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Chapter 3: OSI Reference Model & Layered Communication
Foundation Topics
The OSI, TCP/IP, and NetWare Protocol Architectures
Four topics of particular importance for the CCNA exam are covered in this chapter:
·
The OSI model
--Expect questions on the functions of each layer and examples at each
layer in the CCNA exam.
·
Data link protocols
--This section is important to properly understand LAN switching.
·
Network layer protocols
--This section is important to properly understand routing.
·
Transport layer protocols
--This section is important to properly understand end-to-end
transport.
The last three sections all use the terminology discussed in the first section.
OSI: Origin and Evolution
To pass the CCNA exam, you must be conversant in a protocol specification with which you are
very unlikely to have any hands-on experience. The difficulty these days when using the OSI
protocol specifications as a point of reference is that almost no one uses those specifications.
You cannot typically walk down the hall and see a computer whose main, or even optional,
networking protocols are defined by OSI.
OSI is the Open Systems Interconnection reference model for communications. OSI is a rather
well-defined set of protocol specifications with many options for accomplishing similar tasks.
Some participants in OSI's creation and development wanted it to become
the
networking
protocol used by all applications. The U.S. government went so far as to require OSI support
on every computer it would buy (as of a certain date in the early 1990s) via an edict called the
Government OSI Profile (GOSIP), which certainly gave vendors some incentive to write OSI
code. In fact, in my old IBM days, the company even had charts showing how the TCP/IP
installed base would start declining by 1994, how OSI installations would take off, and how OSI
would be
the
protocol from which the twenty-first century Internet was built. (In IBM's defense,
moving the world to OSI may have been yet another case of "You just can't get there from
here.")
What is OSI today? Well, the protocols are still in existence and are used around the world, to
some degree. The U.S. government reversed its GOSIP directive officially in May 1994, which
was probably the final blow to the possibility of pervasive OSI implementations. Cisco routers
will route OSI. OSI NSAP addresses are used in Cisco ATM devices for signaling. Digital
Equipment's DECnet Phase V uses several portions of OSI, including the network layer (Layer
3) addressing and routing concepts. More often than not, however, the OSI model now is mainly
used as a point of reference for discussing other protocol specifications.
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