What are data link layer design issues?

The Data Link layer is located between physical and network layers. It provides services to the Network layer and receives services from the physical layer. The scope of the data link layer is node-to-node communication.

The following are the key design issues in the Data Link Layer:

• Services provided to the Network layer
• Framing
• Error control
• Flow control

Services to the Network Layer

In OSI each layer uses the services of the layer below and provides services to the layer above. The main function of the data link layer is to provide a well-defined service interface to the network layer.

Types of Services

The data link layer provides three types of services:

• Unacknowledged connectionless service − The sender transmits frames without expecting acknowledgments. This is suitable for low error rate channels and real-time applications.

• Acknowledged connectionless service − Each frame sent is individually acknowledged by the receiver. This provides reliability without the overhead of connection establishment.

• Acknowledged connection-oriented service − A connection is established before data transfer, each frame is acknowledged, and the connection is released afterward. This provides the highest reliability.

Framing

Framing is the process of packaging network layer data into frames for transmission over the physical medium. It provides a way to delimit the beginning and end of each frame.

A typical frame structure contains:

• Frame Header − Contains control information and addressing
• Payload Field − Holds the network layer packet
• Frame Trailer − Contains error detection codes

Framing Methods

Three main framing methods are used in the data link layer:

• Byte-oriented framing − Uses special byte patterns to mark frame boundaries
• Bit-oriented framing − Uses specific bit patterns like flags to delimit frames
• Clock-based framing − Relies on timing information to determine frame boundaries

Error Control

During transmission, electromagnetic signals are susceptible to interference and noise, which can corrupt frames. Error control mechanisms detect and handle these transmission errors.

The error control process involves two main functions:

• Error Detection − Using techniques like checksums, CRC, or parity bits to identify corrupted frames
• Error Correction − Either correcting errors at the receiver or requesting retransmission from the sender

Flow Control

Flow control manages the rate of data transmission between sender and receiver to prevent buffer overflow. It ensures that a fast sender does not overwhelm a slower receiver with more data than it can process.

Flow Control Methods

Two primary methods are used for flow control:

• Feedback-based Flow Control − The receiver sends feedback to the sender about its readiness to accept more data
• Rate-based Flow Control − The sender limits its transmission rate to a predetermined value

Conclusion

The data link layer addresses four critical design issues: providing reliable services to the network layer, framing data for transmission, controlling errors in the communication channel, and managing flow between sender and receiver to ensure efficient data transfer.