What Is a Large-Scale Data Communications Network?


OSPF and IS-IS are uniquely appropriate routing protocols for large-scale data communication networks. But how exactly do we characterize such a network? It might be useful to approach a definition by remembering that data communication networks and ideas about them have been around since long before the advent of computers.

For example, when Alexander Graham Bell and Theodore Vail founded American Telephone and Telegraph Company (AT&T) in 1885, they had no intention of providing telegraph services. But they understood that the network they were building could be used for transmitting more than just telephony signals. They had no clear vision of what those other signals might be, other than knowing that they would be, like telephony signals, representations of information. "Telegraph" reflected Bell's and Vail's anticipation of data communication networks within the understanding of their time.

Wide-area data communication has been around for as long as man has had a need to share information over a range greater than voice can cover. Many ancient civilizations used signal fires to communicate quickly over a long distance. In feudal Japan, villages sent paper lanterns aloft in the evenings, rising on hot air created by the same fire that illuminated them, to notify nearby villages of their safety. Throughout the southwestern United States you can find petroglyphscarvings of figures and symbols on the sides of rockscreated by hunting, warring, or traveling parties of Native Americans over the centuries. Although some might have been intended as merely decorative, many petroglyphs are thought to be signals and messages left by one party for other parties expected to pass that way. These carvings put an interesting twist on data communications: The signal remains stationary while the transmitting and receiving nodes move around.

Telegraph networks were the first data communications networks using electrical, digital signals. And although they were certainly wide-areaconnecting countries and even spanning continents via transoceanic cablesthey did not entail the complexity that we assume today when we talk about large-scale networks. Signals were easily originated, routed, and received by human operators, and the network was maintained through human monitoring and intervention.

How, then, do we define a large-scale network? Although the number of nodes and links certainly influences the definition, a definition based solely on numbers is too narrow. Instead, the scale of a network is defined by its complexity. A small-scale network is one that can be easily managed by direct human intervention; hence early telegraph networks could be considered small-scale even though they covered a very large geographic area. In terms of IP networks, a small-scale network is one that can be routed statically and requires no automated management systems. Notice I say can be: A given small-scale network might in fact run a routing protocol or automated management software; just because it can be routed statically does not mean it must be.

As a network grows in complexity, automation becomes more necessary. A mid-sized network is one in which static routing and management by direct human monitoring and intervention are no longer practical. In IP networks, a routing protocol such as RIP is required to maintain forwarding over many paths.

As the network continues to grow, however, the capabilities of automated systems themselves become an issue. As Chapter 2 explains, simple routing protocols such as RIP present problems in complex networks with many routing variables. A large-scale network, then, is one in which the automated network systems must be able to manage the network as a single entity rather than managing individual connections between nodes. Factors to consider in a large-scale network include the following:

  • Complex interactions between individual nodes

  • Complex path diversity requiring load balancing, traffic monitoring and distribution, and strong loop avoidance

  • Complex link metrics

  • Diverse data transport requirements

  • Stringent requirements for security and reliability

OSPF and IS-IS can easily be run in a network of any size. But their true value is in their capability to perform consistently as their network domain grows large. No other IGP for IP networks can reliably route the world's largest networks.




OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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