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Re: Understanding WANS [1:11755] posted 01/23/2003
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>Thanks, that makes it pretty clear.
>One thing though, and this is just for my information, being that it is
>above the scope of the CCNA,
>If Circut Switched is a Train system and Packet Switched is a Highway with
>Cars, then he data would be people.
>Can the people (data) take the train cross country ( ISDN ) then hop into a
>car ( Frame Relay ) and go to their office.  Or vise versa.   Or would this
>not ever happen.

Yes, they can, but a router is required.  This would occur if your remote
user had an ISDN connection to one location, and a router at that location
had a Frame Relay connection to the final destination.  Here's what happens:

UserLocationA ---------(ISDN)------------- HubOffice ---------(Frame
Relay)--------- MainOffice

1.  A device at Location A determines that an IP packet is destined for the
MainOffice and initiates a call to the HubOffice in order to forward the

2. HubOffice router accepts the incoming ISDN call, and then subsequently
sees the arrival of a PPP frame.

3. HubOffice router de-encapsulates the PPP frame to see an IP packet

4.  It determines that the destination is the MainOffice, and since that
link is Frame Relay the router encapsulates the IP packet in a FR Frame and
sends it along to the MainOffice router

5. MainOffice router sees an incoming Frame Relay frame.  It de-encapsulates
it to find an IP packet.  After determining the actual destination it
forwards the packet appropriately.  Let's say that the destination device is
on an Ethernet LAN attached to MainOffice router.

6.  MainOffice router will send an ARP request for the destination in order
to determine the MAC address of the recipient.

7.  When recipient responds to the ARP request with an ARP reply containing
its MAC address, the MainOffice router encapsulates the IP packet in an
Ethernet frame and sends it out to the destination device.


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