Figure 6-1.The OSI model showing the seven layers.
all seven software layers
implemented. Although there is no requirement for
any hardware or software vendor to adhere to the
principles set forth in the OSI model, there is a
worldwide trend in the computer industry toward
acceptance and conformance to these standards.
Ideally, if the hardware, network software,
application software, and cabling were all supplied by
the same manufacturer, there would be relatively few
problems for users to contend with when designing
and implementing a network. Everything would work
together rather smoothly. However, a computer
manufacturers architecture can make it difficult to
interconnect hardware offered by other competing
manufacturers or vendors. The protocols used by
communications devices are also highly complex and
are often completely different from one manufacturer
Then there is the network software.
Usually, the network software from one LAN vendor
will not work with that of a competitor; neither will
the application programs. Even the cabling must be
selected for a specific local-area network.
The hardware level contains the first two layers of
the OSI reference model. They are the physical layer
and the data-link layer.
These are concerned
primarily with the actual hardware used in a network.
The physical layer is concerned with the
transmission of the unstructured raw bit stream over
a physical medium.
It describes the electrical,
mechanical, and functional interfaces to the carrier.
The physical layer carries the signals for all the higher
layers as follows:
Voltages and pulse encoding of bits
connectors, NIC, and so on)
Line discipline (full- or half-duplex)
The data-link layer provides error-free
transmission of information over the physical
medium. This allows the next higher layer to assume
virtually error-free transmission over the link. The
data-link layer is responsible for getting data
packaged and onto the network cable. It manages the
flow of the data bit stream into and out of each
network node as follows:
Creates and recognizes frame boundaries
Checks received messages for integrity
Manages channel access and flow control
Ensures correct sequence of transmitted data
The data-link layer detects and, when possible,
corrects errors that occur in the physical layer without
using the functions of the upper layers.
provides flow-control techniques to ensure link-buffer
capacity is not exceeded.
The next three layers of the OSI reference model
make up the transport level, also known as the subnet.
The transport level defines the software protocols