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Lesson 2
Selecting Data-Link Layer Protocols
1-13
The seven layers of the OSI model define functions that are implemented in
various
networking protocols, such as Ethernet and TCP/IP. The functions
corresponding to
the layers are as follows:
■
Physical The physical layer defines the nature of the network medium—the
actual fabric of the network that joins all the computers together—and the
nature
of the signals transmitted over the medium. In most cases, the network
medium is
a form of copper cable that uses electric currents for signaling, but
fiber-optic and
wireless media are becoming increasingly prevalent.
■
Data-Link The data-link layer defines the interface between the network
medium and the software running on the computer. Among the data-link layer
functions are packet addressing (which allows computers to direct traffic to
spe-
cific destinations on the local network); media access control (which allows
mul-
tiple computers to share a single network medium without conflicting); and
formatting the frame used to encapsulate data for transmission over the
network.
The data-link layer divides into two sublayers. The logical link control
(LLC) sub-
layer controls elements such as error checking from node to node on the same
LAN, frame synchronization, and flow control. The media access control (MAC)
sublayer controls the movement of data packets to and from one network
inter-
face card (NIC) to another across a shared channel.
■
Network The network layer defines the functions that provide end-to-end com-
munications between computers on different networks. Chief among these func-
tions is routing, which enables computers to relay traffic through
intermediate
networks to a destination on a remote network any distance away. Other func-
tions include packet sequencing, end-to-end error detection from the sender
to
the recipient, congestion control, and addressing. While the data-link layer
is
responsible for local traffic on a single network, the network layer is
responsible
for directing traffic to its ultimate destination.
■
Transport The transport layer provides functions that complement those of
the
network layer, including guaranteed delivery (which uses packet acknowledg-
ments to ensure data is received), flow control (which regulates
transmission
speed to avoid dropped packets), and end-to-end error detection (which
enables
the receiving system to detect damaged packets).
■
Session The session layer provides many functions involved in the regulation
of
the dialog between two computers communicating over the network. For exam-
ple, the session layer sets up, regulates, and terminates exchanges between
the
applications at each end of the communication.
■
Presentation The presentation layer (sometimes referred to as the syntax
layer)
is responsible for translating each computer’s native syntax into a common
trans-
fer syntax readable by the other computers on the network. In some cases,
the
transfer syntax can provide functions such as data compression and
encryption.
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