Information Technology Reference
In-Depth Information
The following sections provide a description and sample protocols of each OSI layer.
Physical Layer (OSI Layer 1)
The physical layer describes the transportation of raw bits over the physical media. It defines
signaling specifications and cable types and interfaces. The physical layer also describes
voltage levels, physical data rates, and maximum transmission distances. In general, the
physical layer deals with the electrical, mechanical, functional, and procedural specifications
for physical links between networked systems.
Examples of physical layer specifications are the following:
EIA/TIA-232
EIA/TIA-449
V.35
RJ-45
Maximum cable distances of the Ethernet family, Token Ring, and FDDI
Data-Link Layer (OSI Layer 2)
This layer is concerned with the reliable transport of data across a physical link. Data at this
layer is formatted into frames. Data-link specifications include the following: sequencing of
frames, flow control, synchronization, error notification, physical network topology, and
physical addressing. This layer converts frames into bits when sending information and
converts bits into frames when receiving information from the physical media. Bridges and
switches operate in the data-link layer.
Because of the complexity of this OSI layer, the IEEE subdivides the data-link layer into two
sublayers for local-area networks. Figure 2-2 shows how Layer 2 is subdivided. The upper layer
is the Logical Link Control (LLC) sublayer, which manages the communications between
devices. The lower layer is the Media Access Control (MAC) sublayer, which manages protocol
access to the physical media. Devices that operate in this layer can contain a unique physical
MAC address. These sublayers are discussed in more detail in Chapter 4, “Local-Area
Networks and LAN Switching.”
IEEE Data-Link Sublayers
Figure 2-2
IEEE 802
Specifications
OSI Model
LLC Sublayer
Data-Link Layer
MAC Sublayer
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