Lesson 2: How Networks Send Data
At first, one might assume that data is sent as a continuous stream of
ones and zeros from one computer to another. In fact, data is broken down
into small, manageable packets, each wrapped with the essential information
needed to get it from its source to the correct destination. This lesson
introduces the concept of packets as the basic building blocks of network
The Function of Packets in Network Communications
Data usually exists as rather large files. However, networks cannot operate
if computers put large amounts of data on the cable at the same time. As
you see in Figure 3.5, a computer sending large amounts of data causes
other computers to wait (increasing the frustration of the other users)
while the data is being moved. This is not called "sharing"; it is called
"monopolizing the network." There are two reasons why putting large chunks
of data on the cable at one time slows down the network:
These effects are minimized when the large data units are reformatted into
smaller packages for better management of error correction in transmission.
This way, only a small section of data is affected, and, therefore, only
a small amount of data must be retransmitted, making it relatively easy
to recover from the error.
Large amounts of data sent as one large unit tie up the network and make
timely interaction and communications impossible because one computer is
flooding the cable with data.
The impact of retransmitting large units of data further multiplies network
Figure 3.5 Large continuous streams of data
slow down the network
In order for many users at once to transmit data quickly and easily
across the network, the data must be broken into small, manageable chunks.
This way, users each get their share of access to the network. These chunks
are called packets, or frames. Although the terms "packet" and "frame"
are often used interchangeably, there are some differences based on the
type of network. This lesson uses the term "packet," meaning "a unit of
information transmitted as a whole from one device to another on a network."
"Device" is a generic term for a computer subsystem. Printers,
serial ports, and disk drives are often referred to as devices; such subsystems
frequently require their own controlling software, called device drivers.
Packets are the basic units of network communication. Figure 3.6 shows
data that is being broken into packets. With data divided into packets,
individual transmissions are speeded up so that every computer on the network
has more opportunities to transmit and receive data. At the target (receiving)
computer, the packets are collected and reassembled in the order of the
Figure 3.6 Breaking data into packets
When the network operating system at the sending computer breaks the
data into packets, it adds special control information to each frame. This
makes it possible to:
Send the original, disassembled data in small chunks.
Reassemble the data in the proper order when it reaches its destination.
Check the data for errors after it has been reassembled.
Packets can contain several types of data including:
Information, such as messages or files.
Certain types of computer control data and commands, such as service requests.
Session control codes, such as error correction, that indicate the need
for a retransmission.
All packets have certain components in common. These include:
Figure 3.7 shows these packet components grouped into three sections: header,
data, and trailer.
A source address that identifies the sending computer.
The data that is intended for transmission.
A destination address that identifies the recipient.
Instructions that tell network components how to pass the data along.
Information that tells the receiving computer how to connect the packet
to other packets in order to reassemble the complete data package.
Error-checking information to ensure that the data arrives intact.
Figure 3.7 Packet components
The header includes:
An alert signal to indicate that the packet is being transmitted.
The source address.
The destination address.
Clock information to synchronize transmission.
This describes the actual data being sent. This part of the packet varies
in size, depending on the network. The data section on most networks varies
from 512 bytes—or 0.5 kilobytes (KB)—to 4 KB.
Because most original data strings are much longer than 4k, data must
be broken into chunks small enough to be put into packets. It takes many
packets to complete the transmission of a large file.
The exact content of the trailer varies depending on the communication
method, or 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 the data has changed
during the transmission. In that case, the CRC routine signals the source
computer to retransmit the data.
Different networks have differing formats for the packets and allow different-sized
packets. The packet-size limits determine how many packets the network
operating system can create from one large piece of data.
A protocol is a set of rules or standards designed to enable computers
to connect with one another and to exchange information with as little
error as possible.
Example: Packets in Printing
The following example illustrates, step-by-step, how packets are used in
A large print job must be sent from a computer to a print server.
In Figure 3.8, the sending computer establishes a connection with the print
Figure 3.8 Establishing a connection with a
In Figure 3.9, the computer next breaks the large print job into packets.
Each packet contains the destination address, the source address, the data,
and control information.
Figure 3.9 Creating packets
In Figure 3.10, the network interface card (NIC) in each computer examines
the receiver's address on all frames sent on its segment of the network.
However, because each NIC has its own address, the card does not interrupt
the computer until it detects a frame addressed specifically to it.
Figure 3.10 Examining the receiver's address
In Figure 3.11, the destination computer is the print server. The packets
enter through the cable into the NIC.
Figure 3.11 Network interface card (NIC) accepts
packets addressed to the print server
The network software processes the frame stored in the NIC's receive buffer.
Sufficient processing power to receive and examine each incoming frame
is built into the NIC. This means that no computer resources are used until
the NIC identifies a frame addressed to itself.
In Figure 3.12, the network operating system in the receiving computer
reassembles the packets back into the original text file and moves the
file into the computer's memory. From there the file is sent to the printer.
Figure 3.12 Reassembled packets sent to the
The following points summarize the main elements of this lesson:
Data on a network is not sent in one continuous stream. It is divided up
into smaller, more manageable packets. These packets, or chunks, of data
make timely interaction and communications on a network possible.
All packets have these basic components:
information with which to reassemble the data package
Packet components are grouped into three sections:
A header that contains clock information
A trailer that contains the error-checking component