Hi Farhan
PRI is supported on the 2811
You can go for any one of the below modules
NM-1CE1T1-PRI 1-port Channelized E1/T1/ISDN-PRI network module
NM-2CE1T1-PRI 2-port Channelized E1/T1/ISDN-PRI network module
If you insist on a serial termination, you will have to use a TA which in most cases is recommended by your telecom service provider.
For serial connectivity you can use any one of the following:
Serial Connectivity Network Modules
NM-1HSSI 1-port High-Speed Serial Interface (HSSI) network module
NM-4A/S 4-port asynchronous/synchronous serial network module
NM-8A/S 8-port asynchronous/synchronous serial network module
NM-16A/S 16-port asynchronous/synchronous serial network nodule
NM-16A 16-port asynchronous serial network module
NM-32A 32-port asynchronous serial network module
The complete list is available on
http://www.cisco.com/en/US/products/ps5854/products_relevant_interfaces_and_modules.html
Now all you need to decide is how many ports do you need 🙂
HTH
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Regards
Arvind