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Networking tutorial – part 1 / Layer 2- the data network layer February 25, 2011

Posted by TelUS Consulting Services in Data Networking catagory.

The data-link layer is responsible for getting data to the physical layer so that it can be transmitted over the network. The data-link layer is also responsible for error detection, error correction, and hardware addressing. The term frame is used to describe the logical grouping of data at the data-link layer.

The data-link layer has two distinct sublayers the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer.

  • MAC layer The MAC address is defined at this layer. The MAC address is the physical or hardware address burned into each NIC. The MAC sublayer also controls access to network media. The MAC layer specification is included in the IEEE802.1 standard.
  • LLC layer The LLC layer is responsible for the error and flow-control mechanisms of the data-link layer. The LLC layer is specified in the 802.2 standard.

The primary responsibility of the network layer is routing, providing mechanisms by which data can be passed from one network system to another. It does not specify how the data is passed, but rather provides the mechanisms to do so. Functionality at the network layer is provided through protocols, which are software components.

Protocols at the network layer are also responsible for route selection, which refers to determining the best path for the data to take throughout the network. In contrast to the data-link layer, which uses MAC addresses to communicate on the LAN, network protocols use software configured addresses and special routing protocols to communicate on the network. The term packet is used to describe the logical grouping of data at the data-link layer.

The basic function of the transport layer is to provide mechanisms to transport data between network devices. Primarily it does this in three ways:

  • Error checking Protocols at the transport layer ensure that data is sent or received correctly.
  • Service addressing Protocols such as TCP/IP support many network services. The transport layer makes sure that data is passed to the right service at the upper layers of the OSI model.
  • Segmentation To traverse the network, blocks of data need to be broken down into packets that are of a manageable size for the lower layers to handle. This process, called segmentation, is the responsibility of the transport layer.

Protocols that operate at the transport layer can either be connectionless, such as the User Datagram Protocol (UDP) , or connection oriented, such as Transmission Control Protocol (TCP). For a further discussion of these protocols, and of the difference between connection oriented and connectionless protocols, we will refer to the information on network protocols later.

Next week we will discuss flow control…

Joe Buck, NCE



1. kyle - March 2, 2011

yeah nice

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