Framing in Data-Link Layer

5 Questions

What is the purpose of framing in the data-link layer?

To pack bits into frames to distinguish them from each other.

In fixed-size framing, delimiter needs to be defined for the frames.

False

What is bit stuffing in bit-oriented framing?

adding one extra 0 whenever five consecutive 1s follow a 0 in the data

Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before __________ for acknowledgment.

waiting

Match the error control methods with their descriptions:

Silently discard corrupted frames = Method used mostly in wired LANs such as Ethernet Send acknowledgment if frame is not corrupted = Method involving automatic repeat request

Study Notes

Framing

In the data-link layer, framing separates messages from different sources to destinations by adding sender and destination addresses.
Framing is necessary to distinguish one frame from another.
Frames can be of fixed size or variable size.

Fixed-Size Framing

Fixed-size framing does not require defining boundaries, as the frame size can be used as a delimiter.
An example of fixed-size framing is the ATM WAN, which uses fixed-size cells.

Variable-Size Framing

Variable-size framing requires a way to define the end of one frame and the beginning of the next.
There are two approaches: character-oriented framing and bit-oriented framing.

Character-Oriented Framing

In character-oriented framing, a header and trailer are added to the frame, carrying source and destination addresses and control information.
A flag (special character) is added at the beginning and end of a frame to signal the start or end of a frame.
Byte-stuffing is used to avoid flags being mistaken for data in the middle of a frame.

Bit-Oriented Framing

In bit-oriented framing, the data section of a frame is a sequence of bits interpreted by the upper layer as text, graphics, audio, video, etc.
Bit-stuffing is used to avoid flags being mistaken for data in the middle of a frame.

Flow Control

Flow control restricts the amount of data that the sender can send before waiting for acknowledgment.
Flow control is necessary to prevent the receiving node from becoming overwhelmed with frames.
Flow control can be achieved through feedback from the receiving node to the sending node to stop or slow down pushing frames.

Error Control

Error control at the data link layer is based on automatic repeat request (ARQ), which involves retransmission of data.
There are two common methods of error control:
- Frames are silently discarded if corrupted, and packets are delivered to the network layer if not corrupted.
- Frames are silently discarded if corrupted, and acknowledgments are sent to the sender if not corrupted.