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: 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