Network-Wide Capacity Planning

Link capacity planning might be enough in most cases when a network administrator knows about a bottleneck in the network. After a link’s bandwidth is upgraded, the network administrator should identify the next bottleneck—this is a continuous process! In addition, most networks are designed with economical justifications, which means that very little overprovisioning is done. The term “network over-subscription” describes an abundance of bandwidth in the network, so that under normal circumstances, performance limitations are not caused by a lack of link capacity. Put another way, the only restriction that one application sees when communicating with another application is in the network’s inherent physical limitations. In contrast, the term “network overprovisioning” describes a network design with more traffic than bandwidth. This means that, even under normal circumstances, not enough bandwidth is provided for all users to use the network to perform their tasks at the same time, using their maximum allocated bandwidth. The network over-subscription concept is obviously a more cost-effective approach than network overprovisioning, because it assumes that not all users will use their fully dedicated bandwidth at the same time. However, capacity planning is more complex in this case. The computation of what constitutes adequate provisioning, without gross overprovisioning, depends on accurate core capacity planning along with realistic assumptions about what group of users will use what applications and services at key time periods. Another approach is to do networkwide capacity planning by collecting the “core traffic matrix.” The core traffic matrix is a table that provides the traffic volumes between the origin and destination in a network. To collect this for all the network’s entry points, we need usage information (in number of bytes and/ or number of packets per unit of time) per exit point in the core network. Figure 1-9 shows the required bandwidth from Rome to all the other PoPs; Table 1-4 shows the results.

Figure 1-9. Core Traffic Matrix Topology

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