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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  OSI Reference Model Layers

OSI Reference Model Layers 1 2 Data Link Layer (Layer 2)

Relationship Between the Physical Layer and Data Link Layer

It's important to point out that while the physical layer of a network technology primarily defines the hardware it uses, the physical layer is closely related to the data link layer. Thus, it is not generally possible to define hardware at the physical layer “independently” of the technology being used at the data link layer. For example, Ethernet is a technology that describes specific types of cables and network hardware, but the physical layer of Ethernet can only be isolated from its data link layer aspects to a point. While Ethernet cables are “physical layer”, for example, their maximum length is related closely to message format rules that exist at the data link layer.

Furthermore, some technologies perform functions at the physical layer that are normally more closely associated with the data link layer. For example, it is common to have the physical layer perform low-level (bit level) repackaging of data link layer frames for transmission. Error detection and correction may also be done at layer 1 in some cases. Most people would consider these “layer two functions”.

In many technologies, a number of physical layers can be used with a data link layer. Again here, the classic example is Ethernet, where dozens of different physical layer implementations exist, each of which uses the same data link layer (possibly with slight variations.)

Finally, many technologies further subdivide the physical layer into sublayers. In order to increase performance, physical layer encoding and transmission methods have become more complex over time. The physical layer may be broken into layers to allow different network media to be supported by the same technology, while sharing other functions at the physical layer that are common between the various media. A good example of this is the physical layer architecture used for Fast Ethernet, Gigabit Ethernet and 10-Gigabit Ethernet.

OSI Reference Model Layers 1 2 Data Link Layer (Layer 2)