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Integrated Services Digital Network ( Isdn )

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Before we get into the discussion, let’s briefly introduce some of the technologies used in WANs and the different characteristics of each one:

Point-to-point links
A so called point-to-point link (Fig. 2.0) provides a pre-established WAN communication path from the user’s location trough a telephone company’s network, also known as the carrier. For this kind of line, the carrier dedicates pairs of wire and hardware to that line only. This kind of circuits are priced depending on bandwidth needed and distance to be covered.

(Figure 2.0 – Point-to-point link)

Circuit Switching
Work like a normal telephone line. Data connections can be started when needed and terminated when the communication is over. An example of circuit switching is called Integrated Services Digital Network (ISDN). When a router receives data that’s intended to go to a specific remote site, the switched circuit (Fig. 2.1) is initiated with a specific circuit number for that particular network. As soon the two networks that are trying to connect get the authentication done, it is possible to transfer data. When data transmission is over, the communication gets terminated.

(Figure 2.1 – Switched Circuit)

Packet Switching
Here users share common carrier resources (Fig. 2.2). This is a more cost effective solution compared for example to point-to-point lines. The carrier can make more efficient use of its infrastructure. Connections into the carrier’s network are shared between many customers and

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