Framing at the Data Link Layer

Marking Where a Message Starts and Ends

The Physical layer only deals with a continuous stream of raw bits - it has no inherent concept of where one message ends and the next begins. Framing is the Data Link layer's job of organizing that raw bit stream into distinct, identifiable units called frames.

The Basic Structure of a Frame

[ Header (addresses, control info) | Payload (the actual data) | Trailer (error checking, e.g. CRC) ]

A frame's header typically carries source and destination MAC addresses (covered in the previous note), while its trailer typically carries an error-checking value like a CRC (covered earlier in this project) to detect corruption during transmission.

Why Clearly Marked Boundaries Are Essential

Without a clear, agreed-upon way to identify where a frame starts and ends, a receiver would have no reliable way to separate one message from the next in a continuous stream of bits - framing is what makes organized, distinguishable communication possible at all, rather than one indistinguishable blur of signals.

Common Ways to Mark Frame Boundaries

• Length-based framing - the header explicitly states how many bytes the frame contains.
• Special delimiter sequences - specific bit patterns mark the start and end of each frame.

A Concrete Example: Ethernet Framing

An Ethernet frame includes a preamble (a pattern helping receivers synchronize), source and destination MAC addresses, a type field, the actual payload data, and a trailing CRC value - a complete real-world example of exactly the structure described above, in active, everyday use.