Elementary Data Link Protocols are important because they show how a very simple communication system gradually becomes a reliable protocol.
First we assume that everything is perfect. Then we add a slow receiver. Finally we add transmission errors.
That gives us three important protocols: Unrestricted Simplex, Simplex Stop-and-Wait and Simplex Protocol for a Noisy Channel.
Elementary Data Link Protocols are simplified protocols used to understand the basic problems of flow control and error control.
The three commonly studied protocols are:
1. Unrestricted Simplex Protocol – perfect channel and receiver always ready.
2. Simplex Stop-and-Wait Protocol – perfect channel but receiver may need time.
3. Simplex Protocol for a Noisy Channel – frames or acknowledgements may be lost or damaged.
The noisy-channel version requires mechanisms such as ACK, timer, retransmission and sequence numbers.
What Are Elementary Data Link Protocols?
Elementary Data Link Protocols are simplified theoretical protocols used to understand how reliable communication can be developed step by step.
Instead of directly studying a complicated protocol, we begin with unrealistic but simple assumptions.
Then one problem is added at a time.
Perfect channel
Fast receiver
No waiting
Perfect channel
Receiver may be slow
Flow control added
Frames may fail
ACK may fail
Error control added
Why Do We Study Elementary Data Link Protocols?
These protocols teach the basic logic behind reliable Data Link Layer communication.
They help students understand:
Frame Transfer
How a sender transfers frames to a receiver.
Flow Control
Why the sender sometimes needs to wait.
Acknowledgement
How the receiver confirms successful reception.
Timers
How the sender detects missing acknowledgements.
Retransmission
How lost or damaged frames are sent again.
Sequence Numbers
How duplicate frames are identified.
Error Control
How communication remains reliable on an imperfect channel.
Efficiency
Why waiting can reduce link utilization.
Basic Assumptions Used in Elementary Protocols
One sender and one receiver are considered.
Information is transferred using Data Link Layer frames.
Sender obtains packets from its Network Layer.
Receiver passes correct packet data upward.
Each protocol removes one unrealistic assumption.
What Does Simplex Mean in These Protocols?
In these textbook elementary protocols, user data moves in one direction:
However, some later protocols may still send acknowledgement/control frames in the reverse direction.
Protocol 1: Unrestricted Simplex Protocol
The unrestricted simplex protocol is the simplest possible Data Link Layer protocol.
It assumes an almost perfect environment.
Receiver is always ready.
Channel never loses or damages frames.
Because no problem is expected, the sender can continuously send frames without waiting.
Assumptions of Unrestricted Simplex Protocol
- The communication channel is perfect.
- Frames are never lost.
- Frames are never damaged.
- The receiver can process incoming data immediately.
- The receiver has effectively unlimited processing ability for the model.
- The sender always has data ready.
- Only one-way user data communication is considered.
- No acknowledgement is required.
- No timer is required.
- No sequence number is required.
Working of Unrestricted Simplex Protocol
Get Packet
Sender obtains a packet from the Network Layer.
Create Frame
The packet is placed inside a Data Link Layer frame.
Send Frame
Frame is immediately transmitted to the receiver.
Receive Frame
Receiver accepts the frame.
Deliver Packet
Receiver extracts the packet and delivers it to the Network Layer.
Repeat
Sender immediately sends the next frame.
Unrestricted Simplex Sender and Receiver Algorithm
Limitations of Unrestricted Simplex Protocol
- Assumes an error-free channel.
- Assumes the receiver is always ready.
- No flow control.
- No error control.
- No acknowledgement.
- No retransmission.
- No timer.
- No duplicate detection.
- Not suitable for realistic networks.
Protocol 2: Simplex Stop-and-Wait Protocol
Now we remove one unrealistic assumption.
The channel is still assumed to be error-free, but the receiver may not be able to process frames infinitely fast.
If the sender continues transmitting without waiting, the receiver may be overloaded.
So flow control is introduced.
Why Is Stop-and-Wait Needed?
Suppose:
- Sender can create 100 frames per second.
- Receiver can process only 20 frames per second.
If the sender transmits continuously, the receiver's buffers may overflow.
Stop-and-Wait solves this by allowing only one outstanding frame at a time.
Working of Simplex Stop-and-Wait Protocol
Sender Creates One Frame
A packet is obtained and placed in a frame.
Sender Transmits
One frame is sent.
Sender Stops
It does not send another data frame immediately.
Receiver Processes Frame
Receiver accepts the frame and delivers its packet upward.
Receiver Sends Acknowledgement
A control frame tells the sender that it may continue.
Next Frame Is Sent
Sender receives the acknowledgement and repeats the process.
Stop-and-Wait Sender and Receiver Algorithm
How Stop-and-Wait Provides Flow Control
The sender is not allowed to continuously transmit frames.
It sends one frame and waits.
Therefore the receiver gets time to:
- Receive the frame
- Process it
- Pass the packet upward
- Prepare for the next frame
Efficiency of Stop-and-Wait Protocol
Stop-and-Wait is simple but can waste bandwidth.
The sender transmits a frame and then remains idle while the frame and acknowledgement travel across the network.
Important Terms
Tt = frame transmission time
Tp = one-way propagation delay
Define:
Ignoring ACK transmission time and processing overhead, Stop-and-Wait utilization is approximately:
Why 2a?
The frame travels from sender to receiver:
The acknowledgement travels back:
Therefore the sender waits approximately:
Stop-and-Wait Solved Numerical
Suppose:
- Frame transmission time = 10 ms
- One-way propagation delay = 20 ms
Step 1: Calculate a
Step 2: Calculate Utilization
This shows why Stop-and-Wait becomes inefficient when propagation delay is large compared with frame transmission time.
Limitations of Simplex Stop-and-Wait
- Only one frame is in transit at a time.
- The sender spends significant time waiting.
- Bandwidth can be wasted.
- Efficiency becomes poor on long-distance links.
- This simple version still assumes an error-free channel.
- It does not yet solve lost frames.
- It does not yet solve damaged frames.
- It does not yet solve lost acknowledgements.
Protocol 3: Simplex Protocol for a Noisy Channel
Now we make the model more realistic.
The communication channel may introduce errors.
A data frame can:
- Arrive correctly
- Be damaged
- Be lost
The acknowledgement can also be lost or damaged.
Therefore simple Stop-and-Wait is no longer enough.
Problems That Occur on a Noisy Channel
Lost Frame
Sender transmits but receiver never receives the frame.
Damaged Frame
Frame reaches receiver with corrupted bits.
Lost ACK
Receiver gets the frame but acknowledgement disappears.
Duplicate Frame
Retransmission may cause the receiver to see the same frame twice.
How the Noisy Channel Protocol Works
Frame
Damaged
Occurs
Frame
Create Frame With Sequence Number
Sender labels the frame, commonly using sequence number 0 or 1.
Send Frame
Frame is transmitted to the receiver.
Start Timer
Sender starts a timer after transmission.
Receiver Checks Frame
Receiver verifies whether the frame is correct and whether its sequence number is expected.
Correct New Frame
Receiver delivers the packet to the Network Layer.
Send ACK
Receiver sends acknowledgement.
ACK Arrives
Sender stops the timer and changes the sequence number for the next frame.
Role of Acknowledgement in Noisy Channel Protocol
An acknowledgement tells the sender that the receiver successfully obtained the required frame.
Without an acknowledgement, the sender cannot know whether:
- The frame arrived
- The frame was lost
- The frame was damaged
← ACK
Timeout and Retransmission
The sender cannot wait forever for an ACK.
So a timer is started after sending the frame.
Frame is transmitted.
Sender waits for acknowledgement.
Timer eventually expires.
Same frame is sent again.
Why Are Sequence Numbers Required?
Timers solve one problem but create another.
Suppose:
- Sender transmits Frame A.
- Receiver receives Frame A successfully.
- Receiver sends ACK.
- ACK is lost.
- Sender's timer expires.
- Sender retransmits Frame A.
Now the receiver sees Frame A again.
Without sequence numbers, it may deliver the same packet twice.
Therefore frames are numbered.
Check Number
Why Are Only Sequence Numbers 0 and 1 Enough?
In Stop-and-Wait, only one data frame can be outstanding at a time.
The receiver only needs to know:
Is this the new frame or the previous frame again?
Therefore a 1-bit sequence number is sufficient:
Case 1: Data Frame Is Lost
Suppose Frame 0 never reaches the receiver.
Sender Sends Frame 0
The timer starts.
Frame Is Lost
The receiver receives nothing.
No ACK Arrives
The sender keeps waiting.
Timer Expires
The sender assumes recovery is required.
Frame 0 Is Retransmitted
The receiver now gets the expected frame.
Case 2: Data Frame Is Damaged
If a frame reaches the receiver but fails the error check, the receiver must not deliver corrupted data to its Network Layer.
Depending on the protocol model:
- The damaged frame may simply be discarded.
- Or a negative acknowledgement may be used.
If no valid ACK reaches the sender, timeout causes retransmission.
Case 3: Acknowledgement Is Lost
This case explains exactly why sequence numbers are necessary.
Receiver successfully receives Frame 0 and passes its packet upward.
Receiver sends ACK.
ACK is lost.
Sender does not know that Frame 0 was received.
Timer expires.
Sender sends Frame 0 again.
Receiver checks the sequence number and realizes: “I have already received Frame 0.”
The duplicate frame is not delivered to the Network Layer again.
How Duplicate Frames Are Handled
The receiver keeps track of the sequence number it expects next.
Suppose the expected number is:
But another Frame 0 arrives.
That means Frame 0 is probably a duplicate retransmission.
The receiver:
- Does not deliver the packet again.
- May repeat the relevant acknowledgement.
- Continues waiting for Frame 1.
Alternating Bit Protocol
The Stop-and-Wait ARQ mechanism using sequence numbers 0 and 1 is commonly called the Alternating Bit Protocol.
Frames alternate:
This single bit is enough because the sender never has more than one unacknowledged data frame outstanding.
PAR – Positive Acknowledgement With Retransmission
The noisy-channel Stop-and-Wait idea is also commonly described as:
The logic is:
↓
Wait for Positive ACK
↓
ACK Arrives → Send Next
ACK Missing → Timeout → Retransmit
Relation With Stop-and-Wait ARQ
The same basic technique is widely referred to as Stop-and-Wait ARQ.
ARQ means:
Noisy Channel Sender Algorithm
Noisy Channel Receiver Algorithm
ACK and NAK in Data Link Protocols
ACK
ACK means Acknowledgement.
It confirms successful reception.
NAK
NAK means Negative Acknowledgement.
It indicates that a frame should be retransmitted.
Flow Control vs Error Control
| Feature | Flow Control | Error Control |
|---|---|---|
| Main Problem | Receiver may be slower than sender | Frames may be lost or damaged |
| Goal | Prevent receiver overload | Provide reliable delivery |
| Main Technique | Control transmission rate | ACK, timeout, retransmission, sequence numbers |
| Basic Example | Stop-and-Wait | Stop-and-Wait ARQ |
| Main Question | “Can the receiver handle another frame?” | “Was the frame delivered correctly?” |
Comparison of the Three Elementary Data Link Protocols
| Feature | Unrestricted Simplex | Simplex Stop-and-Wait | Noisy Channel Protocol |
|---|---|---|---|
| Channel | Perfect | Perfect | Noisy / unreliable |
| Receiver Speed | Assumed always ready | May be slower | May be slower |
| Flow Control | No | Yes | Yes |
| Error Control | No | No | Yes |
| ACK Required | No | Yes for pacing | Yes for reliability |
| Timer | No | Not required under perfect-channel assumption | Yes |
| Retransmission | No | No | Yes |
| Sequence Number | No | No | Yes |
| Duplicate Detection | No | No | Yes |
| Main Concept | Basic frame transfer | Flow control | Flow + error control |
| Realism | Very low | Medium theoretical model | Much more realistic |
How the Three Protocols Evolve Step by Step
“Everything is perfect, just send.”
Protocol 2
“Receiver may be slow, so send one and wait.”
Protocol 3
“Channel may fail, so add ACK + Timer + Retransmission + Sequence Number.”
Transmission Delay vs Propagation Delay
This difference is extremely important for Stop-and-Wait numericals.
Transmission Delay
Time required to push all frame bits onto the communication link.
Propagation Delay
Time required for the signal to physically travel from sender to receiver.
Exam Numerical: Calculate Stop-and-Wait Utilization
A link has:
- Frame size = 1000 bits
- Data rate = 100 kbps
- Propagation delay = 20 ms
Step 1 – Transmission Time
Step 2 – Calculate a
Step 3 – Utilization
Complete Lost-ACK Example With Sequence Numbers
| Step | Sender | Receiver |
|---|---|---|
| 1 | Sends Frame 0 | Waiting for Frame 0 |
| 2 | Starts timer | Receives Frame 0 correctly |
| 3 | Waits | Delivers packet and sends ACK |
| 4 | ACK never arrives | Now expects Frame 1 |
| 5 | Timer expires | Still expects Frame 1 |
| 6 | Retransmits Frame 0 | Receives Frame 0 again |
| 7 | Waits | Recognizes duplicate because it expects 1 |
| 8 | Receives ACK again | Does not deliver duplicate packet |
| 9 | Changes sequence to 1 | Continues waiting for Frame 1 |
Important Exam Terms
Acknowledgement
Confirms successful reception.
Negative ACK
Indicates that retransmission is required.
Timeout Control
Prevents the sender from waiting forever.
Automatic Repeat Request
Uses feedback and retransmission for reliability.
Sequence Number
Distinguishes new frames from duplicates.
Timer Expiry
Triggers retransmission when expected ACK does not arrive.
Transmission Delay
Time required to place all frame bits on the link.
Propagation Delay
Time required for the signal to travel across the link.
Utilization
Fraction of link time effectively used for sending useful data.
Common Student Mistakes
No. It assumes the receiver is always ready and the channel never fails, so acknowledgements are unnecessary.
Not necessarily. Basic simplex Stop-and-Wait is first introduced as a flow-control protocol over an error-free channel. Stop-and-Wait ARQ adds error recovery.
No. The receiver may have successfully received the frame while only the acknowledgement was lost.
No. Timer enables retransmission. Sequence numbers are needed to recognize duplicate frames.
No. The sequence numbers are reused repeatedly: 0, 1, 0, 1, and so on.
No. Only one data frame is outstanding at a time.
No. Transmission delay is based on frame size and rate, while propagation delay is based on distance and propagation speed.
No. A correctly designed reliable protocol identifies the duplicate and avoids delivering the same packet twice.
No. Timeout and missing ACK can also trigger retransmission without an explicit NAK.
It is simple but may be inefficient when propagation delay is large.
Easy Memory Tricks
Elementary Data Link Protocols Quick Revision Notes
- Elementary protocols explain Data Link Layer communication step by step.
- Three important protocols are unrestricted simplex, simplex Stop-and-Wait and simplex protocol for a noisy channel.
- Unrestricted simplex assumes a perfect channel.
- Its receiver is assumed to be always ready.
- It does not require ACK, timer or sequence number.
- Unrestricted simplex has no flow control.
- Unrestricted simplex has no error control.
- Simplex Stop-and-Wait introduces flow control.
- The sender sends one frame and waits before sending the next.
- Stop-and-Wait prevents receiver overload.
- Basic Stop-and-Wait still assumes an error-free channel.
- The noisy-channel protocol adds error-control mechanisms.
- A data frame may be lost or damaged.
- An acknowledgement may also be lost.
- A timer prevents the sender from waiting forever.
- Timeout normally causes retransmission.
- Sequence numbers prevent duplicate delivery.
- In Stop-and-Wait ARQ, a 1-bit sequence number is sufficient.
- The sequence alternates between 0 and 1.
- This is called the Alternating Bit Protocol.
- ARQ stands for Automatic Repeat Request.
- PAR stands for Positive Acknowledgement with Retransmission.
- ACK confirms successful frame reception.
- NAK may request retransmission.
- NAK is not always required when timeout recovery is available.
- Transmission delay depends on frame size and data rate.
- Propagation delay depends on distance and propagation speed.
- a = Tp / Tt.
- Ignoring ACK transmission time, Stop-and-Wait utilization is approximately 1/(1+2a).
- Stop-and-Wait performs poorly on links with large propagation delay.
Important Exam Questions
Very Short Answer Questions
Short Answer Questions
Long Answer Questions
Elementary Data Link Protocol MCQs With Answers
A. Error-free channel
B. Very noisy channel
C. Multiple senders
D. Sliding window
A. Neither flow nor error control
B. Only error control
C. Sliding window
D. Routing
A. Flow control
B. Routing
C. Encryption
D. Address resolution
A. One
B. Two
C. Eight
D. Unlimited
A. Lost or damaged frames
B. Only perfect delivery
C. No delay
D. No retransmission
A. Acknowledgement
B. Address Control Key
C. Automatic Channel Key
D. Access Check
A. Detect missing ACK
B. Calculate MAC address
C. Find IP route
D. Create frame header
A. Retransmits the frame
B. Changes IP address
C. Closes the network permanently
D. Sends DNS request
A. Duplicate frames
B. Cable length
C. Signal frequency
D. Router hops
A. 1 bit
B. 8 bits
C. 16 bits
D. 32 bits
A. 0, 1, 0, 1
B. 1, 2, 3, 4
C. 0, 2, 4, 6
D. A, B, C, D
A. Automatic Repeat Request
B. Automatic Routing Query
C. Address Repeat Queue
D. Acknowledged Routing Request
A. Positive Acknowledgement with Retransmission
B. Packet Address Routing
C. Physical Access Recovery
D. Protocol Automatic Routing
A. Retransmit after timeout
B. Assume success automatically forever
C. Never send again
D. Change MAC address
A. Recognize it as duplicate
B. Deliver it again as new data
C. Restart the computer
D. Change the IP address
A. Frame size and link rate
B. Distance only
C. Number of routers only
D. DNS server
A. Distance and propagation speed
B. Frame size only
C. MAC address length
D. Application protocol
A. Tp/Tt
B. Tt/Tp always
C. Frame size × rate
D. ACK size only
A. 1/(1+2a)
B. 1/a
C. 2a
D. a²
A. Simplex protocol for a noisy channel
B. Unrestricted simplex
C. Protocol with no ACK and no errors
D. None
Frequently Asked Questions
What are the three elementary Data Link protocols?
The three commonly studied protocols are unrestricted simplex protocol, simplex Stop-and-Wait protocol and simplex protocol for a noisy channel.
What is unrestricted simplex protocol?
It is a theoretical protocol that assumes a perfect communication channel, an always-ready receiver and continuous one-way data transfer without acknowledgements.
What is Simplex Stop-and-Wait protocol?
It is a flow-control protocol in which the sender transmits one frame and waits for acknowledgement or permission before transmitting the next frame.
What is the noisy-channel protocol?
It extends Stop-and-Wait by handling lost or damaged frames using acknowledgements, timers, retransmission and sequence numbers.
Why is a timer required?
A timer prevents the sender from waiting forever when a frame or acknowledgement is lost.
Why are sequence numbers required?
Sequence numbers help the receiver distinguish a new frame from a retransmitted duplicate.
Why are only 0 and 1 used in Stop-and-Wait ARQ?
Only one frame can be outstanding, so the receiver only needs to distinguish the current frame from the previous frame. One bit is therefore sufficient.
What is the Alternating Bit Protocol?
It is a Stop-and-Wait ARQ protocol in which sequence numbers alternate between 0 and 1.
What happens when an ACK is lost?
The sender eventually times out and retransmits the frame. The receiver uses the sequence number to recognize the retransmission as a duplicate and avoids delivering the packet twice.
What is the formula for Stop-and-Wait utilization?
Ignoring ACK transmission time and processing overhead, the utilization is approximately U = 1/(1+2a), where a = propagation delay divided by frame transmission time.
What is the difference between Stop-and-Wait and Stop-and-Wait ARQ?
Basic Stop-and-Wait mainly solves flow control on an assumed error-free channel. Stop-and-Wait ARQ adds timers, retransmission and sequence numbers to handle transmission errors.
Conclusion
Elementary Data Link Protocols show how reliable communication is developed one problem at a time.
The easiest way to remember the complete chapter is:
Protocol 1 → Everything Perfect → Just Send
Protocol 2 → Receiver Can Be Slow → Send One and Wait
Protocol 3 → Channel Can Fail → Add ACK, Timer, Retransmission and Sequence Number
Unrestricted Simplex teaches basic frame transfer.
Simplex Stop-and-Wait introduces flow control.
The noisy-channel protocol introduces error control.
For Stop-and-Wait ARQ, the most important mechanism is:
Frame 0 → ACK → Frame 1 → ACK → Frame 0 → ACK...
If the ACK does not arrive:
Timeout → Retransmit → Sequence Number Detects Duplicate
For numericals remember:
a = Tp / Tt
U = 1 / (1 + 2a)
The final memory line is:
Simplex = Send
Stop-and-Wait = Send + Wait
Noisy Channel = Send + Wait + Timer + Retry + Sequence
Next Chapter: Chapter 18 – Sliding Window Protocols: One-Bit, Go-Back-N & Selective Repeat ARQ Explained
