Making WAN Connections
A company's first WAN is usually driven by a particular application, such as an
accounting system. Then once the WAN is operational, the company begins to use the
WAN for other applications.
If you fail to take into account all the uses that the company might have for
the WAN, you could find that you've invested a lot of money in a solution that
doesn't meet all of your needs. Here are some questions to help you determine the
requirements for your company's WAN:
What are the locations that will participate in the WAN and what kind of WAN
services are available to them? A sales office in Tahiti, for instance, is unlikely
to be able to purchase the latest xDSL line.
How much data needs to be transferred from each site to each other site, and in
what time frame?
How quickly does the data need to be transferred?
Does the data transfer need to be synchronous or can it be asynchronous? For
example, a warehouse clerk who is entering records directly into an accounting
system located at another site requires a synchronous (real-time) connection,
while a restaurant that needs to upload sales data to its headquarters at some
time each night needs only an asynchronous connection.
When do the data transfers need to be accomplished? Do they need to occur 24
hours a day, 7 days a week? Or do they need to occur once every 30 minutes, or
follow some other schedule?
What are the budget constraints, and what are the costs of the different
Once you have the answers to these questions, you can determine whether you
need a switched or dedicated link, and if it should be public or private. These issues are
discussed in the following sections.
Switched or Dedicated?
A switched WAN link is one that is not active all the time. For instance, a dial-up
modem connection or an ISDN connection from one location to another is a switched
connection. These are connections that are formed only when you need them, and you
usually pay for the time the connection is open, rather than the amount of data you're
able to transmit over the connection. Figure 7-1 is an example of a switched WAN link.
Switched links can be either connection-based or packet-based. A connection-based
switched link forms a connection as needed and makes a fixed amount of bandwidth
available over that link. A packet-based switched link sends data packets into a network
cloud in which they can follow a number of paths to their destination, and then emerge
from the cloud. Packet-switched networks can be more reliable because the data can
take many different paths, but you are not guaranteed that each packet will arrive in