Layer 2 - Data Link layer

The Data Link layer has three primary jobs:

  1. Bus Arbitration: The Data Link layer controls "whose turn it is to talk?" on the medium.
  2. Framing of the bits: it puts the bits in proper order and fields
  3. Error detection and correction at the bit level

The Data Link layer is in charge of whose turn it is to talk on the wire. It will have a method for determining how its going to control the communication.

The Data Link layer has the job putting the bits in the correct order for sending out on the wire. The Data Link layer has the job of organizing the data in a frame. The frame consists of sections called fields. Each field will have a specific function such as Destination Address which identifies the host for which the data is being sent to. Other fields will be used for synchronizing source and destination clocks and others for error checking.

Fig. 1 Data Link Layer

The Data Link layer resides in the firmware layer of the network interface card. Firmware is software that is burnt into a read only memory. The node will have a unique address that identifies it from all other nodes. This address is called a physical address because it is burnt into the firmware. Ethernet network interface cards' unique address is typically called a MAC address after a Data Link sub-layer called the Media Access Control (MAC) layer.

The Data Link layer takes the packets and puts them into frames of bits: 1s and 0s for transmission and assembles received frames into packets. The Data Link layer works at the bit level and is concerned about bit sequence. Error checking is at the bit level and frames with errors are discarded and a request for re-transmission is sent out.

The Layer 2 - Data Link Layer is divided into the following subsections:

Synchronous Communications - Synchronous transmission is a method of data communication that requires the source and destination to synchronize their clocks together. This synchronization of the clocks can occur externally to the data information or be incorporated with the data information.

Basic Synchronous Frame Structure - When dealing with synchronous communications, all synchronous protocols follow a basic frame structure. This section discusses the components that make up the basic frame structure.

IEEE802.2 - Logical Link Control (LLC) layer - The Logical Link Control Layer resides in the upper portion of the Data Link Layer. One of its original purposes is to provide method for directing traffic to a specific network operating system stack in a multiprotocol environment.

IEEE802.3 - Ethernet - The IEEE-802.3 Protocol is based on the Xerox Network Standard (XNS) called Ethernet. The IEEE-802.3 Protocol is commonly called Ethernet and there are many flavours of it.

IEEE802.5 - Token Ring - Token Ring is a token passing bus arbitration topology for the Physical and Data Link Layers. It is a logical ring and a physical star topology. Token Ring is pretty well not used anymore and is present here for archival purposes and because there is an industrial controls version in use that may benefit from this information.

Packet Sniffing - Capturing packets as they transverse through a network is called packet sniffing. This page shows how to interpret a captured hexadecimal packet and break it down to its component protocols.

Network Interface Cards (NIC) - Network Interface Cards reside partially in the Data Link layer and partially in the Physical layer. Surprisingly, there are a few different interfaces available for controling the network card.

Bridges - Bridges are both hardware and software devices. They can be standalone devices - separate boxes specifically designed for bridging applications, or they can be dedicated PCs with 2 NICs and bridging software. Most servers software will automatically act as a bridge when a second NIC card is installed.

Switches - Ethernet Switches (sometimes called switching hubs) are devices that have changed the way ethernet's bus arbitration works. Ethernet switches change that. They are based on circuit switching technology.

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Copyright July 2013 Eugene Blanchard