often involves upgrading, extending, or modifying
an existing WAN. Much of the data needed can come from existing
network management statistics. Knowing the various end points
allows the selection of a topology or layout for the WAN. The
topology will be influenced by geographic considerations but
also by requirements such as availability. A high requirement
for availability will require extra links that provide
alternative data paths for redundancy and load balancing. With
the end points and the links chosen, the necessary bandwidth
can be estimated. Traffic on the links may have varying
requirements for latency and jitter. With the bandwidth
availability already determined, suitable link technologies
must be selected. Finally, installation and operational costs
for the WAN can be determined and compared with the business
need driving the WAN provision. In practice, following the
steps shown in Figure is seldom a linear process. Several
modifications may be necessary before a design is finalized.
Continued monitoring and re-evaluation are also required after
installation of the WAN to maintain optimal performance.
Interactive Media Activity Drag and Drop: WAN Design Steps
When the student has completed this activity, the student will
be able to identify the basic steps in designing a WAN.
Content 2.3 WAN Design 2.3.3 How to
identify and select networking capabilities Designing a WAN
essentially consists of the following: - Selecting an
interconnection pattern or layout for the links between the
various locations
- Selecting the technologies for those
links to meet the enterprise requirements at an acceptable
cost
Many WANs use a star topology. As the
enterprise grows and new branches are added, the branches are
connected back to the head office, producing a traditional star
topology. Star end-points are sometimes cross-connected,
creating a mesh or partial mesh topology. This provides for
many possible combinations for interconnections. When
designing, re-evaluating, or modifying a WAN, a topology that
meets the design requirements must be selected. In selecting a
layout, there are several factors to consider. More links will
increase the cost of the network services, and having multiple
paths between destinations increases reliability. Adding more
network devices to the data path will increase latency and
decrease reliability. Generally, each packet must be completely
received at one node before it can be passed to the next. A
range of dedicated technologies with different features is
available for the data links. Technologies that require the
establishment of a connection before data can be transmitted,
such as basic telephone, ISDN, or X.25, are not suitable for
WANs that require rapid response time or low latency. Once
established, ISDN and other dialup services are low latency,
low jitter circuits. ISDN is often the application of choice
for connecting a small office or home office (SOHO) network to
the enterprise network, providing reliable connectivity and
adaptable bandwidth. Unlike cable and DSL, ISDN is an option
wherever modern telephone service is available. ISDN is also
useful as a backup link for primary connections and for
providing bandwidth-on-demand connections in parallel with a
primary connection. A feature of these technologies is that the
enterprise is only charged a fee when the circuit is in use.
The different parts of the enterprise may be directly connected
with leased lines, or they may be connected with an access link
to the nearest point-of-presence (POP) of a shared network.
X.25, Frame Relay, and ATM are examples of shared networks.
Leased lines will generally be much longer and therefore more
expensive than access links, but are available at virtually any
bandwidth. They provide very low latency and jitter. ATM, Frame
Relay, and X.25 networks carry traffic from several customers
over the same internal links. The enterprise has no control
over the number of links or hops that data must traverse in the
shared network. It cannot control the time data must wait at
each node before moving to the next link. This uncertainty in
latency and jitter makes these technologies unsuitable for some
types of network traffic. However, the disadvantages of a
shared network may often be outweighed by the reduced cost.
Because several customers are sharing the link, the cost to
each will generally be less than the cost of a direct link of
the same capacity. Although ATM is a shared network, it has
been designed to produce minimal latency and jitter through the
use of high-speed internal links sending easily manageable
units of data, called cells. ATM cells have a fixed length of
53 bytes, 48 for data and 5 for the header. ATM is widely used
for carrying delay-sensitive traffic. Frame Relay may also be
used for delay-sensitive traffic, often using QoS mechanisms to
give priority to the more sensitive data. A typical WAN uses a
combination of technologies that are usually chosen based on
traffic type and volume. ISDN, DSL, Frame Relay, or leased
lines are used to connect individual branches into an area.
Frame Relay, ATM, or leased lines are used to connect external
areas back to the backbone. ATM or leased lines form the WAN
backbone.
Content 2.3 WAN Design
2.3.4 Three-layer design model A systematic approach is
needed when many locations must be joined. A hierarchical
solution with three layers offers many advantages. Imagine an
enterprise that is operational in every country of the European
Union and has a branch in every town with a population over
10,000. Each branch has a LAN, and it has been decided to
interconnect the branches. A mesh network is clearly not
feasible because nearly 500,000 links would be needed for the
900 centers. A simple star will be very difficult to implement
because it needs a router with 900 interfaces at the hub or a
single interface that carries 900 virtual circuits to a
packet-switched network. Instead, consider a hierarchical
design model. A group of LANs in an area are interconnected,
several areas are interconnected to form a region, and the
various regions are interconnected to form the core of the WAN.
The area could be based on the number of locations to be
connected with an upper limit of between 30 and 50. The area
would have a star topology, with the hubs of the stars linked
to form the region. Regions could be geographic, connecting
between three and ten areas, and the hub of each region could
be linked point-to-point. This three-layer model follows the
hierarchical design used in telephone systems. The links
connecting the various sites in an area that provide access to
the enterprise network are called the access links or access
layer of the WAN. Traffic between areas is distributed by the
distribution links, and is moved onto the core links for
transfer to other regions, when necessary. This hierarchy is
often useful when the network traffic mirrors the enterprise
branch structure and is divided into regions, areas, and
branches. It is also useful when there is a central service to
which all branches must have access, but traffic levels are
insufficient to justify direct connection of a branch to the
service. The LAN at the center of the area may have servers
providing area-based as well as local service. Depending on the
traffic volumes and types, the access connections may be dial
up, leased, or Frame Relay. Frame Relay facilitates some
meshing for redundancy without requiring additional physical
connections. Distribution links could be Frame Relay or ATM,
and the network core could be ATM or leased line.
Content
2.3 WAN Design 2.3.5 Other layered design
models Many networks do not require the complexity of a full
three-layer hierarchy. Simpler hierarchies may be used. An
enterprise with several relatively small branches that require
minimal inter-branch traffic may choose a one-layer design.
Historically this has not been popular because of the length of
the leased lines. Frame Relay, where charges are not distance
related, is now making this a feasible design solution. If