Hi Mohd,
These kinds of questions are tough because i assume there is more information needed. Legacy ATMs, those with SDLC interfaces, generally transfer very small amounts of data, so don't require much processing power. I suspect that the smallest router that could accomodate the 20 serial interfaces would have plenty of power to drive the ATM traffic. Perhaps a 2600 or 2800 would be appropriate. The problem is that you say that these are for regional offices, so I suspect there may be many other things these routers would be expected to do in addition to driving the SDLC lines only for the ATMs. In the same way, I could guess that the appropriate size for the data center would be a pair of 3745 or 3800 routers (always wanting to design redundancy here), but again, there may be other functions that it would make sense to run here.
Considerations for the overall design includes quite a few issues, so it would be best to look through the DLSw+ design guide before proceeding.
http://cisco.com/en/US/tech/tk331/tk336/technologies_design_guide09186a008007ce40.shtml
You may also want to consider deploying the SNASw feature at the regional level with EE links to the data center. This could eliminate the need for data center DLSw+ routers, provide better QoS, higher availability, and direct SNA routing to the correct application host.
http://cisco.com/en/US/tech/tk331/tk897/technologies_design_guide09186a008007ca60.shtml
Rgds, Dan
