Data Link Control

Data Link Control

Data Link Control (DLC) is a service provided by the Data Link Layer (Layer 2) of the OSI Model. Its primary goal is to ensure reliable data transmission over a physical communication medium.

Why is Data Link Control Needed?

When data is transmitted across a network:

• Devices may try to send data simultaneously.
• The receiver may not process data as quickly as the sender.
• Frames may be damaged or lost during transmission.

Data Link Control solves these problems by:

•  Coordinating communication between devices.
•  Regulating the speed of transmission.
•  Detecting and correcting transmission errors.

Main Functions of Data Link Control

1. Line Discipline

What is Line Discipline?

Line Discipline is the process of coordinating communication between devices connected through a communication link.

It answers two important questions:

• Which device can send data?
• When can it send data?

Without proper coordination, devices may transmit simultaneously, causing collisions and loss of information.

Methods of Line Discipline

There are two commonly used methods:

• ENQ/ACK
• Poll/Select

ENQ/ACK Method

What is ENQ/ACK?

ENQ (Enquiry) and ACK (Acknowledgement) are control messages used when there is a dedicated communication path between two devices.

Before sending data, the sender checks whether the receiver is ready.

Working of ENQ/ACK

Step 1: Sender Sends ENQ

The sender transmits an ENQ frame asking:

       "Are you ready to receive data?"

Step 2: Receiver Responds

The receiver sends either:

• ACK → Ready to receive
• NACK → Not ready to receive

Step 3: Data Transmission

If ACK is received:

• Sender transmits data.
• Transmission ends with an EOT (End of Transmission) frame.

If NACK is received:

• Sender stops transmission.
• Tries again later.

If no response is received:

• Sender assumes the ENQ was lost.
• Retries several times before terminating communication.

Real-World Example

Think of calling someone on the phone:

• "Hello, can you hear me?" → ENQ
• "Yes, go ahead." → ACK
• Then the conversation starts.

Poll/Select Method

What is Poll/Select?

This method is used in networks where:

• One device acts as the Primary Station.
• Other devices act as Secondary Stations.

The primary station controls all communication.

Example

A classroom teacher controls who can speak.

• Teacher = Primary Station
• Students = Secondary Stations

Students speak only when permitted.

Select Operation

The Select operation is used when the primary station wants to send data.

Steps

1. Primary sends a SEL (Select) frame.
2. Frame contains the address of the target device.
3. Secondary device responds with ACK.
4. Primary sends data.
5. Secondary acknowledges successful reception.

Example

A teacher says:

    "Student A, please listen."

Student A responds:

    "I'm ready."

The teacher then provides information.

Poll Operation

The Poll operation is used when the primary station wants to receive data.

Steps

1. Primary asks each secondary device one by one.
2. Devices respond:   ACK → Data available , NACK → No data available
3. Devices with data are allowed to transmit.
   

Example

Teacher asks:

       "Does anyone have a question?"

Students respond accordingly.

2. Flow Control

What is Flow Control?

Flow Control is a mechanism that regulates the rate of data transmission between sender and receiver.

Its purpose is to prevent the sender from overwhelming the receiver.

Why is Flow Control Needed?

Consider:

• Sender speed = 100 Mbps
• Receiver speed = 10 Mbps

If data arrives too quickly:

• Receiver buffer becomes full.
• Data gets lost.

Flow control ensures that transmission occurs at a manageable speed.

Common Flow Control Techniques

1. Stop-and-Wait
2. Sliding Window

Stop-and-Wait Flow Control

How It Works

The sender:

1. Sends one frame.
2. Waits for acknowledgement.
3. Sends the next frame only after receiving ACK.

This process continues until all data is transmitted.

Example

Sending parcels one at a time:

• Send Parcel 1
• Wait for confirmation
• Send Parcel 2
• Wait again

Advantages

• Easy to implement
• Reliable
• Simple error detection

Disadvantages

• Low efficiency
• Network bandwidth is underutilized
• Long waiting periods

Sliding Window Flow Control

What is Sliding Window?

The Sliding Window technique allows multiple frames to be transmitted before receiving acknowledgements.

Instead of waiting after every frame, the sender continuously sends several frames.

Benefits

• Better bandwidth utilization
• Faster communication
• Higher network efficiency

How It Works

A logical "window" defines how many frames can be sent without waiting for ACK.

For example:

Window Size = 4

Sender can transmit:

• Frame 0
• Frame 1
• Frame 2
• Frame 3

before waiting for acknowledgement.

When ACK arrives:

• Window moves forward (slides).
• New frames can be transmitted.

Hence the name Sliding Window.

Real-World Example

Imagine a courier company.

Instead of sending one parcel and waiting for delivery confirmation, it sends several parcels together.

After receiving confirmations, more parcels are dispatched.

This improves efficiency significantly.

Sender Window

The sender window keeps track of:

• Frames already sent
• Frames awaiting acknowledgement
• Frames ready for transmission

As ACKs arrive, the sender window slides forward and allows new transmissions.

Receiver Window

The receiver window keeps track of:

• Frames expected
• Available buffer space
• Correct frame sequence

It helps ensure that frames are received in the proper order.

3. Error Control

What is Error Control?

Even with reliable communication systems, frames may:

• Become corrupted
• Arrive out of order
• Get lost during transmission

Error Control detects these problems and ensures correct data delivery.

Objectives

• Detect transmission errors
• Recover lost data
• Retransmit damaged frames

Stop-and-Wait ARQ

What is ARQ?

ARQ (Automatic Repeat reQuest) is an error-control technique that uses acknowledgements and retransmissions.

Working

1. Sender transmits a frame.
2. Receiver checks for errors.
3. Receiver sends: ACK if correct  , NAK if damaged
4. Sender retransmits when necessary.

A timer is also used.

If ACK is not received before timeout:

• Sender retransmits the frame.

Scenario 1: Damaged Frame

1. Sender transmits Frame 0.
2. Frame becomes corrupted.
3. Receiver sends NAK.
4. Sender retransmits Frame 0.

Scenario 2: Lost Frame

1. Sender transmits Frame 0.
2. Frame never reaches receiver.
3. No ACK arrives.
4. Timer expires.
5. Sender retransmits Frame 0.

Sliding Window ARQ

Sliding Window ARQ combines:

• Flow Control
• Error Control

It allows continuous transmission while also handling errors.

The sender keeps copies of transmitted frames until acknowledgements are received.

Go-Back-N ARQ

How It Works

If one frame is lost or damaged:

• Receiver rejects that frame.
• All subsequent frames are discarded.
• Sender retransmits the lost frame and every frame after it.

Example

Frames Sent:

 0, 1, 2, 3

If Frame 2 is damaged:

Receiver accepts:

0, 1

Receiver rejects:

2, 3

Sender retransmits:

2, 3

Advantages

• Easy implementation
• Less receiver complexity

Disadvantages

• Many unnecessary retransmissions

Selective Reject ARQ

How It Works

Only the damaged or lost frame is retransmitted.

Correctly received frames are stored temporarily in the receiver buffer.

Example

Frames Sent:

0, 1, 2, 3

If Frame 2 is lost:

Receiver keeps:

0, 1, 3

Sender retransmits only:

2

No need to resend Frames 0, 1, and 3.

Advantages

• Highly efficient
• Fewer retransmissions
• Better bandwidth utilization

Disadvantages

• More complex implementation
• Requires larger receiver memory