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Table Of Contents  CertiGuide to Network+
 9  Chapter 0010:  ISO, OSI, and IEEE Standards
      9  II  Open Systems Interconnect

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Layer 1 - Physical Layer
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Layer 3 - Network Layer
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Layer 2 - Data Link Layer

The Data Link Layer is responsible for flow and error control as well as synchronization for the lower physical layer. Operating on a point-to-point (direct from one piece of hardware to another) basis between devices on the same network, it lies between the physical layer and the network layer.

The MAC (Media Access Control) functionality resides on the Data Link layer. The MAC provides a unique physical address for each piece of hardware on the network, much as the location of a place on a map is identified by its latitude and longitude. This address is generally permanently set in the hardware when the device is manufactured.

Why is this unique address important? In a network setting, many conversations between different pairs of computers (and other devices) on the network are carried simultaneously on a single cable. That makes for a lot of bits traveling on a wire.

When sending out data, a device has to have some way to designate which network device it is sending the data to, so that the information gets to its intended destination. So, part of the conversion process involves associating the physical address of the receiving device, with the data that is being sent. Also, a device on the network has to have some way of knowing whether each set of bits it sees on the wire is intended for its use, or whether those bits can be ignored because they are addressed to another device. So part of the conversion process on the receiving end involves inspecting the address information for each set of data on the cable, and comparing the physical address destination associated with that data, with its own unique physical address. When a match is found, the device knows that that packet of data is meant for itself, so it accepts the data and hands it off to the next network layer to continue the “receive data” process.

Figure 26: Layer 2 - Data Link Layer

 


The data link layer knows only about how to get data from one place to another within a single LAN. Suppose you are in a Wide Area Network environment, and are sending data from a LAN computer to one in another city on your corporate WAN. The data link layer is only in control of getting the data from its source device to the device on your LAN that knows how to reach other WAN sites.

It does not know anything about devices outside the immediate LAN, and instead relies on higher OSI model layers to handle moving data between LANs.

Layer 2

The OSI model does not completely define how Layer 2, the Data Link layer operates. The IEEE 802 sub-committee further defined this functionality. This group refined the 802.x standards. For example Token Ring is defined in 802.5, while Gigabit Ethernet is defined in 802.3a. This reveals the dynamic nature of the 802 sub-committee. We’ll cover the IEEE 802 sub-committee’s standardization work after describing the OSI model.



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Layer 1 - Physical Layer
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Layer 3 - Network Layer
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