Cisco NM-4T : 4-Port Serial 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).
The NM-4T module in any network environment delivers very low price per port and provides higher performance than comparable third-party solutions. For instance, a small or medium-sized Internet service provider (ISP) with high serial density requirements may find this solution very cost-effective per subscriber.