Hello Vinoth,
in your router there is a network module with some A/S serial interfaces.
These interfaces can be used as Asynchronous serial line.
This allows to connect the async line to a dialup modem. An Async interface has no clock signal and sends one octet at a time.
See the following link
http://www.cisco.com/en/US/products/hw/routers/ps274/products_tech_note09186a00800a93f1.shtml
On platforms like C3640 the async number tells in what slot the network module is.
Hope to help
Giuseppe
