Page 91 - Electronic Commerce

Chapter 2

switches. These switches work like the switches you use to turn lights on and off in your
home, except that they open and close much faster, and are controlled by mechanical or
66 electronic devices instead of human hands.
The combination of telephone lines and the closed switches that connect them to
each other is called a circuit. This circuit forms a single electrical path between caller and
receiver. This single path of connected circuits switched into each other is maintained for
the entire length of the call. This type of centrally controlled, single-connection model is
known as circuit switching.
Although circuit switching works well for telephone calls, it does not work as well for
sending data across a large WAN or an interconnected network like the Internet. The
Internet was designed to be resistant to failure. In a circuit-switched network, a failure in
any one of the connected circuits causes the connection to be interrupted and data to be
lost. Instead, the Internet uses packet switching to move data between two points. In a
packet-switched network, files and e-mail messages are broken down into small pieces,
called packets, that are labeled electronically with their origins, sequences, and
destination addresses. Packets travel from computer to computer along the
interconnected networks until they reach their destinations. Each packet can take a
different path through the interconnected networks, and the packets may arrive out of
order. The destination computer collects the packets and reassembles the original file or
e-mail message from the pieces in each packet.

Routing Packets
As an individual packet travels from one network to another, the computers through
which the packet travels determine the most efficient route for getting the packet to its
destination. The most efficient route changes from second to second, depending on how
much traffic each computer on the Internet is handling at each moment. The computers
that decide how best to forward each packet are called routing computers, router
computers, routers, gateway computers (because they act as the gateway from a LAN or
WAN to the Internet), border routers,or edge routers (because they are located at the
border between the organization and the Internet or at the edge of the organization). The
programs on router computers that determine the best path on which to send each packet
contain rules called routing algorithms. The programs apply their routing algorithms to
information they have stored in routing tables or configuration tables. This information
includes lists of connections that lead to particular groups of other routers, rules that
specify which connections to use first, and rules for handling instances of heavy packet
traffic and network congestion.
Individual LANs and WANs can use a variety of different rules and standards for
creating packets within their networks. The network devices that move packets from one
part of a network to another are called hubs, switches, and bridges. Routers are used to
connect networks to other networks. You can take a data communications and networking
class to learn more about these network devices and how they work.
When packets leave a network to travel on the Internet, they must be translated into
a standard format. Routers usually perform this translation function. As you can see,
routers are an important part of the infrastructure of the Internet. When a company or

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