Data Link Layer Overview

Data Link Layer

• The data link layer has two basic functions:
  • It allows the upper layers to access the media using framing.
  • It controls how data is placed on the media and received from the media using media access control (MAC) and logical link control (LLC). LLC handles flow control and error control.

Framing

• The data link layer takes packets from the network layer and encapsulates them into frames for transmission.
• Framing in the data link layer includes source and destination MAC addresses.
• The data link layer translates the physical layer's raw bit stream into discrete units (messages) called frames.
• A frame has a header and a trailer. The header contains information like the start of the frame, address information, and type/length information.
• A trailer contains a frame check sequence (FCS) for error checking. A common method for error checking is Cyclic Redundancy Check (CRC).

Framing: Role of the Header

• A specific bit sequence indicates the start of the frame for the receiving device.
• The header contains the source and destination MAC addresses (physical addresses).
• The header specifies the type (fixed or variable size) or the total length of the frame.

Framing: Role of the Trailer

• Frame Check Sequence (FCS) provides basic error checking, typically using Cyclic Redundancy Check (CRC).
• The sending device calculates the FCS value based on the bits in the header and data portions.
• The receiving device calculates its own FCS value and compares it to the FCS value in the frame trailer.
• If the values match, there is no error. If they don't match, the frame is discarded.
• The trailer also includes a stop sequence, indicating the end of the frame.

Methods of Framing

• There are different methods to perform framing, including:
  • Character count
  • Flag byte with byte stuffing
  • Starting and ending flag with byte stuffing
  • Physical layer coding violations

Framing – Character Count

• The first field in the frame's header is the length of the frame.
• Problems with character count framing include synchronization loss when bits in the count are corrupted during transmission. This corruption leads to the receiver thinking the frame contains more or less data than it actually does.

Framing – Byte Stuffing

• Data frames can have either fixed or variable length.
• A bit pattern is used to delimit the end of one frame and the beginning of the next.
• If the delimiter pattern occurs in the data, a special byte (escape character) is added before every byte that will be transmitted to make sure the delimiter isn't part of the data.
• The receiver detects the escape character and removes it from the received data to retrieve the original data and recreate the frame.

Framing – Byte Stuffing (Sender's Side)

• Each frame starts and ends with an 8-bit flag sequence.
• If a flag sequence appears within the data, an escape sequence is inserted before each flag.
• If an escape sequence is part of the data, another escape sequence is inserted before it for the receiver.
• The Sender sends the stuffed data.

Framing – Byte Stuffing (Receiver's Side)

• The receiver skips the first and last bytes, as they are signaling bytes and not data.
• If two escape sequences are found consecutively, the first one is removed (de-stuffed).
• If an escape sequence is followed by a flag sequence, the escape is removed (de-stuffed).
• This strategy helps the receiver recover the actual sent data accurately.

This quiz covers the fundamental concepts of the data link layer, including its basic functions such as framing and media access control. It explores how data is encapsulated into frames, the role of headers and trailers, and error checking methods like CRC. Test your understanding of these essential networking principles.