Information Technology Reference
In-Depth Information
Data Link Layer (OSI Layer 2)
The data link 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 frame sequenc-
ing, 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 at the data link layer.
Because of the complexity of this OSI layer, the IEEE subdivides the data link layer into
three sublayers for LANs. Figure C-2 shows how Layer 2 is subdivided. The upper layer is
the logical link sublayer, which manages communications between devices. The bridging
layer, defined by IEEE 802.1, is the middle layer. The lowest layer is the Media Access
Control (MAC) sublayer, which manages the protocol access to the physical layer and ul-
timately the actual media. Systems attached to a common data link layer have a unique
address on that data link layer. Be aware that you might find some references describing
this layer as having two sublayers: the Logical Link Control (LLC) sublayer and the MAC
sublayer.
OSI Model
IEEE 802 Specifications
802.2 Logical Link
Data Link Layer
802.1 Bridging
Media Access Control
Figure C-2
IEEE Data Link Sublayers
Examples of data link layer technologies are
Frame Relay
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AT M
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Synchronous Data Link Control (SDLC)
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High-Level Data Link Control (HDLC)
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Point-to-Point Protocol (PPP)
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Ethernet implementations (IEEE 802.3)
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Wireless LAN (IEEE 802.11)
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Network Layer (OSI Layer 3)
The network layer is concerned with routing information and methods to determine
paths to a destination. Information at this layer is called packets. Specifications include
routing protocols, logical network addressing, and packet fragmentation. Routers operate
at this layer.
