100% efficiency, transmission time must be Tt + 2Tp instead of Tt.  This means sender must send the frames in waiting time too.  Now, let us find the maximum number of frames that can be sent in time Tt + 2Tp. We have-  In time Tt, sender sends one frame.  Thus, In time Tt + 2Tp, sender can send (Tt + 2Tp) / Tt frames i.e. 1+2a frames. Thus, to achieve 100% efficiency, window size of the sender must be 1+2a.

Required Sequence Numbers-

 Each sending frame has to be given a unique sequence number.

  1. Go back N Protocol
  2. Selective Repeat Protocol

Efficiency-

Efficiency of any flow control protocol may be expressed as- Example- In Stop and Wait ARQ , sender window size = 1. Thus,

Efficiency of Stop and Wait ARQ = 1 / 1+2a

PRACTICE PROBLEMS BASED ON SLIDING

WINDOW PROTOCOL-

Problem-01:

If transmission delay and propagation delay in a sliding window protocol are 1 msec and 49.5 msec respectively, then-

  1. What should be the sender window size to get the maximum efficiency?
  2. What is the minimum number of bits required in the sequence number field?
  3. If only 6 bits are reserved for sequence numbers, then what will be the efficiency?

Solution-

Given-  Transmission delay = 1 msec  Propagation delay = 49.5 msec Part-01: To get the maximum efficiency, sender window size = 1 + 2a = 1 + 2 x (Tp / Tt) = 1 + 2 x (49.5 msec / 1 msec) = 1 + 2 x 49. = 100 Thus, For maximum efficiency, sender window size = 100 Part-02: Minimum number of bits required in the sequence number field

To get the maximum efficiency, sender window size = 1 + 2a = 1 + 2 x (Tp / Tt) = 1 + 2 x (99.5 msec / 1 msec) = 1 + 2 x 99. = 200 Thus, For maximum efficiency, sender window size = 200 Part-02: Minimum number of bits required in the sequence number field = ⌈log 2 (1+2a)⌉ = ⌈log 2 (200)⌉ = ⌈7.64⌉ = 8 Thus, Minimum number of bits required in the sequence number field = 8 Part-03: If only 6 bits are reserved in the sequence number field, then- Maximum sequence numbers possible = 2^7 = 128 Now, Efficiency = Sender window size in the protocol / Optimal sender window size = 128 / 200 = 0. = 64%