Data Link Layer

As i have mention before the data link layer or layer 2 is the protocol layer which is responsible for the transfer of data within a LAN/WAN or between physically connected nodes on an network such as switches and hubs.
The data link layer provides the means to transfer data between network entities and may also provide the detection and correction of errors that may occur in layer 1.

In this post we will be outlining some of the protocols used, their functions and also we will be talking Switches which are a layer 2 device.

Data Link Protocols:

The Data Link Protocols in general performs three functions:

• Flow control - controls when to transmit data (MAC or media access control)
• Detects and corrects errors (Error Control)
• Identifies the start and end of a message(Message Delineation)

Data Link Layer devices encapsulate layer 3 data with a header and a trailer to create what is called a frame. Layer 2 devices deliver these frames using unique hardware addresses. A frame's header contains source and destination addresses that identify which device originated the frame and which device is expected to receive and process it.

The data link layer is only concerned with local delivery of frames between devices on the same LAN. Data-link frames, as these protocol data units(PDU) are called, do not cross the boundaries of a local network.
When devices attempt to use a medium at the same time, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and can provide ways to reduce or prevent them.

Logical link control (LLC) sublayer:

The first sublayer, LLC, multiplexes protocols running on the data link layer, and can provides flow control, acknowledgment, and error notification. The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the source and destination machines.

Media Access Control (MAC) sublayer:

MAC can refer to the sublayer that determines who is allowed to access the media at any one time or it can also refer to the structured framing of packets of data which are delivered based on the  MAC addresses inside.

There are many many protocols used in layer 2 to complete theses various flow control, framing and error handling functions. Here I am just going to list a few of these protocols

• Address Resolution Protocol(ARP)
• Token Ring 
• Ethernet
• Point-to-point (PPP)
• Fibre Distributed Data Interface(FDDI)
• IEEE 802.11 wireless LAN
• CSMA/CD 
• CSMA/CA