Introduction to DLL
The Data Link Layer (DLL) transforms the physical layer, a raw transmission facility, into a reliable link.
Data Link Sublayers
The IEEE standards divide the Data Link Layer into two distinct sublayers to handle different functionalities.
Logical Link Control (LLC)
Responsible for Flow Control, Error Control, and part of Framing. It provides a standard interface to the Network layer.
Media Access Control (MAC)
Defines the specific access method for each LAN (e.g., CSMA/CD for Ethernet). Handles Physical Addressing and access to the transmission medium.
Responsibilities of DLL
Encapsulating network layer datagrams into frames to distinguish message boundaries.
Adding MAC addresses to the frame header to identify sender and receiver locally.
Preventing the sender from overwhelming the receiver (Stop-and-Wait, Sliding Window).
Detecting and retransmitting lost or damaged frames (CRC, Checksum, ARQ).
Services Provided
Connectionless
- ✓Frames sent without establishing a connection.
- ✓Each frame is independent; no relationship between frames.
- ✓No setup or teardown phases.
- ✓Example: Ethernet, UDP (at transport).
Connection-Oriented
- ✓Logical connection established before transfer.
- ✓Includes Setup, Transfer, and Teardown phases.
- ✓Frames are numbered and sent in order.
- ✓Guarantees reliable delivery (frames are acknowledged).
Packet Delays
Transmission Delay Tt
The time required to push all the bits of the packet onto the transmission medium. If first bit is put on the line at t₁ and the last bit at t₂, then Tt = t₂ - t₁.
Propagation Delay Tp
The time it takes for a bit to travel from point A to point B in the transmission media.
MAC Address (Physical Address)
Media Access Control (MAC) Address
A unique 48-bit identifier assigned to network interfaces for communications on a physical network segment. Often represented in hexadecimal format.
Media Access Control Protocols
When multiple nodes share a common link (broadcast link), we need protocols to coordinate access to avoid collisions.
Switching
Switching is the process of connecting multiple devices on a computer network. The Data Link Layer uses Packet Switching principles where Switches operate.
The Network Switch
A switch is an intelligent device that operates at the Data Link Layer. It uses the MAC address table to decide which port to forward a frame to, unlike a Hub which broadcasts to everyone.