Differentiate multiple USB devices having same Vendor and Product ID

Question

I am writing an application which allows a Linux machine to connect to internet over usb modem. I have multiple usb modems having same vendor and product ID; I cannot differentiate them using lsusb which gives vendor and product ID. Also giving IDs according to their usb ports does not solve my problem either, because I want to be able to use usb modems in whichever port I want.
Is there a unique identifier which I can use to differentiate my usb devices?

Answer 1

Using serial numbers I differetiated my usb modems. Here in this site basic commands are shown.

http://wiki.openmoko.org/wiki/Hardware:AT_Commands#Section_5.2C_General_Commands

I used "ATE+CGSN" command which gives serial number of the device.