Chapter 3 • Telecommunications and Networking 87(^5) These applications and others will be described in Chapters 5 through 7.
computer networks, so it is a good place to begin. The
TCP/IP model, augmented with some other ideas, is the de
factostandard set of protocols for networking in the early
twenty-first century, so we will turn to TCP/IP after
considering the OSI model.
OSI REFERENCE MODEL Because of the importance of
the OSI model, and because it will give us a conceptual
framework to understand how communication takes place
in networks, we will briefly discuss how data move
through the layers in the OSI model and the role of each of
the layers. The OSI model is very complex because it must
support many types of networks (e.g., LANs and WANs)
and many types of communication (e.g., electronic mail,
electronic data interchange, and management reports^5 ).
Let’s consider a communication of some sort, such
as an electronic mail message, moving through a network.
The communication begins at the application layer and
works its way down through the seven layers of Figure 3.11
before it is sent through the network. At each layer, the
communication is processed by the layer to accomplish the
role for which the layer has been designed, and then a
header (and a trailer at one layer) is added to the communi-
cation before it is passed down to the next layer. These
headers provide identifying information about the commu-
nication and its processing, including its routing through
the network. After passing down through all seven layers,
the communication is sent across the network. When the
communication reaches the receiving computer, it works
its way up through the seven layers. At each layer, the
communication is processed by the layer as required, and
the header for that layer (and trailer in one instance) is
stripped off. By the time the communication reaches the
top layer, it is back to the original version of the communi-
cation (e.g., an e-mail message that can be read by the
receiving party).
Application Layer. We will now turn to the role of
each of the seven layers. The uppermost layer deals with
the wide variety of communications-oriented applications
that are directly visible to the user, such as electronic data
interchange, file transfer, electronic mail, and factory floor
control. There will always be differences across different
devices or systems, and a protocol—implemented in soft-
ware—is required for each application to make the various
devices appear the same to the network. For a group of
users to communicate using electronic mail, for example,
the devices they employ must all use the same application
layer/electronic mail protocol. The OSI electronic mail
protocol, known as MOTIS, gained acceptance in some
parts of the world but has largely been replaced by SMTP,
which is at least unofficially part of the TCP/IP model. As
with the layers below it, the application layer attaches a
header to the communication before passing it to the next
layer down.
Presentation Layer. The presentation layer accepts as
input the communication as internally coded by the sending
device and translates it into the standard representation
used by the network. (The presentation layer on the receiv-
ing device reverses this process.) In addition, the data might
be cryptographically encoded if it is especially sensitive.
Session Layer. Through the session layer, users on
different machines may establish sessions between them.
For most applications the session layer is not used, but it
would allow a user to log into a remote computer or to
transfer a file between two computers. The session layer
might provide several services to the users, including
dialog control (if traffic can only move in one direction at a
time) and synchronization (so that a portion of a communi-
cation received need not be retransmitted even if the
network fails).
Transport Layer. The transport layer receives the
communication (of whatever length) from the session
layer, splits it into smaller blocks (called packets) if
necessary, adds special header data defining the network
connection(s) to be used, passes the packet(s) to the
network layer, and checks to make sure that all the packets
arrive correctly at the receiving end. If the network
connection requires multiplexing for its efficient use, the
transport layer also handles this.
Network Layer. The network layer receives a packet
of data from the transport layer and adds special header
data to it to identify the route that the packet is to take to its
destination. This augmented packet becomes the frame
passed on to the data link layer. The primary concern of the
network layer is the routing of the packets. The network
layer often contains an accounting function as well in
order to produce billing information.
Data Link Layer. Data are submitted to the data link
layer in the form of data frames of a few hundred bytes.
Then the data link layer adds special header and trailer data
at the beginning and end of each frame, respectively, so that
it can recognize the frame boundaries. The data link trans-
mits the frames in sequence to the physical layer for actual
transmittal and also processes acknowledgment frames sent
back by the data link layer of the receiving computer and
makes sure that there are no transmission errors.
Physical Layer. The physical layer is concerned with
transmitting bits (a string of zeros and ones) over a physical