ISDN - Thought Experiment

Unanswered Question
Jun 17th, 2007

Whilst studying for the CCNA I have come up with some scenarios with ISDN that I cannot answer, I suspect they are beyond the scope of CCNA but I am still interested in knowing the answer.

Q1 - If I have three sites A, B and C. Each site has an ISDN-2 connection. Can Site A call sites B and C simultaniously, using one B channel for each link? If so can it work inverse with sites B and C initiating the calls to site A?

Q2 - Sites A and B normally communicate via a leased line but have an ISND-2 each counfigured for DDR. There is continuous traffic in both directions on the link. If the link fails A tried to call B and B tries to call A due to "interesting traffic". What happens, are the lines engaged, do the calls complete which site gets the bill for the call?

I have this problem too.
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glengregory Sun, 06/17/2007 - 20:34


A1] An ISDN line consists of 2B channels each of 64kbps. Each channel can be used as a 'separate' line for a connection. So, site A can dial B & C simultaneously to establish 2 different connections. Just think of ISDN as 1 pipe with 2 lines (like your normal Telephone lines).

A2] I wanted to try this out, but never had the chance to. I think that the calls will not clash as only 1 will get through. There will definetely be a basic config to avoid this as this is a commonsense problem and must have been already taken care of. Maybe you can set different timers on each router. The site that calls, gets billed just like a tel cal.



mark.j.hodge Mon, 06/18/2007 - 08:26

Re Q1, I can (just about) get my head around A dialing B and C simultaniously using different channels. Channel One on BRI 0/0:1 and channel two on BRI 0/0:2. But when B calls A how does A know which channel to accept the call on?

Re Q2, I also assume there will be a solution, I would just like to know what it is. just out of accademic interest.

glengregory Mon, 06/18/2007 - 19:27

When B calls A, it probably accepts the connection on the first available channel. If it is not free, A might accept it on the next channel.

Regarding Q2, I am not working right now so I cannot test that scenario. But, I bet it will be mentioned in Cisco docs online. I'll try to search for it.



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