Nimalraj,
The "speed" you are referring to is probably the bandwidth operational parameter of an interface. This does not have anything to do with the real transmission speed of a serial interface and must be manually configured to match the true speed.
On serial interface, the transmission speed can be configured using the clock rate command provided that the HWIC card is the DCE device. In most router-to-modem scenario, this is not the case as the router performs as a DTE device (among other things, it adapts to the clocking signal from the modem and it does not assert its own clocking on the line) while the modem, as a DCE device, provides the clocking. However, when connecting two routers back-to-back using a DCE/DTE serial cable, one of the routers will also provide the clocking of the serial link. The clock rate is given in bits per seconds (actually in Hertz but a single tick of clock allows each router to send a single bit of data, thus the clock rate directly translates to bps). If you want to achieve the 8 Mbps on your HWIC card then you should configure your interface using the command clock rate 8000000.
Please note that the maximal speed of the serial interface depends also on the actual router hardware. With older routers like 2600 series, the maximal supported speed is 4 Mbps although the HWIC card itself is capable of speeds up to 8 Mbps. In recent ISR series (x800, x900), this should not be an issue anymore.
Best regards,
Peter
