The NM-1HSSI is a single port, high-speed network module that provides connectivity to a Wide Area Network. Demand for high capacity corporate backbones, high-speed Internet access, Virtual Private Networks (VPNs), and trunking connections for service provider internetworking has led to an increase demand for clear channel DS3 and E3 connections. Also see the NM-1T3/E3 clear channel T3/E3 Network Module

The NM-4T serial network module has four synchronous serial interfaces. The network module supports a total full-duplex throughput of 8 megabits per second (Mbps), which can be realized over one port (at 8 Mbps) or across all four ports (at 2 Mbps on each port).

You should checkpout following link.

I am focusing on TABLE-6.

http://www.cisco.com/en/US/products/hw/routers/ps274/products_data_sheet09186a0080091b8b.html

Both are important.. where you can use which depand upon scenario.

- Dharmesh Purohit