Information Technology Reference
In-Depth Information
computers divide the work into distinct logical levels, meaning that a
hierarchy of “who's doing what” is established. Each level addresses
specific details and functions, and activities to be accomplished at
higher levels can make use of lowerlevel functions. The idea here is
much the same as discussed in Chapters 1 and 4 on applications and
operating systems, where various details (such as how to manage spe
cific printers or disks) were resolved in libraries of functions, and these
materials then could be used in higherlevel problem solving.
For standard computertocomputer communication, the
International Organization for Standardization (ISO) has specified
an Open Systems Interconnection Reference Model (OSI) with seven
distinct levels. The seven levels identified in the OSI cover the range
of needs for communications, and are outlined in Table 9.2.
Rather than focusing on the details of the OSI model, let's look
at the basic plan outlined by OSI. First, the title OSI highlights the
term “Open Systems.” The open systems premise emphasizes that
communication is possible only if all computers involved agree on
how they will function. With the open systems approach, the work
ings of the communication system are distributed publicly for refer
ence by all manufacturers of computer hardware and software. The
OSI model is not a trade secret; rather, all machines can follow the
same standards, so we can expect that equipment from one manu
facturer will work with that purchased from another source.
Let's consider the communication layers of the OSI model from
the bottom layer upwards. Layer 1, the Physical Layer, handles the
tangible details of transmission from one machine to another. For
Table 9.2 Layers of Computer-to-Computer Communication
Layer
Name
Description
}
7
Application Layer
Layers 5-7 involve
various highlevel capabilities
6
Presentation Layer
for applications
5
Session Layer
4
Transport Layer
Organizes application data into pack
ets; provides delivery mechanism
3
Network Layer
Routes packets (groups of bits) from
source to destination
2
Data Link Layer
Moves frames (groups of bits) from
one machine to the next
1
Physical Layer
Moves bits from machine to machine
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