1. Single VBR video source traffic is considered as LRD traffic. What are the characteristics of multiplexing traffic of multiple sources, (e.g., does the output traffic of an ATM multiplexer have the property of LRD?).

Yes - multiplexed LRD is likewise LRD.

2. Since Internet traffic (i.e., LAN/WAN) is partly considered as LRD traffic, what impact does it have on design of Bridge/Router?

You need a whole lot of buffer to absorb bursts.

3. Where can I find Video traffic database (e.g., film compressed by Mpeg-2)?

I know these exist, but unfortunately can't point you at any.

4. Are there any mail groups discussing LRD and/or ATM?

I don't know of any for LRD. ATM is discussed in the newsgroup comp.dcom.cell-relay. To get more detailed answers, you should dig through the past five years of SIGCOMM proceedings, and also the past three or so years of IEEE/ACM Transactions on Networking, both of which have had a number of LRD-related papers. Vern