there is a need for some geographical
concentration, a two-level design is appropriate. This produces
a “star of stars” pattern. Again, the pattern chosen based on
leased line technology will be considerably different from the
pattern based on Frame Relay technology. When planning simpler
networks, the three-layer model should still be considered as
it may provide for better network scalability. The hub at the
center of a two-layer model is also a core, but with no other
core routers connected to it. Likewise, in a single-layer
solution the area hub serves as the regional hub and the core
hub. This allows easy and rapid future growth as the basic
design can be replicated to add new service areas.
Content 2.3 WAN Design 2.3.6 Other WAN
design considerations Many enterprise WANs will have
connections to the Internet. This poses security problems but
also provides an alternative for inter-branch traffic.Part of
the traffic that must be considered during design is going to
or coming from the Internet. Since the Internet probably exists
everywhere that the enterprise has LANs, there are two
principal ways that this traffic can be carried. Each LAN can
have a connection to its local ISP, or there can be a single
connection from one of the core routers to an ISP. The
advantage of the first method is that traffic is carried on the
Internet rather than on the enterprise network, possibly
leading to smaller WAN links. The disadvantage of permitting
multiple links, is that the whole enterprise WAN is open to
Internet-based attacks. It is also difficult to monitor and
secure the many connection points. A single connection point is
more easily monitored and secured, even though the enterprise
WAN will be carrying some traffic that would otherwise have
been carried on the Internet.If each LAN in the enterprise has
a separate Internet connection, a further possibility is opened
for the enterprise WAN. Where traffic volumes are relatively
small, the Internet can be used as the enterprise WAN with all
inter-branch traffic traversing the Internet. Securing the
various LANs will be an issue, but the saving in WAN
connections may pay for the security.Servers should be placed
closest to the locations that will access them most often.
Replication of servers, with arrangement for off-peak
inter-server updates, will reduce the required link capacity.
Location of Internet-accessible services will depend on the
nature of the service, anticipated traffic, and security
issues. This is a specialized design topic beyond the scope of
this curriculum.
Content Summary An understanding
of the following key points should have been achieved:
- Differences in the geographic areas served between WANs
and LANs
- Similarities in the OSI model layers
involved between WANs and LANs
- Familiarity with WAN
terminology describing equipment, such as CPE, CO, local loop,
DTE, DCE, CSU/DSU, and TA
- Familiarity with WAN
terminology describing services and standards, such as ISDN,
Frame Relay, ATM, T1, HDLC, PPP, POST, BRI, PRI, X.25, and
DSL
- Differences between packet-switched and
circuit-switched networks
- Differences and
similarities between current WAN technologies, including analog
dialup, ISDN, leased line, X.25, Frame Relay, and ATM
services
- Advantages and drawbacks of DSL and cable
modem services
- Ownership and cost associated with WAN
data links
- Capacity requirements and transit times
for various WAN traffic types, such as voice, data, and
video
- Familiarity with WAN topologies, such as
point-to-point, star, and meshed
- Elements of WAN
design, including upgrading, extending, modifying an existing
WAN, and recommending a WAN service to an organization based on
its needs
- Advantages offered with a three-layer
hierarchical WAN design
- Alternatives for interbranch
WAN traffic