Scalability is the measure of a system’s ability to increase or decrease in performance in response to changes in demands. For networks, this means change in the number of users, number of end nodes, the amount of bandwidth, or the demand on any service provided by the network. Networks need to be easily scalable to respond to business demands. Although scalability is often considered in response to growth, networks should also be able scale down gracefully.
Scalability is one of the most important design principles in networks. Scalability is affected by the choice of topology, protocols, or components in the network. A hub-and-spoke topology, for example, is easier to scale than a full-mesh topology. Also some routing protocols can accommodate thousands of routers while others cannot. Scalability is often one of the reasons to employ the modularity principle in network design.
Scalability is accomplished using two approaches:
Scaling out/in (horizontally): Scaling horizontally means increasing (or decreasing) the resources of a network by adding more components of the same functionality. Examples include adding switches and wireless access points to connect more users, or adding another gateway to the Internet to increase throughput.
Scaling up/down (vertically): Scaling vertically means adding resources to (or removing resources from) components in the network. This typically means upgrading devices (or links) to process more packets and offer more throughout. Examples include increasing the bandwidth of a WAN link between two buildings to carry more traffic or upgrading a firewall to handle more VPN sessions.
In many situations scaling out is easier to manage due to the quicker response to demands and reduced outage time (this many not be true in the case on WAN links, though). However, adding more components to the network increases the complexity of network configuration and management.
Design effort does not scale linearly with the size of the network. Some aspects of design my be omitted in small networks but they are more significant in larger networks. In designing a small office network, for example, power, heat, and cabling may not be of major concern. In a network for a large airport, on the hand, these three factors consume significant portion of the design effort.
Read about other Network Design Principles.