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Understanding Data Link Layer 2 Basics
A Data Link Layer or DLL is layer 2 of the 7-layer OSI (open systems interconnection) model. It is the standard under which communication on networks is divided into seven layers. Each layer has a distinct function.
The DLL fits into the OSI model with the physical layer below it and the network layer above it. In essence, what the DLL does is to breakup the incoming packets into bits and put together bits that need to be transmitted into packets. Packets are what get transmitted on the network, while bits are the most basic data unit in computing.
It is hard to overstate the importance of the DLL in today's world where network computing is a necessity. DLL is a part of the TCP/IP protocol used by the internet for packet data transfers. It is important to note that DLL has nothing much to do with global addresses or inter-network connectivity because its functionality is limited to frame data transmission on the same local area network.
Examples of DLL include Token Ring, Ethernet and PPP (Point-to-Point Protocol). For Ethernet and Token Ring, the DLL is a software device driver for the NIC (network interface card). Due to the extremely related nature of the DLL and the physical cards, the cards are often known by the DLL name (like Tokenring cards or Ethernet cards).
There are two parts to the DLL which plug it into the layers above and below it in the OSI model. The first one is the MAC or media access control which has a system for how a computer can transmit date out on to the network and for knowing how data meant for a specific computer reaches it. All data frames have the source/destination MAC address in their headers. This ensures that computers on a network can identify frames meant for them and know which computer sent it.
There are also types of DLL - reliable and unreliable. Unreliable doesn't mean it won't work. It just means that there is no way of knowing whether or not a transmission has been successful, and there is no way of retransmitting an unsuccessful transmission.
On the other hand, a reliable DLL is one which does have an acknowledgment system so that it knows whether the transmission was successful. This is where the second part of the Data Link Layer kicks in, known as the LLC or logical link control. It takes care of frame synchronization, error checking and flow control.
May 20, 2012, 4:22:10 pm, America/Chicago
