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[description], Story, any other text you want to use. Cisco CCNA Certification: Introduction to ISDN, Part I by: Chris Bryant, CCIE #12933
From the CCNA to the CCIE, ISDN is one of the most important technolgies you'll work with. It's also very common in the field; ISDN is frequently used as a backup connection in case an organization's Frame Relay connections go down. Therefore, it's important to know ISDN basics not only for your particular exam, but for job success. ISDN is used between two Cisco routers that have BRI or PRI interfaces. Basically, with ISDN one of the routers places a phone call to the other router. It is vital to understand not only what causes one router to dial another, but what makes the link go down. Why? Since ISDN is basically a phone call from one router to another, you're getting billed for that phone call -- by the minute. If one of your routers dials another, and never hangs up, the connection can theoretically last for days or weeks. The network manager then receives an astronomical phone bill, which leads to bad things for everyone involved! Cisco routers use the concept of interesting traffic to decide when one router should call another. By default, there is no interesting traffic, so if you don't define any, the routers will never call each other. Interesting traffic is defined with the dialer-list command. This command offers many options, so you can tie interesting traffic down not only to what protocols can bring the link up, but what the source, destination, or even port number must be for the line to come up. One common misconception occurs once that link is up. Interesting traffic is required to bring the link up, but by default, any traffic can then cross the ISDN link. What makes the link come down? Again, the concept of interesting traffic is used. Cisco routers have an idle-timeout setting for their dialup interfaces. If interesting traffic does not cross the link for the amount of time specified by the idle-timeout, the link comes down. To summarize: Interesting traffic brings the link up; by default, any traffic can cross the link once it's up; a lack of interesting traffic is what brings the link down. In tomorrow's article, we'll take a look at some common scenarios that make the ISDN link stay up, and what can be done about it. Keep studying, and I'll see you tomorrow! Chris Bryant
ham radio
Many world-class athletes, from all sports, have the ability to get in the right state of mind and when looking for ham radio the state of mind is most important. These athletes visualize breaking records, crossing the finishing line first, having the gold medal draped around their necks, hearing the roar of the crowd. You should have the same attitude in looking for ham radio and we make it easy for you. Such ability is not limited to sports superstars and is easy for anyone to do. In fact, all other great achievers in life, have a crystal-clear vision of the end result of their labors. They make a commitment to get there, they are overwhelmingly positive about their efforts and they know where they are going. This is exactly what you should be doing when looking for ham radio.
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