GBN Protocol Flashcards

Full Name

Go-Back-N Protocol (GBN)

Core Mechanism

Based on sliding window mechanism

Key Differences from Stop-and-Wait

• Sender window: W_T > 1
• Receiver window: W_R = 1

Typical setting: W_T = 3, W_R = 1

Numbering Requirements

At least n bits for frame numbering

Key Condition

W_T + W_R ≤ 2ⁿ

Example

When W_T = 3, W_R = 1 → n ≥ 2 (3 + 1 ≤ 4)

Numbering Cycle

2-bit cycle: 0→1→2→3→0→1...

Definition

Receiver can return only the last frame's ACK for multiple correct frames

Example

After receiving frames 0,1,2 → returns only ACK2

Meaning

ACKi means i and all previous frames were received correctly

Window Sliding

Sender receives ACK2 → confirms 0-2 received → slides window

Data Frame Loss

1. Sender sends frames 3,0,1
2. 0 is lost
3. Receiver expects 0 but gets 1 (illegal)
4. Discards 1, returns ACK3
5. Sender times out, retransmits 0 and all after

ACK Loss

1. Receiver gets 0-2, sends ACK2 (lost)
2. Sender times out, retransmits 0-2
3. Receiver has moved window → discards "old" frames
4. Resends ACK2

Problem Scenario

n=2 (4 numbers), but W_T=4, W_R=1 (4+1>4)

What Happens

1. Sender sends 0-3, ACK3 lost
2. Retransmits 0
3. Receiver window has cycled to new 0

Consequence

Receiver cannot distinguish old and new 0 frames

Solution

Must maintain W_T + W_R ≤ 2ⁿ

Sender Rules

W_T > 1, allows continuous sending

Receiver Rules

W_R = 1, must receive in order

Special Rules

• Cumulative ACK
• Go-back retransmission on timeout
• Illegal frame handling

Frame Numbering

Next frame after i is (i+1) mod 2ⁿ

Advantages

• Implements flow control
• More efficient than Stop-and-Wait

Disadvantages

• Inefficient when receiver is slow (frequent retransmissions)
• Performance drops sharply with high error rates
• Compared to Selective Repeat: less efficient

Key point: Trade-off between simplicity and efficiency