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This chapter is from the book

Label Switching's Legacy: X.25 and Virtual Circuits

For a change of pace here, take a look at a bit of history. The label switching concept began with X.25. In the late 1960s and early 1970s, many data communications networks were created by companies, government agencies, and other organizations. The design and programming of these networks were performed by each organization to fulfill specific business needs. During this time, an organization had no reason to adhere to any common convention for its data communications protocols, since the organization's private network provided services only to itself. Consequently, these networks used specialized protocols that were tailored to satisfy the organization's requirements.

During this period, several companies and telephone administrations in the United States, Canada, and Europe implemented a number of public data networks based on packet switching concepts. These systems were conceived to provide a service for data traffic that paralleled the telephone system's service for voice traffic.

But they did not nail up bandwidth as the telephone system did. Indeed, X.25 represented a major change in viewing service to a user: use a best-effort approach but allow the user to request certain levels of service.

The public network vendors were faced with answering a major question: How can the network best provide the interface for a user's terminal or computer to the network? The potential magnitude of the problem was formidable because each terminal or computer vendor had developed its own set of data communications protocols. Indeed, some companies, such as IBM, had developed scores of different protocols within their own product lines.

X.25 came about largely because the originators of these nascent networks recognized that a common network interface protocol was needed, especially from the perspective of the network service providers.

In 1974, the (former) CCITT issued the first draft of X.25 (the "Gray Book"). It was revised in 1976, 1978, 1980, and again in 1984 with the publication of the "Red Book" recommendation. Until 1988, X.25 was revised and republished every four years. In 1988, the ITU-T announced its intention to publish changes to its recommendations (including X.25) as publishing was warranted, rather than in the four-year cycle previously utilized.

The Logical Channel Number: Precursor to the Label

X.25 identifies each packet in the network with a logical channel number (LCN). The LCN is used to distinguish the different users' traffic that is operating on the same physical link. This idea is to mask from the user the fact that the link is being shared by other users, thus the term virtual circuit (you think you have the full bandwidth of the link, but you don't). A virtual circuit and its label, the logical channel number, are quite similar to the modern label switching network. But I will point out differences as we move into the details of label switching networks.

Frame Relay and ATM: A Rose by Any Other Name Is Still a Rose

The successors to X.25, Frame Relay and ATM, also use the virtual circuit concept. For Frame Relay, the virtual circuit IDs are called data link connection IDs (DLCIs); for ATM, they are called virtual path IDs/virtual channel IDs (VPIs/VCIs). Regardless of their names, they are (a) virtual circuit IDs and (b) label values.

MPLS networks must interwork with these networks, since they are quite prevalent as the principal bearer of services for wide area internets. Fortunately, the MPLS labels correlate rather easily with the ATM and Frame Relay labels, and later chapters explore this subject in considerable detail.

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