Information Technology Reference
In-Depth Information
2.
Data link
. Ensures that the transmitted bits are received in a reliable way, such as adding
extra bits to define the start and end of a data frame, adding extra error detec-
tion/correction bits and ensuring that multiple nodes do not try to access a common com-
munication channel at the same time.
3.
Network
. Routes data frames through a network. If data packets require to go out of a
network then the transport layer routes them through interconnected networks. Its task
may involve, for example, splitting data for transmission and re-assembling it upon
reception. The IP part of TCP/IP is involved with the network layer.
4.
Transport
. Network transparent data transfer and transmission protocol. It supports the
transmission of multiple streams from a single computer. The TCP part of TCP/IP is in-
volved with the transport layer.
5.
Session
. Provides an open communications path with the other system. It involves the
setting up, maintaining and closing down of a session. The communication channel and
the internetworking of the data should be transparent to the session layer. A typical ses-
sion protocol is telnet, which allows for the remote login over a network.
6.
Presentation
. Uses a set of translations that allows the data to be interpreted properly. It
may have to translate between two systems if they use different presentation standards,
such as different character sets or differing character codes. The presentation layer can
also add data encryption for security purposes.
7.
Application
. Provides network services to application programs, such as file transfer and
electronic mail.
Figure 25.8
shows how typical networking systems fit into the OSI model. The data link and
physical layers are covered by networking technologies such as Ethernet, Token Ring and
FDDI. The networking layer is covered by IP (internet protocol) and transport by TCP
(transport control protocol).
Virtual
d
a
t
a
f
low
Application
A
Application
Presentation
P
A
Presentation
Actual
data flow
Session
S
P
A
Session
T
S
P
A
Transport
Transport
N
T
S
P
A
Network
Network
Data link
D
N
T
S
P
A
D
Data link
Physi cal
Physical
Sender
Receiver
Figure 25.7
Seven-layer OSI model
