WAN Concepts

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Washington Elementary School District
WAN Design Concepts

In Semester 4, chapter 3 we learned that WAN communication is characterized by relatively low throughput, high delay, and high error rates. We also learned that WAN infrastructure is often rented from a service provider, and therefore the WAN design should optimize WAN bandwidth in order to minimize cost and maximize the value to end-users, while at the same time allowing for growth in WAN bandwidth requirements. By basing the design on a hierarchical layered model, these goals can be accomplished.

The hierarchical model includes three layers:

Core Layer – Concerned with optimal transport between sites, the core layer consists of fast wide-area connections between geographically remote sites. These connections are generally point-to-point, and should not include any hosts, as the goal of the core layer is to move packets quickly and reliably across the WAN without any packet manipulation or filtering.
Distribution Layer – Concerned with policy-based connectivity, the distribution layer includes the network backbone, and is implemented to interconnect buildings at a single site. This is also where access control lists are implemented. While enterprise servers may be placed at the distribution layer, in general hosts will attach to the network at the access layer.
Access Layer – Concerned with providing workgroup and user access to the network, the access layer is where hosts attach to the network. The access layer allows logical grouping of users into workgroups, and is characterized by shared or switched bandwidth, MAC-layer filtering, and microsegmentation.

Although not all networks will require all three layers, a good network design should plan for future expansion and allow the transition to a three-layer model. This means ensuring that data traffic only travels up the hierarchy as far as it needs to in order to find the destination host, preserving expensive WAN bandwidth for only the amount of traffic which cannot be routed at a lower layer.

Although dedicated links are often used in WAN implementations, they are sometimes impractical due to distance, or can be replaced by an alternative when the end network does not require an "always-on" connection. Frame Relay can be a good alternative to a dedicated link over long distances because distance is not a factor in its pricing. It can also be cheaper in some cases, because price is based on the Committed Information Rate, which guarantees a minimum bandwidth when you need it rather than constantly. ISDN is a good alternative to leased-line when the network is small and has relatively low bandwidth requirements, such as a home office or a remote user.

Application of Concepts to the WAN Design

The Washington Elementary School District WAN consists of three core routers, located at Phoenix N.W., Sunnyslope, and Greenway. Each router is connected to each of the other two routers by four dedicated T1 connections. Each T1 can provide up to 1.544Mbps in bandwidth, which is far less than the 100Mbps provided by the LAN wiring at each school. The four T1 links provide redundancy in case a link goes down. The WAN design also provide for future growth, as each core router can be upgraded to use T3 links in the future by replacing network modules.

The District WAN is based on a two-layer hierarchy consisting of a Core and an Access Layer. Traffic between networks at the same school goes no higher in the hierarchy than the access-layer router at the school. Only traffic destined for another school or for the Internet will enter the Core layer, where it will be routed to its destination and placed back on the Access layer before being filtered through the school's ACL, if destined for another school, or having its network address translated by the NAT router if destined for the Internet. The network design allows for future growth into a three-layer hierarchy, with the routers currently at the Access layer becoming Distribution layer routers, and additional routers placed lower in the hierarchy becoming the new Access layer. This might occur, for example, if each individual building in a campus were subnetted to become its own separate network, or if the district decided to subdivide the administrative network, or if a school outgrew the IP address space currently assigned to it.

At two points in the network, alternatives to dedicated links have been implemented. The connection between the Public Access router and the service provider will be over a Frame Relay connection, which provides cost-savings by providing the same speed as a T1 at a lower cost for a connection that will likely be utilized relatively infrequently. The connection between the Sunnyslope C.O. and the Community School will be over an ISDN connection, as the Community School has few students and no computer lab, and therefore does not need the speed of a T1 or "always-on" connectivity.