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DCE DTE DTE DCE Figure 3-2 A typical beads-o n-a-string architecture of the early 1970s (same as Figure 2-1).. The Beads-on-a-String Model The early research computer networks were not the first networks. The telephone companies had been building networks for nearly a century, and these were large international networks. Clearly, they had developed their own network architecture suited to their needs.

However, the properties of the architecture reflected not only the circuit-switched technology of the telephone networks but also their economic and political environment. From their founding in the 19th century until after the middle of the 20th century, the telephony networks were electrical (physical circuits). Even in the last half of the 20th century as switches used digital communication for control, this control was alongside the telephony network.

Interfaces were always between devices. Although this architecture has never been given an official name, I have always called it the beads-on-a-string model after the figures common in their specifications: different kinds of boxes strung together by wires. The beads-on-a-string model reflects the unique environment occupied by the telephone companies (see Figure 3-2).

It has been primarily promulgated by the CCITT/ITU and the telephone companies and manufacturers closely associated with them. First, until recently, all telephone switching was circuit switched, and hence there was only one layer, the physical layer. Strictly speaking, these networks consisted of two physically distinct networks: one that carried the traffic and a separate one for controlling the switching.

More recently, these are referred to as the data plane and control plane. (Another indication of the Internet s slide into telephony s beads-on-a-string model.) The communication generated by these two planes is sometimes multiplexed onto a common lower layer rather than physically distinct networks.

This split between the voice and switch control was well established. There was no reason for a layered model. This also leads to a very connection-oriented view of the world.

Second, until recently telephone companies were monopolies that either manufactured their own equipment or bought equipment built to their specifications from a very. THE TWO MAJOR ARCHITECTURE PARADIGMS small number of manufacturer s. Hence, where standards were required, they were to define interfaces between boxes, between a provider and someone else (that is, another telephone company or, if absolutely necessary, a customer). In the preferred solution, all equipment used by the customer is owned by the telephone company.

This was the situation prior to deregulation. Therefore, a major purpose of the model is to define who owns what (that is, define markets). In this environment, it is not surprising that a beads-on-a-string model evolved.

And herein lies one of the first differences between the layered and beads-on-a-string model: the definition of interface. In the layered model, an interface is between two layers internal to a system, within a box. In the beadson-a-string model, an interface is between two boxes.

One can imagine the combination of confusion and indignation with which the telephone companies faced the idea of computer networks at the beginning of the 1970s. On the one hand, this was their turf. What were these computer companies doing infringing on their turf, their market On the other hand, these research networks were being built in a way that could not possibly work, but did, and worked better than their own attempts at computer Why Do We Care networks.

In Europe, the telephone companies, known as This is all interesting history PTTs, were part of the government. They made the rules and that may have been important in your day, but it is hardly relevant implemented them. In the early 1970s, it was a definite possito networking today! bility that they would attempt to require that only PTT comOh if that it were the case.

First, puters could be attached to PTT networks. Everyone saw that it is always good to know how communication between computers was going to be a big we got where we are and why we are there. There is a tenbusiness, although it is unlikely that the PTTs had any idea dency in our field to believe that how big.

(As late as the late 1980s, phone company types everything we currently use is a paragon of engineering, rather were still saying that data traffic would never exceed voice than a snapshot of our undertraffic.) The PTTs saw an opportunity for value-added netstanding at the time. We build works, but they did not like the idea of competition or of great myths of spin about how what we have done is the only companies creating their own networks.

way to do it to the point that our The situation in the United States was very different. universities now teach the flaws AT&T was a monopoly, but it was a private corporation. It to students (and professors and textbook authors) who don t thought it understood competition and saw this as a chance know better.

To a large extent, to enter the computer business. But the European PTTs knew even with deregulation, little has they did not like the layered model because as we will soon changed. see, it relegated them to a commodity market.

The layered To be sure, the technology is different, the nomenclature has model had two problems for PTTs. Because most of the new changed, the arguments are (they called them value-added) services were embodied in more subtle; but under it all, it is applications and applications are always in hosts, which are still the same tension. Who gets to sell what Who controls the not part of the network, there is no distinction between hosts account How does this affect owned by one organization and hosts owned by another.

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