8.3 The Backbone Network

---

The corporate network segment known as the “backbone” is one of several components of the entire corporate network. Other components include primary, secondary, and tertiary feeder links. The backbone segment of the network is transparent to individual users, who are most often connected to another network segment by analog modem tail circuits, STDM digital tail circuits, or even wireless links. In fact, most corporate networks consist of smaller regional or subnets tied together via the backbone. The backbone provides high-throughput and traffic concentration capabilities. However, the backbone is important for another reason: It defines the total corporate network architecture and its level of flexibility, efficiency, and control. In essence, the backbone can support existing business requirements and create business opportunities by providing corporations with the means to control their own destinies. In the process, these backbones can enable companies to secure competitive advantages over marketplace rivals.

8.3.1 Private Networking

Carriers offer services that coincide with their business objectives, which frequently are not the same as their corporate customers’ objectives. Traditionally, carriers have segmented traffic according to switched or dedicated, voice or data, and within the category of data, have subdivided this further into analog or digital. This carrier segmentation required users to segment their network traffic into a series of subnets that relied on different lines and services. While the carriers did this to make their infrastructures easier to manage, it also inflated the costs to companies that relied on their services. As a result, in the late 1980s, companies started to implement their own private networks using T-carrier facilities interconnected with multiplexers and DCSs. By combining voice and data over the same network, they realized substantial cost savings.

Another reason companies turned to private networks was to obtain control over communication resources. Following the divestiture of AT&T in 1984, the restructured Bell companies were still local monopolies and were slow to respond to the changing needs of corporate customers, whose reliance on communications, particularly for data, were growing and becoming more advanced. When combining previously segmented traffic from various applications (voice, data, image, video), there was a critical need for a global view of all resources and an immediate response to problems. This meant that companies had to hire technical experts in voice and data, invest in diagnostic tools and management systems, and stock spare parts.

In response, companies set up network operation centers (NOCs) to provide 24/7 active monitoring, diagnostics, and remote control. The center continually tests all customer lines and troubleshoots any problems before they become severe enough to disrupt service. When a service error occurs, technicians immediately reroute network traffic and dispatch local technicians to ensure minimal or no network downtime.

From an administrative perspective, the NOC provides an important vehicle for accommodating change. Equipment and subnets are continually added and deleted, while transmission parameters are continually changed to meet application requirements. Likewise, lines and services are continually added and deleted. All of this activity is not necessarily driven by internal demand; it may be fueled by the seemingly relentless cycle of corporate mergers, acquisitions, and spin-offs. In addition, as carriers compete against each other, new services and more attractive pricing becomes available, forcing companies to continually reassess their networks.

8.3.2 Public Networking

With all the advantages of private networking, why consider public networking at all? Some advantages to using public networks include connectivity, access to services, and economics.

Public networks conform to standards, which leads to a high degree of connectivity among the devices of different manufacturers. Equipment that conforms to public network standards takes advantage of the connectivity potential of the public network, making systems integration easier for economical expansion of the private network. As new technology and services are introduced, companies are better positioned to integrate them into the existing network because their specifications have been written to comply with public network standards. While no longer a problem in TDM-based equipment, some vendors still promote proprietary “boxes” that do not adhere to public network standards. For example, there are wavelength division multiplexers (WDMs) for enterprise networks that increase the bandwidth capacity of fiber but are not compatible with the WDMs used in carrier networks. Even if external conversion devices are available to provide connectivity to the public network, they introduce potential points of failure and weaken the possibility of integrated network management.

The use of private and public networks each have advantages, but the decision to go with one over the other is not always clear. Companies need the efficiency and control of private networking along with the connectivity and access to services that is associated with public networking. Therefore, companies should position themselves to implement the appropriate network architecture (public or private) to realize the best return on investment at any given time.

---