Frame
Components
A
frame (the term for a data packet in the network interface layer)
consists of three components: the header, the data, and the trailer.
Header
The
header includes:
-
An alert signal to indicate that the packet is being
transmitted.
-
The source address.
-
The destination address.
Data
This
is the actual information sent by the application. This component of
the packet varies in size, depending on the size limits set by the
network. The data section on most networks varies from 0.5 kilobytes
(KB) to 4 KB. With Ethernet, the size of the data is approximately
1.5 KB. Because most original data strings are much longer than 4
KB, data must be broken into pieces small enough to be put into
packets. It takes many packets to complete the transmission of a
large file.
Trailer
The
exact content of the trailer varies depending on the network
interface layer
protocol.
However, the trailer usually contains an error-checking component
called a
cyclical redundancy check (CRC).
The CRC is a number produced by a mathematical calculation on the
packet at its source. When the packet arrives at its destination,
the calculation is made again. If the results of both calculations
are the same, this indicates that the data in the packet has
remained stable. If the calculation at the destination differs from
the calculation at the source, this means that the data has changed
during transmission. In that case, the source computer retransmits
the data.
Data
Flow
The
packets of data being transmitted from one computer to another
travel through the layers of the TCP/IP protocol stack. As the
data packets pass through each layer, the protocols in that layer
attach specific information to the header. The information added by
every protocol includes error-checking information, known as the
checksum.
The checksum is used to verify whether the header information added
by the protocol arrived intact at the destination protocol, compared
with the CRC, which verifies the whole packet. The information added
by the protocols in one layer is encapsulated as data by the
protocols in the layer below. When the packet is received at the
destination, the corresponding layer strips off a header and treats
the remaining packet as data. The packet is then passed up the
protocol stack to the appropriate protocol.
Application Layer
The
data transmission process begins at the application layer of the
TCP/IP protocol stack. An application, such as the Ftp utility,
initiates the process at the source computer by preparing the data
in a format that the application at the destination computer
recognizes. The application at the source computer controls the
entire process.
Transport Layer
From
the application layer, the data moves to the transport layer. This
layer contains the TCP and UDP protocols. The application initiating
the transmission request selects which protocol to use—TCP or
UDP—and the checksum is added for both TCP and UDP.
If
selected, TCP:
-
Assigns a sequence number to each segment to be transmitted.
-
Adds acknowledgement information for a connection-oriented
transmission.
-
Adds the TCP port number for the source and destination
applications.
If
selected, UDP:
-
Adds the UDP port number for the source and destination
applications.
