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Chapter 3: OSI Reference Model & Layered Communication
more condensed description of the layer characteristics and examples. This table is taken
directly from Cisco's ICND course, so if you are just not willing to try and remember all of
Table 3-2, the information in Table 3-3 is a good compromise. (ICND is the instructor-led
course in the official CCNA training path.)
Layering Benefits and Concepts
Many benefits can be gained from the process of breaking up the functions or tasks of
networking into smaller chunks, called layers, and defining standard interfaces between these
layers. One obvious benefit is that the individual protocols or layers are less complex and
therefore can be defined in great detail. The following list summarizes the benefits of layered
protocol specifications:
·
Humans can discuss and learn about the many details of a protocol specification easier.
·
Standardized interfaces among layers facilitates modular engineering. Different products
can provide functions of only some layers (such as a router with Layers 1 to 3), or some
products could supply parts of the functions of the protocol (such as Microsoft TCP/IP
built into Win95, or the Eudora e-mail application providing TCP/IP application layer
support).
Table 3-3
OSI Reference Model (Condensed Information)
OSI Layer Name
Functional Description
Examples
Application (Layer 7)
User interface
Telnet, HTTP
Presentation (Layer 6)
How data is presented
Special processing, such as encryption
JPEG, ASCII, EBCDIC
Session (Layer 5)
Keeping data separate from different
applications
Operating systems and
application access
scheduling
Transport (Layer 4)
Reliable or unreliable delivery
Multiplexing
TCP, UDP, SPX
Network (Layer 3)
Logical addressing, which routers use for path
determination
IP, IPX
Data link (Layer 2)
Combination of bits into bytes, and bytes into
frames
Access to the media using MAC address
Error detection and error recovery
802.3/802.2, HDLC
Physical (Layer 1)
Moving of bits between devices
Specification of voltage, wire speed, and cable
pin-outs
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