WANs generally help connecting numerous smaller
networks, including LANs and MANs. WAN is
expected to be stretched because of the
requirement to cover multiple cities, even countries
and continents. Some WANs are immensely
widespread across the globe, but the majority does
not supply accurate worldwide exposure in terms of
coverage and connection. In implementing a WAN,
the following guiding principles are employed:
a) Deploy consistent WAN services.
b) Consider WAN services during branch application
design.
c) Consider the WAN as an end-to-end application
delivery service.
d) Deploy embedded WAN services first, then
special appliances.
e) Appliances last resort: If a WAN service is not
available as an embedded service within branch
and/or aggregated router only deploy the appliance.
f) Broadband First: For new branch offices, seek
broadband connections first before alternative WAN
transport services.
source: http://tutorialslodge.com/principles-wide-area-network-implementation/